single to 3 phase vsd confusion.

[lather]

Confused about single to 3 phase vsd's.
a former electrician warned me against running a 5.5kw motor through a single phase system using a vsd, due to very high current spikes at start up.
These spikes, eventually would lead to cable degradation, also mentioned the cable coming from the power line would need a higher amp rating.
Is this correct! thought that's what they are used for.

[chambezio]

I have a rotary converter which gives me 415 Volt 3 phase from a 240 Volt supply through a 25AMP circuit breaker. I run 5.5 HP motor on my panel saw and 3 HP motors on a thicknesser and jointer. I have been running them since November and so far all seems to be well. No noticeable hot wires either on the 240 or 415 sides. I can't say that a VSD would be the same or not. Every time I use the machines I can't get over being able to run these industrial machines on a 240Volt supply. Its truly great!

[RayG]

Hi Lather,

5.5kw ( 7.5hp ) will need to be permanently wired by an electrician, the supply requirement exceeds power point limits. (similar to a large Aircon or stove )

Voltage spikes are what kills insulation, not current spikes, the circuit breaker for the VFD should be sized to handle the current spike on power up as the capacitors in the drive are charged. Provided the supply cable is sized for the length of the run and the peak currents, and protected by a suitable circuit breaker at the switchboard, you won't have a problem. Your electrician will choose the cable size and circuit breaker.

A vfd will produce voltage spikes on the outputs which depend on a whole host of factors, and especially if you have long cable runs between the vfd and the motor with no dv/dt filtering you might damage the motor insulation, but if your cable run is less than say 50' and it's a relatively new motor you won't have a problem.

One issue not mentioned, is that motors around that size (5hp and up) is where it swaps over from 415V star to 415V delta, if the motor in question is already wired in delta, you can't run it on 240V and get full output power

So before you start, check the motor. if it's 415 star and has 6 wires coming to the terminal strip, then you are in business.. rewire it for delta and run it on 240V 3 phase..

If it's already connected as delta, you have a problem... :)

Regards
Ray

[lather]

my mistake, it was 5.5 hp.

have not purchased the lathe so far, until working out the extra costs involved.
plan on getting a hafco cl-38 lathe.

[Type 28]

I think what he was refering to was large inrush current on bigger motors even though they kinder due to accel times they are not as bad as dol and this is more evident when a single to three phase vfd is used as instead of spreading the load across 3 phase you must supply the current through only 1 phase . To do that you would need seperate circuit with large conductors similar to a stove.

[lather]

thanks for the replies.
the cl-38 most likely does have a 2 speed motor as it has a 2 stage switch similar to the al-1000d

this could be a stupid /confusing question, though could decide if it's worth purchasing the lathe, due to power costs on the increase.
would the larger lathe with a 5.5 hp motor use much more power, compared to a smaller lathe with 2hp if taking the same depth of cut, using the same feed rate.
it's obvious that the 5.5 hp motor will use more power due to more friction e.t.c., on top of the power needed to perform the cut, though overall would the power usage difference be negligible ?

is there any improvement if reducing the max spindle speed 50% by changing the pulleys,( don't use the high speeds),
in regards to reducing the max current running through the cable ?
power to the shed is through a 15 amp cable.

should the vsd have a higher output than the motor rating?

[Keith_W]

Hi lather,
The CL-38A has a 4kW Motor so you would need at least a 50A supply so the cable to the VSD and from the VSD to the Lathe Motor the cable would need to be at least 6mm2 and the MCCB would be 50A.
In setting up my Workshop I took these factors into consideration and ran the 6mm2 Cable, in your post you mentioned that you had a 15A supply which would mean that at best you have 20A Cable.
I suggest that if possible you should look at a new power run to your Workshop to accomodate the larger Load that your equipment is going to need.
If the Motor on the Lathe is Two Speed you need to look at how it is wired as it could be a Star/Delta 415 configeration not suitable for a 240 to 415 VSD.

Regards,
Keith.

[jack620]

Hi lather,
The CL-38A has a 4kW Motor so you would need at least a 50A supply...

Are you sure about that Keith? Seems a huge amount of capacity for a 4kW motor.

[Keith_W]

Hi jack620,
The attached pdf's has the kW Rateings, MCCB Size and Cable Size for a 4kW Motor.
The VSD size I used as an example was a SINUS M 0011 2ST which is a Single Phase in Three Phase out model.

Regards,
Keith.

[lather]

having to rewire power to the shed, plus the cost of at least a 4kw vsd could go over my budget.
looks like the best best option is to get a smaller lathe.

[Graziano]

If the lathe has a dual speed motor then the slower speed will be half the horsepower of the high speed, so running your 5.5Hp lathe at the slower speed will give 2.75Hp. This is just one of those things for any multi speed AC motor, the slower speed gives less Hp.

Three phase motors are much more efficient than single phase motors, single phase units draw close to their full rated power even when there is no mechanical load on the motor. A three phase motor current draw varies with the mechanical load, so if you aren't doing 5.5Hp worth of cutting then the motor won't draw 5.5Hp. There's nothing to stop you setting up the VFD for a slow start to avoid huge start up current and also setting up an upper current limit for the motor to effectively derate the motor.

All that would be left is the short demand for current on powering the VFD up to charge it's capacitors up.

[Pete F]

The former electrician is talking out his watsit when saying inrush current will kill the insulation, unless the wire is so tragically underrated it was getting very hot in the process. That being the case, you'd have bigger things to worry about ... your workshop possibly burning down being one of them! Further more, the VFD effectively eliminates inrush current anyway.

He was quite correct however in that you simply wouldn't run a motor this size off single phase. If you wanted the many benefits of using a VFD, then you would use a 3 phase VFD.

With regards the power consumption difference, unless you're planning on spending 18 hours a day 7 days a week standing at this manual lathe, I wouldn't worry about the difference in power consumption between a 2 or 4 KW motor doing otherwise identical work, the difference compared to all the other costs will be peanuts.

That's a pretty large lathe, what are you planning on doing with it?

Pete

[lather]

Pete,
the lathe would actually be too large, there's not much choice here in Perth.
the lathe in the site below would be preferable, as it's a step up from the models with a 40mm bore, with a wider bed and in between
the H&F range from the al-1000c to the cl-38.

Engine Lathe (CQ6240X1000) - China Engine Lathe,Lathe Machine,Turning Lathe in Machine Tools (http://metalmaster.en.made-in-china.com/product/uejmGJXoZNck/China-Engine-Lathe-CQ6240X1000-.html)

H&F do sell the al-356v with a 51mm bore, though it has disadvantages over the al-1000c as it has variable speed drive with a 2 speed gearbox, smaller width of bed, and travel of the top slide is only 68mm, e.t.c.
don't see the point of taking 1 step forward and many steps back.

anyone know if the CQ6240X1000 is sold over east ?, shipping it here would be cheaper than setting up the cl-38

[RayG]

having to rewire power to the shed, plus the cost of at least a 4kw vsd could go over my budget.
looks like the best best option is to get a smaller lathe.


Hi Lather,

Just some clarifications...

As Graziano already pointed out.. If you cranked up your 5.5hp lathe with no load, you'd find the current is nowhere near the full load rated current.

The circuit breaker supplying the power to the shed will have been sized to protect the cabling, so all that will happen is the breaker supplying your shed will trip when you load it down. Nothing is going to catch fire and burn your shed down... :)

However, I'd suggest that you plan to upgrade your mains power at some future date. 15amps is nowhere near enough..

As to the question of what is the maximum size for a 240V single phase to 240V 3 phase vfd, A number of VFD manufacturers offer single phase 240V input to 240V three phase out up to 10hp (7.5kw) beyond that you generally need to go to 415 3 phase

No forgetting that above 15A it needs to be permanently wired (in Victoria at least) I think NSW you can go to 20A. After that you need a permanently wired connection.

As an example of what you would need, here's my wiring for the surface grinder, which has 3hp spindle motor, and 2hp hydraulic pump motor, 1hp power cross feed and 1hp power elevation

The circuit is supplied from the sub board via a 40amp RCD, through the isolation switch you can see on the wall above the enclosure, the main spindle vfd is 4kw, the others are all 1.5kw

http://www.backsaw.net/pics/Misc/DSCN1919.JPG

In addition there are circuit breakers for each vfd inside the enclosure.

When the spindle is running it rarely gets above a few amps.. no load current as shown on the vfd display is usually about 0.7 0.8 amps..

Regards
Ray

[lather]

ray,
that's a neat setup.

Will be checking out the eastern states stores before selecting a lathe.
The cl-38 is too large and the only reason to purchase it is for the larger bore.

if i get desperate, may just purchase the al-1000c, though i doubt there would be much of a difference from the al-960b, apart from the price.

[Pete F]

The circuit breaker supplying the power to the shed will have been sized to protect the cabling, so all that will happen is the breaker supplying your shed will trip when you load it down. Nothing is going to catch fire and burn your shed down... :)

Ray, that's quite right. In theory. However it has been known for people to illegally replace breakers for much larger capacity ones to "upgrade" the circuit. Also the rating of the cabling may change depending on things like insulation etc being added. I can't recall how much difference this actually makes, as it's been years since I worked in this area, and this was never really my field anyway. However vaguely recall the ratings change IIRC. Nevertheless, yes sorry if my comments about burning the workshop down muddied the water, it was a tongue in cheek comment to mean that if the sparky was suggesting the inrush current was going to damage the insulation, I reckon you've got bigger problems on your hands :D

Pete

[lather]

had been reading stustoys vsd thread, which lead me to ask,
can a 4kw lathe be run using a 3kw vsd with out damaging the vsd or motor, with the only negative being reduced torque ?

