Locked rotor time 4000V

[eemotor]

Hello to all,
I've been going through NEMA and IEEE standards trying to conclude the generally accepted minimum stall time for a 4000V, 3ph., 60HZ, 1250HP induction motor. NEMA specifies that that motors should be able to withstand locked rotor currents for at least 12 seconds. However, this only applies to motors rated at 500HP or less. IEEE 112-1996 specifies that the readings for the locked rotor test should be taken within 5 seconds after voltage is applied. Does anyone know of a standard or specification that might indicate the locked rotor time otherwise. Thank you all.

 

[jbartos]

Suggestion: Larger HP motors usually have their thermal damage curve available. Plotting the motor damage curve and the motor acceleration curve on the same I-t chart will reveal amount of time the motor can stay in the locked rotor mode (stalled) without being damaged.

 

[aolalde]

NEMA MG1 part 20, Large Induction Motors does not recommend a minimum allowable stall time, However it specifies the maximum value of load inertia constant ( W*k^2), reflected at the motor shaft, that the motor should accelerate at full voltage, assuming a load torque which varies proportional to the squared ratio of the speed and equals the full load torque at nominal speed.

 

[aolalde]

The motor should start twice if it is at room temperature, or once if it has the full load operating temperature.

 

[jbartos]

Comment: The motor can be designed for various starting duty. The motor manufacturer tech support should provide this information if it is not included on motor drawings or in the motor data sheet.

 

[eemotor]

Thanks for your input,
I agree that referencing to the thermal damage curve is the proper course of action regarding locked rotor times for large HP machines.