Advantages of Squirrel cage induction motors:
1. Squirrel cage induction motors are comparatively cheaper than wound rotor induction motors and are simple and rugged in construction.
2. A cage rotor needs considerably less conductor material than a wound rotor, no copper loss in the cage the rotor is low and the efficiency of a cage motor is a little more than that of a wound rotor motor.
3. The cage motor is more rugged in construction and needs no slip-rings, brushes etc., therefore, its maintenance cost is low.
4. The cage motor is explosion-proof because the absence of slip-rings and brushes eliminates risk of Sparking.
5. The cage motor can be cooled better because of its bare end rings. Also, there is more space for rotor fans.
6. A squirrel cage induction motor has very small length of overhang, so it has low rotor overhang leakage flux.
This causes standstill reactance X2 low for a cage rotor than wound rotor. So a cage motor has more pull-out torque, greater maximum power output and better operating power factor as compared to a wound rotor motor.
7. Nearly constant speed, high overload capacity and simple starting arrangement which may be made automatic.
Disadvantages of squirrel cage induction motors:
1. Since in cage rotors, rotor bars are permanently short-circuited, it is not possible to insert any external resistance in the rotor circuit for the purpose of increasing starting torque.
Consequently, its starting torque is low with a large starting current:-starting torque varying between 1.5 and 2 times the full-load torque with starting current 5 to 9 times the full-load current. Power factor
is also low at start in case of cage motors.
2. No possibility of speed control.
3. Squirrel cage induction motors are very sensitive to fluctuations in supply voltage.
4. The total energy lost during starting of cage motor is much more than with the slip-ring induction
motor and this fact is very important where frequent starting is required.
Application of Squirrel Cage Induction Motors:
1. Squirrel cage Induction motor is suitable for constant speed industrial drives of small power where speed control is not required and where starting torque requirements are of medium or low value, such as for printing machinery, flour mills and other shaft drives of small power.
2. When the squirrel cage motors are used for crane or hoist work where large starting torque is of more important than high efficiency owing to the intermittent nature of load, the rotor resistance is
increased by employing the end rings, and sometimes the bars as well, of high resistance metal such as German silver.
Advantages of wound rotor (or slip-ring) induction motors:
1. High starting torque with low starting current. By inserting an external resistance of proper value (so that rotor resistance R2 becomes equal to its standstill reactance X2) in each phase of the rotor circuit, maximum torque can be obtained at start.
2. Adjustable speed as speed can be controlled easily in case of a wound rotor motor simply by adding external resistors in the rotor circuit. This is the reason that wound rotor induction motors are considered to be variable speed motors.
3. High over-load capacity, nearly constant speed, smooth acceleration under heavy loads, no abnormal heating during starting, adaptability to automatic starting, and good running characteristics after the external resistances are taken out of the rotor circuit.
Disadvantages of wound rotor induction motors :
1. Initial and maintenance costs are more than those of a squirrel cage motor, because a slip-ring induction motor needs slip-rings, brushes, short-circuiting devices etc.
2. Speed regulation is poor when operated with external resistances in the rotor circuit.
3. Lower efficiency and power factor as compared with cage rotor motor.
4. Low power factor at light loads.
5. Sensitivity to fluctuations in supply voltage.
Application of Wound Rotor motor:
Wound rotor (or slip-ring) induction motors are used for loads requiring severe starting conditions or for loads requiring speed control such as for driving line shafts, lifts, pumps, generators, winding machines, cranes, hoists, elevators, compressors, small electric excavators, printing presses, turn tables, strokers, large ventilating fans, crushers etc.