VFD driven induction motor has flickering currents at lower speed

[BigLe]

I have a 3 hp Dayton induction motor which is rated at 208-230/460V 9.7-9.4A/4.7A at 60hz. 4 pole and sf=1.

when driven with a v-hz type drive with a const v/hz gain 208/60, it behaves strange in the range of 20 to 30hz or so. the three phase currents flickered and the sound from the motor becomes upsetting.. all test were under no load.

Anyone gets similar experience? it was the second motor and second drive we tried.. same behavior.. or is this a normal thing to unloaded motor not desinged for variable speed application?

Thanks

BigLe

 

[Skogsgurra]

No. Not seen that. Not unloaded.

Is your drive connected to a three phase mains or single phase? A single phase supply sometimes makes the DC link ripple a little too much and can cause intermodulation between the resulting 120 Hz ripple and multiples of the drive frequency. If that is the case, you should have the effect also around 60 Hz motor frequency.

Have you checked that all three phases are present at the drive input? Most drives have a built-in supervision, but since I don't know what drive you use it is hard to tell if yours has one.

A severe unbalance in the three-phase mains may also cause this phenomenon. Three or four percent are sometimes sufficient with larger drives.

What happens if you take the V/Hz ratio down? Is the flicker still there? What make is the drive?

Gunnar Englund

http://www.gke.org

 

[BigLe]

Thank you for the analysis, skogsgurra. The drive I was using is a MagneTek GPD205 model CIMR-XCBD21P5. rated for 3hp 0-230Vvac input and 2.7kVA 7A output. max freq 400hz. it was on a 3phase main when the problem occured. I will check the balance of the ac main later then. But the other drive, which was on a magna-power dc supply, gave similar results. When lower the V/HZ ratio to a quater of the 230/60, the unstable region is still there but getting a bit better (mild flucturation still persists). Raise the ratio actually helped remove the flickering at steady state but the transient became nasty.. higher v/hz ratio is not desirable due to the over-excited situation.

 

[petronila]

Hi Bigle,

The noise and vibration could indicate a resonance issue when the motor is operated too close to the natural frequency of the stator teeth, shaft, or the entire motor. You could do a bump test with a vibration analyzer the identify the resonant frequencies of the motor.
Or there could be a problem with the drive itself, perhaps the firing frequency. Are you able to load the motor to see if the problem continues? If not, then it is not a resonance issue - but probably related to the firing frequency of the drive.

An overhauling load can cause regeneration, which could be the cause here. If the motor is coupled to the load, and being slowed down when the noise occurs, that is likely. Motors not designed for VFD use can exhibit trouble, such as increased rotor heating.

Regards

PETRONILA

 

[electricuwe]

The problem you observe may also be related to the effects of the dead-time necessary to avoid shoot through in an inverter phase leg.

Are you able to vary the swicthing frequency ? If the oscillations disappear at lower switching frequency the dead-time-effect is likly to be the cause.

For more information see the following conference paper:

T. Kalker, A. Ackva, U. Jansen: "Novel digital controller for induction machines considering the inverter switching times and a fluctuating DC-link voltage" European Conference on Power Electronics (EPE), Firenze 1991, p. 2-054 to 2-063.

 

[DickDV]

It may be that the drive has slip compensation capability and it is set too high. Instability is usually the result.

In my experience, about half of the motor nameplate slip is all you can compensate out of a V/Hz system.