Types of Three Phase Induction Motor

TYPES OF THREE PHASE INDUCTION MOTOR

1. Squirrel Cage Three Phase Induction Motor:

The rotor of the squirrel cage three phase induction motor is cylindrical in shape and have slots on its periphery. The slots are not made parallel to each other but are bit skewed (skewing is not shown in the figure of squirrel cage rotor beside) as the skewing prevents magnetic locking of stator and rotor teeth and makes the working of motor more smooth and quieter. The squirrel cage rotor consists of aluminum, brass or copper bars (copper bras rotor is shown in the figure beside). This aluminum, brass or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor.

The rotor conductors are permanently shorted by the copper or aluminum rings called the end rings. In order to provide mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as squirrel cage induction motor. The squirrel cage rotor winding is made symmetrical. As the bars are permanently shorted by end rings, the rotor and resistance is very small and it is not possible to add external resistance as the bars are permanently shorted. The absence of slip ring and brushes make the construction of Squirrel cage three phase induction motor very simple and robust and hence widely used three phase induction motor. These motors have the advantage of adapting any number of pole pairs. The below diagram shows squirrel cage induction rotor having aluminum bars short circuit by aluminum end rings.

Advantages of Squirrel Cage Induction Rotor:

1. Its construction is very simple and rugged.

2. As there are no brushes an slip ring, these motors required less maintenance.

Applications: Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing machines etc.

2. Slip Ring or Wound Three Phase Induction Motor:

In this type of three phase induction motor the rotor is wound for the same number of poles a that of stator but it has less number of slots and has less turns per phase of a heavier that of stator but it has less number of slots and has less turns per phase of a heavier conductor. The rotor also carries stator or delta winding similar to that of stator winding. The rotor consists of numbers of slots and rotor winding are placed inside these slots. The three end terminals are connected together to form star connection. As its name indicates three phase slip ring induction motor consists of slip rings connected on same shaft as that of rotor.

The three ends of three phase windings are permanently connected to these slip rings. The external resistance can be easily connected through the brushes and slip rings and hence used for speed control and improved the starting torque of three phase induction motor. The brushes are used to carry current to and from the rotor winding. These brushes are further connected to three phase star connected resistances. At starting, the resistances are connected in rotor circuit and are gradually cut out as the rotor pick up its speed. When the motor is running the slip ring is shorted by connecting a metal collar, which connect all slip ring together and the brushes are also removed. This reduces wear and tear of the brushes. Due to presence of slip rings and brushes the rotor construction becomes somewhat complicated therefore it is less used as compare to squirrel cage induction motor.

Advantages of Slip Ring Induction Motor:

1. It has high starting torque and low starting current.

2. Possibility of adding additional resistance to control speed.

Applications: Slip ring induction motor are used where high starting torque is required. i.e., in hoists, cranes, elevator, etc.