Talk to me about VFDs.

[soundman]

I have just grabed myself a bargin, in the form of an old cast iron lathe.....you can just feel the quality.....I'm realy going to feel the quality when I have to get it off the trailer......its more or less complete, less motor.

So I've managed to ebay myself a near new, 1.1Kw GMC 3 phase motor & i'm doing my homework on VFDs.

If I can lay my hands on a 2hp motor..I'll do so and I'll probably sling the GMC on my drill press.

I know this in the CNC part of the forum...but it seems to be the repository of VFD information.

So a couple of questions.

Who are the reputable VFD suppliers on ebay these days.....I'm watching one of these http://www.ebay.com.au/itm/221229448...84.m1423.l2649.

I know the VFDs will vary from about 0Kz to 400Kz, but what is the reasonable limtation.

I gather there are cooling issues at the low speeds and no doubt there will be balancing and spinning distruction issues at the higher speeds.

Are there limitations caused by the induction in the motors in particular the larger ones.

I gather there are now motors, like these spindle motors that are specifically designed to be run by VFDs......what is the effect and limitations on stock 3 phase motors not specifically designed for use with VFDs.


cheers

[RustyArc]

That Ebay unit is a Huanyang - I wouldn't call it a quality unit, but they're popular so lots of info about them on the net.

If you want a quality unit, look about for an ABB or Danfoss (or Hitachi) unit - even second hand would be fine.

You don't say if you have 3ph - if not you'll need to rewire the motor to star.

I wouldn't run a conventional 3ph motor above 50Hz unless I had good reason to and heat monitoring in place (decent VFDs have inputs for motor temp sensors with associated overtemp alarms and actions).

Oh, and BobL is the resident VFD expert - he's posted quite a bit about them and motors in the electronics forum:
https://www.woodworkforums.com/f271/

[BobL]

That Ebay unit is a Huanyang - I wouldn't call it a quality unit, but they're popular so lots of info about them on the net.
If you want a quality unit, look about for an ABB or Danfoss (or Hitachi) unit - even second hand would be fine..

Huanyangs are cheap bordering on disposable. On a positive note I have but deliberately and accidentally hammered several of mine and they have come though apparently unscathed.

You don't say if you have 3ph - if not you'll need to rewire the motor to star.

I think you mean Delta?

I wouldn't run a conventional 3ph motor above 50Hz unless I had good reason to and heat monitoring in place (decent VFDs have inputs for motor temp sensors with associated overtemp alarms and actions).

Over 50Hz, up to a point heat should not be a problem as the increase in fan speed will usually keep up with the increase in heating. Heatwise the problem is more likely to be at the very low speed side but that depends on the amount of time you run at low speeds. Unless a large motor is used to replace a smaller one then lack of torque tends to limit how low you can go anyway.
A more likely long term problem at high speed is motor structural and bearing capability.

A 2Pole (2880 @ 50Hz) motor can usually be run @60Hz and the most I run mine is at 70Hz (4200) mainly because the bearings on basic motors probably aren;t rated to more than this.
Higher pole motors (lower speed) can be usually be run at higher frequencies.
Eg 4 pole (1440rpm @ 50Hz) motors can usually be run at 100 Hz without much problem. I run my Leeson 4pole on my DP up to 120Hz. Joe Hovel runs his even higher.

Oh, and BobL is the resident VFD expert - he's posted quite a bit about them and motors in the electronics forum:

If an expert is a "drip under pressure" that might be close.
There are folks on this forum with a lot more experience and I still consider myself on the lower part of the VFD learning curve .

[pjt]

I scored myself quite a number of vfd's at auction, I had much the same sort of questions as SM, so after a few phone calls to the OEM I'm a bit better informed, anything I say here is in relation to that brand and may not apply speciffically to your brand, but, in general motor rpm is not the issue, it's the load (current draw) placed on the motor at any given speed and the vfd's ability to deliver that current to the motor at that speed is what will be the limit for a given motor/vfd. When I say motor I mean an ordinary squirrel cage design.
The combination of vfd, motor and type of load all need to be taken into consideration, a variable tourque load (fan) versus a static tourque load (conveyer belt) are 2 completly different animals.
My units have various settings for limiting the current drawn by the motor so if the load is such that the motor is only drawing for e.g. a third the max current at 50 Hz then potentially there's no reason the motor can't be run much faster upto 200Hz (in my case) and asumming the attached equpment could handle that speed, the limit will be the allowable current draw of the motor (from nameplate data) and the ability of the vfd to supply.

