Service factor for electric motors

[Engineer6512]

A quick question regarding the service factor on the nameplate of electric motors. This is the percentage above rated HP the motor can operate safely. My question is whether this means the motor can run at this elevated HP for extended periods? Or is it simply intended for short intervals. What effect would running it for long intervals have on the life of the motor?

Steamguy2

http://www.SteamPlantEngineering.com

- Discussion & resources for professionals in the Power Generation Industry

 

[electricpete]

In theory, running at service factor load increases allowable temperature rise by 10C and reduces the insulation life by half.

Most likely the actual temperature rise will increase by more than 10C going from FLA to SFA.

The halving is an empirical relationship and there are a lot of assumptions built into it.

Most people would consider SF loading is intended for temporary overloads not continuous operation.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[jraef]

Agreed. The generl assumption is that although the motor can run into the service factor and still provide adequate performance, expect a decrease in motor life. NEMA MG-1 states this but does not specify a rate of decreased life expectancy, leaving that open to interpretation by the various mfgrs. The best approach is to nt rely upon SF for continuous operation, keeping it in reserve for small voltage drops throughout the day, or occasional load changes.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[SteveKW]

In the pump industry, it is common to use the motor into its service factor continuously.

 

[dpc]

Another consideration is that all of specified motor efficiency and power factor performance is at the rated horsepower (and on a pure sine wave, BTW). If the motor is operated into the service factor, the efficiency may be less than expected.

 

[aolalde]

In my opinion the service factor should be used only for short periods of overload.
Using the motor continuously into its service factor as in the pump industry is not a recommended practice.
Pump industry providers, having a fierce competition for initial cost, offer a motor-pump sets operating into marginal limits that will result in a problem for the final user.
Since the voltage fluctuations, phase unbalance and harmonic content of the electric power supplied to the motor will increase the motor losses; working on the service factor will result in shortened life expectancies for the motor.

 

[jraef]

I agree with the above. I am right now looking at a motor performance curve, and it shows highest eficiency and best Pf at 75% load, then at 115% load both values drop to being lower than at 100%

3300V 1195HP 50Hz

%Load Pf% Eff%
115 91.2 93.7
100 91.6 94.6
75 91.8 95.8

That loss of eficiency translates to more heat losses, which will result in higher winding temperatures and earlier insulation breakdown. Maybe if a pump application is not really using all of the available HP anyway, the net result would be negligible, but no matter what, heat x time = failure.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Engineer6512]

Thanks for all the responses guys, it was a big help. I'm going to throttle back and run it at 100%.

Steamguy2

http://www.SteamPlantEngineering.com

- Discussion & resources for professionals in the Power Generation Industry