Squirrel Cage Induction Motor: Working Principle & Applications

What is Squirrel Cage Induction Motor

A 3 phase squirrel cage induction motor is a type of three phase induction motor which functions based on the principle of electromagnetism. It is called a ‘squirrel cage’ motor because the rotor inside of it – known as a ‘squirrel cage rotor’ – looks like a squirrel cage.

This rotor is a cylinder of steel laminations, with highly conductive metal (typically aluminum or copper) embedded into its surface. When an alternating current is run through the stator windings, a rotating magnetic field is produced.

This induces a current in the rotor winding, which produces its own magnetic field. The interaction of the magnetic fields produced by the stator and rotor windings produces a torque on the squirrel cage rotor.

One big advantage of a squirrel cage motor is how easily you can change its speed-torque characteristics. This can be done by simply adjusting the shape of the bars in the rotor. Squirrel cage induction motors are used a lot in industry – as they are reliable, self-starting, and easy to adjust.

Squirrel Cage Induction Motor Working Principle

When a 3 phase supply is given to the stator winding it sets up a rotating magnetic field in space. This rotating magnetic field has a speed which is known as the synchronous speed.

This rotating magnetic field induces the voltage in rotor bars and hence short-circuit currents start flowing in the rotor bars. These rotor currents generate their self-magnetic field which will interact with the field of the stator. Now the rotor field will try to oppose its cause, and hence rotor starts following the rotating magnetic field.

The moment rotor catches the rotating magnetic field the rotor current drops to zero as there is no more relative motion between the rotating magnetic field and rotor. Hence, at that moment the rotor experiences zero tangential force hence the rotor decelerates for the moment.

After deceleration of the rotor, the relative motion between the rotor and the rotating magnetic field reestablishes hence rotor current again being induced. So again, the tangential force for rotation of the rotor is restored, and therefore again the rotor starts following rotating magnetic field, and in this way, the rotor maintains a constant speed which is just less than the speed of rotating magnetic field or synchronous speed.

Slip is a measure of the difference between the speed of the rotating magnetic field and rotor speed. The frequency of the rotor current = slip × supply frequency

Squirrel Cage Induction Motor Construction

A squirrel cage induction motor consists of the following parts:

• Stator
• Rotor
• Fan
• Bearings

Stator

It consists of a 3 phase winding with a core and metal housing. Windings are such placed that they are electrically and mechanically 120^o apart from in space. The winding is mounted on the laminated iron core to provide low reluctance path for generated flux by AC currents.

Rotor

It is the part of the motor which will be in a rotation to give mechanical output for a given amount of electrical energy. The rated output of the motor is mentioned on the nameplate in horsepower. It consists of a shaft, short-circuited copper/aluminum bars, and a core.

The rotor core is laminated to avoid power loss from eddy currents and hysteresis. Conductors are skewed to prevent cogging during starting operation and gives better transformation ratio between stator and rotor.

Fan

A fan is attached to the back side of the rotor to provide heat exchange, and hence it maintains the temperature of the motor under a limit.

Bearings

Bearings are provided as the base for rotor motion, and the bearings keep the smooth rotation of the motor.

Application of Squirrel Cage Induction Motor

Squirrel cage induction motors are commonly used in many industrial applications. They are particularly suited for applications where the motor must maintain a constant speed, be self-starting, or there is a desire for low maintenance.

These motors are commonly used in:

• Centrifugal pumps
• Industrial drives (e.g. to run conveyor belts)
• Large blowers and fans
• Machine tools
• Lathes and other turning equipment

Advantages of Squirrel Cage Induction Motor

Some advantages of squirrel cage induction motors are:

• They are low cost
• Require less maintenance (as there are no slip rings or brushes)
• Good speed regulation (they are able to maintain a constant speed)
• High efficiency in converting electrical energy to mechanical energy (while running, not during startup)
• Have better heat regulation (i.e. don’t get as hot)
• Small and lightweight
• Explosion proof (as there are no brushes which eliminate the risks of sparking)

Disadvantages of Squirrel Cage Induction Motor

Although squirrel cage motors are very popular and have many advantages – they also have some downsides. Some disadvantages of squirrel cage induction motors are:

• Very poor speed control
• Although they are energy efficient while running at full load current, they consume a lot of energy on startup
• They are more sensitive to fluctuations in the supply voltage. When the supply voltage is reduced, induction motor draws more current. During voltage surges, increase in voltage saturates the magnetic components of the squirrel cage induction motor
• They have high starting current and poor starting torque (the starting current can be 5-9 times the full load current; the starting torque can be 1.5-2 times the full load torque)

Difference Between Squirrel Cage and Slip Ring Induction Motor

While slip ring induction motors (also known as wound-rotor motor) aren’t as popular as squirrel cage induction motors, they do have a few advantages.

Below is a comparison table of squirrel cage vs wound rotor type motors:

Classification of Squirrel Cage Induction Motor

NEMA (National Electrical Manufacturer’s Association) in United States and IEC in Europe has classified the design of the squirrel cage induction motors based on their speed-torque characteristics into some classes. These classes are Class A, Class B, Class C, Class D, Class E and Class F.

Class A Design

1. A normal starting torque.
2. A normal starting current.
3. Low slip.
4. In this Class, pullout torque is always of 200 to 300 percent of the full-load torque and it occurs at a low slip (it is less than 20 percent).
5. For this Class, the starting torque is equal to rated torque for larger motors and is about 200 percent or more of the rated torque for the smaller motors.

Class B Design

1. Normal starting torque,
2. Lower starting current,
3. Low slip.
4. Induction Motor of this class produces about the same starting torque as the class A induction motor.
5. Pullout torque is always greater than or equal to 200 percent of the rated load torque. But it is less than that of the class A design because it has increased rotor reactance.
6. Again Rotor slip is still relatively low (less than 5 percent) at full load.
7. Applications of Class B design are similar to those for design A. But design B is preferred more because of its lower starting-current requirements.

Class C Design

1. High starting torque.
2. Low starting currents.
3. Low slip at the full load (less than 5 %).
4. Up to 250 percent of the full-load torque, the starting torque is in this class of design.
5. The pullout torque is lower than that for Class A induction motors.
6. In this design the motors are built from double-cage rotors. They are more expensive than motors of Class A and B classes.
7. Class C Designs are used for high-starting-torque loads (loaded pumps, compressors, and conveyors).

Class D Design

1. In this Design of Class motors has very high starting torque (275 percent or more of the rated torque).
2. A low starting current.
3. A high slip at full load.
4. Again in this class of design the high rotor resistance shifts the peak torque to a very low speed.
5. It is even possible at zero speed (100 percent slip) for the highest torque to occur in this class of design.
6. Full-load slip (It is typically 7 to 11 percent, but may go as high as 17 percent or more) in this class of design is quite high because of the high rotor resistance always.

Class E Design

1. Very Low Starting Torque.
2. Normal Starting Current.
3. Low Slip.
4. Compensator or resistance starter are used to control starting current.

Class F Design

1. Low Starting Torque, 1.25 times of full load torque when full voltage is applied.
2. Low Starting Current.
3. Normal Slip.