Working of Electric Motor

An electric motor converts electrical input into mechanical shaft output through electromagnetic torque. Motors have more than three possible classifications, but this article compares three widely used types of electric motor.

1. DC Motor.
2. Induction Motor.
3. Synchronous Motor.

The working principle of an electric motor is the production of force or torque through an interaction between a magnetic field and current. The way each motor creates and controls its stator and rotor fields determines how it starts, develops torque and regulates speed.
The following sections compare the basic operating principles of the three listed motor types.

Working of DC Motor

The Working principle of DC Motor can be analysed with Fleming Left Hand rule. In a basic DC motor, the armature sits between magnetic poles. Applying DC to the armature winding causes current to flow through its conductors. Current-carrying conductors in the field experience forces that form a torque about the shaft. Fleming’s rule gives the force direction for the stated current and field directions.
As a rotor coil moves from beneath one pole to the next, the commutator reverses its connection to the DC supply. This current reversal keeps the electromagnetic torque acting in the same rotational direction.

At a given instant, armature conductors beneath opposite poles carry current in opposite axial directions. Their individual forces form a couple, so they add to continuous unidirectional shaft torque rather than cancelling.

Working of Induction Motor

The Working of electric motor for an induction motor differs from that of a DC motor. A basic single phase induction motor produces a pulsating stator field and has no net starting torque without an auxiliary starting method. A balanced supply to the stator of a three phase induction motor produces a rotating magnetic field at synchronous speed. The rotor may be a squirrel-cage rotor with bars joined by end rings or a wound rotor whose circuit is closed through slip rings or external components. Relative motion makes the stator flux link the closed rotor circuit and induce an electromotive force.

According to Faraday’s law of electromagnetic induction, the induced electromotive force drives rotor current. The rotor current creates a second magnetic field, and its interaction with the stator field produces torque. In accordance with Lenz law, the torque acts in the direction of the rotating field. The rotor accelerates but must retain slip below synchronous speed while motoring: at exact synchronous speed there would be no relative motion, induced rotor current or induction torque.

Working Principle of Three Phase Induction Motor – Video

Working of Synchronous Motor

In a synchronous motor, a balanced three-phase stator supply creates a rotating magnetic field at synchronous speed. A DC-excited winding, permanent magnets or another synchronous rotor principle establishes the rotor field. Once started and synchronised, the rotor field maintains a fixed average angle behind the stator field and the rotor turns at synchronous speed. Increasing shaft load increases this torque angle until the motor reaches a stability or current limit; it does not produce induction-motor slip in normal steady operation.