Squirrel Cage Induction Motor
Table of Contents
Squirrel Cage Induction Motor
Electric motors are devices that Transform Electrical energy into Mechanical energy. These have a Straightforward design, are simple to use, Inexpensive, highly effective, low Maintenance, and reliable. One kind of electric motor that differs from others is the Three-phase Induction motor. The main Distinction is that the rotor winding has no Electrical connection to any supply source. Induction from the stator winding provides the required current and voltage in the rotor circuit. For this reason, it is refere to as an Induction motor.
What is Squirrel Cage Induction Motor?
Definition: One of the different kinds of induction motors is the squirrel cage motor. It hardens electromagnetism to produce motion. As the output shaft is attache to the cage-like inner component of the rotor. So, the name “squirrel cage” was chosen. Rotor bars attach the two circular end caps to one another. These operate in accordance with the EMF, which is produce by the stator. The outer housing, which is constructed of wire coiling and laminated metal sheets, is another source of this EMF. The stator and the rotor are the two essential components of any kind of induction motor. An easy way to pull the electromagnetic induction effect is with a squirrel cage. The image below shows a 4-pole squirrel cage induction motor.
Squirrel Cage Induction Motor Working Principle
Workings of a squirrel induction motor are based on the electromagnetism theory. A rotating magnetic field (RMF) with synchronous speed is created when a three-phase AC is applied to the stator winding. The rotor bars experience voltage induced by this RMF. As a result, that is where the short-circuit current passes. These rotor currents produce a self-magnetizing field that interacts with the stator field. Now, as predicted by the theory, the rotor field starts to work against its cause. The rotor current zeros out when the RMF captures the rotor moment. Without a relative moment, the rotor and RMF wouldn’t move in tandem.
The rotor consequently experiences zero tangential force and temporarily reduces. The reconstruction of the relative motion between the RMF and the rotor induces the rotor current once more following this reduction in the rotor’s moment. As a result, the rotor’s tangential force for rotation is restored, and it begins by starting to follow the RMF. The rotor in this instance maintains a constant speed that is lower than the RMF and synchronous speeds. Here, slip is used to measure the speed disparity between the rotor and the RMF. By multiplying slip and supply frequencies, one can determine the rotor’s final frequency.
Squirrel Cage Induction Motor Construction
The stator, rotor, fan, and bearings are necessary components for the construction of a squirrel cage induction motor. The stator consists of a metal housing and core with three-phase winding that are mechanically and electrically 120 degrees apart. The winding is mounted on the laminated iron core to provide the flux produced by AC current with a low-reluctance path.
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.
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 Motor
|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.|
|Less overhang and better space factor in slots.||Highest overhang and poor space factor in slots.|
|Cost and maintenance are less.||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 current||Poor 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 starting||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
Three-phase Squirrel cage Induction Motors in the range of up to 150KW at various standard Frequencies, Voltages, and speeds are required by the industry. These motors are divided into six different types based on their Electrical Characteristics, as discussed below.
These types of motors are more Efficient at full load and have lower resistance, Reactance, and slip. The main Drawback is a high starting current that, at rated voltage, is 5 to 8 times that of full load. Small ratings of these motors are Frequently use in machine tools, Centrifugal pumps, fans, blowers, etc.
Class B Design
These motors run in the 5-150KW range and have high Reactance. Due to their Characteristics, which are Comparable to Class A motors and share the same starting current, these motors can substituted with Class A motors for new Installations. (about 5 times the rated voltage at full load current).
Class C Design
These motors, which have a high starting torque and a low starting current, are referred to as double cage motors. Driving air Compressors, Conveyors, Reciprocating Pumps, crushers, mixers, large Refrigeration machines, etc. are all applications for class C motors.
Class D Design
These motors are high resistance Squirrel cage motors. They provide high starting torque as a result, and low starting current. These motors are only capable of driving Sporadic loads with high accelerating duty and high impact loads, such as punch presses, shears, Bulldozers, small hoists, etc. They also have a low operating Efficiency.
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.
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.
The Disadvantages of a Squirrel cage Induction motor include the following.
- 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.
The applications of squirrel cage induction motor include the following.
- Suitable for industrial drives of small power where speed control is not require 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.
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.