Induction motor stall time?

[veritas]

Does anybody have some useful hints on how to calculate the hot and cold stall times for induction motors if there is no data readily available? I presume the locked rotor amps is 6 x FLA.

I am looking at a 375hp (285kW), totally enclosed =, fan cooled, threephase induction motor with the following nameplate details :

Hp = 375
RPM = 490
Insulation Class B
Stator Amps = 65
Mass = 5100kg
Manufactured to BS 2613 : 1957
Rating Cont. 80 degree rise (don't quite get what this means)

Thanks and regards.

 

[electricpete]

“Rating Cont. 80 degree rise” means your stator insulation is rated for continuous operation at 80C average rise above 40C ambient. This is standard for class B insulation.

I don’t think there is any easy way for a user to calculate stall time. Stall time and thermal damage curve usually come from the oem. By the way if you were lucky enough to have a thermal damage curve, you can read the stall time off of it.


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

 

[eemotor]

As Electricpete mentioned, you could read the stall time right off of the thermal damage curve, provided you had one. What voltage does this motor operate at? 3300V? If so, chances are your stator may be able to withstand more time under locked rotor condition than your rotor will. It's important however, that the motor doesn't exceed maximum temperature for the rotor and the stator, which would seem to be (80 C + ambient temperature for which the motor was designed for) on the stator. As far as the maximum temperature on the rotor, I suppose I would want to subject copper bars to more than 300 C, less for aluminum.

 

[electricpete]

NEMA MG-1 2003

"12.49 STALL TIME
Polyphase motors having outputs not exceeding 500 horsepower and rated voltage not exceeding 1kV
shall be capable of withstanding locked-rotor current for not less than 12 seconds when the motor is
initially at normal operating temperatures."

Your motor should meet this cold stall time requirement if it was purchased to this standard.

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

 

[Mstrvb19]

I agree with electricpete and eemotor-
parendse-if you are looking for this information for the purposes of protective relaying, and you can't ensure that the motor was bought to NEMA MG-1, you could perform a simple conservative calculation, but this only works if you have room in your coordination scheme to be conservative.
The the safe stall time is usually determined by a test to insulation breakdown, which you don't want to do if you intend to keep using your motor. Instead, try determing the winding resistance of the machine. Keep in mind that it is likely that the skin effect and the proximity effect will probably have significant effects on the resistance for motor windings, so you need to be conservative here. Then you find the thermal conductivity/resistivity of the insulation. Calculate approximately how long it will take to reach 120C at locked rotor current based on the power dissipation in the motor windings and this will give you an idea for your protection curves. Be careful, though, this is tricky, and I would definitely recommend using the 12 second defualt time if you can-this is a last ditch method. The hard part is ensuring that you are being conservative when you don't know the extent of the additional resistance due to the skin effect and proximity effects, nor do you know the thermal characteristics of the machine as a whole. You can take advantage of performing your analysis ignoring the cooling fan and that will gain you something.