Testing a VFD and Motor

[agarb]

Does anybody know of a lab with a dyno that can do testing on some 40HP drive and motor combinations?

Stuff like:
- For line voltage of 80-90% of nominal for a maximum of 20 seconds, verify that we still have sufficient torque.
- Current & torque ripple measurements
- speed vs. torque curves
- speed loop test

Any suggestions?

 

[10BaseT.]

Rockwell Drive Systems in Dierikon Switzerlend can do all of this - but expect to pay !

For the sort of wedge they will charge , it will be cheaper to set up a rig yourself

 

[Gerry M]

Check with your motor manufacturer, they can supply typical curves. If that is not good enough ask them to do a test for you, or google "motor calibration services". I believe www.magtrol.com can also do it for you, they also supply test rigs.

 

[Sparkz]

Stuff like:
- For line voltage of 80-90% of nominal for a maximum of 20 seconds, verify that we still have sufficient torque.
- Current & torque ripple measurements
- speed vs. torque curves
- speed loop test

What exactly are you trying to achieve with a dyno test?

Nearly any standard asynchronous AC motor is capable of delivering 2.5 times its nominal torque, but that is of course without a VFD and if current can rise accordingly.

But in general: Torque (in Nm) = Power (in kW) x 9656 / speed (in rpm)
To deliver the same amount of torque at a 20% voltage drop, current should rise 20%. (I'm not taking speed variation into account.) So you could overcome the torque issue by raising the nominal motorcurrent setting by 20%, because VFDs have the annoying habit of lowering the motor's speed first when the motor is being overloaded (just to keep motor current below the settings). 20 seconds seems very short in motor terms, but just how repetitive are these voltage drops? If the motor is already running near full load, you can not expect it to overcome 20% voltage drops every 5 minutes!

Another issue, just how critical is speed to your application? Also, dyno tests tell you nothing about load inertia. What type of load is the motor driving?

If speed drop could become a problem, check if the VFD is (sensorless) vector control capable. Using vector control, instead of standard U/f control, will improve speed stability, even under overload or undervoltage conditions.

However, chances are the VFD will trip due to a 20% voltage drop...

Now I'm assuming the motor/VFD is powered by a generator ('cause which right-in-his-mind net provider would allow a 20% drop)... Isn't overvoltage going to be something to worry about? I've seen too many VFDs trip because of a voltage overshoot (due to a generator's reaction to sudden load variations).

 

[agarb]

Thanks to all who have responded thus far.

What exactly are you trying to achieve with a dyno test?

Meet the customer's specification to do a bunch of testing.

Funny thing is the mechanical folks just give me a blank stare and start laughing when I ask what the torque requirements actually are.

For now, I'm just trying to find somebody who can do the testing and will give me a price.

 

[10BaseT.]

Controversial I know - but I don't reckon your customer has a clue what they want , but they wish to appear that the do .

It is up to you as OEM suppliers to determine that the drive is fit for the purpose , and if your mechanical guys don't know the exact load characteristics that are liable to be applied , buy a slightly bigger motor and drive .

I would guess that as mentioned here , the data you require is available from the motor manufacturer .

 

[Sparkz]

Funny thing is the mechanical folks just give me a blank stare and start laughing when I ask what the torque requirements actually are.

Why is always up to the electrical engineer to prove a motor's capabilities? Too often I've been struggling to convince mechanics (not to mention some developers)...

I assume the mechanical folks have come up with a certain drivetrain setup, so frankly they should be able to deliver the required data, like nominal torque, max speed variation, etc... Based on those elementary data, you should be able to size the motor & VFD.

 

[10BaseT.]

Why is always up to the electrical engineer to prove a motor's capabilities? Too often I've been struggling to convince mechanics (not to mention some developers)...

I assume the mechanical folks have come up with a certain drivetrain setup, so frankly they should be able to deliver the required data, like nominal torque, max speed variation, etc... Based on those elementary data, you should be able to size the motor & VFD.

It is not , it was what the Rating Plate and issued motor data are for . Half the time mechanics seem to use a VFD as a saviour for all sins of wrong drive ratios etc , and would have you running the thing between 5Hz and 287Hz given the chance -
Ask them to provide the torque characteristics of the load - that should shut them up .

 

[Sparkz]

... Half the time mechanics seem to use a VFD as a saviour for all sins of wrong drive ratios etc , and would have you running the thing between 5Hz and 287Hz given the chance ...

This sounds so familiar...

 

[L D[AR2P#0.0]]

agarb - What is the application for the motor - is it something very critical and hence the mention of torque ripple ?

 

[agarb]

agarb - What is the application for the motor - is it something very critical and hence the mention of torque ripple ?

No, it is a boiler-plate specification from a large engineering firm that somebody should have taken exception to in the quoting process. Now we have the motors and drives and have to get the whole mess tested.

 

[allscott]

Get the characteristic curves from whoever you bought the motor from. What you are trying to do has already been done for you.

Here's a hint for next time, you are supposed to size the motors to the load before you buy them! I'm sure it wasn't your fault though.

 

[agarb]

We have characteristic curves for the motors. We have manufacturer's standard test reports for the drives. We have manufacturer's standard test reports for the motors. But, the customer's spec says they must be tested as a system.

We've been building this and similar type equipment for years. I'm sure they are sized to do the job we need them to do.

But, to meet our contractual obligations, I need test results for the assembled drive and motor system. Silly? Perhaps, but I need to get the piece of paper that shows we did it.

To whoever posted the magtrol quote, thanks. I'll check them out. The Rockwell folks in Switzerland are a little far away.

 

[allscott]

Are you sure they don't want you to do these tests after or during commisioning of the installed system?

Most of the values you are after can be taken out of the drive.

 

[10BaseT.]

No, it is a boiler-plate specification from a large engineering firm that somebody should have taken exception to in the quoting process. Now we have the motors and drives and have to get the whole mess tested.

Ahh you have to deal with idiot salesmen too ! I periodically have to tidy up the mess left behind by two geriatrics who once read a book on engineering in 1906 - they issue qoutes with comment like "BEARINGS :-All bearings on the dryer are of the anti friction type"
For the life of me I can't see why you would want bearings that had a high coefficient of friction , and this rubbish goes on and on !

Still , keeps us in work .....