Motor overamp

[hilltop]

Why will a large motor overamp if you run it without a load?
For instance we were testing a motor we just replaced the bearings on before we put the belts backon.
It is a 3 phase 60HP motor, 480V on a GE soft start. The soft start stopped the motor and the code displayed said it was due to high amps.

 

[jumper]

I would assume an unloaded motor would pull less amps.:?

 

[stew]

typically a 3 phase motor with no load will draw maybe 15 or 20 % of nameplate amps no load.Just enough to magnetize and overcome friction of rotor and windage.

 

[kwired]

It isn't single phasing is it?

Did it accelerate to speed?

A motor with no coupled load usually runs well below FLA. If your 60 HP motor is 480 volt with FLA of 77 you may only see 15-20 amps with nothing connected.

It may have some winding damage - hopefully not from disassembling to fix the bearings - seen that happen before - luckily not on a motor that big.

 

[ptonsparky]

Are you sure the SS parameters are set correctly?

 

[kwired]

Are you sure the SS parameters are set correctly?

I thought about that some - not as hard to mess those up to that extreme as it may be with a VFD so I didn't mention it. Could have a failing SS control. Try starting the unloaded motor across the line and see how it performs. Maybe meg the windings first - just to rule out ground faults - soft starter can dampen those enough that there is no violent current - across the line will just go boom.