What is the effect of slip on torque in an AC induction motor?

Slip (Slip) is an important parameter for AC induction motors, and it significantly affects the motor's torque (Torque). Slip is defined as the ratio of the difference between the synchronous speed and the actual rotor speed to the synchronous speed. Slip can be expressed using the following formula:

where:

s is the slip

ns is the synchronous speed

nr is the actual rotor speed

Impact of Slip on Torque

Slip at Startup

At startup, the rotor is stationary, i.e., 

nr=0, so the slip s=1.

At startup, the rotor current is maximum, and the magnetic flux density is also maximum, resulting in a high starting torque (Starting Torque).

Slip During Operation:

When the motor is running, the rotor speed 

nr is close to but less than the synchronous speed 

ns , so the slip 

s is less than 1 but greater than 0.

The larger the slip, the greater the rotor current, and consequently, the greater the electromagnetic torque. Therefore, slip is directly proportional to torque.

Maximum Torque

There is a specific slip value, known as the critical slip (Critical Slip), at which the motor produces maximum torque (Maximum Torque).

Maximum torque typically occurs at a slip of about 0.2 to 0.3, depending on the motor's design parameters, such as rotor resistance and leakage reactance.

Steady-State Operation

During steady-state operation, the slip is usually small, typically between 0.01 and 0.05.

At this point, the motor's torque is relatively stable but not at its maximum.

Relationship Between Slip and Torque

The relationship between slip and torque can be represented by a curve, which is usually parabolic. The peak of the curve corresponds to the maximum torque, where the slip reaches the critical value.

Factors Affecting Slip

Load

When the load increases, the rotor speed decreases, increasing the slip and the torque, until a new equilibrium is established.

If the load exceeds the load corresponding to the maximum torque, the motor will stall.

Rotor Resistance

Increasing the rotor resistance can increase the maximum torque and starting torque, but it will also reduce the motor's efficiency and operating speed.

Supply Voltage

A decrease in supply voltage results in a decrease in rotor current, thereby reducing the torque. Conversely, increasing the supply voltage can increase the torque.

Summary

Slip significantly affects the torque of an AC induction motor. The greater the slip, the greater the torque, up to the point of maximum torque at the critical slip. Understanding the relationship between slip and torque is crucial for correctly selecting and using AC induction motors.