Graziano, thanks for clearing up the difference between the efficiency of a 3 phase compared to a single phase.
never knew that a single phase uses close to it's rating when not under load.
would it be correct to state that the 3 phase would be cheaper to run when taking slow feed, light finishing cuts.

[Stustoys]

If the lathe has a dual speed motor then the slower speed will be half the horsepower of the high speed, so running your 5.5Hp lathe at the slower speed will give 2.75Hp. This is just one of those things for any multi speed AC motor, the slower speed gives less Hp.
Are you sure about this? Sure you are turning at half the rpm but you have twice as many poles therefore (not inculding any loses) twice the torque??



Three phase motors are much more efficient than single phase motors, single phase units draw close to their full rated power even when there is no mechanical load on the motor. A three phase motor current draw varies with the mechanical load, so if you aren't doing 5.5Hp worth of cutting then the motor won't draw 5.5Hp.
Are you sure about this? The only way I can see it being correct is it the PF goes out the window at low load(which I think it does??). But you don't pay for bad PF on domestic supply so you are using less power. Either that or the motor would have to turn the power that wasnt being used into heat, this would turn a 2hp motor into a nice heater with no load.

What am I missing this time?


can a 4kw lathe be run using a 3kw vsd with out damaging the vsd or motor, with the only negative being reduced torque ?

Well the short answer would seem to be yes, BUT(dont you just hate buts) I'm not sure you'd be able to run even a 3kW VSD on a 15amp supply.

Stuart

[Greg Q]

I'm struggling to see the merits of a 5.5 hp motor in a domestic hobby lathe. Sure, bigger is almost always better, but a three h.p motor running from a (cheaper, and more widely available single phase input vfd) would surely provide the speed range and power you'd need? How big of a DOC are you hoping for?

Used motors in the 2.2 kW size are commonly sold for less than $50.

The common wisdom is that undersizing a vfd is poor practice which can lead to harm to the vfd. This last piece of advice is offered second hand-only what I have read on the internet from possibly equally ignorant sources.

Greg

[Pete F]

I agree Greg, there's a big difference between what you may be able to get away with and what is recommended both by industry practice and the drive manufacturers themselves. Once stepping beyond those boundaries I think it's fair to say that what may work in one case, may not work in another case, yet because it worked once it becomes internet fact. :wink:

One fact that is indisputable however is that the recommended and normal practice is to match drive rating with motor rating, and wire the motor in the appropriate configuration for that drive. When purchased used, 3 phase motors (of the size anyone here is likely to be using) are cheap, as are the drives. Converting from Star to Delta is normally a complete non-event, but for those not confident in doing so, is very inexpensive to have done by someone else. With all that in mind, I honestly can't see any good reason not to use the drives as they were designed to be used. I have a total of 5 VFDs now, and wouldn't go any other way, however I always configure them exactly as they were intended.

Pete

[Stustoys]

Hi Greg,
Just to be clean(maybe someone will come along and read this in 5 years lol), I was using this situation as an example for an idea I had. I was talking about running an "undersized" 240V VSD on a motor still wired for 415V(star if its dual voltage, delta if its 415V start/run). Sometimes changing motors is a big issue.

Stuart

[Stustoys]

Hi Pete,
Changing the motor isn't always an option. Above 3hp the motor may be 415V start/run so digging out the star point(well ok you wouldn't have to dig, it would be out already) wont do you any good. So a full rewire would be in order*(I believe?). Now if you are only going to end up with a 2.2kW motor after a $500 rewind, why not just leave the windings as they are and run it on the 2.2kW VSD? The drive rating and the motor rating will be matched(at least when its running, not so sure about starting), as by running the motor on 240V you have derated it. The VSD should be more than able to look after itself, although I fail to see why it would have to(although I'm sure there would be a point where that no longer applied).

Stuart

*A 415V VSD isnt the answer as we want to limit the power anyway as anything above 2.2kW(?) would need to be hard wired.

[lather]

more confused now then at the beginning of the thread.

Greg, your correct about the lathe being too large as a hobby lathe.
trying to step up from a 40mm bore, problem is that it's difficult to find a lathe that's a mid step up from the 40mm bore lathe up to the cl-38.

tried to get the motor details by talking with a H&F electrician, due to being on the road this will have to wait, the only details supplied is it's a 2 speed motor.
the motor sits inside the stand so may not be worth going in to check the motor details as the plate may not be visible.

the electrician was helpful with a few issues (that may have been mentioned in the thread), when running from a 240v supply.
queried about just using a phase converter, the response was that at times there were problems in trying to get a 2 speed 3 phase motor into 2nd speed,
though works great for a single speed 3 phase motor.
the lathe can draw up to 40 amps depending what gear is used at start up, and since it takes longer to reach running speed would need the higher amp cable.
if using a vsd, the inrush current at start up can be reduced by setting the vsd on a low setting then slowly speed it up.
This is not practical, don't vsd's have a built in system that can be preset to automatically perform the same task ?

[Pete F]

Hey Stuart, sorry mate you completely lost me after you said sometimes motors can't be changed. I agree, as I recall Ken was in that position. Personally I'm a bit reluctant to swap motors myself anyway, I just like to keep things standard, but that's very much just me. I was a relatively early adopter of using VFDs for these purposes, and at the time everyone was telling me I was mad, and why not simply change the 3 phase motor for a cheap single phase! The answer is that these things are motor drives, and the fact they (sometimes) can be configured to convert single to 3 phase is simply a coincidence of how they operate. It was these advantages that I was wanting, something simply not available by swapping the motor.

Anyway that's all an aside. In the examples you gave, if you're wanting to run a 5.5 KW motor, surely you'd be powering it off a 3 phase supply? In which case just use a 3 phase VFD. Off single phase at full rating how much current would it be drawing? There is a limit to the maximum current appliance that can be connected to a single phase, as dictated by the relevant supplier, however I have absolutely no clue of the code in that circumstance, and will possibly vary depending on where you live anyway?

I have absolutely zero experience with very big 3 phase motors, all the 415V motors I've seen, even if single voltage, were configured in a conventional star arrangement and started with conventional motor starters, the star point was in there somewhere, it was just a matter of going in and digging around to find it. Normally it's relatively obvious, but I did have one "problem child" who took a bit of convincing to give it up :D If it could otherwise run off 415 V, I'm having difficulty in imagining a motor that would require a complete $500 rewind simply to be configured in delta.

My professional experience in this area was at the opposite end of the scale, very small but super precise motor drives driving weeny motors but super, SUPER accurately. Indeed the driver part of it wasn't even called a VFD! So big-#### motors are definitely something I'm not really qualified to talk about. However one thing I'll say is to not presume the drive is indestructible. What needs to be remembered is that it forms only one half of the drive system, and the winding to which it is connected forms the other half. The drive expects to see a coil of copper of a certain value behave in a certain way, start screwing around with that basic presumption too much and as far as the drive manufacturer is concerned, you're on your own. Switching a motor out of circuit for example is a bad, BAD idea. Not only does the drive suddenly have his partner MIA, but I can imagine a nasty back EMF spike off a big motor especially. However see above as to my expertise in this area, but I sure as heck wouldn't ever do it.

Pete

[Pete F]

if using a vsd, the inrush current at start up can be reduced by setting the vsd on a low setting then slowly speed it up.
This is not practical, don't vsd's have a built in system that can be preset to automatically perform the same task ?

Yes very much so, the soft-start function of the drive is one of its big advantages. For example I have my surface grinder on a very long start cycle. This is a very small 0.75 kw spindle motor, so not for the same reason you're looking for, but in my case it's to prevent the grinding wheel been spun on the arbour. Likewise when stopping it's programmed to spin down slowly. While my application is very different to yours, the end result is identical.

Pete

[RayG]

more confused now then at the beginning of the thread.



Hi Lather,

Can't say I'm surprised, it seems we have two threads getting intertwined.. a rope? :)

Just so we are all on the same page.. we are talking V/F drives, not Vector drives
.
Now read on...

A 415V motor run at half voltage will give half the power, and in consequence you can use a smaller rated VFD. Sometimes this is a useful way of doing things, for any number of reasons, so in this instance the VFD is not undersized for the job it's being asked to do.

Whether you are happy running a 10hp lathe at 5hp, that's another different question.

It's not a case of it worked for someone somewhere once and it becomes internet fact, it's ohms law.. (at least last time I checked) :rolleyes:

Stuart's got it correct :2tsup:

Regards
Ray

[Pete F]

It's not a case of it worked for someone somewhere once and it becomes internet fact, it's ohms law.. (at least last time I checked) :rolleyes:

Regards
Ray

Ray, I'll spare the eye rolls, but just for the record check again, a motor is not a resistor, they are in fact inductive reactance loads, particularly on starting. Simple ohm's law calculations do not necessarily apply.

Pete

[RayG]

Hi Pete,

Taint so, the fact that it's a complex impedance and sine/cosine voltages and currents doesn't change the fact that if you reduce the voltage then you reduce the available power.

