Which induction motor has a high starting torque and Why ?

Induction motors are classified into different types based on their rotor construction and the method of starting. The two main types that exhibit high starting torque are the Squirrel Cage Induction Motor (SCIM) and the Slip Ring (or Wound Rotor) Induction Motor.

1. Squirrel Cage Induction Motor (SCIM):

  • Starting Torque:
    • SCIMs are known for their moderate-to-high starting torque.
  • Rotor Construction:
    • The rotor of a SCIM consists of conductive bars or shorted aluminum or copper conductors arranged in a cylindrical cage, creating a simple and robust design.
  • High Starting Torque Mechanism:
    • The high starting torque in a SCIM is attributed to the rotor cage design. When the motor is started, the rotor currents induce a magnetic field that interacts with the stator field, creating a torque that initiates the rotation.

2. Slip Ring (Wound Rotor) Induction Motor:

  • Starting Torque:
    • Slip ring induction motors are designed to provide high starting torque.
  • Rotor Construction:
    • The rotor of a slip ring motor is wound with insulated coils, and the rotor windings are connected to external resistors through slip rings.
  • High Starting Torque Mechanism:
    • During startup, external resistors are initially included in the rotor circuit, which reduces the rotor current and allows the motor to start with a high torque. As the motor accelerates, the external resistors are gradually shorted out.

Factors Influencing High Starting Torque:

  • Rotor Resistance:
    • Both SCIMs and slip ring motors achieve high starting torque by having a higher rotor resistance during startup. This higher resistance allows for greater torque production during the initial stages.
  • Adjustable Resistance (Slip Rings):
    • In slip ring motors, the external resistors connected through slip rings are adjustable. This adjustability allows for fine-tuning the starting torque based on the specific requirements of the application.

Advantages and Applications:

  • Advantages of High Starting Torque:
    • Motors with high starting torque are advantageous in applications where the load requires a significant initial force to overcome inertia.
  • Applications:
    • High starting torque induction motors are commonly used in applications such as conveyors, crushers, compressors, pumps, and other equipment where a high starting torque is crucial.


  • Rotor Resistance Control:
    • The control of rotor resistance in slip ring motors provides flexibility in adjusting the torque-speed characteristics based on the load requirements.
  • Efficiency:
    • While these motors offer high starting torque, it’s essential to consider the trade-off with efficiency, as the increased rotor resistance can result in higher losses during normal operation.


Both Squirrel Cage Induction Motors and Slip Ring Induction Motors can be designed to provide high starting torque. The key lies in the rotor construction and the ability to control rotor resistance during startup. The selection between the two types depends on the specific application requirements and the need for adjustable starting torque characteristics. Each motor type has its advantages and trade-offs, and the choice should be made based on the unique demands of the system in which they are employed.

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