The main parts of a induction motor are
It is made of thin sheets arranged as tube. The laminated core has slots cut longitudinally on it parallel to the axis. It is wound for two, four, six poles depending on the required speed.
Based on the construction of the rotor, the three phase induction motor can be classified as
- Squirrel cage induction motor
- Slip ring induction motor
Construction of Squirrel Cage Induction Motor
The rotor of a squirrel cage motor is made of cylindrical laminated core with small openings. These openings receive insulated conductors. The openings are formed parallel to the axis. So, the motor starts to run with a uniform rotating torque. Humming is reduced to a great extent. One bar is placed in each slot and all the bars are short circuited by two end rings. This makes a complete squirrel cage rotor.
There are two types of squirrel cage induction rotors
- Single squirrel cage induction rotors – It has only one winding in the rotor.
- Double squirrel cage induction rotors – It has two windings in the rotor.
Construction of Slip Ring Induction Motor
The rotor of the slip ring motor is actually wound for three phase windings connected in star formation. These windings are placed in insulated rotor slots and the ends are brought out and connected to the three slip rings mounted on the shaft. Brushes mounted on the slip rings connect the rotor windings to external resistance for the purpose of starting.
This motor is started with full additional resistance in the rotor circuit to ensure less starting current and more starting torque. The external resistance is reduced gradually and the required speed is obtained by short-circuiting the three slip rings. The rotor winding will have the same number of poles found in the stator. This motor is also known as wound rotor induction motor.
Differences Between a Squirrel Cage Induction Motor and a Slip-Ring Induction Motor
|Squirrel cage induction motor||Slip ring induction motor|
|1. Simple in construction
2. Low cost
3. Operates at high efficiency
4. There is no chance of sparks and hence it is explosion proof
5. Requires the least maintenance
6. Starting arrangements are simple
7. Starting torque is less
8. Speed control is not easily possible
| 1. Construction is complicated because rotor is also wound with insulated wire
2. The cost is high
3. Efficiency is not high
4. Sparking risk is there because of slip-rings and the brushes riding on them
5. Maintenance is troublesome
6. Requires costly starters
7. High starting torque
8. Speed can be changd earily