A.C. squirrel cage induction motor

[michaelt]

We are currently using two 250 h.p. motors on a coal cutting machine that are rated for 950 volts. Our power supply is 1100 volts, so my question is, is the increase in voltage hence torque going to increase the H.P. the motors develope, because we are encountering overload trips and the motors stalling out on heavy cutting.

 

[electricpete]

For a nema design B motor driving a pump or fan, it would be a good approximation to say that the speed is constant (to within a few percent), and therefore the load (pump) determines the load with little effect from voltage changes. It's true that if you boost the voltage you will go a little bit faster (smaller slip), but not too much (for NEMA B motor driving centrigaul pump/fan).

I'm not sure about your case. If it is a NEMA design D than there can be substantial change in slip and changing voltage would have a big effect on speed and torque. Also I'm not sure of the torque-speec characteristics of your load.

 

[jbartos]

Suggestion: Please, could you post the motor nameplate data. Also, the motor is not supposed to have increased the terminal voltage more than 10% on continuous basis according to NEMA Std MG-1.

 

[busbar]

NEMA ICS1-2000 §4.2

"The 1000-volt utilization voltage applies to equipment installed on 1040Y/600-volt electrical distribution systems typically used in mines."

 

[cbarn24050]

Hi, did I read this right? Your connecting a 950volt motor to a 1100 volt supply and you are suprised that it trips?

 

[busbar]

ehh,
1100/950=1.16
1040/1000=1.04

 

[electricpete]

I was directly addressing the question asked. What is effect on horsepower (output) the motor developes.

cbarn brings up a more relevant point that as you increase voltage above nameplate, at some point the current will start increasing due to increase in exciting current (core saturation).

Also with higher voltage you will always expect higher current during starting (but usually lower starting duration)

 

[buzzp]

Your problem is the high voltage. The current draw goes up with a decrease in nameplate volts and also goes up after about a 5% INCREASE in voltage above the nameplate volts. This is why your overload is tripping. The transformer should be retapped by the power company or if your company is responsible, then your company will have to retap the transformer or put in one with the appropriate ratio. Good luck.

 

[busbar]

Minor passing note.. It sounds like overvoltage is a problem. Has other equipment been found having similar problems? The OP should realize that if his operation has other loads fed from the same service entrance—oops, point of common coupling—that tapchanging upstream of the {maybe skid-mounted} local transformer can upset the apple cart. It may be a single load fed from a single utility-owned transformer, but doesn’t seem likely. At 1kV, this arrangement would be hen’s teeth.