My brand will go up to 200Hz, I have been bench testing a few motors (2-2.2kw and 3-3kw) up to max Hz, a 2p motor will do about 11400rpm at 200Hz no attached load, all these motors ran quite nicely, as far as bearings go, typically for deep groove ball grease filled bearings to suit these size motors will handle these sorts of speeds, the seal type has an influence on the speed rating, e.g. rubber lip type will be approaching their limit at these sorts of speeds, steel non contact type will be faster rated as will oil filled bearings, this sort of info is available from bearing manufacturers so if any doubt check.
If there was balance issues in the rotor/fan that might limit your speed but that's more of a mechanical issue than vfd. All my five motor tests the motors just sat on some small squares of foam and never moved. When connected DOL they jumped a bit at start. A sqirrel cage rotor is a robust and sturdy construction, I'm not sure what speed they might start to fly apart but all my motors gave no indication of anything nasty about to happen at 200Hz.

For a lathe a large range of settable speeds would be desired so I would be looking at at least a 6 pole motor that would give 475 @ 25Hz to 3800rpm @200Hz, If SM is going to be doing large diameter stuff with heavy cuts then maybe even an 8p or more, my units have settings that can increase (upto a limit) the tourque output at slower speeds, ideal for big heavy cuts, but if SM is only doing small diameter stuff fast is the go. I'm sure SM knows all that tho!!

Other knob's of interest that I recall is, a vfd slightly larger or equal to motor is good sizing but essentially there's no reason a 5kw vfd can't drive a 2.2kw motor but using an 80kw on a 2.2kw is not a good idea.
Where I am my line voltage (3ph) is ~430v, my vfd's will output this so if the votage rating on the motor nameplate says 415 I can get a little more power above the nameplate kw rating as long as I haven't reached the current limit.
On mine there are many settable features; motor protection, how the speed reference is selected, relay switching at set speeds, overide features, how fast it starts up and stops, along with options like dynamic braking, (fast stopping of the load), could be ideal if some big and heavy turning got away on a lathe.





Pete

[BobL]

According to theory, the power output of a 50/60Hz rated motor should be constant above the rated frequency but my testing under load does not seem to agree with this.

The highest frequency I have tested a motor for is 130Hz and below is the typical curve I obtained (details @ https://www.woodworkforums.com/showth...182106&page=11)
The HP appears to be constant up to about 90Hz but then it drops off and at 130Hz this motor is only producing about half it's rated HP.
Also significant would be the HP at the nameplate current at each frequency.



It would be interesting to see what happens in practice above 130 Hz.
The drop off in power with frequency for this motor looks like it could be linear, or slow exponent,l but either way it looks like motor power could be below half the motor rating at 200Hz.

The frequency range over which motor HP stays reasonably constant seems to be motor dependent, on one 2P I tested the the HP started dropping of significantly above 60Hz.

Sometime when I have the HP rig out again I will take the above motor at least to 150Hz, above that frequency that motor starts to make some noises that I don't like the sound of.
The other problem at those speeds is vibe from the HP rig brake drums.
I wish I had access to a dynamic balancer that I could balance the drums that would maybe give me the confidence to go to higher frequencies.

[NCArcher]

What sort of lathe SM? Are there multiple pulleys on the headstock? I have converted my lathe with a Powtran VSD (which I can recommend). I've found that the combination of the multiple pulleys and VSD give a wide range of speeds without going to extremes. It sits on one pulley for most stuff but I still have the option of moving the belt if I need a lower or higher speed without pushing the VSD to it's limits. If I need a really low speed I can drop down to the smallest pulley and still run the motor fast enough to provide cooling. The same with a higher than normal speed. The biggest pulley and 60 Hz will give me more speed than I think I will ever need but isn't pushing the VSD/motor all that hard.

[pjt]

According to theory, the power output of a 50/60Hz rated motor should be constant above the rated frequency but my testing under load does not seem to agree with this.

The highest frequency I have tested a motor for is 130Hz and below is the typical curve I obtained (details @ https://www.woodworkforums.com/showth...182106&page=11)
The HP appears to be constant up to about 90Hz but then it drops off and at 130Hz this motor is only producing about half it's rated HP.
Also significant would be the HP at the nameplate current at each frequency.



It would be interesting to see what happens in practice above 130 Hz.
The drop off in power with frequency for this motor looks like it could be linear, or slow exponent,l but either way it looks like motor power could be below half the motor rating at 200Hz.

