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VOOZH | about |
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VOOZH | about |
An electric motor is a machine that converts electrical energy into mechanical energy. The motor consists of two parts: the rotor and the stator. The rotor part is not fixed and thus it moves whereas the stator part is fixed and doesn't move. Magnet and armature are also a part of the electric motor, one of which is attached to the stator and the other is attached to the rotor. There are other types of induction motors, and they are called induction motors because rotor winding is not connected electrically anyhow to any of the source supplies and the current and voltage are present through the induction which is from stator winding. A Squirrel Cage Induction Motor is a type of three-phase induction motor.
Table of Content
A Squirrel Cage Induction Motor is widely used in industrial and commercial areas, it is called a "Squirrel Cage" because the rotor which is placed inside it looks like the "Squirrel Cage". The shaft is connected to the rotor which looks like a cage and it works on the principle of electromagnetism. So it uses the electromagnetic induction effect to convert the electrical energy into rotational energy. Also, the torque-speed characteristics can be changed easily in Squirrel Cage Induction Motor. It is used for continuous-speed work areas such as printing equipment and other small devices. The speed torque characteristics can be easily changed by simply changing the shape of the bars that are present in the rotors. The rotor is in a cylindrical shape which is of steel lamination and as the current passes through the stator windings then the magnetic field is produced, hence producing current in rotor winding and inducing its magnetic field respectively. This magnetic field of the stator and rotor together produces a torque on the Squirrel Cage Induction Motor.
The Squirrel Cage Induction Motor consists of the parts -
Stator has a laminated core with spaced slots , which cover the stator winding and provides the low reluctance path for the flux by AC current .The windings are apart by 120 degree -electrically as well as mechanically .Since the stator winding is connected to AC power supply - produces the rotating magnetic field.
The rotor is the moving part of the induction motor which is also responsible for converting the electrical energy to mechanical output. The rotor core is laminated in order to avoid the eddy current loss and hysteresis loss. The iron cores are shorted at both ends and are generally made up of aluminium or copper. This also gives the better transformation ratio between stator and motor - the reason is that the conductors are skewed , also to prevent cogging. Fan is present at the back of the rotor for the heat exchange to the environment , the heat is propelled by the fan to the environment so the temperature of the motor gets maintained.
Bearing is used basically to reduce friction , heat generation and to avoid metal to metal contact and supports rotating shaft which allows rotor to rotate smoothly- overall used for the smooth rotation of the motor.
The overall working of Squirrel Cage Induction Motor is based on the principle of electromagnetism. As the 3 phase supply is given to the stator winding , a rotating magnetic field is produced and the speed is called the synchronous speed.
Then the voltage is induced in the rotor bars and causes current to flow through it that creates a magnetic field or we can say that a self magnetic field is generated which gets interact with the field of the stator . The interaction between the two generates torque which allows the motor to start and move , the rotor follows the rotating magnetic field .
Since there is zero tangential force then the rotor decelerates for sometime . Then, after this the rotor current gets induced again and tangential force of the rotor is restored , so the rotor maintains the constant speed which is less than the synchronous speed . The speed difference the rotor and stator fields , induces the rotor current - which generates torque and motor continues to run . So in that case the constant speed of rotor is less than the synchronous speed of the motor , so , Slip is a measure of the difference between the speed of the rotating magnetic field and rotor speed .
Based on the electrical characteristics and the need in the commercial and industrial applications , the motor is divided into six categories , which are mentioned below:
This is the most popular and used type of Squirrel Cage Induction Motor. They are widely used in machine tools , fans , etc. They have normal starting toque, low resistance, slip and higher efficiency at full load. Also they have slip which 5%less than the synchronous speed. The main disadvantage is the high starting current at rated voltage.
They operate at high reactance and can be easily replaced with the Class A type for the new installations and requirements , especially when reduced starting voltage is needed. They start at full load and are used in high inertia uses such as large fans , and so on .They have normal starting torque , low slip and the starting current , high pullout torque but still lest than that of class A and Class B design is more preferred somehow because of the low start current characteristics.
These type have low starting current and high starting torque and have low starting current characteristics , they are also called as double cage motors . Most commonly used applications are used in high starting torque applications such as Conveyors , crushers , reciprocating pumps , etc. It has low slip and less pullout torque as compared to than that of class A.
These motors have high resistance and also have high starting torque and low starting current , and a large amount of slip at full load . Common applications for these type of motors are bulldozers, shears , etc. They have low operating efficiency and the high rotor resistance makes these motors strong . In this class of motor, full load slip is high because rotor resistance is always high.
These motors operating at normal starting current and low starting torque , normal starting current and have low slip at rated load . They require compensator because starting current is high for the motors above 5 kW ratings .
These motors operate at low starting current and low starting torque and at normal slip. They maintain a constant speed , self starting and require low maintenance . Most commonly used in centrifugal pumps.
Squirrel Cage Induction Motor | Slip Ring Induction Motor |
|---|---|
The rotor of the motor is Squirrel cage type that is simple and rugged. | The rotor of the motor is not simply constructed , it is slip ring type and consist of slip rings, brushes , etc. |
Cost and maintenance is less. | High maintenance and expensive . |
No External resistance is added to the rotor because the rotor bar is shorted at the end. | External resistance is added to the rotor. |
Simple construction and high efficiency. | The construction is complicated and efficiency is low. |
Poor power factor and small copper losses. | Power factor can be improved at starting. |
The slots of rotor are skewed. | Parallel slots cylindrical laminated core are constructed. |
Some of the commonly used applications are as follows:
Some of the advantages and disadvantages of the Squirrel Cage Induction Motor are as follows:
Squirrel Cage Induction Motor on being versatile, reliable ,efficient makes a good choice for being used widely in may domains .The main advantage of these motors are that the torque speed characteristics can be easily changed by adjusting the shape of the bars simply. After studying all the aspects of the Squirrel Cage Induction Motor , we can say that Squirrel Cage Induction Motor-self starting , reliable , convenient motor to use which is used in many areas . Due to versatility , robustness and other factors these motors have made their path in many areas of applications in electrical engineering.
