Thread: single to 3 phase vsd confusion.

Originally Posted by NCArcher

Or increase the current draw while maintaining the power output.

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

Regards
Ray

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

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

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

Originally Posted by Stustoys

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.

Originally Posted by Stustoys

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.

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

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 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

Originally Posted by Graziano

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
https://www.woodworkforums.com/attach...d-img_8036.jpg

Though the current has gone up a little

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

Originally Posted by Graziano

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

Originally Posted by 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.

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/sho...oducts/12-0110
No idea what the lathe is like but the design looks OK...

Originally Posted by Stustoys

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.

Originally Posted by Stustoys

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
https://www.woodworkforums.com/attach...d-img_8036.jpg

Though the current has gone up a little

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.

Originally Posted by Stustoys

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.

Originally Posted by Pete F

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 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

Originally Posted by 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

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

Originally Posted by Graziano

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

Originally Posted by PDW

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 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/energy...quirements.pdf I know that's for WA, and nobody lives there, but it's likely to be the similar in the civilised world. ....jooookkkiiinnnggg

Originally Posted by Graziano

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.

Originally Posted by RayG

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