Thread: VSD power tests

Originally Posted by jhovel

How are you measuring the current, Dean? Are you just relying on the VFD option to display the current?
I think BobL in his tests found that to be inaccurate to say the least....

I am using an EBay supplied ammeter the same as has been discussed before. A clamp meter is on my wish list. I could try the 10A option on one of my multimeters.

Stuart

Just to be sure. You are running a 240V VSD on a 415V 2 speed motor?

Correct.

Have you kept the "two speed" wiring on your motor?

I only have 3 wires connecting the motor plus earth. I removed all the wires from the conduit to clean up a lot of oil inside it and only replaced those 4. It shouldn't be too hard to put the others back tho. Back then, star connected motors were no good on VFD's.

One thing I noticed today. I turned the control up quickly and the power cut to the motor well before 80Hz. It looked like about 50-60Hz to me. The amps were reading high, but I don't recall how high.

Dean

Originally Posted by jhovel

How are you measuring the current, Dean? Are you just relying on the VFD option to display the current?
I think BobL in his tests found that to be inaccurate to say the least....

It is very difficult to measure the pulse width modulated output voltage or current of a VFD. It is probably impossible to get any meaningful measurement with only a low cost true rms multimeter - it will never match the VFD's own figures. You need a multimeter designed for the particular task of measuring VFD outputs - these have a built in low pass filter. And even then there will be some discrepancy. Remember, generic low cost multimeters are made to measure true RMS of 100 and 120Hz AC signals. Modern VFD's operate at chopper frequencies of 16kHz to reduce audible noise. Normal multimeters go crazy when measuring VFD outputs. If you have no access to a high end Fluke 87 and its successors designed with VFD's in mind, you may get far more usable results using an old fashioned moving needle voltmeter, than a cheap digital multimeter.

here some reading:
http://www.testequipmentdepot.com/ap...289-dmm_an.pdf
How to Measure Output Voltage from a VFD to a Motor
http://www.wtb.tue.nl/woc/ptc/educat...0voltmeter.pdf
http://www.est-aegis.com/datasheets/...ke_appnote.pdf


Also, note that VFD's may internally use a Hall-effect current transducer instead of a current transformer to measure current. It will measure both AC and DC components, whereas a current transformer only measures AC:
Hall effect - Wikipedia, the free encyclopedia
https://www.ohiosemitronics.com/data...itoring(1).pdf

Originally Posted by jhovel

Stu is right, Dean. Your 7.5kW motor is probably rated at 18A at 415V. That would mean (to me) that it can;t draw any more than 11A max at 240V.
The other thing is that your VFD at 17A is really a 3.7kW VFD - (17X220) and may not have the capacitors to sustain mare than 8A into a 415V coil... but I'm out of my depth here - so let's see if others here can put a better explanation forward.

Also, have you actually loaded your machine to the point of stalling or REALLY slowing down your motor? Maybe the drive train doesn't handle anything like 7.5kW?

No I have not tried REALLY slowing down the motor. I found an old rusty drive shaft from a 4WD on the weekend, which I have cut off to use as a test piece if it is suitable. It has a spline inside a tube. I am hoping there is some unhardened shaft inside. When I get time. I was trying to do another job on the lathe.

Maybe the drive train doesn't handle anything like 7.5kW?

Phyisically? I am not sure how this relates to this issue. I have yet to look inside, but from what I have seen I would say it would go pretty close. It does run triple B section belts. Not that I am likely to be able to supply anywhere near the full kw's anyway. Stand alone 3ph solar system is about the only way I could do that. It's only 7.5hp by the way. A bit less than 7.5kw.

Dean

Originally Posted by Oldneweng

I only have 3 wires connecting the motor plus earth. I removed all the wires from the conduit to clean up a lot of oil inside it and only replaced those 4. It shouldn't be too hard to put the others back tho. Back then, star connected motors were no good on VFD's.

I doubt it was the star connect that worried anyone. Just a question of how much power you would get wired for 415V (whether star or delta) on 240V
As best I know, nothing different between a 415V(parallel?) star and 415V (series?) delta as far as running them on 240V. I think you might be asking to much to take a 2 speed 415V motor and run it on 240V with 1 speed. Either is "ok", both is asking a bit to much.(though sometimes we have to take what we can get lol)

Originally Posted by Oldneweng

One thing I noticed today. I turned the control up quickly and the power cut to the motor well before 80Hz. It looked like about 50-60Hz to me. The amps were reading high, but I don't recall how high.

While I think I know whats going on when that happens I'm really just guessing, hopefully someone else can explain. Having said that you can adjust the speed the VSD changes Freq when following the speed pot. PD071


Stuart

QUOTE=Stustoys;1790938]I doubt it was the star connect that worried anyone. Just a question of how much power you would get wired for 415V (whether star or delta) on 240V
As best I know, nothing different between a 415V(parallel?) star and 415V (series?) delta as far as running them on 240V. I think you might be asking to much to take a 2 speed 415V motor and run it on 240V with 1 speed. Either is "ok", both is asking a bit to much.



