In the shaded-pole induction motor, the shading coil fitted on the ________ is called _________.

Question

Examoogle · Accepted Answer

The correct answer is Option C: main pole; shading pole. In a shaded-pole induction motor, the main salient pole is slotted to accommodate a short-circuited copper ring known as the shading coil. The portion of the pole surrounded by this coil is called the shading pole, while the remaining portion is called the unshaded part.

Step-by-Step Solution & Concepts:
In a shaded-pole induction motor, the main salient pole is slotted to accommodate a short-circuited copper ring known as the shading coil. The portion of the pole surrounded by this coil is called the shading pole, while the remaining portion is called the unshaded part.

Working Principle: When alternating current flows through the main field winding, it induces an EMF in the shading coil due to transformer action. This secondary current creates a magnetic flux that opposes the main flux, causing the magnetic flux in the shaded portion to lag behind the flux in the unshaded portion. This phase difference between the shaded and unshaded fluxes creates a rotating magnetic field necessary for starting the motor.

Key Formulas: $\Phi_{sh} \approx \Phi_{m} \sin(\omega t - \alpha)$ — the magnetic flux in the shaded portion lagging behind the main flux; $T_{start} \propto \Phi_{sh} 	imes \Phi_{u} \sin(	heta)$ — starting torque produced by phase displacement $	heta$

Key Points: Shaded-pole motors are self-starting due to the elliptical rotating magnetic field.; The shading coil acts as a short-circuited secondary winding of a transformer.; The efficiency of these motors is generally very low due to I²R losses in the shading ring.; They are typically used for low-power applications like table fans and toys.

Advantages: Extremely simple construction and low cost; Rugged and requires very little maintenance

Disadvantages: Very low starting torque; Poor power factor and low efficiency

Applications: Small household fans; Hair dryers and electric clocks

Additional Info: The shading ring provides the necessary time displacement between the shaded and unshaded fluxes.. Option A and D are incorrect as the shaded pole is a section of the main pole itself, not a separate auxiliary pole.

Answer: C is correct — the shading coil is fitted on the main pole, and that portion of the pole is referred to as the shading pole.

Exam Tip: Always remember that shaded-pole motors do not have a centrifugal switch or a capacitor, making them distinct from other single-phase motors.

Examoogle · Answer

Option A: auxiliary pole; shading pole

Examoogle · Answer

Option B: main pole; non-shading pole

Examoogle · Answer

Option D: auxiliary pole; non-shading pole

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