Definition
A shaded‑pole motor is a single‑phase alternating‑current (AC) electric motor that utilizes a simple rotor and a stator equipped with one or more pole pieces surrounded by copper shading coils. The shading coils create a delayed magnetic flux in a portion of each pole, producing a rotating magnetic field that initiates and sustains rotor motion.
Construction
| Component | Description |
|---|---|
| Stator | Typically consists of laminated iron cores forming a pair of salient poles. Each pole is partially encircled by a copper shading coil (also called a shading ring) embedded in a slot of the pole face. |
| Shading coil | A short‑circuit ring that lags the main magnetic flux due to its inductance, causing a phase shift between the shaded and unshaded portions of the pole. |
| Rotor | Usually a squirrel‑cage type with aluminum or copper bars short‑circuited by end rings. In some low‑power designs the rotor may be a permanent‑magnet type. |
| Enclosure | Motors are commonly housed in a cast‑iron or aluminum frame. Small units may be open‑frame. |
Operating Principle
When AC voltage is applied to the stator winding, a sinusoidal magnetic flux is generated across each pole. The shading coil, due to its inductance and resistance, causes the flux density in the shaded portion of the pole to reach its peak slightly later than the unshaded portion. This temporal flux difference creates a weak rotating magnetic field that drags the rotor in the direction of the delay. The motor thus operates as a single‑phase motor without the need for auxiliary start windings or capacitors.
Performance Characteristics
- Power range: Typically 1 W to 1 kW; most common in the 10 W to 250 W range.
- Speed: Fixed speeds tied to supply frequency (e.g., 1,800 rpm at 60 Hz, 1,500 rpm at 50 Hz).
- Efficiency: Generally low, ranging from 15 % to 35 % for most designs.
- Torque: Low starting torque; suitable for fan, pump, and blower applications where high torque is not required.
- Noise: Relatively quiet due to the simple construction and lack of auxiliary components.
Applications
Shaded‑pole motors are widely employed in low‑power, cost‑sensitive devices, such as:
- Household fans and exhaust blowers
- Small electric pumps (e.g., circulating water pumps)
- Refrigeration fans and air‑handling units
- Automotive accessories (e.g., windshield wiper motors in some low‑cost models)
- Toys and small appliances
Advantages
- Simple and inexpensive to manufacture
- Robust and reliable with few moving parts
- Low maintenance requirements
- Compact size and quiet operation
Disadvantages
- Low efficiency and poor power factor
- Limited starting torque, restricting use to low‑load applications
- Fixed speed tied to mains frequency; speed control requires external methods (e.g., variable frequency drives)
Historical Context
The shaded‑pole principle was patented by Charles F. Bragdon in 1902 and later refined by various manufacturers during the early 20th century. Its simplicity made it a popular choice for mass‑produced electric fans and ventilation equipment throughout the mid‑1900s. While supplanted in many higher‑power roles by capacitor‑start or permanent‑magnet motors, shaded‑pole designs remain in production for niche low‑power markets.
See Also
- Single‑phase induction motor
- Capacitor‑start motor
- Permanent‑magnet synchronous motor
References
- Fitzgerald, A. E., Kingsley, C., & Umans, S. D. Electric Machinery, 6th ed., McGraw‑Hill, 2003.
- D. C. McLallen, “Shaded‑Pole Motors: Design and Application,” IEEE Transactions on Industry Applications, vol. 12, no. 3, 1976, pp. 464‑470.
- IEC 60034‑1:2022, “Rotating electrical machines – Part 1: Rating and performance.”