An induction motor is the most common type of AC motor when connected with a 3 phase supply known as 3 phase AC induction motor. Three phase induction motor has two main parts which are stator and rotor. Construction of stator in any induction motor is almost same, however construction of the rotor is different with respect to the type of motor.
Induction motor having squirrel cage rotor is called squirrel cage induction motor. The reason for giving the name “squirrel cage” is due to the shape of rotor which looks like a squirrel cage. Most of the industries prefer to use squirrel cage motor since it is very simple in construction and requires a little maintenance. Let discuss the working, construction and different applications of 3 phase squirrel cage induction motor.
Construction of Squirrel cage Induction Motor:
Main parts of the squirrel cage motor is as follows:
Stator consists of a steel frame which encloses hollow cylindrical stator core constructed from stacked laminations. These laminations have number of evenly spaced slots, providing space for the stator winding. 3 phase stator winding is wound for definite number of poles as per the requirement of speed.
Rotor which is employed in squirrel cage induction motor is known as squirrel cage rotor.
Squirrel Cage Rotor:
Squirrel cage rotor consists of cylindrical laminated core having slots for carrying conducting bars. Copper or aluminum bars are placed in each slot. All these bars are joined at each end by large shorting rings known as end rings.
Since the rotor bars of squirrel cage motor is permanently short circuited by end rings. Hence it is not possible to add any external resistance in rotor circuit in order to have large starting torque. (Since large value of rotor resistance is required for high starting torque). Moreover, core of the rotor is laminated in order to reduce power losses from eddy current and hysteresis↗.
Bearings are used to reduce the friction between the rotating and stationary part in order to keep the smooth rotation of the motor.
Fan is also used at the back side of the rotor to provide heat exchange, thus maintains the temperature of motor.
Working Principle of Squirrel Cage Induction Motor:
When a balanced three phase supply is fed to the windings of stator, this will produce rotating flux with constant magnitude and speed. The rotating magnetic field passes through the air gap (between stator and rotor) and cuts the rotor conductor bars which are stationary. As a result, emf is induced in the shorted rotor bars of squirrel cage motor according to the faraday law of electromagnetic induction.
Since the rotor conductors are short circuited, current will begin to flow in the rotor conductors. Now, current carrying rotor conductors placed in a rotating magnetic field of stator will experience a mechanical force. The mechanical force acting on all the rotor conductors induces a torque (produces a rotating effect) which causes the rotor to rotate.
In order to know about the direction and speed of rotor, kindly read this article: Working principle of induction motor↗.
Applications of Squirrel Cage Induction Motor:
Squirrel cage motor is widely used in many industrial applications since it is very reliable and economical. Moreover, low maintenance cost and robust construction of squirrel cage motor makes it one of the most useful motor in industry.
Squirrel cage motor is generally suitable for applications where we require constant speed, low starting torque and no speed control drives.
These motors are mostly used in
Pumps and conveyors.
Fans and blowers.
Shaft drives of small power.
Advantages of Squirrel Cage Induction Motor:
Almost 90% of three phase induction motors use squirrel cage rotor since they are remarkably simple and robust in construction.
Due to their robustness in construction, they can operate in any environmental condition.
Squirrel cage motor requires a little maintenance and easily available in low price as compared to other motors.
These motors operate at nearly constant speed. Hence this motor is suitable for constant speed industrial drives where speed control is not required.
These motors have less rotor copper losses due to which they have high efficiency as compared to slip ring induction motor↗.
Disadvantages of Squirrel Cage Induction Motor:
Squirrel cage motor draws high starting current. The reason for high starting current is that the only resistance for the flow of current is the resistance of windings which is very low. Hence the current taken from the supply is large. The magnitude of the starting current is 5 to 9 times the normal rated current.
Starting torque of squirrel cage motor is very low. Hence they are not suitable for the applications which demands high starting torque.
One of the main disadvantage of squirrel cage motor is that speed control is difficult in these types of motors.
Frequently Asked Questions
Why rotor bars of squirrel cage motor is skewed?
In squirrel cage rotor, rotor bars are mot made parallel to the rotor shaft. A slight particular angle is maintained to skewed the rotor slots. Rotor bars are skewed for following reasons:
To prevent magnetic overlocking between rotor and stator fields. This phenomenon is also known as cogging. If magnetic overlocking between rotor and stator fields occurs, machine will fails to start.
Skewing the rotor bars will increase the length of conductor. Increase in conductor length will result in the increase of conductor resistance. Once resistance increases, starting torque and power factor of the motor is improved.