VFD/ Motor question

[pkbrown0138]

Just replaced a 480/3/60- 50hp motor that failed after many years of service. This motor is attached to a VFD drive in a MCC. The electrican replaced the old motor with an identical new motor. Now, the new motor is pulling 73 amps at 60hz. The name plate FLA is 59 amps. The motor connections have been checked for thightness both at the motor and drive. There are two other 50hp motors on this same MCC that are operating fine. Any idea why this motor is amping so much higher?

 

[Jraef]

Just replaced a 480/3/60- 50hp motor that failed after many years of service. This motor is attached to a VFD drive in a MCC. The electrican replaced the old motor with an identical new motor. Now, the new motor is pulling 73 amps at 60hz. The name plate FLA is 59 amps. The motor connections have been checked for thightness both at the motor and drive. There are two other 50hp motors on this same MCC that are operating fine. Any idea why this motor is amping so much higher?

In order of likelihood:

1. You have something set incorrectly in the VFD, i.e. the V/Hz pattern, motor rating, something like that.
2. There is something wrong in the power train components, i.e. a bad coupling, journal or bearing that is NOT in the motor itself.
3. There is something wrong with the VFD, i.e. a failing transistor, bad firing circuit etc..

The easiest way to check this is in your situation to swap with one of the known good VFDs and see if the problem is still there. If it isn't, the problem is in the VFD. If it is, the problem is in the power train.

 

[Besoeker]

Just replaced a 480/3/60- 50hp motor that failed after many years of service. This motor is attached to a VFD drive in a MCC. The electrican replaced the old motor with an identical new motor. Now, the new motor is pulling 73 amps at 60hz. The name plate FLA is 59 amps. The motor connections have been checked for thightness both at the motor and drive. There are two other 50hp motors on this same MCC that are operating fine. Any idea why this motor is amping so much higher?

Maybe something on the load has changed and that's what caused the original motor to fail?

 

[TxEngr]

Break the coupling to the load (pump, fan, etc.) then run the motor. If it still shows high loads, or at least high loads for an unloaded motor, look at the motor and drive. If not, then it's the actual load and probably what killed the first motor. Note that in most VFDs you can limit the current to 100% of FLA to prevent what you have happening. A quick test to narrow down the problem.

Txengr

 

[Besoeker]

Break the coupling to the load (pump, fan, etc.) then run the motor. If it still shows high loads, or at least high loads for an unloaded motor, look at the motor and drive. If not, then it's the actual load and probably what killed the first motor. Txengr

Good idea. Just this afternoon, we had a similar sort of problem reported - a motor taking far too much current. Our project engineer suggested exactly what you did.

At the time the drive was tested in our works, the drive default settings were used. What we didn't know then, but do know now, is that the motor being installed was rated at half that of the drive. Had we known, we would have set up the drive accordingly. Hindsight......
However, we do send out comprehensive O&M manual and a record of all the drive settings. However, the guy commissioning the plant is mechanical.....'nuff said.