Shaded Pole Motor — Construction, Working Principle & Applications
What is a Shaded Pole Motor?
A shaded pole motor is the simplest and most economical type of single-phase induction motor. It is self-starting and requires no external capacitor or centrifugal switch. The motor uses a short-circuited copper ring (called a shading coil) embedded in a portion of each stator pole to produce a sweeping magnetic field that generates starting torque.
These motors are manufactured in fractional horsepower ratings — typically from 1/200 HP to 1/6 HP — making them ideal for low-torque, constant-speed applications like fans, blowers, and small household appliances.
Construction
A shaded pole motor consists of the following main components:
- Stator: Made of laminated silicon steel with salient (projecting) poles. Each pole has a slot cut across one side, dividing it into a larger unshaded portion and a smaller shaded portion.
- Shading Coil: A short-circuited copper ring (or band) fitted around the smaller portion of each pole. This is the key element that produces the phase-shifted flux needed for starting.
- Rotor: A standard squirrel cage rotor with aluminium or copper bars short-circuited by end rings. Identical to rotors used in other induction motors.
- Main Winding: A concentrated coil wound around each salient pole, connected directly to the AC supply.
Fig: Construction of a Shaded Pole Motor showing shading coil placement
Working Principle
The shaded pole motor works on the principle of a sweeping (non-uniform rotating) magnetic field created by the interaction between the main flux and the flux in the shaded portion of the pole.
Step-by-step operation:
- Step 1: When AC supply is applied, alternating current flows through the main winding, producing an alternating flux in the stator poles.
- Step 2: A portion of this flux passes through the shaded part of the pole and links with the shading coil (copper ring).
- Step 3: By Faraday's law, an EMF is induced in the shading coil. Since the coil is short-circuited, a current flows through it.
- Step 4: This induced current produces its own flux (by Lenz's law) that opposes the change in main flux passing through the shaded portion.
- Step 5: The result is that flux in the shaded portion lags behind the flux in the unshaded portion — creating a time and space displacement between the two fluxes.
- Step 6: This displacement (less than 90°) produces a sweeping field that moves from the unshaded portion toward the shaded portion, generating starting torque in the rotor.
Phase lag angle: α = tan⁻¹(ωL/R) where L, R are shading coil parameters
Starting torque: T_start ∝ Φ₁ × Φ₂ × sin(α)
Key point: The direction of rotation is always from the unshaded portion to the shaded portion. Reversal of rotation is not possible without physically repositioning the shading coils or using two sets of shading coils with a switching mechanism.
Torque-Speed Characteristics
The torque-speed curve of a shaded pole motor is similar to other induction motors but with notably lower starting torque (typically 25–75% of full-load torque). The motor operates in a narrow speed range near synchronous speed.
For 2-pole, 50 Hz: Ns = 120 × 50 / 2 = 3000 RPM
Typical operating slip: 10–20% (higher than capacitor motors)
Advantages & Disadvantages
Applications
Due to their low cost and simplicity, shaded pole motors are widely used in applications where low starting torque is acceptable:
- Cooling fans: Table fans, exhaust fans, ceiling fan regulators, refrigerator evaporator fans
- Blowers: Small centrifugal blowers in heaters and hair dryers
- Office equipment: Photocopiers, projectors, vending machines
- Home appliances: Microwave turntable motors, humidifiers, small pumps
- Instrumentation: Record players, tape recorders, timing devices
Comparison with Other Single-Phase Motors
Frequently Asked Questions
1. Why is a shaded pole motor self-starting?
The shading coil creates a time-lagging flux in the shaded portion of the pole. This phase difference between shaded and unshaded flux produces a sweeping magnetic field that generates enough starting torque to rotate the squirrel cage rotor without any external starting mechanism.
2. Can we reverse the direction of a shaded pole motor?
Not by simply reversing the supply connections. The direction is fixed from unshaded to shaded portion. Reversal requires either physically repositioning the shading coils or using a design with two sets of shading coils on opposite sides of each pole with a selector switch.
3. Why is the efficiency of a shaded pole motor so low?
The shading coil is permanently short-circuited, so it continuously dissipates energy as I²R copper losses even at no load. Additionally, the phase angle between the two fluxes is much less than 90°, resulting in a weak rotating field and high slip operation.
4. What is the typical power rating of a shaded pole motor?
Shaded pole motors are built in very small sizes, typically ranging from 1/200 HP (about 4 watts) to 1/6 HP (about 125 watts). They are not manufactured in larger sizes because their inherently low efficiency would make them uneconomical.
5. What is the difference between a shading coil and an auxiliary winding?
A shading coil is a single short-circuited copper ring embedded in the pole face — it has no external connections and cannot be controlled. An auxiliary winding (used in split-phase or capacitor motors) is a separate distributed winding with external connections that can be switched out after starting or used with a capacitor for better performance.