Induction Motor Speed–Torque Characteristics
The speed–torque characteristics and torque–slip characteristics of an induction motor are important parameters for determining the performance of the motor. Typical speed–torque characteristic and torque–slip characteristics of a three-phase induction motor are shown in Figure 4.2.
It can be seen that when a motor starts from zero speed, the start torque is lower than the full load torque and the motor can start at light-to-no load.
The normal full load torque is achieved at a point where the rotor speed is only 5% less than the synchronous speed. From this point onwards, the torque drops to zero value since there is no relative motion or slip between the stator and the rotor.
In order to achieve a high starting torque, the rotor is made with high-resistance conductors or else an external resistance is inserted in the rotor circuit.
The nature of the characteristic curve can be changed, in case of a slip-ring type induction motor, by inserting an external resistance in the rotor circuit. If the rotor resistance is increased from r1 to r2, r3, r4 (r1 < r2 < r3 < r4), then the maximum torque remains the same, but the slip at which the maximum torque occurs is shifted, as shown in Figure 4.2. The method of introducing an external resistance in rotor circuit is used, to obtain a higher starting torque, as required, up to the maximum torque limit that the motor can produce. This method of increasing the starting torque can be used only in the case of slip ring or wound rotor induction motors.