While I think I know whats going on when that happens I'm really just guessing, hopefully someone else can explain. Having said that you can adjust the speed the VSD changes Freq when following the speed pot. PD071


Stuart[/QUOTE]

The first paragraph appears contradictory to me. Are you saying that both star and delta should be utilized, separately to achieve the best speed range/power combination?

I mentioned the issue with the fast speed adjustment only as a comment. It will not be a problem. I have a multiturn pot to install.

Re taking what we can get, you have got that right. I have no idea whether I will find any limitations with the way this lathe runs at the moment, but one can never tell what will happen in the future so I am just trying to find the real practical limits of my setup, and the best way to operate it, now while the information is hot.

Dean

Originally Posted by Oldneweng

The first paragraph appears contradictory to me.

I wonder it you're confusing your motor with "normal" single speed 240V delta motors. While you've connected your motor in Delta (I believe) its series Delta..i.e. its still wired for 415V.* Both your "star connected" and "delta connected" are capable of being supplied with 415V, each equally "no good" for used on a 240V VSD.

Originally Posted by Oldneweng

Are you saying that both star and delta should be utilized, separately to achieve the best speed range/power combination?

In your case......yes, thats what I'm saying.**
If you had a 415V VSD, as you would have full power, you could likely get the speed range you are after with only the one motor speed.
In your case a 415V VSD isnt an option as you cant supply the power to run it. Which is why I suggested running the motor on 240V "as wired" as this limits the power required.
With a 240V VSD you're going to be down 50% on hp to start with, so will have a smaller usable range of speed adjustment with the VSD(mostly downwards according to Bob tests)


As always.... not sure I have made things any clearer

Stuart




*Now in theory I believe that you could change series delta to parallel delta then get full power(only on that speed, I cant see you can do anything about the parallel star) which is one of the things I set out to test. Of course this is no good to you as you can't supply full power.

**I've not fully tested a 415V two speed motor on a 240V VSD, and few tests I did seem a little "suss".

Originally Posted by Stustoys

I wonder it you're confusing your motor with "normal" single speed 240V delta motors. While you've connected your motor in Delta (I believe) its series Delta..i.e. its still wired for 415V.* Both your "star connected" and "delta connected" are capable of being supplied with 415V, each equally "no good" for used on a 240V VSD.


In your case......yes, thats what I'm saying.**
If you had a 415V VSD, as you would have full power, you could likely get the speed range you are after with only the one motor speed.
In your case a 415V VSD isnt an option as you cant supply the power to run it. Which is why I suggested running the motor on 240V "as wired" as this limits the power required.
With a 240V VSD you're going to be down 50% on hp to start with, so will have a smaller usable range of speed adjustment with the VSD(mostly downwards according to Bob tests)


As always.... not sure I have made things any clearer

Stuart




*Now in theory I believe that you could change series delta to parallel delta then get full power(only on that speed, I cant see you can do anything about the parallel star) which is one of the things I set out to test. Of course this is no good to you as you can't supply full power.

**I've not fully tested a 415V two speed motor on a 240V VSD, and few tests I did seem a little "suss".

You have added a few bits to the puzzle. I believe that often people in the know do not think to explain basic principals because they don't think it is required. A person who knows nothing does not even know that they are missing bits of the puzzle.

As I have said, I only wish to make maximum use of what I have at the moment and ensure that I have not overlooked a simple, cheap improvement.

Dean

I have a small 1440RPM@ 50Hz 3phase ∆ connected CMG motor I bought a few years ago for $20.
Here's a pic of it with the home made brake drum from my De Prony Rig on it.
The shaft is 11m with a 4 mm keyway so I made up a 19-11 mm bush with a 4mm keyway cut into it


The motor has no name plate but based on its physical size and coil resistance I guessed it would around 1/5 HP and decided to check it on my de Prony rig.



Looks like a 1/5HP motor.

BTW this time during all the measurements I monitored the "VSD displayed current" and the "current in a single phase" out of the VSD to the motor and they consistently read the same value within display uncertainty.

Nothing new with this one - it's just nice to see things coming out as expected.

This is a so called 1000W 1440 RPM SP motor from my budget Chinese DP. This motor has been floating around my shed since I replaced it with a Leeson 1150W 3P motor.

By monitoring input current the power drawn by the motor can be calculated and compared with the output power so the motor efficiency can be determined.
The graph shows the HP (LHS vertical scale) and the Efficiency (RHS Vertical Scale) versus the RPM of the motor.

The rating of 1kW appears about right although it's only doing ~1390 RPM when outputting that power.
Interesting that it is at its greatest efficiency (70%) at it's rated output.
An efficiency of 70% is about all a small motor like this can achieve.

