Squirrel Cage Induction Motor: Working Principle & Applications

What is Squirrel Cage Induction Motor

A 3 phase squirrel cage induction motor is a three phase induction motor that runs by electromagnetism. The rotor bars and end rings form a cylinder that looks like a squirrel cage, which is how the machine got its name.

The rotor is a cylindrical stack of steel laminations with aluminium or copper bars. When alternating current flows in the stator windings, the stator produces a rotating magnetic field.

That field induces current in the rotor bars. The rotor then has its own magnetic field, which reacts with the stator field and produces torque.

The torque-speed curve depends on rotor-bar shape and resistivity. That choice is made in the factory as a NEMA or IEC design class. A user does not reshape the bars on a finished motor.

These motors are common in industry because they are robust, self-starting on a three-phase supply and cheap to maintain. Speed on a fixed-frequency supply stays a little below synchronous speed. A variable-frequency drive is what changes speed in most new plants.

Squirrel Cage Induction Motor Working Principle

A three-phase stator supply sets up a rotating magnetic field in the air gap. The speed of that rotating magnetic field is the synchronous speed: 120 times the supply frequency in hertz, divided by the number of poles, in revolutions per minute.

The rotating magnetic field induces voltage in the rotor bars, so short-circuit currents flow in the cage. Those currents produce a rotor field that reacts with the stator field. By Lenz’s law the rotor tries to cancel the relative motion, so it accelerates in the same direction as the rotating magnetic field.

If the rotor ever reached synchronous speed, rotor current would fall to zero because there would be no relative motion. With no torque, load and friction would slow the rotor. In a real machine the rotor never sits at synchronous speed under load. It runs with a small slip so that enough rotor current remains to supply the load torque.

As soon as slip appears, voltage is induced in the bars again and torque returns. The rotor therefore settles at a steady speed just below synchronous speed, set by the load.

Slip is the fractional difference between synchronous speed and rotor speed. Rotor-current frequency is slip times supply frequency.

Squirrel Cage Induction Motor Construction

A squirrel cage induction motor consists of the following parts:

• Stator
• Rotor
• Fan
• Bearings

Stator

The stator holds a three-phase winding, a laminated core and a metal housing. The coils are placed 120^o apart in space, electrically and mechanically. The winding sits on a laminated iron core so the flux from the AC currents has a low-reluctance path.

Rotor

The rotor turns and delivers mechanical output from the electrical input. The rated output is on the nameplate, in horsepower or kilowatts depending on the market. The rotor has a shaft, short-circuited copper or aluminium bars and a core.

The rotor core is laminated to cut eddy currents and hysteresis loss. Conductors are skewed to reduce cogging at start and to reduce harmonic locking. Skew also changes the coupling between stator and rotor slightly.

Fan

A fan on the non-drive end of the rotor moves air over the frame so the windings stay within their temperature class.

Bearings

Bearings support the shaft and let the rotor turn with low friction when they are fitted and lubricated correctly.

Application of Squirrel Cage Induction Motor

Squirrel cage induction motors are common in industry. They suit loads that run at nearly constant speed on a fixed-frequency supply, that must self-start or that should need little routine maintenance. A variable-frequency drive is used when the same motor must run over a speed range.

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 compared with wound-rotor machines of similar rating
• Require less maintenance (as there are no slip rings or brushes)
• Good speed regulation on a fixed-frequency supply (they hold a nearly constant speed against load)
• 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
• No brush sparking at the rotor. Explosion protection still needs a certified enclosure. Absence of brushes does not make the motor explosion-proof by itself

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:

• Poor inherent speed control on a fixed-frequency supply. A variable-frequency drive is the usual modern method of varying speed
• 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 modest starting torque (locked-rotor current is often about 5 to 7 times full-load current for a Design B machine, sometimes quoted up to about 8 or 9 times; locked-rotor torque is often about 1.5 to 2 times full-load torque, depending on design class)

Difference Between Squirrel Cage and Slip Ring Induction Motor

Wound-rotor (slip-ring) induction motors are less common than squirrel cage machines. They still have a use where high starting torque with limited line current, or rotor-resistance speed control, is needed. A squirrel cage motor plus a variable-frequency drive now covers many of those jobs.

The table below is a textbook contrast on a fixed-frequency supply, without a drive. Running efficiency of a modern wound-rotor machine is not always poor; the extra losses depend on how the rotor circuit is used.

	Squirrel Cage Motor	Slip Ring Motor
Cost	Low	High
Maintenance	Low	High
Speed Control	Poor	Good
Efficiency on startup	Poor	Good
Efficiency during operation	Good	Poor
Heat regulation	Good	Poor
Inrush current & torque	High	Low

Classification of Squirrel Cage Induction Motor

NEMA (National Electrical Manufacturers Association) in the United States groups squirrel-cage motors by speed-torque behaviour into Designs A, B, C and D in current MG-1. IEC 60034-12 uses different design letters (N, H and related). Older NEMA editions also listed Designs E and F; those labels are historical and are not in current MG-1. The class notes below keep the page’s original A to F headings.

Class A Design

1. A normal starting torque.
2. A normal starting current.
3. Low slip.
4. Pullout torque is typically 200 to 300 percent of full-load torque and occurs at a low slip (often 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 use double-cage rotors and cost more than Class A and B machines.
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 is high because of the high rotor resistance. Typical values are about 7 to 11 percent; some designs reach about 17 percent or more.

Class E Design

1. Very Low Starting Torque.
2. Normal Starting Current.
3. Low Slip.
4. An autotransformer (compensator) or resistance starter is used to limit 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.