The squirrel cage induction motor is the most popular type of AC motor. It is very commonly used in industries because it is very cheap, robust, efficient, and reliable. The slip ring motor has very little application in industries. Rarely 5% – 10% slip ring motors are used in industries because it has several disadvantages like it required frequent maintenance, having a high copper loss, etc.
One of the major difference between the slip ring and the squirrel cage motor is that the slip ring motor has an external resistance circuit for controlling the speed of the motor. Whereas in squirrel cage motor, it is not possible to add any external circuit because the bar of the motor is permanently slotted at the end of the ring. Some other differences between them are explained below in the comparison chart.
Comparison Chart: Slip Ring V/s Squirrel Cage Motor
|Basis For Comparison||Slip Ring Motor||Squirrel Cage motor|
|Defintion||The rotor of the motor is constructed as a slip ring type.||The rotor of the motor is a squirrel cage type.|
|Rotor||Cylindrical laminated core with parallel slots and each slot consist one bar.||The slots of the rotor are not parallel, but are skewed.|
|Other name||Phase wound rotor||Cage motor|
|Resistance||Added external to the rotor||The rotor bar is permanently shorted at the end of the ring, thus it is not possible to add any external resistance.|
|Starter||The rotor resistance starter can be used.||Rotor resistance starter can not be used.|
|Maintenance||Frequent maintenance required||Less maintenance required|
|Speed Control||Possible||Not Possible|
|Uses||Use in hoist, cranes, elevator where high torque is required.||Use in lathe machines, fan, blower, profiting machines, etc.|
Definition of Slip Ring Motor
The motor which employing the wound rotor is known as a slip ring induction motor or phase wound motor. It consists laminated cylindrical core which has a semi-closed slot at the outer periphery and carries three-phase insulated winding. The rotor is wound for the same number of poles as that of the stator.
The three finish terminals are connected forming star point, and the three start terminals are connected to three copper slip rings fixed on the shaft. The mild steel shaft is passed through the centre of the rotor and fixed to the key. The purpose of the shaft is to send mechanical power.
Definition of Squirrel Cage Motor
The motor which employing squirrel cage type rotor is known as the squirrel cage motor. The construction of the rotor is rugged and simple. The rotor of the motor consists the cylindrical laminated core having semi-closed circular slots and short circuit at each end by copper or aluminium ring, called short circuiting ring. It is not possible to add any external resistance in the rotor of the circuit.
The rotor slots are not parallel but are skewed. The skewing of the rotor has the following advantages.
- Its reduces humming and thus ensuring the quiet running of a motor.
- The skewed rotor gives smooth torque curves for different positions of the rotor.
- It reduces the magnetic locking of the stator and rotor.
- It increases the rotor resistance due to the increased length of the rotor bar conductors.
Key Differences Between Slip Ring & Squirrel Cage Induction Motor
- The motor whose rotor is wound type such type of motor is called slip ring induction motor, whereas the squirrel cage motor, has a squirrel cage type rotor.
- The rotor of the slip ring motor has a cylindrical core with parallel slots, and each slot consists each bar. The slot of the squirrel cage motor is not parallel to each other.
- The slip ring motor is also called phase wound rotor. The another name of the squirrel cage motor is cage motor.
- The construction of the slip-ring motor is complicated because it consists slip ring and brushes whereas the construction of the squirrel cage motor is simple.
- The phase wound motor consists external resistance circuit, whereas in squirrel cage motor it is not possible to add any external resistance circuit because their rotor bars are permanently slotted.
- For starting the slip ring motor the rotor resistance starter is used, whereas the slip ring motor does not require any starter.
- The starting torque of the slip-ring motor is high, whereas in squirrel cage motor it is low.
- The maintenance cost of the slip ring motor is high as compared to squirrel cage motor because the slip ring motor consists brushes and rings.
- The copper loss is more in slip ring motor as compared to squirrel cage motor.
- The slip ring motor has brushes for transferring the power whereas the squirrel cage motor is brushless.
- The copper loss in the phase wound motor is high as compared to squirrel cage motor.
- The efficiency of the slip ring motor is low whereas the squirrel cage motor has high efficiency.
- The speed of the phase wound motor is controlled by using the resistance circuit. It is impossible to control the speed of the squirrel cage motor.
- The slip ring motor has low power factor as compared to squirrel cage motor.
- The cost of the phase wound rotor is high because it consists brushes. The squirrel cage motor is cheap.
- The starting current of the phase wound rotor is low because it is controlled by resistance circuit whereas it is high in squirrel cage motor.
- The phase wound motor is mostly used in places where high starting torque is required like a hoist, cranes, etc. The squirrel cage motor is used in a drilling machine, lathe machine, etc.,
The overloading capacity of the slip ring motor is high as compared to squirrel cage motor and it is smoothly running under heavy loads. It is less sensitive and also has no abnormal heating during the starting.