Regards
Ray

[lather]

planned to purchase a larger lathe when starting this thread, though when reading some of the replies it's made me realize that a lathe of that size is not realistic
for hobby use.
need to weigh up my options and determine if the extra benefits of a larger lathe, is worth the extra cost.

if deciding to purchase the lathe, it's no problem running the lathe at half the kw rating, as long as it performs a similar DOC of a 1.5kw single phase lathe.
don't intend to run the lathe on the high speeds, the motor will only ever be used on the lower setting.

ray, much of the thread is lost to me,
will need to do a little research about vsd's to understand it.
most likely it's a V/F drive, unsure what a vector drive is, so whichever drive is suitable for the situation at a reasonable price.

[NCArcher]

Hi Pete,

Taint so, the fact that it's a complex impedance and sine/cosine voltages and currents doesn't change the fact that if you reduce the voltage then you reduce the available power.

Regards
Ray

Or increase the current draw while maintaining the power output. :D
Does the lathe need a 5.5HP motor? Can it be ordered with a 3 HP motor as an option?
I know nothing about lathes. :no:

[Pete F]

Or increase the current draw while maintaining the power output. :D

Indeed, ha ha :D I thought with all the eye rolling and misquoting going on, a pedantic correction was possibly in order :p While the end result is appropriate, it sure aint derived from that law :wink:

Excellent point about the motor btw. If you've decided a smaller lathe is needed because of the motor size, why not just get a smaller motor. Is this a production lathe where DOC is critical?

Pete

[RayG]

Or increase the current draw while maintaining the power output. :D


Funnily enough that's exactly what happens when you change from star to delta... :rolleyes:

Regards
Ray

PS... and yes Pete Ohms law still applies...

[PDW]

I hate to bring this up yet again, but why don't you just get 415V 3 phase power and run whatever you want? I've never found it that big a deal, and I've done it 3 times for 3 different houses.

As for star/delta, Pete F must be playing with different motors to mine as all mine are 3 phase 415V and don't have *anything* except 4 wires going in. I've had a look in some of the control boxes, no jumpers to be rearranged. Talking of starters on a 3 phase motor lost me totally I'm afraid, I've never seen such a thing. The 3 phase motors I'm familiar with are straight induction/repulsion devices and start due to the phase shift between phases. That's what makes them so useful for hard starting devices like pumps, air compressors etc etc.

So I too am confused.....

As for lathe size I see nothing wrong with a machine that big for a hobby machine and I'm sure RC doesn't either. Big spindle bores are useful, I wish my Monarch had a 2" bore instead of the 1.5" it does have. Almost bought a Colchester once simply because it had a 3" bore but fortunately I got over it.

On that subject the Nuttall lathes all the NSW TAFE colleges used to have were short bed machines with a 2" bore, camlock chuck mount, clutch drive so you didn't have to start/stop the motor and a pretty good speed range. One of those not flogged to death would be a very nice machine for a home shop; they come up on Ebay every so often. I've got an inch leadscrew & split nuts for one floating about the place somewhere, gift from a TAFE teacher when they converted all the machines to metric. What a c-f that must have been.....

PDW

[Greg Q]

Just to be clear, I don't think hobbyists with the space and power should limit themselves at all. But with a 15A single phase service I think you have to accept some limitations. I agree that getting 3 phase is the answer...a15A service is too small for either a big VFD (and they get crazy expensive for single phase input/ high hp output) or an RPC.

I had a five h.p. RPC powering my woodworking gear in the last house. I had to get a 30A circuit installed for reliable starting of the idler.

When we shift south I hope there will be a chunky lathe in the container too.

Greg

[Graziano]

Are you sure about this? Sure you are turning at half the rpm but you have twice as many poles therefore (not inculding any loses) twice the torque??


Yes I'm sure, take a look at the data plate for the next AC two speed motor you see, there'll be two HP ratings, e.g. my Colchester Bantam motor is 2Hp/1Hp. The lower the RPM of the AC motor the more iron you need to prevent magnetic saturation of the stator and rotor, a 720 RPM motor of a given HP will need 4 times the cross sectional area of the magnetic material in the rotor as a 2880 rpm motor of the same HP as the field rotates at 1/4 the speed.

So if you halve the rpm of a given motor, all things being equal you have to derate it by half at the same time. Power transformers operate under the exact same constraint: if you halve the frequency of a transformer you have to halve the VA rating.




Are you sure about this? The only way I can see it being correct is it the PF goes out the window at low load(which I think it does??). But you don't pay for bad PF on domestic supply so you are using less power. Either that or the motor would have to turn the power that wasnt being used into heat, this would turn a 2hp motor into a nice heater with no load.

What am I missing this time?

Stuart

A single phase motor under no load has poor efficiency compared to full load, in other words load or no load it sucks the same power, I could be wrong but I understand that under no load one winding fights the other winding to keep the RPM constant. An example I have are some Ebay single phase 2Hp motors, under no load they consume 6.8A current which works out to 1632 Watts. This is under no load for a motor that should use 1500W when loaded.

[Pete F]

Ray I'm not going to get into an argument with you about this. If you feel a 3 phase motor behaves strictly according to a simplistic AC version of Ohm's Law, I'll leave you with it. I post to the forum to try to help others if I feel it may be of some value, and don't see the forum as a contest. If you wish to continue off-line and put your case as to why motor load, power factor, motor speed, and efficiency are apparently not important in your calculations then I guess that would be ok. But in the meanwhile I don't feel continuing is in the interests of anyone here.

Peter, all the motors I have received have been single voltage star configuration. Exactly as you described. I needed to dig out the star point. As far as the existing wiring, I honestly couldn't tell you, as I simply ripped it all out and replaced it, literally without taking any notice. I have another sitting here to do so will take more note this time :D

I have 3 phase on my place here, but we only use 2 phases. You're quite right, I could use the extra phase, but I think I told you the story when you were around here, they wanted a fortune to convert the meter box since it had asbestos in it :doh: Besides, as I mentioned, VFDs provide many advantages, and 3 phase conversion is just one of them. I've heard that some suppliers want a fortune to connect 3 phase to a house, some people are on SWER lines, etc etc. I agree though, 3 phase in a house would be the way to go, however I would still want my machines controlled by VFDs.

Pete

[Stustoys]

Yes I'm sure, take a look at the data plate for the next AC two speed motor you see, there'll be two HP ratings, e.g. my Colchester Bantam motor is 2Hp/1Hp. The lower the RPM of the AC motor the more iron you need to prevent magnetic saturation of the stator and rotor, a 720 RPM motor of a given HP will need 4 times the cross sectional area of the magnetic material in the rotor as a 2880 rpm motor of the same HP as the field rotates at 1/4 the speed.

So if you halve the rpm of a given motor, all things being equal you have to derate it by half at the same time. Power transformers operate under the exact same constraint: if you halve the frequency of a transformer you have to halve the VA rating.

But all things arent equal, you havent halved the freq, you've halved the rpm by doubling the number of poles.

Here is a 4 speed motor The slowest speed about a 1/6th of the highest speed but its
0.9,0.9,0.9,0.9
http://www.woodworkforums.com/attachments/f65/199141d1329957017-urgent-help-please-drill-vsd-img_8036.jpg

Though the current has gone up a little

:C no idea, just wondering and I likely wont understand the answer.



A single phase motor under no load has poor efficiency compared to full load, in other words load or no load it sucks the same power, I could be wrong but I understand that under no load one winding fights the other winding to keep the RPM constant. An example I have are some Ebay single phase 2Hp motors, under no load they consume 6.8A current which works out to 1632 Watts. This is under no load for a motor that should use 1500W when loaded.
Is that true power or apparent power?
Surely at 1632W something would have to be getting hot if it is true power.
Again........ no idea.

Ok Ray, how about a 5.5kW 415V start/run(no idea of their correct name) motor on a 240V VSD in star? then you'd only need 1.5kW?

Stuart

[.RC.]

planned to purchase a larger lathe when starting this thread, though when reading some of the replies it's made me realize that a lathe of that size is not realistic
for hobby use.
need to weigh up my options and determine if the extra benefits of a larger lathe, is worth the extra cost.



What do you plan to do with it?

Here is a nice looking 14" swing 40 inch centres lathe weight 1440kg with a 3hp 3phase motor... Might be out of your price range though
http://www.assetplant.com/epages/shop.sf/en_AU/?ObjectPath=/Shops/shop/Products/12-0110
No idea what the lathe is like but the design looks OK...

[Graziano]

But all things arent equal, you havent halved the freq, you've halved the rpm by doubling the number of poles.



For an AC induction motor, the rotor sees a rotating magnetic field at half the normal speed, i.e. half the frequency.



Here is a 4 speed motor The slowest speed about a 1/6th of the highest speed but its
0.9,0.9,0.9,0.9
http://www.woodworkforums.com/attachments/f65/199141d1329957017-urgent-help-please-drill-vsd-img_8036.jpg

Though the current has gone up a little

:C no idea, just wondering and I likely wont understand the answer.


No idea here either, there's more than one type of AC motor out there.





Is that true power or apparent power?
Surely at 1632W something would have to be getting hot if it is true power.
Again........ no idea.


It would be the same temperature as the fully load motor as the power consumed is the roughly the same, just one is doing no work and one is doing work at rated power.

I'm pretty sure the no load efficiency of a single phase induction motor is 0%. I'll see if I can find a reference somewhere about it.

