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Squirrel Cage Induction Motor
In
induction motors there are two types of rotor constructions used i.e. Squirrel
Cage Rotor and Slip Ring or Wound Rotor. This article is
specially designed to complete all aspects about squirrel cage rotor induction
motor including construction of squirrel cage rotor, advantage of using
squirrel cage induction motor, uses of squirrel cage induction motor and lastly
specifications of squirrel cage induction motor.
A
Squirrel Cage Induction motor is an induction motor which employs a squirrel
cage rotor. The application of the squirrel cage rotor makes these types of
motor robust and low maintenance operation. The name squirrel cage is derived
from its cage-like structure. The working principle of the squirrel cage
induction motor is the same as that of the working principle of three phase induction motor.
Squirrel Cage Rotor Induction Motor Construction
Squirrel Cage Induction motor Stator Construction
The
stator construction for these types of motors is same as the induction motor.
It consists of a stator frame, stator core, slots and winding. (refer induction motor basics for details)
Squirrel Cage Rotor Construction
The
rotor of an induction motor consists of rotor core and rotor shaft as shown in
the given figure.
The
rotor shaft is a cylindrical rod as shown in picture which drives the load by
mechanically coupled with them.
The
rotor core is a cylindrical stack of thick laminated steel plates. The
laminated steel plates are punched together in the form of cylindrical
structure to make the core.
In the rotor core slots are provided. Generally, in low rating squirrel cage induction motor closed slots are used while in high rating induction motor semi closed or semi opened slots are preferred.
In
squirrel cage rotor aluminium or copper conductor bars are placed in rotor
slots instead of winding. These conductor bars are shorted by end-rings at both
the ends shown in figure. (This is the special constructional feature of the
squirrel cage rotor).
These conductor bars are inclined or skewed at some angle with respect to the rotor shaft axis. This practice is done to reduce the cogging and crawling in the motor.
The use of aluminium or copper conductor bars make these types of rotors robust.
Some Specifications of Squirrel Cage Induction Motor
- The rotor of a squirrel cage induction motor is permanently short circuited hence additional resistance cannot be added to the rotor circuit. So, the starting current cannot be controlled by the rotor resistance control starting method that’s why it has high starting current and low starting torque. However, starting current can be controlled by the reduced voltage starting methods on the stator side like Auto Transformer method, Stator Resistance Control method or Star Delta Starting method.
- The rotor circuit’s resistance of the squirrel cage induction motor is designed to give excellent running characteristics.
- As additional resistance cannot be added to this type of rotor so the rotor resistance control method of speed control cannot be employed on these types of rotors.
- Windings are not present in the squirrel cage rotor that's why it has no orientation of poles. Means they don't have their own poles. But this type of rotor automatically creates the image of stator poles and can react to any number of poles. That’s why the pole changing method of speed control can be employed effectively on this type of motor.
Advantage of Squirrel Cage Induction Motor
- Squirrel cage rotor has robust construction.
- This type of rotor does not have any brush or slip rings that’s why they require less maintenance.
- These types of motor have excellent running characteristics means the variation of speed from no load to full load is low.
Applications Of Squirrel Cage
Induction Motor
- The squirrel cage induction motors are very popular in industry due to its simple and rugged design, constant speed from no load to full load, and less maintenance operation.
- Due to its simple design these types of motor are most widely used in small toys and low rating application, where setting up the winding on rotor slots is difficult.
- These types of motor have excellent running characteristics, means the change of speed of the motor from no load to full load is low. This property makes these types of motor suitable for constant speed applications like printing machines, flour mills, conveyor belt etc.
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#_torque slip characteristics of three phase induction motor
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