The shaded-pole induction motor usually has salient poles with one portion of each pole surrounded by a short-circuited turn of copper called a shading coil. A schematic is shown in figure below. Induced currents in the shading coil cause the flux in the shaded portion of the pole to lag the flux in other portion. The result is similiar to a rotating field moving in the direction from the unshaded to shaded portion of the pole causing induction of current in squirrel-cage rotor and a low starting torque is produced.
ϕm – component of main flux in unshaded portion
ϕ'm – component of main flux in shaded portion
Us – voltage induced in shading coil
Is – current in shading coil
ϕ's – flux produced by Is
ϕs = ϕ'm + ϕ's – resultant flux in shaded portion
θ – space displacement between two fluxes ϕm and ϕs ≈ 90°
α – time displacement between two fluxes ϕm and ϕs (30° to 40°; ideally should be 90°)
Effect of Magnetic Bridge: By bridging the opposite poles magnetically, leakage component of the main flux through the shaded portion is much increased. This leakage flux although does not contribute to torque production, but it increases the component of main flux through the shading coil, so Us and therefore Is increases. ϕ's due to Is also increases and the angle between ϕ'm (component of main flux through shaded portion which crosses the air-gap) and ϕs increases and the torque of the motor increases.
One method of analysing the performance of shaded-pole induction motor is following the two axes theory (d-q axes theory or cross-field theory). The flux ϕm is taken as d-axis component and ϕs as q-axis component. The squirrel cage rotor is represented by two pseudo-stationary coils, one each in d-axis and q-axis. With these, equations can be written down for estimating the performance of motors with the following assumptions:
- Sinusoidal distribution of air gap flux
- No magnetic bridging
- Time phase and space phase difference between the two components of flux are 90°
Therefore the circuits can be represented as shown in figure—
On account of conplication in any analytical treatment, the design of such motors largely depend on trial and error. Performance characteristics of these motors are shown below—
Effect of rotor resistance: It is desirable to work near the maximum starting torque region since even then the starting torque is less than the full-load torque.
Effect of shading coil resistance: A low resistance increases very much the value of shading coil current and therefore losses giving a poor efficiency, so it is generally desirable to use a high resistance than that corresponding to maximum starting torque.
Amount of rotor shading: Amount of shading is also a compromise between good starting and good running characteristics, and about one-third of the pole is shaded in usual motors.
These motors are constructed in small sizes, generally not exceeding 50 W. A set of typical characterisctics are shown below:
Efficiency is quite low, but this is of little importance for the small sizes for which these motors are built and being the least expensive type of subfractional-kilowatt motor. Gramophone motors have efficiencies between 4 % to 6 %.
Starting torque is usually between 0.4 to 0.9 times of full-load torque, and the maximum torque is between 1.1 to 1.3 times of full-load torque.