[PDW]

I have 3 phase on my place here, but we only use 2 phases. You're quite right, I could use the extra phase, but I think I told you the story when you were around here, they wanted a fortune to convert the meter box since it had asbestos in it :doh: Besides, as I mentioned, VFDs provide many advantages, and 3 phase conversion is just one of them. I've heard that some suppliers want a fortune to connect 3 phase to a house, some people are on SWER lines, etc etc. I agree though, 3 phase in a house would be the way to go, however I would still want my machines controlled by VFDs.

Pete

I had to replace the entire meter box at my Sydney house. That was back in 1986 so I don't remember the cost, wasn't too bad, maybe $1000 including circuit breakers and running cable to the small workshop. VFD's weren't really an option back then so you ran 415V or you forgot about buying big machinery. I was running a big stick welder, still have it in fact as you can't kill those things even if you use a brick. Running 6mm iron powder rods makes short work of welding stuff together.

Quite agree on the utility of VFD's. I have 3 myself, 415-415 units though. Was looking at the single phase ones but every motor I have here can't be jumpered and I have insufficient need to dig around in them. Pity because I'd like to put a 3 phase motor & VFD on a drill press.

Back on lathes, swapping the motor out for a smaller one would work fine provided it wasn't built in & impossible to extract. Also lathes with 2 speed motors would be better avoided. A lathe with clutch would also be a better choice as you could bring the spindle up to speed in high gear more gently. One of those ex-TAFE Nuttalls would be a good choice IMO unless you needed more distance between centres.

That lathe RC linked to is only 40mm bore so no advantage.

PDW

[PDW]

Just to be clear, I don't think hobbyists with the space and power should limit themselves at all. But with a 15A single phase service I think you have to accept some limitations. I agree that getting 3 phase is the answer...a15A service is too small for either a big VFD (and they get crazy expensive for single phase input/ high hp output) or an RPC.

I had a five h.p. RPC powering my woodworking gear in the last house. I had to get a 30A circuit installed for reliable starting of the idler.

When we shift south I hope there will be a chunky lathe in the container too.

Greg

I guess it comes down to how difficult it is to run another cable from an additional CB on the main board. There shouldn't be a problem getting 32A because that's what a stove uses.

My standard practice is to run a fat multi-core cable to a sub-board and pick off the circuits I need from there. It's tidier and gives better control over what's on and what's off. 6mm 5 core cable was approx $7/m last time I bought some.

Bring a big lathe with you because you're not likely to find one here. That said I did come across a nice condition short bed (30" BC) Monarch CY with TA in Launceston once. Passed on it because I was still building & had nowhere to put it. Later I imported my 54" bed CY from Wollongong, cost more in freight than I paid for the machine!

PDW

[Stustoys]

It would be the same temperature as the fully load motor as the power consumed is the roughly the same, just one is doing no work and one is doing work at rated power.

I'd thought about that, but at full load isnt most of the power is going to the spindle and being turned into heat there? The heat in the motor is some % of the input(depending on the efficiency of the motor)?

Stuart

[Pete F]

Quite agree on the utility of VFD's. I have 3 myself, 415-415 units though. Was looking at the single phase ones but every motor I have here can't be jumpered and I have insufficient need to dig around in them. Pity because I'd like to put a 3 phase motor & VFD on a drill press.


PDW

Peter I think you're right and I should simply have the 3 phase connected and be done with it. The house has only 2 power circuits and too many GPOs on them, so it really needs another circuit for that. I think your idea of a sub-board for the workshop would be good too.

Honestly, on most motors I've seen pulling out the star point is no big deal. They have to have the windings connected when made, so you're simply "unmaking" a bit :D Next time you're over I don't mind pulling a motor off a machine and showing you what to look for, or if I have a motor that needs doing I'll do it while you're here. As I said, 30 mins is normally it it takes so while Anna gets the bickies ready for coffee we'll do the motor, heck bring your motor over and we'll do that one if you like! Otherwise any motor rewinding service down your way will do it quite cheaply.

Pete

Edit: Oh just regarding the 32A cable etc. As I mentioned, just be aware that I believe at least some power authorities won't let you throw any old thing on just one of their phases. Ranges etc are an exception and I believe have special rules, but this was never an area I worked in, so may not be quite right. I do know they get a bit funny about phase balancing though.

Edit: #2 Ah ok, here's an example of what I was quacking on about. Have a look at page 11 http://www.commerce.wa.gov.au/energysafety/PDF/Publications/WA_Electrical_Requirements.pdf I know that's for WA, and nobody lives there, but it's likely to be the similar in the civilised world. ....jooookkkiiinnnggg

[RayG]

So if you halve the rpm of a given motor, all things being equal you have to derate it by half at the same time. Power transformers operate under the exact same constraint: if you halve the frequency of a transformer you have to halve the VA rating.


Hi Graziano, with multipole switching it also depends on how you do the switching, you can do constant torque or constant horsepower.

If you are interested I could dig out some reference books and scan the relevant pages.


Hi PDW,
I suspect PeteF was talking about soft starters, the supply authority sets a limit of what size motor you can start dol, I think above 15-22 kw you will find soft starters are the preferred option these days.

Hi PeteF,
No problem, I'm always happy to help...

Regards
Ray

PS Stuart, efficiency of cap start single phase motors is something I vaguely remember... I'll see what I can find.

[Pete F]

Hi PDW,
I suspect PeteF was talking about soft starters, the supply authority sets a limit of what size motor you can start dol, I think above 15-22 kw you will find soft starters are the preferred option these days.


Ah no, actually I wasn't, I was talking about the maximum single phase appliance load. However the suppliers also specify a maximum inrush current, and that would by default mean a soft-start on a big motor ... well unless it was one of those Ohm's Law motors :wink:

[Dave J]

Hi Ray, Stuart,
Thanks for the info as it was something I never knew could be done, but if Ray has his shaper hooked up that way it seems to work fine.
After talking to Lather over the past few months through pm's, it looks like this would be the way to go for a larger motor running off single phase power, when he doesn't need the full power of the lathe, only the spindle bore.

Dave

[lather]

RC.
basically half way through the process of building a prototype device, which through my hobby, had discovered a relationship that deals with force, balance and resistance,
won't say much more for obvious reasons.

The fact is that perhaps it was the experience with using my previous lathe, that has scared me off purchasing another lathe around a similar size.
no longer have the lathe due to defects causing premature wear and tear.
a DOC of 2mm (4mm o.d.) would do.

not having a lathe at this point is frustrating, and would purchase the al-1000c, though their out of stock.

wasted enough time in searching for a S/H lathe or thinking about importing a lathe and decided against it,
the main options at the moment, is either waiting for the al-1000c to arrive, or purchase the cl-38a.

prefer a cheaper lathe, though with limited choice, the al-1000c would be the only other option with a 40mm bore, noticing the price difference from that lathe to the cl-38a, makes the cl-38a look tempting.
now when adding up all the extra costs involved with the cl-38a, funds for the project would be reduced.

need to decide if the time the cl-38 saves in pissing about when performing certain jobs is worth it.

[Graziano]

I'd thought about that, but at full load isnt most of the power is going to the spindle and being turned into heat there? The heat in the motor is some % of the input(depending on the efficiency of the motor)?

Stuart

Motor manufacturer's seem to go to some trouble to cool your bog stock common, cap start, cap run, single phase motor with fans and fins on the side, or in the case of older motors blowing air through the windings.

[Graziano]

Hi Graziano, with multipole switching it also depends on how you do the switching, you can do constant torque or constant horsepower.

If you are interested I could dig out some reference books and scan the relevant pages.



Did you have a copy of "Electric Motor Repair" by Rosenthal?, that's about the only text I have access to at the moment.




Hi PDW,
I suspect PeteF was talking about soft starters, the supply authority sets a limit of what size motor you can start dol, I think above 15-22 kw you will find soft starters are the preferred option these days.


Doesn't a VFD function as a soft starter anyway with it's slow starting and programmable current limits?. At least a lathe won't need huge starting currents to get going compared with, say an air compressor.




PS Stuart, efficiency of cap start single phase motors is something I vaguely remember... I'll see what I can find.


I'd be interested also, as I can't find anything on the net that's specific to single phase induction motors.

[PDW]

Hi PDW,
I suspect PeteF was talking about soft starters, the supply authority sets a limit of what size motor you can start dol, I think above 15-22 kw you will find soft starters are the preferred option these days.

Yeah, that explains the big cabinet full of electronic thingies that came attached to my 25kW air compressor motor. The seller told me it ran off a 20A 3 phase supply. Fortunately I didn't believe him as the cabinet was full of stuff with 32A breakers when I had a look. I gave it its own dedicated 6mm line to the sub board. Needless to say I don't fire it up to put air in the wheelbarrow tyre.

PeteF, there are all sorts of rules :-) I do my best to balance load across the phases, never had any problems from the supply authorities. Using a 32A single phase line to feed a big single phase VFD *might* technically be breaking the rules but there are some big heat pumps/air conditioners out there fed via a dedicated line (in fact 1 good excuse for getting 3 phase installed is to tell them you want to but in a 3 phase air con plant so as to balance the load). I can't see that there is any difference. Main thing is to comply with *all* the rules governing wiring and over current protection. For anything over a domestic GPO, I typically run wire 1 size up from the standard ie 4mm for a dedicated 15A circuit, same for 20A and then 6mm for 32A. Wire & properly sized breakers are cheap in comparison with a fire.

