Construction of Three Phase Induction Motor

The three phase induction motor is an AC electrical motor built from a stator and a rotor. Plants use three phase induction motors for much of their shaft power because the construction is simple and rugged. There is no commutator. Slip keeps speed fairly steady with load. Power crosses the air gap from the stator winding to the rotor by induction. The induction motor is asynchronous, unlike a synchronous motor: it runs below synchronous speed when motoring.

3 Phase Induction Motor Construction

A 3 phase induction motor is built from two main parts, the stator and the rotor:

1. Stator: The stator is the stationary part. It holds the stator windings that take the three-phase supply.
2. Rotor: The rotor turns with the shaft and drives the mechanical load.

The rotor of a three phase induction motor is classed as

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

Depending on that rotor, the three phase induction motor is classed as:

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

The stator construction is the same for both kinds of three phase induction motor and is described below. Use suitable electrical tools if you strip a motor.

The other parts of a 3 phase induction motor are:

1. Shaft for transmitting the torque to the load. This shaft is steel.
2. Bearings for supporting the rotating shaft.
3. The windings heat the machine in service. A shaft-mounted or enclosure fan removes that heat.
4. A terminal box takes the external electrical connections.
5. The clearance between rotor and stator is the air gap. On typical machines it is about 0.4 mm to 4 mm, smaller on small motors and larger on big frames.

You can check related points in the electrical engineering MCQ.

Stator of Three Phase Induction Motor

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

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

Stator Frame

This is the outer part of the three phase induction motor. It supports the stator core and the field winding. It covers the active parts and gives the machine mechanical strength. The frame is die-cast or fabricated steel. It must stay rigid because the air gap is small. If the rotor is not concentric with the stator, unbalanced magnetic pull appears.

Stator Core

The stator core carries the alternating flux. To cut eddy current loss, the core is laminated from stampings typically about 0.4 to 0.5 mm thick. Those stampings are stacked to form the core and housed in the frame. Silicon steel stampings also cut the hysteresis loss 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. Three-phase AC is applied to that winding. The three phases are connected in star or delta according to the starting method. Squirrel-cage motors are also started DOL, with an autotransformer, a soft starter or a VFD. Star-delta start needs six terminals and a running delta; it is not the only cage-motor start. Slip-ring motors are started by inserting rotor resistances, so the stator may be star or delta. The stator winding is also called the field winding. When it is excited from three-phase AC it produces a rotating magnetic field.

Types of Three Phase Induction Motor

Squirrel Cage Three Phase Induction Motor

The rotor of the squirrel cage three phase induction motor is cylindrical and has slots on its periphery. The slots are skewed rather than parallel to the shaft (skew is not shown in the figure beside). Skew reduces magnetic locking of stator and rotor teeth and makes torque smoother and quieter. The squirrel cage uses aluminum, brass or copper bars (a copper-bar rotor is shown beside). Those bars are the rotor conductors and sit in the rotor slots. End rings of copper or aluminum permanently short the bars. The bars are braced to the rings for strength, so the circuit looks like a cage and gives the motor its name. The cage winding is symmetrical. Because the rings short the bars, rotor resistance is low and extra resistance cannot be added. With no slip rings or brushes, the squirrel-cage three-phase induction motor is simple and robust, so it is the usual industrial choice. A cage rotor follows the stator pole count; a wound rotor must be wound for the same poles. The diagram shows a squirrel-cage rotor with aluminum bars shorted by aluminum end rings.

Advantages of Squirrel Cage Induction Rotor

1. Its construction is very simple and rugged.
2. There are no brushes or slip rings, so these motors need less maintenance.

Applications of Squirrel Cage Induction Rotor

Squirrel cage induction motors drive lathes, drilling machines, fans, blowers and printing machines.

Slip Ring or Wound Rotor Three Phase Induction Motor

In this type of three phase induction motor the rotor is wound for the same number of poles as the stator. It has fewer slots and fewer turns per phase of a heavier conductor. The rotor winding may be star or delta, like the stator.

The rotor has slots that hold the rotor winding. The three finish ends are joined to form a star connection. Three slip rings sit on the same shaft as the rotor, which is why the machine is called a slip-ring motor.

The three ends of the three-phase windings are permanently connected to these slip rings. External resistance can be connected through the brushes and slip rings and is used for speed control and for raising starting torque of three phase induction motor. The brushes carry current to and from the rotor winding. Those brushes go to a three-phase star-connected resistance. An electrical diagram of a slip ring three phase induction motor is shown below:

At starting, resistance is in the rotor circuit and is cut out as the rotor picks up speed. On some machines, once the motor is running the slip rings are shorted by a metal collar and the brushes are lifted. That reduces brush wear. Slip rings and brushes make the rotor more complicated, so this construction is used less than the squirrel cage.

Advantages of Slip Ring Induction Motor

1. It has high starting torque and low starting current.
2. External rotor resistance can be added to control speed.

Application of Slip Ring Induction Motor

Slip ring induction motors are used where high starting torque is required, for example in hoists, cranes and elevators.

Difference between Slip Ring and Squirrel Cage Induction Motor

Slip ring or phase wound Induction motor	Squirrel cage induction motor
Construction is complicated due to presence of slip ring and brushes	Construction is very simple
The rotor winding is similar to the stator winding	The rotor consists of rotor bars which are permanently shorted with the help of end rings
We can easily add rotor resistance by using slip ring and brushes	Since the rotor bars are permanently shorted, its not possible to add external resistance
Starting torque can be raised by adding rotor resistance	Starting torque is set by the cage design and cannot be raised by adding rotor resistance
Slip ring and brushes are present	Slip ring and brushes are absent
Frequent maintenance is required due to presence of brushes	Less maintenance is required
The construction is complicated and the brushes and slip rings make the motor more costly	The construction is simple and robust and it is cheap compared to a slip ring induction motor
Slip-ring motors are a smaller share of industrial drives	Due to its simple construction and low cost the squirrel cage induction motor is widely used
Rotor copper loss is higher while starting resistance is in circuit so running efficiency is lower if that resistance stays in	Rotor I²R is lower in normal running so efficiency is typically higher
Speed control by rotor resistance method is possible	Speed control by rotor resistance method is not possible
Slip ring induction motors are used where high starting torque is required, for example in hoists, cranes and elevators	Squirrel cage induction motors are used in lathes, drilling machines, fans, blowers and printing machines