According to the supply used for the motors, the electrical motors are classified into two types; AC motor and DC motor. The induction motors are considered as AC motors. Because it needs an alternating voltage as the source of energy for the operation.
The induction motor can operate on either
single-phase or three-phase AC supply. However, the single-phase induction
motors are suitable only for few applications. In almost 85% applications the
three-phase induction motors are preferred.
According to the type of rotor, the induction motor is classified into two types;
- Slip ring induction motor
- Squirrel cage induction motor
Construction of 3-Phase Induction Motor
There are two main parts of a 3-phase induction motor.
- Stator
- Rotor
Stator
As the name suggests, it is the stationary
part of the motor. The stator is made of silicon steel stamping (i.e. thin
sheets). The stamping are slotted.
When the complete stator is assembled from
the stamping, the through slots are formed on the inner side of the stator. The
slots may be closed type, semi-open type, or open type. In these slots, a
3-phase winding is accommodated and it is made of super enameled copper.
This winding is known as stator winding. It
may be a star or delta connected. The three ends of this winding are brought
out into the terminal box where a three-phase AC supply can be connected.
The stator winding, stator, and AC supply
connected to the stator winding are shown in the following figure.
Rotor
According to the construction of the rotor,
the induction motor is classified into two types.
- Squirrel cage type
- Wound rotor type
Squirrel Cage Type Rotor
The squirrel cage type of induction motor
is the simplest and most rugged construction. The rotor consists of a
cylindrical laminated core with skewed rotor slots. The rotor body is made of
silicon steel stamping.
The rotor conductors are thick copper bars.
These bars are placed in these slots and are brazed or welded to and rings.
Thus, the rotor conductors are permanently short-circuited. Therefore, it is
not possible to add any external resistance in the rotor circuit.
The construction of the rotor looks like a
cage of ‘squirrel’ and hence called squirrel cage type motor.
Wound Rotor or Slip Ring Induction Motor
In this type of induction motor, the rotor
is wound for the same number of poles as that on the stator.
The rotor is made up of lamination with
slots on the outer periphery. In this area, a three-phase rotor winding is
placed. The three phases are connected in star connection internally. The
remaining three terminals are brought out and connected to the slip-rings. The slip-rings
are mounted on the shaft.
When the motor running under normal conditions,
the slip-rings are short-circuited by a metal collar. The metal collar is
pushed along the shaft and the brushes are lifted from the slip-rings to reduce
the frictional losses and wear.
As a regular three-phase winding is used
for the rotor, this type is called a wound rotor type. It is also called a
slip-ring type induction motor as it uses the slip-rings.
A cross-section view of a slip-ring type three-phase induction motor is as shown in the following figure.
Components of Induction Motor with Function
The induction motor is made of different
component. Here we will discuss all major components with their function.
Stator Frame
The stator frame is made up of cast iron.
It is used to support the core and provides mechanical strength to the inner
parts.
Stator Core
It is made of silicon steel. And it used to
provide housing to the stator winding.
Stator Winding
Generally, stator winding is made of
copper. But in some cases, to reduce the cost of machine stator winding is made
of aluminium. It is used to produce rotating magnetic field.
Rotor Core
The rotor core is made of silicon steel.
Similar to the stator core, rotor core is also used to provide housing to the
rotor winding.
Rotor Winding
It is made from copper or aluminium. It is
used to produce rotor current.
Air Gap
There is a small gap between rotor and
stator. This gap is known as Air gap. It provides to avoid friction between
rotor and stator. The area of air gap chosen wisely.
Air Duct
Due to rotation of rotor, some amount of
energy is converted into heat. Hence, the air duct available in the motor to
remove heat and make motor cool.
Cooling Fan
The function of cooling fan is same. It is
used to remove the heat from the motor.
Slip-ring
It is made of phosphorus bronze or copper.
It is used to connect resistance to rotor circuit via brushes.
Brushes
Generally, the brushes are made of carbon.
It makes connection between resistance and slip-ring.
Shaft
The shaft made of MS (mild steel). It is used to support rotor. And it is connected with the load.
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