This type of motor consists of a squirrel cage rotor and stator consisting of salient poles i.e. projected poles. The poles are shaded i.e. each pole carries a copper band on one of its unequally divided part called shading ban Fig.1(a) shows 4 pole shaded pole construction while Fig. 1(b) shows a single pole consisting of copper shading band.
|Fig. 2 (a) Waveform of stator flux|
|Fig. 2 Production of rotating magnetic field|
|Fig. 3 Torque-speed characteristics of shaded pole motor|
- The starting torque is poor.
- The power factor is very low.
- Due to I2R, copper losses in the shading ring the efficiency is very low.
- The speed reversal is very difficult. To achieve the speed reversal, the additional set of shading rings is required. By opening one set and closing other, direction can be reversed but the method is complicated and expensive.
- The size and power rating of these motors is very small. These motors are usually available in a range of 1/300 to 1/20 kW.
These motors are cheap but have very low starting torque, low power factor and low efficiency. These motors are commonly used for the small fans, by motors, advertising displays, film projectors, record players, gramophones, hair dryers, photo copying machines etc.