Three Phase Induction Motor | Electrical Machines

 

What is Induction motor ? Define Three phase induction motor and its construction.And what are the advantages and disadvantages.

Three Phase Induction Motors

1.0 Introduction

The three-phase induction motors, most widely used in Industry, run at essentially constant speed from no-load to full-load. The speed being frequency dependent these motors are not easily adapted to speed control. However, these motors are simple, rugged, economical, having low maintenance cost.  

2.0 CONSTRUCTION

2.1 Stator

The stator is a steel frame enclosure consisting of a hollow, cylindrical core made up of thin laminations of silicon steel to reduce hysteresis and eddy current losses. A number of evenly spaced slots are provided on the inner periphery of the laminations .

The insulated windings placed in these slots are connected to form a balanced 3-phase star or delta circuit (Fig. 3). The 3-phase stator winding is wound for a definite number of poles as per requirement of speed. Greater the number of poles, lesser is the speed of the motor and vice-versa. When 3-phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced. This rotating field induces currents in the rotor by electromagnetic induction.

2.2 Rotor

The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery. The winding placed in these slots may be Squirrel Cage type or Wound type.

i) Squirrel cage rotor 

It consists of a laminated cylindrical core having parallel slots on its outer periphery. A copper or aluminum bar is placed in each slot. All these bars are joined at each end by metal rings called end rings (Fig. 4) making a permanently short-circuited winding. The entire construction (bars and end rings) resembles a squirrel cage. The rotor has current induced by transformer action from the stator. 

Most of 3-phase induction motors are squirrel cage type  as these are simple in construction and can operate in adverse condition. However, their limitation is low starting torque because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit.

ii) Wound rotor

Wound rotor consists of a laminated cylindrical core and carries a 3- phase winding (Fig. 5). The rotor winding is usually star-connected. The open ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft.  The three brushes, resting on the slip rings, are connected to a 3-phase star-connected rheostat (Fig. 6). At starting, the external resistances are included in the rotor circuit during starting period, to give high starting torque, and are gradually reduced to zero.

When the motor attains normal speed, the three brushes are short-circuited so the wound rotor operates like a squirrel cage rotor. 

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.

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