- Three Phase Induction Motor Definition: A three-phase induction motor is an asynchronous motor that operates at a speed different from the synchronous speed, using three-phase power.
- Stator: The stator is the stationary part of the motor that receives the three-phase supply to generate a rotating magnetic field.
- Rotor: The rotor, connected to the mechanical load, rotates within the stator’s magnetic field.
- Types of Rotors: There are two types of rotors in three-phase induction motors: squirrel cage rotor and slip ring rotor.
- Types of Three Phase Induction Motor: Three-phase induction motors are classified as squirrel cage induction motors and slip ring induction motors based on the rotor type.
An induction motor is an asynchronous motor: on a three-phase supply it motors below synchronous speed. Like other electrical motors, it has a stator and a rotor. Three-phase machines of this kind are built with a squirrel-cage rotor or a slip-ring rotor.
Stator
The stator is the stationary part of the induction motor. The three-phase supply is connected to the stator winding.
Rotor
The rotor turns. It is coupled to the mechanical load through the shaft. Rotors of a three phase induction motor are classed as:
- Squirrel Cage Rotor
- Slip Ring Rotor or Wound Rotor or Phase Wound Rotor
The motor is named after that rotor:
- Squirrel Cage Induction Motor
- Slip Ring Induction Motor or Wound Rotor Induction Motor or Phase Wound Induction Motor
Both kinds of three phase induction motor use the same stator arrangement. The outline below covers that stator.
Stator of Three Phase Induction Motor
The stator of the three phase induction motor has three main parts:
- Stator frame
- Stator core
- Stator winding or field winding
Stator Frame
The stator frame is the outer body of the three-phase induction motor. It holds the stator core and the winding, and it gives the inner parts mechanical protection. Frames are die-cast or fabricated steel. They have to be stiff enough to keep the small air gap even, so unbalanced magnetic pull does not pull the rotor to one side.
Stator Core
The stator core carries the alternating flux. It is laminated so eddy current loss stays down. Each lamination is typically about 0.4 to 0.5 mm thick. The stampings are stacked into the stator core and fitted in the frame. Silicon steel is used to keep hysteresis loss down.
Stator Winding or Field Winding
Slots around the stator core hold the three-phase winding. That winding is fed from a three-phase AC supply. The three phases are connected in star or in delta, depending on the starting method.
A squirrel-cage motor is often started direct-on-line. Star-delta start is used on some sizes that bring out six terminals and run in delta. Autotransformer, soft-starter and converter starts are also used. A slip-ring three phase induction motor is started by inserting resistances in the rotor circuit, so the stator winding can be star or delta. The stator winding is sometimes called the field winding. When it is fed from three-phase AC it produces a rotating magnetic field.