PDW

[Pete F]

Peter, yes I agree. The trouble also is, as I understand it, the supplier themselves will dictate what can and can't be put on their supply, and under what conditions. The brochure I attached after posting that comment was simply the first that I could find to support the statement. I'm not suggesting that would apply everywhere, maybe just something members may want to look into before rushing out and buying an XYZ lathe etc with a big motor and simply presuming they will be allowed to drop it on a single phase. On that note, I must confess all the larger VFDs I've seen were 3 phase in anyway, maybe I just need to get out more and just haven't seen others, but I'm not sure if all this is a moot point.

Likewise I over-specify the wiring when I do it. As you say, the difference is peanuts when considering the overall cost of things.

That's one big compressor you have there! It's a shame Anna and the family have already done our "Tassie Experience" recently as I'd have loved to call in to see your "humble" workshop. :) Do you have any idea of the CFM of a compressor that size? I checked mine and it free-flows 12 cfm (I'd previously thought it was only 8!), but it's too small. I think I may have to be happy with 16 cfm and simply look on at others like yourself with air-to-spare with admiration. I do like air tools!

Pete

[jack620]

A single phase motor under no load has poor efficiency compared to full load, in other words load or no load it sucks the same power, I could be wrong but I understand that under no load one winding fights the other winding to keep the RPM constant. An example I have are some Ebay single phase 2Hp motors, under no load they consume 6.8A current which works out to 1632 Watts. This is under no load for a motor that should use 1500W when loaded.

I believe that when a single-phase motor is not loaded the power factor drops significantly. So, while the current drawn may be near full rating, it is significantly out of phase with the voltage waveform. Thus the "real power" consumed by the motor is quite low. As Stuart pointed out, if this were not the case the motor would basically become a heater and get very hot. The "reactive power" is stored in the magnetic field and released back to the power distribution system each cycle. This causes problems for the power supplier and is the reason why some single-phase motors have a 'run' capacitor to increase the PF.

How all of this affects VFD rating I have no idea!

[Pete F]

How all of this affects VFD rating I have no idea!

Surely you'd be running a 3 phase motor off the VFD?

[RayG]

I believe that when a single-phase motor is not loaded the power factor drops significantly. So, while the current drawn may be near full rating, it is significantly out of phase with the voltage waveform. Thus the "real power" consumed by the motor is quite low. As Stuart pointed out, if this were not the case the motor would basically become a heater and get very hot. The "reactive power" is stored in the magnetic field and released back to the power distribution system each cycle. This causes problems for the power supplier and is the reason why some single-phase motors have a 'run' capacitor to increase the PF.

How all of this affects VFD rating I have no idea!

Hi Jack,

Yes, that makes sense, and seperates the magnetizing current (no load current) from the torque producing current, a bit similar to vector drives (see attached ppt)


Hi Graziano,

I can't find anything in Rosenberg that relates directly to single phase induction motor efficiency, but the multipole switching is in table 3-143.

Here's a power point presentation on vector drives, the vector stuff is not strictly relevant to this discussion, but the slides relating to basic ac motor principles are.

The forum won't let me upload ppt as an attachment, so I've made it downloadable from here.

http://www.etech.net.au/TorqueProdACDrives.ppt

Regards
Ray

[jack620]

Pete F,
I don't understand your question.

Sorry, now I get it. Correct, you wouldn't be running a single phase motor off a vfd. What I meant to say was the single phase power discussion is unrelated to the vfd question. :)

[Pete F]

The forum won't let me upload ppt as an attachment, so I've made it downloadable from here.

http://www.etech.net.au/TorqueProdACDrives.ppt

Regards
Ray

Nice find Ray, it explains things very well.

Pete

[Graziano]

This causes problems for the power supplier and is the reason why some single-phase motors have a 'run' capacitor to increase the PF.


Isn't the run capacitor providing enough phase delay to get the motor running as a two phase motor?. I'm pretty sure it's to provide a higher torque motor than any concern about power factor, although they do have a higher power factor than a cap start only motor.



How all of this affects VFD rating I have no idea!

Before things went off topic into single phase motors, the original poster mentioned that the lathe likely had a two speed three phase motor. If that is the case then at the lower speed it's likely to be running at half the rated horsepower. It would save a lot of trouble if he just ran the lathe at the lower speed and used less current. I was also wrong about the current demand halving, apparently as the RPM decreases with the switch from two pole to four pole and higher the motor's efficiency drops off as well so the current doesn't decrease as expected.

I'm attaching a couple of three phase motor data plates that illustrate the loss of horsepower as the speed goes down. One is off a Bantam lathe and the other is off a Wadkin power saw motor.

http://www.woodworkforums.com/attachment.php?attachmentid=204001&stc=1&d=1333620181


http://www.woodworkforums.com/attachment.php?attachmentid=204002&stc=1&d=1333620186

[Pete F]

I was also wrong about the current demand halving, apparently as the RPM decreases with the switch from two pole to four pole and higher the motor's efficiency drops off as well so the current doesn't decrease as expected.

Hallelujah! Thank you :rolleyes:

[Graziano]

Hallelujah! Thank you :rolleyes:

Ugh, so instead of the 50% I originally claimed, it's a 33% reduction in current demand. If the original poster's lathe has a similar motor, that's still a significant reduction without any modification to the lathe.



Hi Graziano,

I can't find anything in Rosenberg that relates directly to single phase induction motor efficiency, but the multipole switching is in table 3-143.

Regards
Ray

Hi Ray, what I was looking for in Rosenberg's book yesterday was a table of expected current draws for single phase motors vs HP when under no load, if I find it I'll pass it on.

Cheers

[Pete F]

Correct, my comments were not especially directed at you. My previous point was that motors are actually quite complex little buggers from an electrical perspective, one reason being that they're interacting between the physical environment and the electrical environment, and the interaction is two way. Distilling that down to explain some things with simple, albeit inappropriate, formula may well be quite convenient, however as it turns out it's also quite wrong. But hey, what would I know :rolleyes:

Sorry for the confusion.

Pete

[Graziano]

Correct, my comments were not especially directed at you. My previous point was that motors are actually quite complex little buggers from an electrical perspective, one reason being that they're interacting between the physical environment and the electrical environment, and the interaction is two way. Distilling that down to explain some things with simple, albeit inappropriate, formula may well be quite convenient, however as it turns out it's also quite wrong. But hey, what would I know :rolleyes:

Sorry for the confusion.

Pete


My apologies too Pete, by now I should know not to make posts until the morning coffee has sunk in :doh: . BTW I was looking at an old "GMF Cadet" motor I'm using for my bench grinder spindle project: 1/2Hp output or 375W but it consumes 3.6A or 864W of power!. The thing runs hot even with a fan fitted but at least I'll only be using it for short periods, I guess efficiency was low on their list of priorities.

[RayG]

Hi Pete,

Just for the record, it's apples and oranges, Graziano was discussing the difference in current between 2 pole and 4 pole.

My comments were discussing the difference in current between star and delta..

I'm not entirely surprised you didn't pick up on that difference.

Regards
Ray

[lather]

received the call from the electrician regarding the cl-38a motor, though was unable to supply the details, the only other details supplied is that the
average amps are 6.4A / 3.3A.

[Pete F]

No worries Ray, as I'm sure you know full well know Ohm's Law is a law dealing with resistance, any bastardisation of it is not technically Ohm's Law at all, indeed virtually no part of Ohm's Law is in any way applicable to AC motors from any practical standpoint. I guess that's unremarkable given that the gentleman who gave his name to it died decades before AC current was even invented! Nevertheless I can appreciate that it is sometimes a convenient way to think about the topic for those not properly familiar with the subject.

Pete

[RayG]

No worries Ray, as I'm sure you know full well know Ohm's Law is a law dealing with resistance, any bastardisation of it is not technically Ohm's Law at all, indeed virtually no part of Ohm's Law is in any way applicable to AC motors from any practical standpoint. I guess that's unremarkable given that the gentleman who gave his name to it died decades before AC current was even invented! Nevertheless I can appreciate that it is sometimes a convenient way to think about the topic for those not properly familiar with the subject.

Pete

Wow!... stunning statements...

Ohms law applies equally to AC circuits just as much as DC circuits.. Interestingly Ohms Law also has a equivalent in magnetic circuits.

Here's a question, what do you understand by the term VA rating?, and how do you calculate it?

Regards
Ray

[Pete F]

G'day Ray, well I wouldn't say they're really very stunning statements at all, actually very basic electrical theory. I've said it above but will confirm it again, I'm not going to argue with you about this. This is Ohm's Law, as developed by Georg Ohm, V = I*R No more. No less. If you want to believe otherwise please feel free to do so.

As I mentioned, considering the problem in an overly simplistic manner is sometimes convenient for those who don't properly understand a subject, however the caveat in doing so is that while that inappropriate process may well provide a correct solution to that particular problem, it can lead to completely incorrect solutions to what, at face value, may appear quite similar problems. Indeed that has already happened in this thread.

Cheers,
Pete

*No eye rolls were harmed in the making of this post

[RayG]

Hi Pete,

I've pretty much stated the case, in as many ways as I can, for the reduced available power when running a motor off 240V when compared with 415V, and yes it's ohms law...

I don't see much point in continuing to feed your peculiar need to argue the point ad infinitum.

