What is a Squirrel Cage Induction Motor and Its Working

A machine that converts electrical energy into mechanical energy is called electric motor. These are simple in design, easily used, low cost, high efficiency, low maintenance, and reliable. Three-phase induction motors are one of the types and different from other types of electric motors. The main difference is that there is no electrical connection from the rotor winding to any source of supply. The required current and voltage in the rotor circuit are provided by induction from the stator winding. This is the reason to call is as an induction motor. This article describes the Squirrel cage induction motor, which is one of the types of three-phase induction motor.

What is Squirrel Cage Induction Motor?

Definition: Squirrel cage motor is one of the types of induction motors. In order to generate motion, it hardens electromagnetism. As the output shaft is connected to the rotor inner component which is looking like a cage. Hence it is called squirrel cage. The two-end caps i.e, circular in shape are joined by rotor bars. These are acted based on the EMF i.e, generated by the stator. This EMF is also generated outer housing that is made of laminated metal sheets and wire coiling. The two main parts of any type of induction motor are the stator and the rotor. The squirrel cage is a simple method of pulling an electromagnetic induction effect. A 4-pole squirrel cage induction motor is shown below.

Squirrel Cage Induction Motor
Squirrel Cage Induction Motor

Squirrel Cage Induction Motor Working Principle

Squirrel induction motor working is based on the principle of electromagnetism. When the stator winding is supplied with a three-phase AC, it produces a rotating magnetic field (RMF) which has a speed called synchronous speed. This RMF causes voltage induced in the rotor bars. So, that short-circuit current flows through that. Due to these rotor currents, a self magnetic field is generated which interacts with the stator field. Now, as per the principle, the rotor field starts opposing its cause. when the RMF catches the rotor moment, the rotor current drops to zero. Then there would be no relative moment between the rotor and RMF.

Hence, the zero tangential force is experienced by the rotor and reduces for a moment. After this reduction in the moment of the rotor, the rotor current is induced again by the reconstruction of relative motion between the RMF and the rotor. Hence the tangential force of the rotor for the rotation is restored and starts by following the RMF. In this case, the rotor maintains a constant speed, which is less than the speed of RMF and synchronous speed. Here, the difference between the speed of RMF and the rotor is measured in the form of slip. The final frequency of the rotor can be obtained by the multiplication of slip and supply frequency.

Squirrel Cage Induction Motor Construction

Parts that are required for the construction of squirrel cage induction motor are stator, rotor, fan, bearings. The stator consists of mechanically and electrically 120 degrees apart three-phase winding with metal housing and core. In order to provide the path of low reluctance for flux generated by AC current, the winding is mounted on the laminated iron core.

Motor Parts
Motor Parts

Rotor converts given electrical energy into mechanical output. The shaft, a core, short-circuited copper bars are the parts of the rotor. In order to avoid hysteresis and eddy currents that are leading to power loss, the rotor is laminated. And I order to prevent cogging, conductors are skewed which also helps to give a good transformation ratio.

Motor Construction
Motor Construction

A fan attached at the back of the rotor for heat exchange helps in maintaining under a limit of the temperature of the motor. For the smooth rotation, bearings are provided in the motor.

Difference between Squirrel Cage Induction Motor and Slip Ring Induction Motors.

Squirrel Cage Induction Motor

Slip Ring Induction Motor

The construction of squirrel cage induction simple and rugged.Construction of slip ring induction motors needs slip rings, brushes, short-circuiting device, etc.
This type of motor has less overhang and better space factor in slots.These motors have the highest overhang and poor space factor in slots.
Cost and maintenance are less.The cost is more.
Higher efficiency (in case of machines, not designed for high starting torque)Low efficiency and more copper losses.
Small copper losses and better power factor.Poor power factor and can be improved at the start.
The cooling factor is better because of its bare end rings and the availability of more space for rotor fans.The cooling factor is not quite efficient.
These motors have better speed regulation, simple starting and low staring torque with high starting currentPoor speed regulation when operated with external resistances in the rotor circuit. The motor needs slip rings, brush gear, short-circuiting device and starting resistors, etc. Possibility of increasing starting torque due to external resistances in the rotor circuit.
Power factor is poor at startingThe power factor can be improved.
There is no possibility of speed control.Speed control is possible by the insertion of external resistors in the rotor circuit.
Explosion-proof against protection.Explosion-proof against protection.

Classification of Squirrel Cage Induction Motor

In order to meet the industry requirements, three-phase squirrel cage induction motors in a range up to 150KW at various standard frequencies, voltages and, speeds. According to their electrical characteristics, of these motors are divided into 6 types as discussed below,

Class-A Design

These type motors have low resistance, reactance, slip, and higher efficiency at full load. The main disadvantage is high starting current which is 5 to 8 times of full-load current at rated voltage. These motors are widely used in small ratings for machine tools, centrifugal pumps, fans, blowers, etc.

Class B Design

These motors have high reactance and operate 5-150KW range. These motors can be replaced with class A motors for new installations because of its characteristics which are similar to Class A motors and have the same staring current. (around 5 times the full-load current at rated voltage).

Class C Design

These motors are known as double cage motors that high starting torque with the low starting current. Applications of class C motors are, driving air compressors, conveyors, reciprocating pumps, crushers, mixers, large refrigerating machines, etc.

Class D Design

These motors are squirrel cage motors with high resistance. Hence, they give high starting torque with the low starting current. These motors have low operating efficiency and limited to drive intermittent loads involved in high accelerating duty and high-impact loads such as punch presses, shears, bulldozers, small hoists, etc.

Class E Design

These motors operate with low starting torque, normal starting current, and also low slip at rated load.

Class F Design

These motors are operated with low starting torque, low starting current, and normal slip.

Advantages

The advantages of a squirrel cage induction motor include the following.

  • Simple and rugged construction.
  • The low initial as well as maintenance cost.
  • Maintains constant speed.
  • The overload capacity is high.
  • Simple starting arrangement.
  • High power factor.
  • Low rotor copper loss.
  • High efficiency.

Disadvantages

The disadvantages of a squirrel cage induction motor include the following.

  • Motor
  • High starting current
  • Very sensitive to fluctuations in supply voltage
  • Low power factor at light loads.
  • Speed control is very difficult
  • Very poor starting torque due to its low rotor resistance.

Applications

The applications of squirrel cage induction motor include the following.

  • Suitable for industrial drives of small power where speed control is not required such as for printing machinery, flour mills, and other shaft drives of small power.
  • Centrifugal pumps, fans, blowers, etc
  • In driving air compressors, conveyors, reciprocating pumps, crushers, mixers, large refrigerating machines, etc.
  • Punch presses, shears, bulldozers, small hoists, etc.

FAQs

1) Why is it called squirrel cage induction motor?

As it has a rotor which is a squirrel cage in shape, called squirrel cage induction motor.

2) What is the difference between a squirrel cage motor and an induction motor?

The difference between squirrel cage induction motor and the induction motor is the type of rotor used for construction.

3) What is the purpose of Squirrel Cage Induction Motor?

It is used to increase the starting torque of the motor and to decrease time to accelerate.

4) Is a squirrel cage motor AC or DC?

It is AC squirrel cage induction motor

5) Why do motors use laminations?

In order to reduce eddy currents, motors use laminations.

Thus, this is all about the squirrel cage induction motor – definition, working, working principle, construction, differences between squirrel cage and slip ring induction motors, classification, advantages, disadvantage, and applications. Here is a question for you, ” What is the working of slip-ring induction motors?”

Add Comment