The frequency range over which motor HP stays reasonably constant seems to be motor dependent, on one 2P I tested the the HP started dropping of significantly above 60Hz.

Bob, Would it be the tourque dropping off? and an increase in rotor losses which consumes power in the form of heat?
Different design rotor resistance will give vastly different tourque curves so there maybe something there with differing results with different motors.


Pete

[BobL]

Bob, Would it be the tourque dropping off? and an increase in rotor losses which consumes power in the form of heat?
Different design rotor resistance will give vastly different tourque curves so there maybe something there with differing results with different motors.

The motors I tested all had significant torque losses at higher RPM and were only developing the power by virtue of the higher RPMs.
I agree resistance will be motor dependent but none of the ones I looked at were doing that well above about 90 Hz.
Of course the simple way around this is to use a bigger motor.

[soundman]

As I suspected there are some significant limitations in the whole power speed thing.

seems arround -30% +100% is about the reasonable limit with an ordinary motor.

I suspect in the lower end the poles simply wont be passing quickly enough to operate efficiently......at the upper end...the size of the poles probably comes into play too.......as someone mentioned a motor with more and smaller poles would operate over a wider window.

ANYway, that amount of variation, would give me 950 to 3000 rpm shaft speed out of a 1440 motor.......that seems pretty usefull for a wood lathe.

bring into play the existing 3 speed pully arrangement and I should get some pretty serious tractability

It would be good to get down into the low hundreds in the bottom end.

I have not worked out the ratios on the pullies, but the middle speed is 1:1.....the steps are not great.

There are some issues, both the pully stacks have the end face of the largerst pully broken....george arround the corner may be able to work his magic with that.

It occurs to me that a bigger motor and VFD may be worth while.

most of the medium to large lathes only seem to have 1.5Hp motors..though the powermatic has a 3hp....AGH AGH AGH

The existing pully set is a double V belt...so it must have had some power to it in its day.

I have not looked close at the headstock yet.....I suspect it may be white metal bearings.....this may account for some frictional losses in comparison to a roller or ball bearing headstock.

Another thaught at the low end is to impliment some cooling independent of motor speed in the form of an electric fan.....I'm not short of a few DC fans arround here.

thanks for the information.

What VFD brands and suppliers are people using.

The Hangyang I linked earlier being "disposable" is not too much of a worry at $135 dollars......hell that is lees than a brand name DOL starter.....and if someone where to bring it to me to fix...there would be a minimum $80 fee....so yep disposable.

cheers

[pjt]

The motors I tested all had significant torque losses at higher RPM and were only developing the power by virtue of the higher RPMs.
I agree resistance will be motor dependent but none of the ones I looked at were doing that well above about 90 Hz.
Of course the simple way around this is to use a bigger motor.

Just me thinking out loud here, but I wonder wether there would be different test results if you compared a 3ph supply in with 1ph supply in and then star v delta connections.
Also, how much detail do these cheapie vfd give via the display, Is it just Hz? or more? On the Zener it will display either Hz, amps, %load, Kw and a few more.



Pete

[BobL]

Just me thinking out loud here, but I wonder wether there would be different test results if you compared a 3ph supply in with 1ph supply in and then star v delta connections.

I haven't checked the wave profile of the Output V at all frequencies but the one s I have checked have all be pretty clean so I can't see this being a problem. The most significant thing seems to be motor size with small motors having a narrow usable power range

Also, how much detail do these cheapie vfd give via the display, Is it just Hz? or more? On the Zener it will display either Hz, amps, %load, Kw and a few more.
Pete

The following can be displayed on the cheap HY VFD
Output Freq
Set Freq
Output Current
Output RPM
DCV
SCV
Temp
Something called scouter value
Target Value if PID
Feedback Value of PID
Present time of power on
Total Time of power on.

[soundman]

Just a few more thaughts and questions.

It occurs to me that its probably not a bad idea to over rate the VFD in relation to the motor..so say 4Hp VFD for a 3 HP motor.....like would be practice in other switch gear.

Now the interestng question.

Do these VFDs have the ability to current limit.
So going the other way would it be possible to run a 4 Hp motor off a 3HP VFD buy applying some sort of electronic limitation.

After all in many situations a motor may be a given rating but may never be required to deliver its rated output due to the load applied

cheers

[BobL]

It occurs to me that its probably not a bad idea to over rate the VFD in relation to the motor..so say 4Hp VFD for a 3 HP motor....

