What parameters affect the starting torque of a motor?

Starting torque is a crucial performance parameter for motors , especially for direct-start motors. It is expressed as a multiple of the rated torque, and for direct-start squirrel-cage motors, a starting torque of approximately twice the rated torque is generally required. The starting torque requirements for hoisting and metallurgical motors are even higher.

When the power supply remains constant, i.e., both the power supply voltage and frequency remain constant, the starting torque is negatively correlated with the stator and rotor. That is, the larger the leakage reactance of the stator and rotor, the smaller the starting torque. Otherwise, effectively controlling and reducing the leakage reactance of the stator and rotor can improve the starting torque of the motor.

For squirrel-cage motors, deep-slot rotors or double-cage rotors can be used to increase starting torque by utilizing the skin effect of the current. For wound-rotor motors, the starting torque can be increased by increasing the rotor resistance; when the total resistance of the rotor circuit is increased to the total leakage reactance, the starting torque can reach the maximum torque, which is also the upper limit of the motor torque.

Similarly, the starting torque of a motor is inversely proportional to the power supply frequency. That is, the higher the power supply frequency, the lower the starting torque of the motor, and vice versa. This is the essence of the use of low-frequency starting for variable frequency motors. In other words, the motor operates at constant torque when the frequency is below the base frequency, and at constant power when the frequency is above the base frequency.

Analyzing the condition of constant motor power, the basic principle of variable frequency motor operation is high torque and low speed at low speed, and high speed and low torque at high frequency.