--- title: "Shaded pole motor operating principle" subject: "Electric Machines II" chapter: 3 key: "semester_5/electric_machines_ii:shaded-pole-motor" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/shaded-pole-motor site: BCT Engineering --- # Shaded pole motor operating principle Electric Machines II · Chapter 3: Fractional Kilowatt Motors - Asked in: 2076 Asoj, 6 marks - Video: [Shaded Pole single phase Induction Motor (Electrical Machines) 1st year engineering) BEE (EEE)](https://www.youtube.com/watch?v=HO0-p9z5yuI) — Gautam Varde - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/shaded-pole-motor - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3 ## Question Explain the principle of operation of shaded pole motor. ## Short answer A shaded-pole motor has a short-circuited copper ring on part of each salient pole. The ring delays flux in the shaded portion, causing the flux axis to sweep from unshaded to shaded each half-cycle, producing a weak rotating field. Starting torque is low (40–50% of full-load); efficiency is poor; used in small fans, hair dryers, record players. ## Answer A **shaded-pole motor** is a simple single-phase induction motor in which the starting rotating field is produced by a short-circuited copper ring (the **shading ring**) placed on part of each salient pole. #### Construction - **Stator:** salient poles with a concentrated exciting winding fed from the single-phase supply. Each pole is slotted, and about one-third of the pole face is surrounded by a heavy copper ring (shading coil). - **Rotor:** ordinary squirrel-cage rotor. - No capacitor, no centrifugal switch, no auxiliary winding. ```text +----------------------+ | unshaded | shaded | | part | [ring] | pole face +----------------------+ ( rotor ) field moves: unshaded ---> shaded ``` #### Principle of operation The flux in the shaded part is made to **lag** the flux in the unshaded part. Consider one half cycle of the current: 1. **Current rising quickly (near zero):** the rapidly changing flux induces a large current in the shading ring. By Lenz's law it opposes the change, so most flux passes through the **unshaded** part. Flux axis is at the unshaded part. 2. **Current near peak (little change):** almost no emf is induced in the ring, so flux is spread uniformly over the pole. Flux axis is at the **centre** of the pole. 3. **Current falling:** the ring current now opposes the decrease, so flux is concentrated in the **shaded** part. Flux axis moves to the shaded part. Thus the flux axis **sweeps from the unshaded to the shaded part** every half cycle. This moving field (two fluxes displaced in space and in time) behaves like a weak rotating field and induces currents in the cage rotor, producing torque in the direction **unshaded → shaded**. #### Characteristics - Very low starting torque (about 40–50% of full-load torque) - Low power factor and low efficiency (5–35%) because of copper loss in the ring - Direction fixed by construction; cannot be reversed electrically - Speed fairly constant; ratings up to about 1/20 kW (some up to 1/4 HP) #### Applications Table and ceiling fans of small size, exhaust fans, hair dryers, record players, small blowers, toys, advertising displays and electric clocks. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What produces the moving field in a shaded-pole motor? A. An auxiliary winding with a capacitor B. A short-circuited copper ring on part of each pole C. A centrifugal switch D. A permanent magnet rotor Answer: B. Current induced in the shading ring delays the flux in the shaded part, giving flux displaced in space and time. (Key point: Role of shading ring) 2. In which direction does the field sweep and the rotor turn? A. From shaded to unshaded part B. From unshaded to shaded part C. Either, depending on supply polarity D. Along the shaft axis Answer: B. Flux in the shaded part lags, so the flux axis moves from the unshaded part to the shaded part. (Key point: Direction of motion) 3. When the current is rising quickly, where does most flux pass? A. Through the shaded part B. Equally through both parts C. Through the unshaded part D. Outside the pole Answer: C. The ring current opposes the change in flux, pushing flux into the unshaded part. (Key point: Lenz's law in ring) 4. Which is true of a shaded-pole motor? A. High starting torque and high efficiency B. Low starting torque and low efficiency; direction cannot be reversed electrically C. Needs a capacitor and centrifugal switch D. Used for large compressors Answer: B. Ring copper loss makes efficiency low, starting torque is small, and direction is fixed by construction. 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