In general yes

Do these VFDs have the ability to current limit.
So going the other way would it be possible to run a 4 Hp motor off a 3HP VFD buy applying some sort of electronic limitation.

Some VFDs have a fixed current limit while others are programmable.
Also bear in mind that most VFDs can for a short period deliver up to ~50% above their rated current limit before tripping out.
I'm not sure how many times this can be done but I on the VFD I do my HP motor testing it must have tripped more than 20 times and it is still working fine.

The question of which way to do it is indeed interesting and depends on the application.

In a low use situation like a DIY shed if a 3HP motor only rarely uses its 3HP capacity then using a 4HP VFD and current limiting the VFD to 3HP may be unnecessary - just get the 3HP VFD to start with.

In a commercial situation or if the motor sits on its 3HP generation capability for a significant period of its use then an over rated VFD should handle any higher VFD temperatures better than a lower rated unit.

My motor/VFD pairings are
Both my MW and WW lathes are use a 1HP motor / 2HP VFD
8" Grinder: 3/4HP motor / 2HP VFD
DP: 1.5HP motor / 2HP VFD
1100 CFM extraction fan: 1/4HP motor/ 1 HP VFD

My 150mm wide x 1m long Linisher uses 3HP motor / 3HP VFD.
I initially used the 3HP VFD because I already had it in my shed and meanwhile I ordered a 4HP VFD. Monitoring current draw using the 3HP VFD I found 90% of my linisher use was drawing <6A. I run the linisher at half speed (25Hz) and find I get much better control of the sanding process To make the motor draw 10A I have to use it at 50Hz with a 4" wide full belt length long rough sawn piece of hardwood and push hard onto a 60 grit belt. I don't use it like this, so for the moment I have left it on the 3HP VFD.

Compressor: 5HP motor / 5HP VFD.
The VFD can put out a mac 18A for extended period before tripping whereas the max current drawn by VFD during the charging cycle is 15A@240 which is for the last 15 seconds or so of the air charging cycle. The charging cycle currents are weird. I use a soft (20s) start (90 PSI). At the end of the 20s it draws around 9A, slowly rises to 11A, and then at around 105PSI it drops back to around 8A and then increases slowly up to 15A for the last part of the cycle (135 PSI)

[soundman]

A couple of background examples.

I have a single phase twon cylinder compressor that in the old money clamed 13CFM.
I have a high accuracy average reading and peak holding clamp meter.

When it came to me new it was fitted a motor and a 10 amp plug....and the pressure switch set fairly low and a pretty wide window ( normal).
Above this machine there where two larger machines a twinn cylinder and a tripple...both fitted with what I am lead to believe is the same motor but different pumps.

the two larger machines came with 15 amp plugs.

I have measured the run cucle of my machine after I'd set the pressure switch properly cut in around 95 and cut out around 110.

from memory it strats around 8 amps and steadily rises to about 13 amps at the end of run.....still auguably with in the 10 amp nominal current draw and still running on a 10 amp plug.

So on my compressor I believe the motor to be well and truvly under run.

on the other hand my saw bench a low end commercial machine , that has a claimed 3Hp motor....this is a physicaly large motor compred to what you will see on the TSC10HB and the unisaw..both claimed as 3hp 15 amp machines.

It came fitted with a 15 amp plug, but after a bit of measuring it got fitted with a 20.

Free running with no load on the blade it is pulling around 12 amps.
On start up it peaks at around ..... 56 amps ( remember peak hold reading).
If I stuff a piece of 90mm KD hardwood into a 60 tooth blade as hard as I dare so the motor is making bogging sounds,
Its pulling up in the 30's

Now this is typical single phase DOL behaviour.

I have a "big blue lathe" wood lathe project in the pipeline.

(those who have been on the board a while will remeber the big blue lathe saga, almost as memorable as Barry Sumpters account of gluing his socks to he feet with polyeurathane......new commer will no know what the hell I'm taklimg about...but I digress)


I'm looking at what I am going to drive it with.
This machine is a big lump of cast iron with a banjo that weighs almost as much as my whole existing lathe, and a headstock I struggle to lift on my own.
white metal bearings, twin V belt drives and all
She will have a 305mm centre height.when she is going.

So I don't want to underpower it.
I have on hand a 2Hp 1240rpm motor that is near new....but I think it might be a little underpowered.

I have a nice italian 3hp 2400ish rpm motor but I think it might be a bit speedy.

So I've been looking arround and there are a few 4 HP three phase motors arround and well priced.

