Induction Motor

Induction machines are also called asynchronous machines i.e., the machines which never run at a synchronous speed. Whenever we say induction machine we mean to say induction motor. Induction motors may be single-phase or three-phase.

The single-phase induction motors are usually built in small size (up to 3 H.P). The three-phase induction motors are the most commonly used a.c. motors in the industry because they have simple and rugged construction, low cost, high efficiency, reasonably good power factor, self-starting torque, and low maintenance. Almost more than 90% of the mechanical power used in industry is provided by three-phase induction motors.

Although, 3-phase induction motors are invariably applied in the industry for bulk power conversion from electrical to mechanical. But for small power conversion, 1-phase induction motors are mostly used. These motors perform a variety of services in the home, office, business concerns, factories, etc. Almost in all the domestic appliances such as refrigerators, fans, washing machines, hair driers, mixers, etc., only 1-phase induction motors are applied.

DC Motor – Definition, Working Principle, Types

Principle of Operation

The principle of operation of the induction motor is based on generating a rotating constant magnetic field. Within a 3 phase motor, this is achieved by orientating the three coils 120 physical degrees apart in space, and imposing 3 phase voltages on the windings also separated in time by 120 electrical degrees. As will be shown later, this results in a constant amplitude magnetic flux rotating at the frequency of the supply for a two-pole motor.

This rotating magnetic field interacts with a set of conductors arranged on the rotor, and is short-circuited at the ends with two rings. These are shown in Figure, and as it looks like a squirrel cage, it gives the motor its name (Squirrel cage induction motor, SCIM). This interaction between the magnetic fields and the conductor induces a current in the bars. This induction action is what gives the motor its name, and makes it similar to the transformer action, in the fact that a voltage is induced into the rotor (sometimes called secondary) by current flowing in the stator (sometimes called the primary). A current flows in the conductors of the rotor, through the short-circuiting rings at the end. This current in turn produces a magnetic field. It is the Interaction between the rotor magnetic field and the squirrel cage bars that induces the torque and causes rotation.

But in induction motor, we give only one supply, so it is really interesting to know how It works. It is very simple, from the name itself we can understand that there is an induction process that occurred. Actually, when we are giving the supply to the stator winding, the flux will generate in the coil due to the flow of current in the coil. Now the rotor winding is arranged in such a way that it becomes short-circuited in the rotor itself. The flux from the stator will cut the coil in the rotor and since the rotor coils are short-circuited, according to Faraday’s law of electromagnetic induction, the current will start flowing in the coil of the rotor.

When the current will flow, another r flux will get generated in the rotor. Now there will be two fluxes, one is stator flux and another is torque flux and the rotor flux will be lagging to the stator flux. Due to this, the rotor will feel a torque which will make the rotor rotate in the direction of rotating magnetic flux. So the speed of the rotor will be depending upon the ac supply and the speed can be controlled by varying the input supply. This is the working principle of an induction motor of either type.

Types Induction Motor

Single Phase Induction Motor

1. Split phase motor
2. Capacitor start motor
3. Capacitor start capacitor run motor
4. Shaded pole motor  

Three Phase Induction Motor

1. Squirrel cage motor
2. Slip ring motor 

A Single-phase induction motor is not self-starting and a three-phase induction motor is self-starting. So what is self-starting? When the machine starts running automatically without any external force to the machine, then it is called self-starting. For example, we see that when we press the key the fan starts to rotate automatically, so it is self-starting. Point to be noted that fan used in home appliances is single phase induction motor but it is self-starting. How? We will discuss how.

In a three-phase system, there are three single-phase lines with a 120° phase difference. So the rotating magnetic field is having the same phase difference which will make the rotor move. If we consider three phases a, b, and c, when phase a is magnetized, the rotor will move towards the phase a winding, in the next moment phase b will get magnetized and it will attract the rotor and then phase c. So the rotor will continue to rotate.

Construction of 3-Phase Induction Motors

The three-phase induction motor is the most widely used electrical motor. Almost 80% of the mechanical power used by industries is provided by three-phase induction motors because of their simple and rugged construction, low cost, good operating characteristics, absence of a commutator, and good speed regulation. In a three-phase induction motor, the power is transferred from stator to rotor winding through induction. The induction motor is also called the asynchronous motor as it runs at a speed other than the synchronous speed.

