Induction motors stalling

Interrupting an induction motor under a stalled condition which is almost a locked rotor condition. 

There are two important time periods that are critical in the application of induction motors. One is the allowable run-up or starting time and the other is the maximum stalling time. 

It may be seen that if this method is used to control the speed of a standard, almost constant-speed type of induction motor, then the actual range of speed control obtained will, in fact, be small before stalling occurs. The situation could be improved by using a motor with a high rotor resistance as shown in Figure 14.2. The rotor resistance at full-load has been increased by a factor of 10 in order to demonstrate the effect on the torque-speed characteristic. 

Motor stall torque Induction motors can tolerate 100% stall torque indefinitely because current is shared equally by all three phases at slip frequency. A synchronous motor will require a DC current at stall, and therefore thermal capacity is reduced in both motor and inverter at stall conditions. 

If the load-torque requirement ever exceeded the maximum torque capability of the induction motor, the motor would not have enough torque to accelerate the load and would stall. For instance, if the load line required more torque than the motor could produce at the pullup torque point, i.e., 170 percent load torque versus 140 percent pullup torque, the motor would not increase in speed past the pullup torque speed and would not be able to accelerate the load. This would cause the motor to overheat. It is, therefore, important to ensure that the motor has adequate accelerating torque to reach full speed. 

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