Induction Motor Components: Internal Structure and Types of Three-Phase Motors

Electric motors convert electrical energy into mechanical energy. In this article, we will discuss the operation and applications of different types of three-phase induction motors.

Nearly 80% of all motors used in industry are three-phase induction motors, making them the most important type among all electric motors.

Internal Composition

A three-phase motor is an AC induction motor that operates on a three-phase power supply, as opposed to a single-phase induction motor that requires a single-phase supply. The three-phase supply current generates a rotating electromagnetic field in the stator windings, which in turn induces torque in the rotor through magnetic interaction.

The induction motor has a simple and robust construction, consisting of two main parts: the stator and the rotor.

Stator

The stator is the stationary part of the motor. It consists of three main components:

1. Stator Frame

The stator frame forms the outer structure of the motor. Its primary function is to provide mechanical support for the stator core and stator windings, giving structural strength to the internal components. Depending on the application, the frame can be made of die-cast or fabricated steel, aluminum/aluminum alloy, or stainless steel.

2. Stator Core

The stator core carries the alternating magnetic flux, which produces hysteresis and eddy current losses. The core is built from thin laminations insulated from each other by varnish. These laminations are stacked together to form the stator core and are securely fixed within the stator frame.

3. Stator Windings

The stator windings are placed in the slots of the stator core. Three-phase windings are arranged as the stator windings and connected to a three-phase power supply. The number of poles of the motor is determined by the internal connections of the stator windings, which in turn determines the motor's synchronous speed.

Rotor

The rotor is the rotating part of the motor. Based on the rotor construction, induction motors are classified into two types: squirrel-cage induction motors and slip-ring (wound rotor) induction motors. Both types share the same stator construction. The specific rotor types used in three-phase induction motors are discussed in the following section.

Types of Three-Phase Induction Motors

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

Squirrel-Cage Motors

The rotor of this motor resembles a squirrel cage, hence the name "squirrel-cage induction motor." This rotor design is extremely simple and robust, which is why approximately 80% of all induction motors are of the squirrel-cage type.

The rotor consists of a cylindrical laminated iron core with slots around its outer periphery. The slots are not parallel but are skewed at an angle. This skewing helps prevent magnetic locking between the stator and rotor teeth, ensuring smooth operation and reducing noise.

Instead of windings, the squirrel-cage rotor uses rotor bars made of aluminum, brass, or copper. This type of rotor does not require slip rings or brushes, making its construction simpler and more rugged.

Slip-Ring Motors

Slip-ring motors, also known as wound rotor motors, have a rotor consisting of a laminated cylindrical core with slots on the outer periphery. Rotor windings are placed in these slots.

The rotor windings are arranged so that the number of poles in the rotor winding matches the number of poles in the stator winding. The rotor windings can be connected in either a star (wye) or delta configuration. The ends of the rotor windings are connected to slip rings, which is why this motor is called a slip-ring induction motor.

When the motor operates near its rated speed, the slip rings are short-circuited by a metal ring. This arrangement removes the brushes and external resistors from the rotor circuit, reducing rotor copper losses as well as brush friction. Compared to squirrel-cage motors, the rotor construction of slip-ring motors is somewhat more complex.