Like any other electrical motor induction motor also have two main parts namely rotor and stator :

1. Stator: As its name indicates stator is a stationary part of the induction motor. A stator winding is placed in the stator of the induction motor and the three-phase supply is given to it.

2. Rotor: The rotor is a rotating part of an induction motor. The rotor is connected to the mechanical load through the shaft.

The rotor of the three-phase induction motor is further classified as:

1. Squirrel cage rotor
2. Slip ring rotor or wound rotor or phase wound rotor 

Depending upon the type of rotor construction used the three-phase induction motor are classified as:

1. Squirrel cage induction motor
2. Slip ring induction motor or wound induction motor or phase wound induction motor

The other parts, which are required to complete the induction motor, are:

1. Shaft for transmitting the torque to the load. This shaft is made up of steel.
2. Bearing for supporting the rotating shaft.
3. One of the problems with electrical motor is the production of that during its rotation. In order to overcome this problem we need fan for cooling.
4. For receiving external electrical connection Terminal box is needed.
5. There is a small distance between rotor and stator which usually from 0.4 mm to 4 mm. Such a distance is called air gap.

The stator of Three Phase Induction Motor

The stator of the three-phase induction motor consists of three main parts:

1. Stator frame
2. Stator core
3. Stator winding or field winding 

A detailed discussion of these main parts is given below.

Stator Frame

It is the outermost part of the three-phase induction motor. Its main function is to support the stator core and the field winding. It acts as a covering and provides protection and mechanical strength to all the inner parts of the induction motor. The frame is either made up of die-cast or fabricated steel. The frame of the three-phase induction motor should be very strong and rigid as the air gap length of the three-phase induction motor is very small, otherwise, the rotor will not remain concentric with the stator, which will give rise to unbalanced magnetic pull.

 Stator Core

The main function of the stator core is to carry the alternating flux. In order to reduce the eddy current loss, the stator core is laminated. These laminated types of structure are made up of stamping which is about 0.4 to 0.5 mm thick. All the stamping is stamped together to form a stator core, which is then housed in a stator frame. The stamping is generally made up of silicon steel, Which helps to reduce the hysteresis loss occurring in the motor.

Stator Winding or Field Winding

The slots on the periphery of the stator core of the three-phase induction motor carry three-phase windings. This three-phase winding is supplied by a three-phase ac supply. The three phases of the Winding are connected either in star or delta depending upon which type of starting method is used. The squirrel cage motor is mostly started by stator star delta stater and hence the stator of squirrel cage motor is delta connected. The slip ring three-phase induction motor is started by inserting resistances so, the stator winding of the slip ring induction motor can be either in star or delta. The winding wound on the stator of a three-phase induction motor is also called field winding and when this winding is excited by a three-phase ac supply it produces a rotating magnetic field.

Types of Rotor

Squirrel Cage Rotor:

The rotor of the squirrel cage three-phase induction motor is cylindrical in shape and has slots on its periphery. The slots are not made parallel to is e other but are a bit skewed (skewing is not shown in the figure below of squirrel cadge rotor beside) as the skewing prevents magnetic locking of stator and rotor teeth and makes the working f motor more smooth and quieter. The squirrel cage rotor consists of aluminium, brass or copper – bars. These aluminium, 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 conductors are braced to the end ring and hence form a complete closed circuit resembling 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 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 the squirrel cage induction rotor having aluminum bars short-circuited by aluminum end rings.

Advantages of squirrel cage induction rotor:

1. Its construction is very simple and rugged.
2. As there are no brushes and slip ring, these motors requires less maintenance.

Applications:

Squirrel cage rotor is used in lathes, drilling machines, fans, blower printing machines, etc

Slip ring or wound rotor:

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

The external resistance can be easily connected through brushes and slip rings and hence used for speed control and improving the starting torque of the 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 resistance is connected to the rotor circuit and is gradually cut out as the rotor picks up its speed. When the motor is running the slip ring is shorted by connecting a metal collar, which connects all slip rings together and the brushes are also removed. This reduces the wear and tear of the brushes. Due to the presence of slip rings and brushes the rotor construction becomes somewhat complicated therefore it is less used as compared to squirrel cage induction motor.

Advantages of slip ring rotor:

1. It has high starting torque and low starting current.
2. Possibility of adding additional resistance to control speed.

Application:

Slip ring induction motors are used where high starting torque is required i.e., in hoists, cranes, elevators, etc.