Introduction Motors ( Advantages & Disadvantages)

Three Phase Induction Motors Introduction

Introduction

The three-phase induction motors are the most widely used electric motors in

industry. They run at essentially constant speed from no-load to full-load.

However, the speed is frequency dependent and consequently these motors are

not easily adapted to speed control. We usually prefer d.c. motors when large

speed variations are required. Nevertheless, the 3-phase induction motors are

simple, rugged, low-priced, easy to maintain and can be manufactured with

characteristics to suit most industrial requirements. In this chapter, we shall

focus our attention on the general principles of 3-phase induction motors.

Three-Phase Induction Motor

Like any electric motor, a 3-phase induction motor has a stator and a rotor. The

stator carries a 3-phase winding (called stator winding) while the rotor carries a

short-circuited winding (called rotor winding). Only the stator winding is fed

from 3-phase supply. The rotor winding derives its voltage and power from the

externally energized stator winding through electromagnetic induction and

hence the name. The induction motor may be considered to be a transformer

with a rotating secondary and it can, therefore, be described as a “transformertype”

a.c. machine in which electrical energy is converted into mechanical

energy.

Advantages

(i) It has simple and rugged construction.

(ii) It is relatively cheap.

(iii) It requires little maintenance.

(iv) It has high efficiency and reasonably good power factor.

(v) It has self starting torque.

Disadvantages

(i) It is essentially a constant speed motor and its speed cannot be changed

easily.

(ii) Its starting torque is inferior to d.c. shunt motor.