Torque at Standstill

Author: E.E. Kimberly

The torque of a squirrel-cage motor is produced by the mutual reaction of the main rotating field and the rotor field which is produced by the rotor currents. The rotor bars and the end rings are both resistive and inductive, and therefore the bar currents lag the induced electromotive forces which produce them. When the motor is at standstill, the frequency of the rotor currents is the same as that of the stator current. Between standstill and synchronous speed the rotor-current frequency is proportional to the slip. Therefore, the rotor reactance is

It may be showN₃ that the maximum torque is developed in a rotor when the rotor reactance is approximately equal to the rotor resistance. If a motor be designed so that the rotor resistance and reactance are equal at standstill, the maximum torque will occur at standstill and the speed-torque curve will be similar to curve (a) of Fig. 18-9.

If the rotor resistance be made somewhat less, but the inductance be kept as before, the resistance and reactance will become equal at some speed other than zero and the maximum torque will occur at that speed, as indicated in Fig. 18-9 by curve (b). If then the resistance be still further reduced as far as practicable, a characteristic curve similar to curve (c) may be obtained. It is significant that the maximum values of curves (a), (b), and (c) are all equal] and that the change in rotor resistance produces only a change in the speed at which the maximum torque occurs, and also a change in torque at standstill or starting torque. This phenomenon is of great industrial importance in adapting motor characteristics to special applications.

Fig. 18-9. Induction Motor Characteristic Speed-Torque Curves