Good luck to you .:rolleyes:

Regards
Ray

[Pete F]

G'day Ray, I agree let's leave it at that, as I mentioned you can believe whatever you want, and I wouldn't let the fact that the rest of the world thinks something different worry you one little bit. :wink:

Happy Easter mate :2tsup:

Pete

[Stustoys]

G'day Ray, I agree let's leave it at that, as I mentioned you can believe whatever you want, and I wouldn't let the fact that the rest of the world thinks something different worry you one little bit. :wink:

Happy Easter mate :2tsup:

Pete

Some may say speaking for the rest of the world is a little over the top, you havent even asked them. At best its "the rest of the world minus one"

And a happy easter to you.

Stuart

[Dave J]

Pete,
Back in post 36 you stated,


Ray I'm not going to get into an argument with you about this. If you feel a 3 phase motor behaves strictly according to a simplistic AC version of Ohm's Law, I'll leave you with it. I post to the forum to try to help others if I feel it may be of some value, and don't see the forum as a contest.

We are all here to have fun and learn from each other, if you feel you don't understand something or think something maybe explained wrong, ask a question politely of how the member thinks it works, and I am sure any members will accommodate you in explaining things.

We really don't need any members to have condescending or sarcastic posts, or having to have the last say because someone thinks there idea is right and the other person is wrong, it just spoils the thread.
This is a great forum, lets not let it drop down to the level of some other forums.

Over the years I have always found Ray to be nothing but helpful and knowledgeable, even if it means going out of his way to help. From what I know of Ray, he is an electrics guru and still does contract work in the field.

I am sure I speak for many other members here, in that we are not here to ague with each other, or win contests. We are here to talk shop among fellow enthusiasts.

I learnt a lot in this thread, but sadly it will now probably not be added to.

Dave

[jack620]

But hang on Dave, the rest of the world agrees with Pete. :wink:

[Pete F]

Sorry Stuart, quite correct. Obviously I was speaking metaphorically to mean those who actually know something about the topic. I take it you'd like me to remove your name from the "rest of the world" list then? :wink:

Dave while it's good to see the clique back together, I would like to put this nonsense to bed once and for all, because personally I have better things to do with my time, and I'd be surprised if others don't too. Though obviously, as has been pointed out, I can't speak for others.

It would appear from the tone of responses that several days ago I may have said something that Ray has taken in quite a different manner to how it was intended, I have no idea how I pi$$ed in his porridge, but however, it was definitely not intentional. We had several similar threads running at the same time and quite a few crossings. I actually penned a lengthy PM to Ray to try to get to the bottom of it, however given the continuing tone of the responses simply thought "suit yourself" and deleted it.

Electronics is actually my trade Dave, and I then went on to study further until I (literally) ran out of courses to take at the Regency Park School of Electronic Engineering back when I lived in Adelaide. While electronics and electrical are really somewhat different fields, and VERY different trades, the electrical theory is of course the same, and I also found myself having to cross over into electrical when I worked in the marine field. Nevertheless on this board it would be Ray who I would turn to to fill in the very considerable gaps in my knowledge, as that was a long time ago and I have forgotten a huge amount in that time. Indeed I literally did turn to him on a number of occasions within these threads where I thought he may be more up to speed with the electrical code. I would never, ever , and will never hold myself up as a "guru" in any subject, least of all this one. I don't mention the above to "big-time" myself, that was merely my education and occupation, seemingly a lifetime ago, before a very radical career change, self-funded incidentally by working my guts out in that field.

Within the numerous threads I made a general comment, not really aimed at anything or definitely not anyone, that just because someone gets something to work in one situation, it doesn't mean it will work in another situation ie the case that it worked once becomes an internet fact. What I had in mind was that some of the configurations being suggested for VFDs within these threads seemed to be straying further and further from the manufacturer's recommended practices. That doesn't mean they won't work, but as a rule I try to K.I.S.S; ie do what the manufacturers' tell you, because the chances are they know a little about what they designed. The trouble is that somebody else comes along with a different manufacturer's product and it may not work in the same way.

So, what happened? Well I happen to agree with you Dave, and I don't feel sarcastic and condescending posts for absolutely no reason have a place here. I personally found Ray's post #26 condescendingly written to the board members here. Maybe it wasn't intended that way, I don't know, I frankly don't care. What I did care about was that it misquoted what I said above, complete with sarcastic "smilie". Now as you, of all people, know Dave, if you want to take a swing my way, that's fine, but as you know expect me to swing back! Because quite unfortunately it contained a number of errors, the most obvious being the reference to Ohm's Law. Now I knew what Ray meant. I think he knew what he meant, however as it stands that statement is wrong. I really don't care what anyone wants to believe, it is wrong, end of story. Georg Ohm's Law deals with resistance, yes there are other similar laws, but they are not Ohm's Law. Joule's Law for example was thrown in there for good measure a few posts ago (#65), for God knows what reason. Now the big problem with making incorrect statements about laws is that people then apply them in other situations that seem to be similar but are in fact not. Normally I wouldn't have said anything, but given the circumstances, intentional or not, of the context in which it was written I pointed that error out. Indeed it can be seen quite clearly that continuing to turn to Ohm's Law when it is not appropriate can lead to all sorts of trouble, as quite correctly pointed out by member "Graziano".

Now if in fact the big problem here is that I'm stepping on some self-proclaimed guru's hallowed ground, then I'm more than happy to simply not post from now on whenever I see a person with a problem regarding any electrical or electronic issue. At the moment I am frankly flat-out with work (working all Easter of course) and given that a number of my posts were intended to try to help you Stuart, I'm more than happy to spend my very valuable time on more enjoyable pursuits! I didn't join a machinists board to talk about electrical or electronics, but if I'm able to draw on almost forgotten experience to help somebody, especially join in with other members who have some knowledge of the field and collectively we can work together to solve somebody's problem, them I'm more than happy to give up that time and do the best I can.

It's always a shame to see the same old familiar names pop up the instant they smell some possible blood in the water and try to drag others down to their apparent level. But I should by now have learnt to expect nothing more :(( Personally I'd put it behind and moved on, indeed going as far as encouraging comments about the powerpoint presentation, but if it's the last word you're concerned about David, then you should probably note that the author of post #62, the one that bought all this up again, was not myself :((

Now hopefully that truly IS the end of that :((

Pete

[Dave J]

I would just like to set the record strait that Ray has not self-proclaimed to be a guru, that is of my opinion he is one.
From the work I have seen and seen him do and the help I have got, I proclaim he is one as he is very knowledgeable in not only that field.
Also from what I have seen he is both good at electronics and electrics, but I am not saying he is the best and we all should bow down to him, but if we cross reference any member here about something, it should be done politely as you would like done to you, not in a condescending way because you don't believe it.

Pete you do claim everyone is out to get you when ever something like this pops up, but it is not the case at all. We all like to live on a peaceful forum but you seem to take it those few steps further forward to get your point across, instead of doing like the rest of us and take a step back.
As for me knowing you, I don't at all, honestly, I gave that away a long time ago. Going by what I see on the forum I do find you get over involved, being condescending to members and you like to push your point across and have the last say. I am not the only one that thinks this way either, and I am not talking about any names in this thread what you call the old familiar names.
There is no clique on the forum. These old familiar names like to talk to each other and discuss tooling etc so they post a lot, and when they see stuff they think is out of line they speak up, thats all. Think of how a new member would read your posts and if they would join in, not likely. Thats not to mention how the person feels that you are addressing.

I thought I put my last post together in the best word possible for this situation as I was enjoying the thread and learning a bit from it. I did not attack you in any way, nor did I go after blood as you say. I just wanted to let you know yet another thread has died because you wanted to put your point across no matter what. Ray was putting out some good info even if you do say it was wrong.

Hopefully we can all move on and just remember the values of this forum and what it and it's members gives us.

Dave

[Steamwhisperer]

Well said Dave.
Pete, I don't know if you have personally met Ray but I have been lucky, and I mean lucky to have spent two days at close quarters with Ray. The first thing you notice is what a nice bloke he is. I absolutely wouldn't use the word 'condescending' and Ray in the same sentence. Over the two days at close quarters I troubled Ray for advice and not once did he make me feel like it was too much trouble or that I was being annoying, instead he was always more than happy to stop what he was doing to give me a hand. Not everybody can articulate what they want to say in writing and I fear most people fall into this category on occasion, you and I included. I can assure you, from personal experience Ray does not deserve a tirade, well maybe tirade is too strong a word but on this forum I suppose your comments would 'almost' qualify as one seeing as how this particular forum is one of the few left where 'prima donnas' who seem to think that their opinion is the only one that counts, seem to leave us alone (thank goodness).
Last thing, for every action there is an equal and opposite reaction, Ray reacted, appropriately. This is just my opinion, which may be way off base but is still, just my opinion.

Phil

[Bushmiller]

I have followed this thread for two reasons. Firstly I am most interested in the option of using VSDs to drive 3ph motors from a 1ph supply. I would like to return to some of these aspects in a separate post.

Secondly the debate over various aspects has been.......interesting:).

I participate in another section of these forums where debate seems to all too easily become heated. In fact the moderators have had to step in on a number of occasions deleteing posts, issuing thinly veiled cautions and at times weilding the axe. I would not wish to see that here. An unfortunate outcome is that one or more parties go off in a huff!

To some extent heated debate comes about because of a driving passion within the people themselves. That is coupled with the written word which so often does not convey our full meaning. While ever we can keep it civil the thread does not degenerate into a mele.

I am impressed by the amount of knowledge displayed in this thread. Let's leave it at that as I would like to get back to the original post shortly.

