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Three-Phase Induction Motors: Internal Structure And Types

Sep. 02, 2021

Electric motors are used to convert electrical energy in the form of electrical energy into mechanical form. In today's post we will discuss the work and applications of the different types of three-phase motors.

Almost 80% of all motors used in industry are three-phase induction motors. Induction motors are therefore the most important of all other types of motors.

Internal composition

A three-phase motor is a type of AC induction motor that operates on a three-phase supply as opposed to a single-phase induction motor that requires a single-phase supply to operate. The three-phase supply current produces an electromagnetic field in the stator winding, which in turn produces torque in the rotor winding of a three-phase induction motor with a magnetic field.

The induction motor has a very simple and robust construction. It has two main parts: the stator and the rotor.

Three-Phase Induction Motors: Internal Structure And Types


As the name implies, the stator is the stationary part of the motor. The stator of an induction motor consists of three main parts.

1,Stator frame

The stator frame is the exterior of the motor. The function of the stator frame is to provide support for the stator core and the stator windings. It provides mechanical strength to the internal parts of the motor. Depending on the application, the frame is made of die-cast or assembled steel, aluminium/aluminium alloy or stainless steel.

2,Stator cores

The function of the stator core is to carry the alternating magnetic flux that generates hysteresis and eddy current losses. These stampings are insulated from each other by means of varnish. All stampings are stamped together in the shape of the stator core and are fixed to the stator frame.

3,Stator winding

The stator windings are placed in the available stator slots inside the stator core. Three-phase windings are placed as stator windings. A three-phase power supply is also provided for the stator windings. The number of poles of the motor depends on the internal connections of the stator winding, which will determine the speed of the motor.


As the name implies, the rotor is the rotating part of the motor. Depending on the type of rotor, induction motors are divided into squirrel-cage induction motors and slip-ring induction motors. Both types of induction motors have the same stator construction. We will discuss the type of rotor used in three phase induction motors in the following section on three phase induction motor types.

Three-Phase Induction Motors: Internal Structure And Types

Types of three-phase motors

Three-phase motors are divided into two main categories based on rotor windings, namely squirrel-cage and slip-ring.

Squirrel cage motors

The shape of this rotor resembles that of a squirrel cage. This is why the motor is called a squirrel cage induction motor. The construction of this rotor is very simple and robust. For this reason, almost 80% of induction motors are squirrel-cage motors.

The rotor consists of a cylindrical laminated iron core with slots around the outer circumference. The slots are not parallel, but are angled at an angle. It helps to prevent magnetic locking between the stator and rotor teeth. It allows for smooth running and reduces hum.

The squirrel cage rotor consists of a rotor bar instead of a rotor winding. The rotor bars are made of aluminium, brass or copper. In this type of rotor, slip rings and brushes are not used. The construction of this type of motor is therefore simpler and more robust.

Slip-ring motors

Slip ring motors are also known as wound rotor motors. The rotor consists of a laminated cylindrical core with slots on the outer circumference. The rotor windings are placed in the slots.

In this type of rotor, the rotor winding is wound in such a way that the number of poles of the rotor winding is the same as the number of poles of the stator winding. The rotor windings can be connected in a star or triangular shape. The end of the rotor winding is connected to a slip ring. This type of motor is therefore called a slip-ring induction motor.

When the motor is running near the actual speed, the slip ring is short-circuited by the metal ring. With this arrangement, the brushes and external resistors are removed from the rotor circuit. This reduces rotor copper losses as well as brush friction. Compared to squirrel-cage motors, the rotor construction is somewhat more complex.

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