In this article we will learn about the construction of
induction motor. We will discuss about the rotor , stater of induction motor.
Construction of induction motor
Like any electric motor, a 3-phase induction motor has a
stater and a rotor. The stater carries a 3-phase winding (called stater winding)
while the rotor carries a short circuited winding (called rotor winding.) Only
the stater
winding is fed from 3-phase supply. The rotor 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 considered as a “transformer type” a.c machine in which electrical energy is
converted into mechanical energy. Construction of induction motor is simple and
maintenance less.Advantage Of Induction Motor
1. It has simple and rugged construction
2. It is relatively cheap
3. It requires little maintenance.
4. It has high efficiency
5. It has self-starting torque.
6. construction of induction motor is very simple.
Disadvantage Of Induction motor
1. It is essentially a constant speed motor and its speed cannot
be changed easily.
2. Its starting torque is inferior to d.c shunt motor.
Stater of induction motor
It consists of a steel frame which encloses 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 connected to form a balanced
three phase star or delta connected circuit. The three 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 is produced. This
rotating field induces currents in the rotor by electromagnetic induction.it is
static part of construction of induction motor.
Rotor of induction motor
The rotor, mounted on a shaft, is a hollow laminated core
having slots on its outer periphery. The winding placed in these slots (called
rotor winding) may be one of the following two types: it is rotating part of induction motor
construction.
1. Squirrel cage type 2. Wound type
1. Squirrel cage rotor of Induction Motor
It consist s of a laminated cylindrical core having parallel
slots on its outer periphery. One copper or aluminum bar is placed in each
slot. All these bars are joined at each end by metal rings called end rings.
This forms a permanently short-circuited winding which is indestructible. The
entire construction (bars and end rings) resembles a squirrel cage and hence
the name. The rotor is not connected to electrically to the supply but has
current induced in it by transformer action form the stater.
Those induction motors which employ squirrel cage rotor are called squirrel cage induction motors. Most of 3-phase induction motors use squirrel cage rotor as it has a remarkably simple and robust construction enabling it to operate in the most adverse circumstances. However, it suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit to have a large starting torque.
2. Wound Rotor Of Induction Motor
It consists of a laminated cylindrical core and carries a
3-phase winding, similar to the one on the stater. The rotor winding is
uniformly distributed in the slots and is usually star-connected .the open end
of the rotor winding are brought out and joined to three insulated slip rings
mounted on the rotor shaft with one brush resting on each slip ring. The three
brushes are connected to a three phase star-connected rheostat. At starting,
the external resistances are included in the rotor circuit to give a large
starting torque. These resistances are gradually reduced to zero as the motor
runs up to speed.
The external resistances are used during starting period
only. When the motor attains normal speed, the three brushes are
short-circuited so that the wound rotor runs like a squirrel cage rotor.