Why does Induction Motor Torque increase when the Current Decreases?

The correlation between slip, torque, & current in an induction motor explains why torque increases as current decreases.

Here’s a simple explanation:

Slip & Torque Relationship

Slip (s) in an induction motor is described as the difference between the synchronous and rotor speeds, given as a fraction of the synchronous speed:

𝑠 = ( 𝑁 𝑠 βˆ’ 𝑁 π‘Ÿ ) / 𝑁 𝑠

Torque (T) is proportional to slip in the linear section of the torque-slip curve, particularly for low slip values (near synchronous speed). Torque grows roughly linearly from 0 to the greatest torque point (breakdown torque).

Current & Torque Relationship

The rotor current (Ir) is slip-dependent. Slip increases the relative speed between the revolving magnetic field and the rotor, causing a larger electromotive force (EMF) in the rotor and increasing the rotor current.

In the linear area of the torque-slip curve, torque (T) is proportional to rotor current (Ir).


When the motor is operating at low slip (near synchronous speed), any increase in load causes the slip to increase, increasing the rotor current and, as a result, torque.

However, after the slip exceeds a certain amount, the motor enters a region where an increase in current does not linearly increase torque due to reasons such as higher rotor resistance and core losses.

The maximum torque of an induction motor occurs at a specific slip angle. If the motor is working in the zone where the slip is less than the maximum torque point, reducing the slip (by lowering the load) results in a decrease in current while retaining relatively high torque. when a result, in this zone, when the current reduces (due to lower slip), the torque either remains relatively high or increases until it achieves peak torque.

To summarize, torque rise with current decrease can occur when the motor is operating near the point of maximum torque (breakdown torque) & then slightly reducing the load, which reduces current while maintaining high torque due to the torque-slip curve’s characteristics.