Shaded Pole Induction Motors
We know that 'single phase induction motors are not self-starting' .To make them self starting we use capacitors,shaded poles etc.In this article we going to see shaded pole induction motors-working, construction.Shaded pole induction motors are very popular for low ratings about 0.05 HP (~ 40 W) because of its extremely simple construction and high performance.
Shaded pole induction motor consists of salient poles(projected poles) on stator and a squirrel cage rotor.Poles are shaded i.e. each pole carries a copper band on one of its unequally divided part called shading ban.You can observe 4 pole shaded poles in below figure.
Construction of Shaded Pole Induction Motor
The stator of shaded pole induction motors are also similar to normal motor's stator construction. Usually 2 to 4 salient poles are used. A coil is wounded on it and part of each pole is wrapped by a copper coil.These poles are shaded by copper band or ring which is inductive in nature. The poles are divided into two unequal halves. The smaller portion carries the copper band and is called as shaded portion of the pole.The slot is approximately one third distance from the edge of the pole.
Speed of motor depends on number of poles for example at 50Hz supply 2 pole stator gives a synchronous speed of 3000 rpm where a 4 poled stator speed will be 1500rpm.
Squirrel cage type rotor is used in shaded pole induction motors.Rotor bars are skewed in 60 degrees to obtain efficient starting torque and for limiting the torque dip during run up.Air gap length will be around 0.25 to 0.5 mm between stator and rotor.It doenst have any extra make ups like commutator, brushes, collector rings etc.This is very cheap in cost, rugged in construction and reliable. Absence of centrifugal switch eliminates the possibility of motor failure due to faulty centrifugal switch mechanisms.
Working of Shaded Pole Induction Motor
The working operation of shaded pole motor can be easily understood.Our main aim is to produce rotating magnetic field with single phase AC supply this is possible with shaded poles.How ? lets see
Alternating flux is produced by input ac supply.We can observe waveform of the flux in below figure.The distribution of this flux in the pole area is greatly influenced by the role of copper shading band.Now assume our ac supply as sine wave as shown in 1.
Advantages & Applications of Shaded Pole Induction Motor
- These motors are commonly used in
- Small fans
- Advertising displays
- Film projectors
- Record players
- Hair dryers
- Photo copying machines etc.
- Low starting torque, low power factor and low efficiency, copper losses are high due to presence of copper band.[ Disadvantage :-( ]