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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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The rotor slots skewed to _________ degree in a Shaded Pole motor to ______________
A) 20, Improve power factor
B) 45, Improve efficiency
C) 60, obtain an optimum starting torque and for limiting the torque dip during the run-up
D) 0, Reducing the locking tendency
60, obtain an optimum starting torque and for limiting the torque dip during the run-up
In a shaded pole single-phase induction motor, the rotor slots are skewed by approximately 60 electrical degrees. Skewing the rotor slots reduces harmonic torques, minimizes cogging (magnetic locking), and eliminates unwanted dips in the torque-speed curve to ensure smooth acceleration and optimal starting torque.
In a shaded pole single-phase induction motor, the rotor slots are skewed by approximately 60 electrical degrees. Skewing the rotor slots reduces harmonic torques, minimizes cogging (magnetic locking), and eliminates unwanted dips in the torque-speed curve to ensure smooth acceleration and optimal starting torque.
θskew=60° — Rotor slot skew angle in electrical degrees for optimal harmonic attenuation
Ks=γ/2sin(γ/2) — Skew factor reducing the effective magnitude of harmonic voltages
Shaded pole motors produce a weak, highly non-sinusoidal rotating magnetic field due to phase displacement between the unshaded and shaded regions of the stator pole. This non-sinusoidal field creates significant space harmonics (especially 3rd and 5th harmonics) that cause parasitic torques and severe torque dips during run-up. Skewing the rotor slots by 60 electrical degrees averages out these high-order space harmonics along the length of the rotor, reducing crawl torque and preventing magnetic locking.
Shaded pole induction motors inherently have low starting torque and low efficiency.
The magnetic field produced by a shaded pole motor contains heavy space harmonics due to salient pole construction and shading ring arrangement.
Rotor skewing helps in reducing noise, vibration, magnetic locking (cogging), and crawling.
A 60° skew specifically targets the harmonic torque dips that occur during the motor's acceleration phase.
Prevents torque dips during motor acceleration (run-up).
Ensures smooth starting and reduces acoustic noise during operation.
Eliminates magnetic locking tendency between stator and rotor slots.
Slightly reduces the fundamental induced voltage in the rotor bars.
Increases leakage reactance of the rotor winding.
Small fans and blowers
Record players and turntables
Hair dryers and small household appliances
Option A (20°): Incorrect angle; standard skewing angles in conventional 3-phase motors are usually around 1 slot pitch (~15°-30°), but shaded pole motors require ~60° due to heavy harmonic content.
Option B (45°): Incorrect magnitude; skewing does not directly improve overall efficiency, as it slightly increases leakage reactance.
Option D (0°): Zero skewing would exacerbate magnetic locking and harmonic torque dips rather than reducing them.
C is correct — The rotor slots in a shaded pole motor are skewed to 60 degrees to obtain optimum starting torque and limit torque dips during run-up.
For competitive exams, remember that conventional 3-phase induction motors skew rotor slots by 1 slot pitch to reduce cogging and noise, but single-phase shaded pole motors specifically use ~60° skewing to counter severe harmonic torque dips during run-up.