Hi Bob,

ok now it starts to get interesting. Does any of that current represent apparent power? or is there really 600W turning to heat at 0.2hp. If its apparent power you don't have to pay for it.(well for the minute at least)

How do VSD's deal with apparent power?

Stuart

For any of you that have been following this thread I've posted something on True RMS current measurement over in the electronics Forum.
https://www.woodworkforums.com/186933...-measurements/

I'm interested in measuring the HP of some bigger motors so I made up a new bigger brake drum so the luggage scales I have won't top out above their range.
Also bigger motors have bigger shafts so I would have to bore out the old drum if I wanted to use it anyway

Here you can see the new drum which has a radius of 80 mm compared to the 60 mm for the first one I made.
I didn't paint the new one so its not so much of an issue if it really heats up.

While welding the hub to the disc I got a bit too enthusiastic with the extent of the welding and this distorted the disc so that it was no longer perpendicular to the hub.
To get around this I put it on my lathe held by the hub and turned the edge of the disc parallel to the hub so that I could press the drum over the disc so that the drum was parallel to the hub.
Before I welded it I ran it on the lathe to see the drum ran true (which after a few taps of a hammer it did) and then welded the drum to the disc using 3 short (2 cm) runs on each side.
The result was much better than I expected and the drum could be run on the motor for a short period up to 80Hz without much vibe - I'd be reluctant to run the motor above this anyway.

The test motor is a 2 pole (2880 rpm @ 50Hz) 24 mm shaft CMG motor.
This never used notor is from a liquidation of a mining company - could be a bit more of that coming up in the next few years.
The nameplate says it is a ∆ wound 240V 2HP 3Phase motor



Here is the HP - RPM graph.
Nothing dramatic to report and it seems to meet spec.
I didn't want to take it over 70 Hz as there were too many sparks coming off the brake drum above that speed,

All of the motors I have tested so far have been unmodified 440V Y / 220V ∆ motors but up until now I have not tested any for which I have done the Y to ∆ conversion.

One motor I wanted to test was an ABB 3HP 440V Y motor that I converted to ∆. This is a very clean motor that looks near new. It cost me $30 and came out of a large air conditioner.

One thing that held me up was a suitable 28 mm bore brake drum which I finished making yesterday. The drum is basically the same as the 24mm bore drum I used on the 2HP motor in previous tests, except the diameter is 2 mm greater and the disc that attaches the drum to the hub is made of 12 mm thick steel plate compared to 6 mm thick plate for the 19 and 24mm bore drums. I used thicker disc steel this time mainly because I had some 12mm thick blank discs already close to size and I thought the extra metal might act like a heat sink during the braking process.

Here's the setup - basically the same as last time.
One difference is that the VFD is a 3kW unit.

If you look closely at the currents shown on the yellow ammeter (17.81A) the VFD (15.8A) and the blue LED V/I meter (26.1A) you can see this sucker (at least briefly can really chew some juice. BTW at those currents the motor is according to the torque and RPM (briefly) putting out close to 4.5HP! by cranking the brake on a little harder I got it just over the 4.5HP mark. These currents are well over the VFD current rating and at the next crank point the VFD just shut down.

The tension upwards on the right had side of the drum is nearly 40kg while on the other side its nearly 10kg. The difference between these two times the radius of the drum is the motor torque.




Here is the HP/RPM curve for that motor on 240V.
I only did it at 50Hz because I just wanted to know the HP rating of the ∆ connected motor


The name plate current for the 3HP rating at 440V in Y connection is 5.5A.
Interestingly the current drawn by the motor at 240V input and ∆ connection when developing 3HP is 11 A which is twice the current at 440V.

The results shown above represent the cumulative values from about 4 runs.
The first run stopped abruptly at about 2.8HP when the power board circuit breaker dropped out the AC power to the VFD.
Then I hooked the VFD up directly to a 15A line and it got up to about 3.4 HP before the wire cable connected to one of the force meters slipped.
After tightening the cable clamps I managed to get the rig working properly but then the VFD tripped out as it went over current.
By then I figure I had enough data to confirm that the motor was still a genuine 3HP motor even after modification.

These measurements are a bit hair raising even if the motor is only @ ~1500 rpm, and I'm glad I made the rig as robust as I did as it all strains and rattles around a fair bit when its braking >3HP. The bike hand brake is managing the 40kg forces OK - it's like really hoiking on a racing bike when braking going steeply downhill. The drum does get quite hot - but not smoking as this drum, like the 24mm bore drum, is unpainted. I can do one set of measurements and then I have to wait for the fan to cool the disc/drum down. I'm not looking forward to any 3000 PRM testing of these bigger motors. I was hoping to test a couple of 4HP motors but I would need a 5HP VFD and force sensors that read over 50kg, or go for an even bigger drum. I have a 5HP VFD on my compressor but I don't want to pull it out of the compressor circuitry.

It goes without saying that these measurements are potentially very dangerous and should not be attempted unless you have a good idea about what is going on.