Breakdown Torque

[butelja]

Is there a rule of thumb for the ratio of breakdown torque to full load running torque for NEMA B 3 phase motors? I've been told it is aroung 3.0 +/-.

 

[redtrumpet]

Breakdown torques for motors are given in NEMA MG 1, Part 12. The value varies with speed and motor size. The breakdown torque is given in percent of full-load torque. For NEMA B motors 250 - 500 hp, minimum breakdown torque is 175% of full-load torque. For the vast majority of NEMA B motors 5 - 200 hp, the minimum breakdown torque is 200% of full-load torque. For motors smaller than 5 hp, minimum breakdown torque ranges from 200 - 300%.

Of course, there is nothing to stop a manufacturer from designing a motor with more than the minimum breakdown torque.

 

[redtrumpet]

Addition to my previous post - looking at performance data published by Toshiba for their 575 V EQP III+ premium efficiency TEFC motor, their breakdown torques handily beat the NEMA minimums, ranging from a smoking 400% for 1.5 and 2 hp, 3600 rpm motors to 225% for a 100 hp, 1200 rpm motor.

I don't know if you could derive a rule of thumb that the ratio is 300% (breakdown to full-load torque). In general, the smaller motors have higher breakdown torque than the big ones.

 

[butelja]

Thanks for the info. For the preliminary work, I'll use 200%-400%, as most motors I deal with are below 100 HP. For final work, I'll get the data sheet from the manufacturer.

 

[jbartos]

Suggestion: It can also be calculated, if you know other suitable parameters entering the relationships. Visit

http://www.textrolinc.com/formulas.html

for more info.