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Induction motor: Definition, Structure, Types and Reason for self-starting.

Scheme: Induction motor

An induction motor is a machine. Any machine that converts electrical energy into mechanical energy is called a motor. An induction motor works according to the principle of electromagnetic induction. Let us know why it is called induction motor. In a d.c motor electrical energy is conducted directly to the armature (i.e rotating part of the motor) through brushes and Commutator. Hence the d.c motor can be called the conduction motor. Like this in a.c motors, the rotor does not connect to the electrical energy by conduction instead by induction. In this sense, such an electric motors are called induction motors.

An induction motor is also called a rotating transformer because this operating principle is consistent with the transformer operating principle. That is, the primary winding is stationary but the secondary winding is rotable.

Structure of an Induction motor:

The structure of an induction motor consists of two very important parts, i.e

(a) a stator and           (b) a rotor

Let us see the description as below

(a) Stator:

Scheme: Stator
The stator of the induction motor corresponds to the stator system of the synchronous motor or generator. The stator is designed to fit into a system called a yoke or frame. Yoke or frame this gives the motor a shape. It is made up of a number of stampings, which are slotted this slot is designed to fit the winding. This winding is designed to fit a 3 phase supply. These windings are circled depending on the number of poles. How many poles you need depends on the motor speed. If the number of poles is high, the motor speed will be low. If the number of poles is low, the motor speed will be high. When the stator winding is connected to a 3 phase supply the winding produces magnetic flux, which is of constant magnitude the speed of magnetic flux lines are equal to synchronous speed.

i.e synchronous speed= 120f/p

where, f- frequency in Hz

p- Number of poles.

According to the principle of mutual induction, whenever the magnetic flux linked with circuit changes an emf always induced in it.

(b) Rotor:

1. Squirrel cage rotor and     2. Phase wound or wound rotor

1. Squirrel cage rotor:

The motor most often used in factories is the motor equipped with squirrel gauge rotor. Thus these are called squirrel cage induction motor. The simple and very rough construction is unimaginably strong and almost indestructible. These rotors are cylindrical in shape and well laminated. They also have parallel slots, Also these parallel slots are fitted with rotor windings. A special feature of this is that the rotor winding is not a coil wrapped in copper wire but is made of strong strip copper, aluminium or alloys. A bar is mounted on each of the rotor slots. The rotor bars are bolted or soldered or brazed and short circuited with two heavy and strong end rings. Hence the reason this name (Squirrel cage) came up. One thing to note is that the rotor windings are permanently shortened with end rings so no additional windings can be added to the rotor.

In fact these rotor slots are not parallel to the shaft but have a slightly twisted shaft-like structure. These is so called skewing. This system is designed for some important reason.

a. Skewing are provided mainly to reduce magnetic humming effect. And it make the motor to run smoothly.

b. Second, skewing protects the rotor from magnetic locking. Magnetic locking it can be said like this, when the number of teeth of the rotor stick directly to the number of teeth of the stator due to magnetic attraction and prevent the rotor from rotating. The small capacity induction motor rotor is slightly modified:

c. Accordingly the rotor is made of a solid steel in which there is no slot and no conductor. This type of motor speed depends on the amount of eddy current produced by the rotor.

d. It consist of placing the entire rotor core in a mould and casting all the rotor bars and end rings in single piece. The most commonly used metal name for this casting is aluminium alloy. 

Scheme: Rotor
2. Phase wound or wound rotor:

Phase wound rotor is commonly spin with 3 phases, double layer, distributed winding. In this type of rotor, the rotor is designed to have as many poles as the stator has. It should be noted that although the stator is designed for two phases, the rotors are always designed for only three phases. The three phases are connected to each other with a star connection. The other three phase terminals are brought separately outside and connected to insulated slip-rings mounted on the shaft with brushes. Further these three brushes are connected to the externally mounted star connected rheostat. This rheostat adds more resistance to the motor rotor when the motor is started. This addition adds resistance increases motor starting torque and for changing speed-torque or speed-current characteristics. Normally when the motor starts, these slip-rings are automatically short-circuited by a short-circuited metal collar. This action is held by pushes the shaft forward and connects all the ring together. These brush packs are disconnected from the circuit when the motor rotor reaches 90% speed. This action protects the motor against wear, tear and mechanical losses. From this we learn that once the motor reaches 100% speed the rotor is short circuited within itself which is compatible with the squirrel gauge rotor operation.

Let's look at a few more important parts of an induction motor

3. Frame:

It is also called frame or yoke. It is this frame that gives the motor a shape. Rough to look, have of greater mechanical strength. This gives the motor the necessary cooling by conduction.

4. Fan´s:

The fan is used to cool the motor. The fans used for cooling are made of lightweight aluminium or fibres. These are fitted with Non drive end of the motor to rotate along with the motor shaft.

5. Bearing´s:

Ball and roller bearings are used which are selected as suitable for motor use. The bearings are attached to the respective housings (i.e D.E/N.D.E).

6. Eye bolt:

Eye bolt, it is attached to the motor frame. It is used to move the motor easily from one place to another.

Categories of induction motor:

(A). Synchronous motor

Classified as follows depending on how the rotor is magnetized:

1. Non-excited             2. Direct-current excited

(B). Asynchronous motor

(a). Induction motors

    i. Squirrel cage induction motor:   a. single cage      b. double cage

    ii. Slip-ring/ external resistance induction motor

(b). Commutator motors:      i. single phase             ii. universal

Classified as follows depending on the type of voltage to be connected:

1. Single phase            2. Three phase

Classified as follows depending on the speed:

1. Constant speed       2. Variable speed        3. Adjustable speed

Classified as follows depending on the structure:

1.Open           2. Enclosed      3. Semi-enclosed         4. Ventilated

5. Pipe-ventilated                    6. Riverted frame eye

Why three phase induction motor is self-starting:

Suppose the stationary winding is made of three phase wound. The stator can be connected to a three phase supply as appropriate. These three windings are mounted at an angle of 120 degrees to each other.

As mentioned above the stator produces a rotating magnetic field when the stator is connected to a three phase supply. Initially the rotor is stationary without rotation and thus emf is generated according to the transformer mutual induction principle. As we have read before, the rotor bar is short circuited with end rings because short circuit current is produced in it. It produces more induced emf.

According to the lenzs law stator flux and rotor flux flows in the opposite direction. Since the stator flux resists the rotor flux, the rotor tries to rotate in the direction of rotating magnetic field. Therefore the induction motor is usually self-starting motor. No additional motor or excitation is required to start induction motors.

Benefits of induction motor:

a. The price is very cheap, and reliable.

b. Low cost for maintenance.

c. It has a simple design and is sturdy.

d. Will work on the required efficiency and with the required power factor.

e. The induction motor is a self-starting motor. So these do not require any additional starting motor.

Drawbacks of induction motor:

a. Controlling the induction motor speed is a bit difficult. In addition resistance must be added to control the speed.

b. Its starting torque is less than that of a DC shunt motor.

c. Just like a D.C shunt motor, its speed decreases with increase in load.

Notes taken from:

1. Electrical machines- volume 2 by B.L Theraja.

2. https://en.wikipedia.org/wiki/Induction_motor

3. Electrical machines-ii by M.V.Bakshi, U.A.Bakshi


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Location: India

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