Regards
Paul

[Bushmiller]

May I return to Lather's post #1 where he queried the viability of runing a large 3ph motor off 1ph via a VSD. He made reference to 5.5kw, which he later corrected as he meant 5HP.

Although I work in an industrial electrical environment I am not an electrician. Consequently I have a little electrical knowledge, which arguably makes me more dangerous than those to whom electricity is a complete mystery:wink:. I am quite prepared to be shot down in flames and in fact want incorrect statements corrected.

I know that as this thread progressed Lether became more confused (he said so) than he was at the start. Frankly I'm not surprised. I've been trying to get a handle on this for a long time.

Can we take a 5HP (4KW) 3ph motor and consider hooking it up to the 240V supply.

We have two options: Hard wire or via a 20amp power outlet

Problems are that some states don't allow this, this only allows for the continuous running current and assumes that the installed cabling and circuit breakers are of a sufficient size.

The problem is that on start up an induction motor will draw between four and seven times it's normal running current, although this will only be momentary.

My impression is that the number of poles will affect that start up time. The reason I say this is that at another place of work we had large motors. The largest, slightly over 9MW took around five seconds to run up to speed. But the biggest problem was a smaller motor (8MW) which had 6 poles. It took 28secs to reach full speed!

I am using this example to illustrate that things are far from simple in the electrical world and indeed what will work in one situation may not work in another.

Back to the start up current. Can that high starting draw be mitigated by the VSD? Some reference was made in a previous thread to a soft start.

The connection pattern of the motor is another issue and one of which I was completely unaware. I have a jointer with a 3ph motor connected as star/ delta. It is started on star and once running is switched to delta where there is a noticeable increase in power. I believe that the star connection is there to reduce the starting current. Would that preculde the option of using a VSD to run it off a 1ph supply?

Lastly, my impression is that VSDs are effective down to about half the normal speed (frequency) only.

Regards
Paul

[Bushmiller]

Here's a question, what do you understand by the term VA rating?, and how do you calculate it?

Regards
Ray

Ray

On the subject of electricity, Vars are even more of an intangible. I descibe vars as "wattless power." It does explain why, for example, a 3KVA generator will only run a motor of approx 2500watts.

I found this link which might help:

Power Calculator for Generators: Convert kVA to kW, kW to kVA, kW to HP (http://dieselserviceandsupply.com/power_calculator.aspx)

Regards
Paul

[jack620]

Going by what I see on the forum I do find you get over involved, being condescending to members and you like to push your point across and have the last say.

Pete F,
Read this and absorb it. It's spot on.
Chris

[Pete F]

David I have no idea as to what you're talking about, I do not think the whole world is out to get me :rolleyes: If you read back through this thread you'll see that the sarcasm and generous use of eye rolling smilies did not come from me. Unremarkably I don't appreciate that, nevertheless as far as I was concerned the matter was over and I had moved on, it was only when a "for the record ..." comment was posted that the whole matter was raised again. That comment did not come from me. So how in God's name you can now blame me for killing a thread is a complete mystery.

Phil, you're quite correct, and no thus far I have not had the pleasure of meeting Ray in person, but I am sure that it will indeed be a pleasure when I do. As far as any tirade, nothing could be further from the truth, I posted a lengthy explanation of what happened here as I saw it and what my background in the field may be in the hope that it would stop this going any further. Sadly these things tend to get rolled over into other threads and in my experience that is when real problems begin to emerge on boards. I believed it was best to "clear the air" as it were and get it over with, so that others may move on too. The comments leading up to it by others were unhelpful given the circumstances to say the least, and the next thing I could see was the moderators having to take action. You're absolutely right though Phil, every action does indeed generate a reaction, hence why I pointed out the initial "action" at post #26. I don't know where that came from nor the reason for it, only that it, and the generous use of the eye-rolling "smilie" was not appreciated. Other than the correction to the statement made, a correction that despite stating precisely why it is incorrect and the dangers in doing so, does not seem to be accepted, I have absolutely no desire to try to take anything away from Ray's knowledge in this field and take my hat off to the time he also spends trying to help others.

Now, if everyone has finished with their opinion of my writing style and my clearly excessive personality flaws can we move on. Anyone else ... Beuller ... Beuller ... anyone? Perhaps some may also do a bit of self-reflection of their own and consider when they may feel a well worded PM is often a better option for all concerned!

Paul, you make some extremely good points in your posts, as they were spot on. Motors are very complex. You have some good questions and I hope you get them answered to your satisfaction.

Pete

[Stustoys]

Sorry Stuart, quite correct. Obviously I was speaking metaphorically to mean those who actually know something about the topic. I take it you'd like me to remove your name from the "rest of the world" list then? :wink:
No I meant it was a little over the top that you assume the rest of the world "those who actually know something about the topic" agree with you.


I actually penned a lengthy PM to Ray to try to get to the bottom of it, however given the continuing tone of the responses simply thought "suit yourself" and deleted it.
I also wrote a lengthy reply but realised all my points are covered in this thread for anyone that would like to take the time to go back, read and understand all the posts in order.

Ok now that that is over with onto Paul,

Bearing in mind that these answers don't necessarily apply to running a 5hp on single phase, but I wasn't sure if you meant the questions to or not.

"Can that high starting draw be mitigated by the VSD?" Yes, the ramp time(on the huanyang inverter I have) can be set to 6500 seconds. How much this effects current I havent measured but it would be less than DOL starting(but if you were talking single phase thats not going to happen anyway).

"Would that preculde the option of using a VSD to run it off a 1ph supply?" No, But you would remove the star start contactor, timer etc, wire the motor in delta and let the VSD take care of the starting.

"Lastly, my impression is that VSDs are effective down to about half the normal speed (frequency) only." ummmm yes and no, it depends what you are tyring to do. Vector drive VSDs will run the motor at full torque down to and at 0 rpm. Other VSDs wont do as well. But if you only want enough power to jog the machine over slowly while setting up they may well do the job. Setting the machine up so you run the motor over speed maybe a better option if you are worried about having full Hp at all times(for 1440rpm motors).

Stuart

[RayG]

Hi All,

Let me start by apologizing to Pete if he thought I was being condescending, I admit to being startled (several times) by his assertion that ohms law didn't apply... it's the fundamental equation for all circuit analysis.. motors included...

The question is what happens if you have a lathe with a 415 V 3 phase star connected 5.5hp motor (4kw) and you connect it to a 240V 3 phase supply..

Look at the motor name plate and for 415V it will be something around 7 amps or so...

Remembering Ohms Law, Motor Z = 415/7 = 240/x, so x = 7*240/415 = 4 amps

The power will have dropped substantially. But the lathe will run, and you won't trip the circuit breakers.. For some situations that might be a suitable solution.

If you rewired the motor as delta, then you will get full power again.

Let me go back to basics, and try to describe how an AC induction motor works, (I'll probably end up confusing everybody... ) I'll try and do this without reference to field equations, and stick with just text..

The stator is wound in such a way as to produce a rotating magnetic field, the speed that this field rotates depends on the number of poles and the frequency of the AC signal... Now we put a rotor inside the stator, the rotor consists of low impedance single turn coils, which acts a bit like a transformer, (at zero rpm at least) the current in the rotor coils generates a magnetic field and the rotor turns driven by the stator fields.

If the rotor is turning at exactly the same angular velocity as the stator field, then the slip is zero, the rotor frequency is also zero, since the rotor coils aren't cutting the flux lines of the stator field, so torque is zero.

Now we apply a mechanical load to the motor shaft, the shaft slows slightly and the rotor frequency increases, (because the rotor coils are cutting the stator flux lines more) and torque increases... so the torque in an ac motor automatically increases as the load is applied..

Now what happens when the rotor is stationary, like on power up, the rotor frequency is the same as the line frequency and slip will be 100% and current will be maximum, could be 5 or 6 times full load amps, you also get a similar situation when you stall the motor and the rotor is not turning, you might see this as locked rotor amps in the data sheets..

So how do we avoid tripping the circuit breaker on power up?
If we are running on a vfd, you can program the acceleration ramp to start at lower frequency, and for VFD's that means lower voltage, so the rotor starts turning and slip is reduced, so that you avoid the big inrush.. Soft starters work much the same way.
Starting torque is also less. But you can't have everything.. :)

On the subject of "wattless power" motor impedances are complex, (that is complex in the sense of complex numbers), there is a resistive component and a reactive component, the wattless power refers to the reactive component of the motor Z.

I managed to dig out a fair enough sort of article on Induction Motor Theory, the maths is not too heavy, and I'd like to point to the use of Ohms law throughout the circuit analysis..

http://www.etech.net.au/InductionMotorTheory.pdf

Apologies if this is coming across like a lecture, and for those who already know this stuff, perhaps you can correct any dumb screw ups I might have made.


Regards
Ray

PS A belated thanks for all the messages of support, I feel like I don't deserve it.. but it's nice to know.

[RayG]

Hi All,

Just in case some might think PeteF has disappeared for some other reason, I received a PM from him to the effect that he was off to Singapore.

The disagreement between Pete and myself, eventually came down to what is the definition of ohm's law.

Pete's position is that ohm's law only applies to resistance, that is V=IR, my position is that you use the same formula substituting Impedance Z for resistance R and that, of course is the correct approach and is universal practice in circuit analysis, can you still call it "ohms law"?, I know that, it's a widespread practice and a lot of people do. Right or wrong the equation is the same, whatever name you choose to call it..

