A slip ring induction motor is an asynchronous motor, since the rotor never operates at synchronous speed with the stator poles. We will understand the construction and operation of the slip ring induction motor:
Construction of Slip ring induction motor:
The stator construction is the same for both the squirrel cage and the slip ring induction motor. The main difference in the slip ring induction motor is in the construction and use of the rotor. Some changes in the stator can be found when using a slip ring motor in a cascade system since the supply of the slave motor is controlled by the supply of the rotor of another slip ring motor with external resistance mounted on its rotor.
Slip ring induction motors usually have a “coiled phase” rotor. This type of rotor is provided with a distributed three-phase double-layer winding consisting of coils used in alternators. The rotor core is formed by steel laminates that have grooves to accommodate single-phase, 3-phase windings. These windings are placed at 120 degrees of electrical separation.
Wound rotor type, which comprises 3 sets of insulated windings with connections brought out to 3 slip rings mounted on the shaft. The external connections to the rotating part are made via brushes onto the slip rings. Consequently, this type of motor is often referred to as a slip ring motor.
High starting torque with low starting current. The maximum starting can be achieved in the slip ring motor compared to the squirrel-cage motor by inserting an external resistor in each phase of the rotor circuit and cutting the resistance during start-up.
The main advantage of slip ring induction motor is that its speed can be controlled easily
The speeds can be adjusted in the case of the slip ring induction motor (wound rotor) by inserting a resistor. Therefore, slip ring motors are considered variable speed motors.
A squirrel cage induction motor takes 600% to 700% of the full load current. But a slip ring induction motor takes a very low starting current approximately 250% to 350% of the full load current.
The initial and maintenance cost is more compared to the squirrel cage motor due to the presence of slip rings, brushes, short circuit devices, etc.
The speed regulation is deficient when operating with external resistors in the rotor circuit.
The efficiency and power factor of the slip ring motor are lower compared to the squirrel cage induction motor.
Sensitivity to fluctuations in supply voltage