What is the difference between squirrel cage and wound rotor induction motor?

What is an induction motor?

A motor can be described as an electric machine that is capable to convert mechanical energy into electrical energy. The induction motor is a three-phase AC motor and it is widely used in many industries the major feature of an induction motor is that its construction is simple and rugged. It also has high reliability and efficiency, and also it doesn’t need to be synchronized.

Major parts of induction motor

There are two major parts of an induction motor one is the stator and the other is the rotor. The stator would be composed of a steel frame and this would support the cylindrical core.

The rotor is the rotating part, the rotor winding could be three-phase windings which is conventional. There could be a squirrel cage type too, which is done by shorting the aluminum bus bars together at the end of the ring, and thus it would look like a squirrel cage circuit.

Operating principle of induction motor

The induction motor would create torque by using electromagnetic induction. When we apply three-phase voltage, to the stator winding then it would reach the three interconnected phase windings. The rotating magnetic field cuts the winding of the rotor and due to this, the rotor current would create another magnetic field. So torque will be created due to the interaction between these two magnetic fields.

Classification of the induction motor according to the rotor windings

Wound rotor induction motor

Working principle of Wound rotor induction motor

Wound rotor induction motor is a conventional type of induction motor which has a conventional three-phase winding. This would be made up of insulated wire and it would be usually Y connected. The ends of the winding will be connected to a resistor through a slip ring and brushes. This would be composed of the slotted armature and also has insulated conductor arranged in slots thus a three-phase winding similar to the stator will be created. The winding is connected to the resistor or power supply to decrease the starting current and also to do the speed control.

When we apply power to the stator windings then it would create a magnetic flux. Because of this, the rotor winding would create a magnetic flux.

What are the advantages of a wound rotor induction motor?

• We can add external resistance to this type of induction motor
• Starting current and speed can be controlled
• High starting torque will be available with a low starting current
• Power factor improvement can be done
• This type of induction motor can be utilized to accelerate high inertia loads that would require a certain time to gain speed.

What are the disadvantages of a wound rotor induction motor?

• Expensive due to the maintenance cost, like upkeeping the slip rings, rotor windings, and also the carbon brush
• The speed regulation is not good while operating with an external resistor in the rotor circuit.
• The power factor and efficiency of the wound rotor motor is not good when compared to the squirrel cage motor

What are the applications of the wound rotor induction motor?

This type of induction motor can be used where the load would need high starting torque

• Hoists
• Elevators
• Driving lifts and pumps

Working principle of Squirrel cage induction motor 

Squirrel cage induction motor

A squirrel cage induction motor will be composed of the squirrel cage windings. Aluminum bars would be molded on the slots and they will be short-circuited at the end rings. In order to reduce noise, the bars would be slanted in a small rotor. In the squirrel cage type, there won’t be any slip rings but it could have conductors like copper, and also it will be galvanically isolated. So the rotor inside this would look like a squirrel cage. The rotor is a cylindrical steel lamination with a conductive material embedded in its surface. So when an AC current passes through the stator windings then a rotating electromagnetic field will be created.

When AC power is applied to the stator windings then there will be a rotating magnetic field. This would create a current in the windings of the rotor and it would create its own magnetic field. The magnetic fields produced by the stator and the rotor would interact and this would create torque in the squirrel cage rotor.

Skewed rotor slots

The skewed rotor slots would decrease the magnetic hum and also reduce the vibration of the motor. The locking and the cogging tendency of the rotor can be reduced in the rotor, which would be created because of the magnetic attraction of the stator. It would also increase the transformation between the stator and the rotor.

What are the advantages of the squirrel cage induction motor?

• Construction is simple and rugged
• Low maintenance because there is no slip rings or brushes
• Low cost
• The speed regulation is really good
• Good heat regulation
• Lightweight and small
• There won’t be any explosion due to the lack of slip rings and brushes

What are the disadvantages of the squirrel cage induction motor?

• Poor speed control
• A lot of power would be consumed during the starting time
• High starting current and poor starting torque
• These motors are really sensitive to the supply voltage variations/ fluctuations. A motor would draw more current if there is a reduction in supply voltage and if there is an increase in voltage then it would saturate the magnetic components of the squirrel cage motor

What are the applications of the squirrel cage induction motor?

• Drilling machines
• Lathe
• Blower printing machines
• Centrifugal pumps
• Large fans
• It is used in industrial drives to run the conveyor belt

Difference between squirrel cage induction motor and wound rotor induction motor

We could only change the rotor resistance in a wound rotor induction motor by adding extra resistance. The extra resistance could affect the motor efficiency, so it would be used for a short period of time.

What are the applications of induction motors?

• Conveyors
• HOIST
• Compressors
• Positive displacement pumps
• Milling machine
• Lathe machine