Two motor problem on a single VFD

[ccaissie]

Hi:

We have 12 flocculators running at a Water Treatment plant, and have multiple and repetitive failures of the motor shaft and gearbox shaft. Via tandem gearboxes, we operate the 90" impellers in water.

The 3hp 1200rpm 3 phase motors operate in pairs, two per VFD, and run at about 70% speed most of the time. Summer time puts them up to 100%.

The failure mode involves gradual destruction of the keyway of the motor, and sometimes the hollow quill on the gearbox, and disintegration of the key itself.

We are believing that the destruction is from a constant acceleration/deceleration of the motor and a "chucking" of the shaft and quill. We thought the waveform of the pulse width modulation in the VFD might be a factor, and recently someone suggested that the power sharing of two motors on a single VFD might cause imbalances and alternating loads between the two motors.

This one has us stumped. We have hundreds of these units out there and mechanically they have been bulletproof until this job...

Any insight into the role of VFD's in this problem? How do we begin to unravel the root cause here??

Colin S. Caissie
Designer,
UET Mixers Inc.

 

[DickDV]

It's extremely unlikely that the carrier frequency pulses would be responsible. To test this, you could set the carrier frequency up much higher and check for "chucking" but I doubt this is the problem.

Oscillations between the two motors are unlikely as well.

Is it possible that these drives have been set up as sensorless vector controllers. If they are, it may be that the P and I gains are set too high causing some shaft oscillations. Actually, with multiple motors, sensorless vector isn't going to get you anything in performance so I would use V/Hz only.

You could have oscillations in your speed reference. It would be interesting to see what happens when you use fixed speed inputs for speed control instead of an analog speed signal. Fixed speeds eliminate any chance of speed reference noise being involved.

 

[jraef]

Dick beat me to it. That is exactly what happens when you try to run multiple motors in Open Loop Vector mode. Someone probably forgot to change it to V/Hz mode in setup. The drive output is hunting, trying to stabilize on a moving target of motor feedback.


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[LionelHutz]

I'll say the same. Sounds like a sensorless vector problem. Make sure the VFD is set to use V/hz mode.