locked rotor code using a VFD 1

[sgtcmh10]

I had to do a locked rotor test but unfortunately i was also using a VFD. The application was 415 V, 50Hz and the hp was 29.72. Locked rotor current recorded as the drive attained 50 Hz was 125A. Seemed low and due to the drive it wasn't a true instantaneous reading. Any suggestions?

 

[jraef]

The term "Locked Rotor Test" on a motor would be meaningless behind a drive, the drive will go into current limit and/or trip off line long before you get anywhere near the Locked Rotor Current of the motor. Either remove the drive and start across the line (DOL), or reevaluate your need for a locked rotor test. Why are you doing that anyway?

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[aolalde]

As Raef stated the LRC does not make sense if a VFD will power the motor.
See IEEE Std 112 part 7 for details on how to perform that test. Normally the motor manufacturer makes this test for quality purposes.

 

[sgtcmh10]

Sorry I omitted some useful information. The VFD was not specifically sized for this motor. Typically I would agree with the overload remark. However, this is a test lab VFD with a current rating output up to 260 amps. The test was being performed to identify the locked rotor current so we could note the Locked rotor code on our nameplate. The motor is a canned motor. I'm thinking the test might be ok since the readings were taken with 10 seconds upon motor start. The acceleration of the drive got to 50 Hz prior to the 10s criteria set forth by IEEE.

I appreciate the replies, please post more. Thanks

 

[DickDV]

Locked Rotor Current assumes a sine wave Power supply with near infinite source current.

That certainly does not describe an inverter. I would expect any data you would get on an inverter source to be invalid when operating on sine wave power regardless of the ampacity of the inverter.

 

[sgtcmh10]

I think what i'll need to do is perform some test on some 60 Hz motors with and without the VFD to compare results. I was hoping that some one might have run into this before and been able to provide some validation in or out of favor. Thanks again for all the posts.

 

[jraef]

Plenty of motor shops use inverters for creating test power, but they are not doing LRC testing on new motor designs. I agree with DickDV, this kind of a test behind an inverter would be suspect. Your numbers don't seem right either, although you don't give us all the info such as FLA and torque-speed curve design. If it is a design B curve, that LRC value seemes extremely low. 22kW @ 415V would I think be around 40A, which means your measured LRC is only 313%. I would suspect the VFD has something to do with that.

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