How does the rotor current change when the load increases in an induction motor?

When the load of induction motor increases, the rotor current changes. The working principle of an induction motor is based on the interaction between the rotating magnetic field generated by the stator windings and the induced current in the rotor windings. The following explains how the rotor current changes as the load increases:

How it works when the load increases

• Load increase: When the load of the induction motor increases, it means that the motor needs to do more work to overcome greater resistance or drive a heavier load.

• Increased torque demand: Increased load causes the motor to need to generate greater torque to maintain the same speed.

• Electromagnetic torque: The electromagnetic torque of the induction motor is determined by the amperage force generated by the stator magnetic field and the rotor current. In order to increase the torque, the rotor current must be increased.

Changes in rotor current

• Slip rate: Slip rate is an important parameter of induction motor, defined as the ratio of the difference between the synchronous speed and the actual speed and the synchronous speed, that is, s= (ns−n) /ns, where ns is the synchronous speed and n is the actual speed.

• Rotor current increase: When the load increases, the actual speed will decrease, resulting in an increase in slip. According to the rotor current formula  I2=k⋅s⋅I1, where I2 is the rotor current, I1 is the stator current, and k is a constant. It can be seen that with the increase of slip rate s, the rotor current will also increase.

• Stator current change: As the load increases, the stator current will also increase accordingly, because the motor requires more electrical energy to produce greater torque.

Motor response

• Voltage adjustment: In order to maintain the normal operation of the motor, the control system may adjust the input voltage or frequency to keep the motor speed close to the synchronous speed.

• Thermal effect: As the rotor current increases, the heat inside the motor will also increase, so the motor may heat up. The motor needs to be designed with heat dissipation in mind to ensure that it does not overheat when the load is increased.

Motor efficiency

Efficiency change: As the load increases, the efficiency of the motor may decrease slightly because a portion of the energy is converted to thermal energy rather than mechanical energy. However, motors are usually most efficient when they are close to full load.

Motor protection

Overload protection: In order to prevent damage to the motor due to overload, overload protection devices are usually installed, such as thermal relays or current protectors, which automatically cut off the power supply when the rotor current is too large.

Sum up

When the load of the induction motor increases, the rotor current increases in order to generate greater torque to overcome the increased load. This process causes the actual speed of the motor to temporarily decrease and the slip rate to increase, which in turn leads to a further increase in the rotor current. The motor control system will maintain the speed of the motor as close to the synchronous speed as possible by adjusting the input voltage or frequency, and ensure that the motor will not be damaged by overload.