4160 VOLT MOTOR RUNNING @ 480 VOLT? 3

[wolfie1a]

I feel kinda of stupid asking this question, but here goes. We recently sent one of our 4160V motors out to a repair shop. We typically have the motor run on a dyne after the repair. The motor shop told us the dyne was out but they test ran the motor on 480 Volt. When I first heard this I thought is was incorrect info but the motor repair shop assured me this was correct. Seems to me if you attempted to run 4160 volt motor at 480 the current would be too high? Any one heard of such a testing method?

 

[LionelHutz]

They didn't run it under load so it must have been an uncoupled rotation test. The current would have been about 8.7 times less than the 4160V current.

 

[edison123]

Not all rewind shops have the facility to run motors at their rated volage given the wide range of motor operating voltages.

A LV run is normally done to check for mechanical troubles like vibration, high bearing temperatures etc.

As LionelHutaz says, the motor no-load current at 480 V will only be a fraction of that at 4160 V.

 

[oldfieldguy]

I worked in motor repair ofr a while myself. Our shop's test stand had 480 volts only, and we regularly ran medium-voltage motors on it at full RPM for vibration analysis after rebuild.

They don't accelerate very fast, but they do get to full speed.

old field guy

 

[Skokian]

The starting current is down by the voltage ration, the load poriton of the running current is increased by the voltage ratio.

The no load current is mostly inductive, and mostly saturation currents. These will be down dramatically with lower applied voltage, expecially the first 20% or 30% reduction since this gets the iron out of saturation. After that, the no load current will continue to drop with lower applied voltage due to reduced hysteresis (core loss) and eddy current losses.

Hope this helps.

Regards . . .

Jim S. Nasby

 

[electricpete]

I agree with the above description of the current behavior.

I agree it is common practice to test motors at reduced voltage in the repair shop.

We have encountered one occasion where a slow speed vertical motor ran OK at the repair shop and then vibrated upon installation at a frequency of 1x running speed. The cause ended up being excess clearance between upper thrust runner and the shaft. The vibration in this case much lower under reduced voltage than under full voltage. Our specification now requires test run at full voltage. Sometimes we pay a premium for this (very few shops can accomodate 13.2kv), but it's worth it to identify a problem at the shop than after installation at the plant imo.


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[panter]

Hello everyone,
In our repair shop we have ability to ran motors on maximum 760V and sometime we can't reach full speed for some 2 pole 6kV motors .Very often , like Pete said, this type of motor has different behavior(vibration, temperature of bearings, . . . ) under reduced voltage and after installation on 6kV.We solve this problem with practice that after first testing in repair shop we make another testing in one pump station where we have 6kV cable and installation and where motor has nominal voltage and speed .If motor pass that testing ,we can guarantee good repair work and installation at the plant .