How can we reduce the starting torque of an induction motor?

(1) Stator winding series resistance or reactance start

Principle: By connecting a resistor or reactor in series with the stator winding of the motor, the voltage drop across the resistor or reactor reduces the voltage applied to the motor winding below the source voltage, thereby reducing the starting torque. After starting, the resistor or reactor is short-circuited to allow the motor to operate at rated voltage. This method is suitable for cage-type induction motors with moderate capacity requiring smooth starting. However, the starting resistor consumes a certain amount of power and should not be started frequently. Additionally, the starting torque decreases due to the reduced starting current.

(II) Using the Reduced Voltage Starting Method

1. Autotransformer Voltage Reduction Starting

Principle: When starting the electric motor, connect the three-phase AC power supply to the electric motor through an autotransformer. The autotransformer can be selected with different transformer voltage taps according to the allowed starting current and the required starting torque, reducing the voltage applied to the electric motor and thus reducing the starting torque. After the start-up is complete, the autotransformer is disconnected, allowing the electric motor to be directly connected to the three-phase power supply for normal operation. It is suitable for larger capacity motors and has the advantages of compact line structure and no restrictions on the wiring mode of the motor windings.

2. Y-Δ Starting (for three-phase induction motors)

Principle: For three-phase induction motors that are normally connected in a delta configuration during operation, the starting process begins with a Y-shaped connection. At this point, the voltage applied to each phase winding is One third of the square root times the normal operating voltage, resulting in a lower voltage and, consequently, a reduction in starting current and torque. After the start is complete, the motor is switched back to the delta configuration for normal operation. This method is simple and economical, but it significantly reduces the starting torque, making it suitable for light load or no-load starting scenarios.

(3) Adjust the load characteristics of the motor

Principle: If the inertia of the load driven by the motor is large or the torque characteristic of the load can be adjusted during startup, increasing the resistance torque of the load appropriately (e.g., using a braking device for certain mechanical loads to apply resistance at the moment of startup) can relatively reduce the effective torque output of the motor during startup, thus achieving the effect of reducing the starting torque. However, this method requires careful operation based on the specific load conditions to avoid any adverse effects on the motor and load equipment.