The relationship between rotor resistance and starting torque of an induction motor

There is a close relationship between the rotor resistance of induction motor and its starting torque. Starting torque refers to the torque generated when the motor is started in a static state, which is an important indicator to measure the starting performance of the motor. The following is a detailed explanation of the relationship between rotor resistance and starting torque:

Equivalent circuit model at startup

In order to understand the effect of rotor resistance on starting torque, it is first necessary to understand the equivalent circuit model of induction motor at starting. At the start of the motor, the speed is zero, and the equivalent circuit can be simplified to a circuit containing the stator winding and the rotor winding.

Torque expression at startup

When starting, the torque T of the induction motor can be expressed by the following formula:

• Es is the stator voltage;

• R 'r is the rotor resistance (converted to the stator side);

• Rs is the stator resistance;

• Xs is stator reactance;

• X 'r is the rotor reactance (converted to the stator side);

• k is a constant factor that is related to the physical size and design of the motor.

Effect of rotor resistance

Starting torque is proportional to rotor resistance: As can be seen from the above formula, starting torque is proportional to rotor resistance R 'r. In other words, increasing the rotor resistance can increase the starting torque.

The starting current Is inversely proportional to the rotor resistance: The starting current is inversely proportional to the rotor resistance R 'r, that is, increasing the rotor resistance will cause the starting current to decrease.

Concrete impact

• Increase in starting torque: Increasing the rotor resistance can increase the starting torque, which is very important in applications where large starting torques are required.

• Reduced starting current: Increasing rotor resistance can also reduce starting current, which helps protect the grid from large current shocks, especially if multiple motors are started at the same time.

•   Efficiency impact:Increasing the rotor resistance improves the starting torque, but during the operation of the motor, too much rotor resistance will lead to a decrease in efficiency because of the increased energy loss.

Coil rotor induction motor (WRIM)

Wire-wound rotor induction motors (WRIM) allow external resistance through slip rings and brushes, which dynamically adjust rotor resistance to obtain a large starting torque at startup. After starting, the normal operation efficiency of the motor can be restored by gradually reducing the additional resistance.

Sum up

There is a proportional relationship between the rotor resistance of induction motor and its starting torque. Increasing the rotor resistance can improve the starting torque, but it also affects the starting current and operating efficiency. Therefore, when designing and selecting a motor, factors such as starting torque, starting current and operating efficiency need to be considered comprehensively to achieve the best performance balance.