I've been contemplating mounting up one of these and doing some sort of current or power limitation, both to keep the current demand within reason and to controll the massive torque this thing may produce.

There are some other things I am considering...because these VF drives are so cheap and so are used 3 phse motors.......and simgle phase motors are such poorly mannered and inefficient things....a few things round here may get converted.

The 2Hp might go in the drill press for example.

so..thaughts.

cheers

[BobL]

on the other hand my saw bench a low end commercial machine , that has a claimed 3Hp motor....this is a physicaly large motor compred to what you will see on the TSC10HB and the unisaw..both claimed as 3hp 15 amp machines.

Free running with no load on the blade it is pulling around 12 amps.
On start up it peaks at around ..... 56 amps ( remember peak hold reading).
If I stuff a piece of 90mm KD hardwood into a 60 tooth blade as hard as I dare so the motor is making bogging sounds,
Its pulling up in the 30's

Doesn't sound like a 3HP motor.
Free running with minimal load 12A is high even for a 5HP motor.
What size is the blade?

My 3HP/12" TS is a chinese contractor saw and on start up draws around 24A
It's definitely a 3HP motor (more like 3.2 HP) because I have measured this on my HP meter.
It's draws ~ 3A with no load on the blade and it's thermal cut out switch kicks in after about 20s running at ~17A
Of course it depends on cutting rate, saw blade type, sharpness etc but 6A will cut 3/4" pine, 10A will cut 2" jarrah and 12A will cut 75 mm jarrah

My 5HP compressor motor (comes from a table saw) free running (no load) draws 0.6A @ 50Hz.
Driving the compressor at near zero PSI (which would still be a greater load that a naked saw blade) it only draws about 5A

My 3HP Taiwanese DC draws around 57A on start up and max load (all blast gates open) draws around 9.2A.

I have on hand a 2Hp 1240rpm motor that is near new....but I think it might be a little underpowered.
I have a nice italian 3hp 2400ish rpm motor but I think it might be a bit speedy.

In general (assuming they have the same HP) I have found 4 pole (~1440 rpm) motors more versatile than 2 pole (3000 rpm @50Hz) motors to drive machines, like lathes ,via a VFD.

Most 2 and 4 pole motors,( either the 50 or 60Hz models) all use the same bearings so all of these motors should physically not have a problem running at 3600 rpm.

One of the limiting steps on basic motors will be the bearings and on most basic motors if you want them to last they cannot be spun at indefinite speeds
At higher speeds the HP also drops of, quite dramatically for smaller motors, and less so for larger ones

At double the 50Hz frequency of a 4 pole motor ft's still only doing 2880 rpm, but if the frequency of a two pole motor is doubled its now close to 6000 rpm which puts it in my "don't know what's going to happen long term" category. Of course there are motors around that will go much faster than this but the bearings will have been designed to cope with this.

This is why I limit my two pole motors to around 1.5x their rated frequency and 4 pole motors to about 3x rated frequency.

Now going back the other way, the HP of a 2pole at 0.5x the rated frequency will be about 0.5 the rated HP, and the same applies to the 4 pole motor so you cannot go too low without sacrificing significant power.

Comparing like for like, the working frequency range for a 2 pole motor is for me nominally 0.5 to 1.5 x or a range of 3x, for a 4 pole motor its 0.5 to 3, or a factor of 6x.
The 4pole thus has twice the working frequency range where it will deliver approx the same HP.

The does not mean I don't run VFDs lower than 0.5x or 0.25x the motor's rated frequency, of course I do go lower but the lower I go the less time I run things at those speeds as cooling can become a problem. The only machine this doesn't matter on is the grinder which has no fan in it anyway. If power is not and issue and you wish to run at low speeds for extended periods then fitting auxiliary cooling is recommended.

Of course none of this is absolute. you can decide your bearings are good at higher speeds and your own working range will be something like 0.25 to 2x the motors rated frequency for a 2 pole. To match the same RPM at the high end that would be 0.25 to 4x for a 4 pole
Now the 2 pole has a working range of 7x while the 4 pole has a range of 15x.

Of course comparing different powered motors then that also has to be taken into a count.
You could for example replace a 1HP motor with a
4 pole 2HP motor and have more than 1HP over the frequency range 0.5 to 3 x (range of 6x)
or a
2 pole 4HP motor and have more than 1HP over the frequency range 0.25 to 1.5 x (range of 7x)
or a
4 pole 4HP motor and more that 1HP over 0.25 to 3x (range of 15)
or a
ETC