Induction machine | Construction | Principle of operation | Induction generator
Induction machine has a wide rage of application as motors in industries. Single phase induction motors are used for domestic purposes. We can see induction motors every where in the form of fans, pumps etc. More than 85% of motors used in industries are induction motors. It has a constant speed operation. It is a singly fed machine i.e. the rotor does not need to be excited as in the case of synchronous motor. It runs at the speed little less than the synchronous speed. Hence it is called as asynchronous machine. A wide range of speed control is possible in induction motor by using power electronic circuits.
Induction machine is a rotating machine. Hence it has a stationary part and a rotary part. The stationary part is called as stator and the rotary part is called as rotor.
The stator of the induction machine is similar to that of the synchronous machine. It consists of winding housed in the slots cut in the stator.
The two types of rotor constructions are employed for the rotor are:
- Wound rotor.
- Squirrel cage rotor
The rotor core is made up if laminated steel to reduce eddy current loses. It contains semi enclosed slots suitably punched over it to accommodate the rotor winding in case of wound rotor and rotor bars in case of squirrel cage rotor. The semi enclosed rotor slots increases the permeance per pole and reduces the magnetizing current.
The winding of a wound rotor are similar to the winding of the stator except that the number of slots and turns are lesser and the conductor is thicker than the stator winding. In three leads of the three phase winding are taken out through the slip rings and are connected in star. The slip rings are tapped using carbon brushes. External variable resistance can be included in the winding to reduce the starting current and improve the starting torque and to control the speed.
Squirrel cage rotor
The squirrel cage rotor has solid bars of conducting material placed in the rotor slots. These bars are permanently short circuited at the both end using end rings. In large machines alloyed copper bars with copper end rings and in small machines diecast aluminum rods and end bars are used. The squirrel cage induction motors have low starting torque. The starting torque of the motor can be increased by using double cage rotor or deep bar rotor.
The number of rotor slots are kept less than the number of stator slots to prevent magnetic locking of the rotor. The rotor teeth are skewed slightly.
Principle of operation
Consider that the stator windings are connected to a 3 phase ac supply. The stator current set up a rotating magnetic field in the air gap rotating at the synchronous speed. This rotating field induces emf in the rotor winding. The frequency of the induced emf will be same as that of the stator.
Suppose if the rotor winding is not short circuited no current flows and no mmf is produced. No torque will be developed in the rotor and it continues to be stationary.
Now consider that the rotor windings are short circuited and are blocked from rotation. The rotor now carries three phase currents creating MMF rotating in the same direction as that of the stator. Let the short circuited rotor is now allowed to rotate. Now the rotor runs in the direction of stator field and acquires a steady speed N which is less than the synchronous speed.
When a 3 phase ac supply is connected to the stator windings, a rotating magnetic field is created in the air gap rotating at the synchronous speed that induces emf in the rotor winding. Since the rotor windings are short circuited, current flows through the rotor winding. This current produces a magnetic field. The interaction between the two fields develops a torque and the rotor starts rotating in the same direction as that of the stator.
As the rotor picks up speed, the frequency and magnitude of the induced emf in the rotor decreases.
The rotor tries to catch up with the rotating magnetic field. However it cannot do so because if it does so the relative motion between the stator and the rotor becomes zero and no emf will be induced in the rotor. If no emf is induced no current will flow and the torque will fall to zero.
The difference between the speed of the rotor and the speed of the rotating magnetic field (synchronous speed) is known as slip speed. slip is defined as the ratio of slip speed to that of the synchronous speed.
Cogging and crawling
If the number of stator slots is equal to or integral multiple of the rotor slots, strong alignment forces are produced during starting. These forces may alignment forces may create an alignment torque greater than the accelerating torque and the motor refuses to start. This phenomenon is known as cogging. Such design of slots must be avoided during the design itself.
The tendency of the induction machine to run stably at speeds as low as one seventh of the synchronous speed with a low pitched howling speed is called crawling.
Induction motors always runs at sub-synchronous speeds for a constant frequency supply. Suppose the rotor is driven at speeds higher than its synchronous speed, it acts as a generator. Such a generator is known as an induction generator. It is used in wind turbines and small hydro power plants.