Induction motors Maximum torque

The magnetic field rotates at a synchronous speed, so it should also rotate the rotor. But this is not so in an induction motor. During start-up, the rate of cutting of llux is the maximum and so is the induced e.m.f. in the rotor circuit. It diminishes with motor speed due to the reduced relative speed between the rotor and the stator flux. At a synchronous speed, there is no linkage of flux and thus no induced e.m.f. in the rotor circuit, consequently the torque developed is zero. 

Typically in the lower torque range (geared hoist drives), the preference is for squirrel-cage induction motors. Variable speed drive application allows for Optimum Pole design of the motors, where the number of motor poles is selected by taking into account the torque density, power factor and speed range, rather than the maximum operating speed for a fixed frequency. 

FIGURE 3-14. Performance Characteristics of Motors. For typical squirrel-cage induction motors. Points (I) show the speed-torque limits during initial acceleration. Area is the maximum torque range recommended for short-term overloads. Point is the reverse-curvature point on the speed-torque curve and is the point of minimum accelerating torque. 

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. 

The maximum torque for an induction motor is limited by the adjustable-frequency controller current rating. In order to determine the maximum torque that would be available from an induction motor, it would be necessary to define the motor torque at the controller s maximum current rating. 

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