Why increasing resistance in induction motor increases torque?

Increasing the resistance of an induction motor can enhance the starting torque, as it leads to an increase in the power factor and the active component of the rotor current. Specifically, augmenting the rotor resistance can improve the power factor, even though the rotor current decreases. Due to the significant enhancement of the power factor, the overall product of torque actually increases. However, it's important to note that the rotor resistance should not be excessively high; it needs to fall within an appropriate range of resistance values to ensure optimal performance.

In addition, for wound-rotor induction motors, the motor starting process can achieve small current and large torque by adjusting the rotor resistance. After the motor starts, the external resistance will be cut off to meet the performance requirements of normal operation of the motor. This technology allows for higher torque at the starting stage while maintaining lower starting current, thus protecting the motor and power grid.

In summary, increasing the resistance of an induction motor can increase torque under specific conditions (such as during startup), but it is necessary to adjust the resistance value within an appropriate range to balance various performance parameters.