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In shaded pole single-phase induction motor. The displacement between the unshaded and shaded portion varies between _________.
5°-10°
10°-20°
20°-30°
35°-55°
20°-30°
Quick Summary: In a shaded pole motor, the shaded pole creates an auxiliary flux that lags behind the main pole flux, producing a rotating magnetic field. The geometric displacement between the unshaded portion and the shaded copper ring is typically maintained between 20° and 30° to ensure effective starting torque generation.
In a shaded pole motor, the shaded pole creates an auxiliary flux that lags behind the main pole flux, producing a rotating magnetic field. The geometric displacement between the unshaded portion and the shaded copper ring is typically maintained between 20° and 30° to ensure effective starting torque generation.
Φs=Φmsin(ωt−α) — where Φs is the flux in the shaded pole and α is the phase shift angle.
Ts≈kΦmΦssin(θ) — represents starting torque production due to the phase lag θ between pole fluxes.
When the main AC current flows through the pole winding, a time-varying flux is produced. The copper shading ring encircling a portion of the pole acts as a short-circuited secondary winding; the induced current in this ring opposes the main flux, causing the flux in the shaded portion to lag behind the flux in the unshaded portion. This phase difference, combined with the geometric displacement, simulates a rotating magnetic field necessary for motor self-starting.
Shaded pole motors are self-starting, unlike simple single-phase induction motors.
The shading ring is a heavy copper loop causing the magnetic flux to lag.
Efficiency is generally low (typically 5-35%) due to I2R losses in the shading ring.
Starting torque is low, typically around 50% of full-load torque.
Extremely simple and rugged construction
Low manufacturing cost
Reliable starting mechanism
Very low starting torque
Low power factor and efficiency
Limited to small power ratings (fractional HP)
Small fans and blowers
Hair dryers and electric clocks
Relays and small office equipment
The displacement angle is crucial: if it were 0°, no rotating field would be produced; if too high, efficiency drops significantly.
Option A (5°-10°) is insufficient for creating enough starting torque for practical loads.
Option D (35°-55°) would require excessive shading ring material and cause high resistive losses.
C is correct — The geometric displacement between the unshaded and shaded portions is optimally maintained at 20°-30° to provide the necessary phase shift for a rotating magnetic field.
Remember that the shading ring effectively acts as a short-circuited turn, similar to the secondary of a transformer, introducing a delay in the flux of that region.