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Breakdown Torque

Breakdown Torque

Breakdown Torque

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 +/-.  

RE: Breakdown Torque

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.

RE: Breakdown Torque

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.

RE: Breakdown Torque

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.

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