So that's that finally sorted and this marks the end of the disagreement, perhaps I could have been a bit more diplomatic ... maybe next time. :)

When I see disagreements on other forums, it often degenerates quickly out of control into personal abuse and name calling, threads get locked and deleted, none of that sort of stuff seems to happen here (well not much), which is good to see. I guess that means the moderators have trained us well. :)

Regards
Ray

[Greg Q]

Phew...good on both of youse.:)

[Ropetangler]

Hi All,


So that's that finally sorted and this marks the end of the disagreement, perhaps I could have been a bit more diplomatic ... maybe next time. :)

When I see disagreements on other forums, it often degenerates quickly out of control into personal abuse and name calling, threads get locked and deleted, none of that sort of stuff seems to happen here (well not much), which is good to see. I guess that means the moderators have trained us well. :)

Regards
Ray
I'm sure that we are all pleased to see it all put behind us, It behoves us all to be temperate and moderate in our choice of words, it can be much easier to misinterpret the written word as we have none of the cues present in spoken language to guide us in determining the intent of the writer. Some too can be more sensitive to perceived slights, when maybe none were intended, so we need to be extra careful to avoid statements which can have other interpretations, or where the ''force'' of a word is difficult to know, because some will take the best interpretation while others see it the worst possible light.

To return to the OPs issue, that of running 4Kw motor off single phase, with a V.F.D. I imagine that it could be done, that is it could be done without too much trouble and extra expense, providing that there are no complicating factors, such as 2 speed motors, and for an out of the box solution, it will be the easiest and probably cheapest to implement. However if there are complicating factors like 2 speed motors, or the need to switch motors off downstream from the VFD, there may be an argument for a Rotary Phase Converter.
These can be home brewed quite cheaply if you are a good scrounger, and have good metal bashing skills and facilities. As the RPC is generally started first without a load, inrush currents are less, and could be reduced further by utilising a (small fractional H.P, single phase) pony motor to get the RPC up to speed, and then starting your 3 phase motor from that. Commercial off the shelf solutions are out there for RPCs, but they are pretty expensive. For those without an electrical background of any sort, RayG has posted links on this and other threads which have good info to help get you up to speed, and I would recommend Practical Machinist forums which have a sub forum dedicated to this topic, and 2 useful books are Three-Phase Conversion, by Graham Astbury, No.47 in The Workshop Practice Series (available from The Book Depository for under eleven dollars AU and post free in about 5 days) and The A.R.R.L. Handbook , for Amateur Radio Operators - this tome is updated annually and a 20year old copy will be just as informative as the latest edition, but instead of costing over $100.00, you can pick up a second hand copy from the likes of Amazon or Abe Books for about $6.00 and about the same for postage. Besides the electrical theory which is covered in reasonably simple plain language without too much complex mathematics, there is a good section on rewinding transformers for anyone wanting to generate their own 415 V 3 phase. Just remember to get your theory checked by a suitably qualified person, before you build it and then get your handiwork checked after you build but before you power it up, if you are working outside your qualifications. Good luck whichever way you decide to go,
Rob.

[Stustoys]

To return to the OPs issue, that of running 4Kw motor off single phase, with a V.F.D. I imagine that it could be done
Remember he only has a 15amp supply.

Stuart

[Ropetangler]

Thanks for pointing that out Stuart, I had read it but it had slipped my feeble old brain.:B With a 15A supply there is perhaps a chance it could be run, but it certainly won't give anything like full power, and even to start it would be problematical. For hard to start loads as I understand it you would want something like a 100 A supply for a DOL starter, but for light load starts, you would get away with much less, particularly if a soft start is an option.
Rob.

[lather]

gave up on the idea of running a 4kw motor off single phase.
would a 2.4 kw 3 phase lathe be o.k to run off a VSD with a 15 amp power supply.

if it is,any recommendations for a reliable make-model vsd ?

[Jekyll and Hyde]

gave up on the idea of running a 4kw motor off single phase.
would a 2.4 kw 3 phase lathe be o.k to run off a VSD with a 15 amp power supply.

if it is,any recommendations for a reliable make-model vsd ?

eBay Australia: Buy new & used fashion, electronics & home d (http://www.ebay.com.au/itm/UPDATED-NEW-VARIABLE-FREQUENCY-DRIVE-INVERTER-VFD-4HP-3KW-13A-220-250V-e3-/190630319679?pt=BI_Electrical_Equipment_Tools&hash=item2c6273d63f)

Looks like no worries to me, that Huanyang I've linked suggests it will pull 13A to run a 3kw motor. Their quoted current draws in the listing sounds about right, from memory my 2.2kw Huanyang on the mill pulls around 7-8 amps under full load (about 4A just spinning the spindle over without a cut) running a 1.5kw motor. And as far as a reliable make, there are a lot of guys on here running these Huanyangs (because they're so bloody cheap!), and I don't think anyone has managed to let the smoke out of one yet?

And even if you do let the smoke out of one, you can probably buy another to replace it, and still be ahead over buying a 'known' brand! Of course, if you're planning to use the VSD to actually vary the speed of the lathe significantly, you might want one of them fancy fandangled vector drives to preserve torque at low speeds, but I get the impression you simply want the VSD for its 'phase converter' abilities...

[lather]

Thanks, will look into it.
won't be needing to reduce the speed with the vsd, the slowest speed on the lathe is 50 rpm.

[Dave J]

Lather
Haven't you got a new lathe yet,:D
I bet your getting machining withdrawals going without a lathe for this long. Any longer and you may need to spend money on counseling, LOL

Looking forward to seeing what lathe you get.:2tsup:

Dave

[lather]

Stressing out at the moment, due to not being able to complete my project.

[RayG]

gave up on the idea of running a 4kw motor off single phase.
would a 2.4 kw 3 phase lathe be o.k to run off a VSD with a 15 amp power supply.

if it is,any recommendations for a reliable make-model vsd ?

Hi Lather,

I'd like to see the motor nameplate and also confirm that the motor can be connected as delta, but assuming that's all ok..

Yes, in practice you should be fine with a 2.2kw drive.

If you want a "better" brand, I'd go for Danfoss, VLT51 micro drive Model Number 132F 0007 is the one for 240 2.2kw single phase input. You should be able to pick up one for around the $350-450 mark.

The actual current draw will be fairly low under low or light loads, and 10-11 amps under full load, which you will probably never see in normal operation.

The current inrush on starting, is not an issue with VFD's

Regards
Ray

[lather]

Thanks ray,
will pop in and check the name plate.
at the moment looking at a hafco al-540,

do the cheap vsd's as in the site below have an adjustable overload setting, e.g. setting an amp value, which would cutout if the motor is overloaded ?
if using a 2.4kw motor, would a 2.4kw vsd be needed or would a 3kw vsd do, though run at a lower setting ?
UPDATED NEW VARIABLE FREQUENCY DRIVE INVERTER VFD 4HP 3KW 13A 220-250V e3 | eBay (http://www.ebay.com.au/itm/UPDATED-NEW-VARIABLE-FREQUENCY-DRIVE-INVERTER-VFD-4HP-3KW-13A-220-250V-e3-/190630319679?pt=BI_Electrical_Equipment_Tools&hash=item2c6273d63f)

[RayG]

Hi Lather,

Yes, you just set the motor current on the VFD to match the name plate rating, no reason you can't set it to a lower value.

I just had a quick look at the AL-540, while you are in there looking at it, check what voltage the coolant pump is running, it might be 415 as well.

Also, you are going to need to allow for re-wiring the start/stop fwd/rev and jog buttons to the VFD controls.

Regards
Ray

[lather]

you lost me ray,
thought that the original 3 phase plug wires were connected to the vsd, which allowed the use of the other switches.

[RayG]

Hi Lather,

When you do a vfd conversion, the vfd is wired direct to the motor, and the switching is done up stream of the vfd.

The vfd has input terminals for start/stop forward reverse and jog.

The best way of doing it, is to rewire the switches to connect to the vfd inputs. The e-stop/power-on switching will most likely connect to a latching contactor, and that supplies power to everything.

If you're not sure of how to go about it, I could draw up some circuits.

You don't have to do it that way of course, you could always just use the keypad on the drive to start and stop and not use the controls on the lathe.. but the vfd must still be directly connected to the motor.

Regards
Ray

[lather]

Ray
looks like all the switches will need to be wired up, as i prefer to use the standard lathe switches.

[Stustoys]

On my mill I just removed the 3 phase wriing from the contactor and ran the VSD control wires to the contactor instead. You'd need the stepdown tranformer to have a 240V tap on the input though.

Stuart

[RayG]

Hi Stuart,

Yep, that's the way to I do it too. I've done exactly that on the lathe as well. :)

With the AL540, there are a few extra's to sort out, the foot brake, jog switch, coolant pump, etc, etc, I'm thinking that they would be running 24v coils on the contactors, but could just as easily be something else.. there should be a 240v transformer with 24v ac and maybe 12v tapping for the worklight. The DRO would be 240v input as well. I'll wait till I see the wiring diagram.

I'm starting to think It might be a worthwhile documentation exercise to do a write up on how to do the wiring on a vfd conversion..

Regards
Ray

[Bushmiller]

I'm starting to think It might be a worthwhile documentation exercise to do a write up on how to do the wiring on a vfd conversion..



Ray

I think that would be well received, but remember to issue the usual disclaimers and advise all checking of work:).

From my part I will look forward to it.

Regards
Paul