Induction Motors: Key Facts & Types
Induction motors (asynchronous motors) are the most widely used AC motors, operating via electromagnetic induction—the rotor current is induced by the stator’s rotating magnetic field (RMF), requiring no brushes (in squirrel-cage types). The rotor always runs slightly slower than the RMF, a difference known as slip.
Main Motor Types
| Classification | Motor Type | Key Application & Feature |
| By Phase | Three-Phase | Self-starting. Used in industrial drives, pumps, and compressors for high power and efficiency. |
| Single-Phase | Not self-starting (requires capacitor/winding). Used in household appliances like fans and small pumps. | |
| By Rotor | Squirrel Cage | Simple, rugged, low maintenance, and cost-effective. The industry standard. |
| Wound Rotor (Slip Ring) | Provides high starting torque and speed control; used in heavy-duty applications (cranes, hoists). |
How They Work
- Rotating Magnetic Field (RMF): The stator windings, energized by an AC supply (three-phase or single-phase with a capacitor), create a magnetic field that rotates at the synchronous speed ($N_s$).
- Induction: The RMF cuts the conductors in the rotor, inducing a current (Faraday’s Law).
- Torque & Slip: The interaction between the induced rotor current and the RMF creates a force (torque) that rotates the rotor. The rotor must run slightly slower than the RMF—a difference called slip—for continuous current induction and torque production.
Key Applications
Induction motors are favored for their ruggedness, low maintenance, and cost-effectiveness.
| Motor Type | Starting Feature | Primary Applications |
| Three-Phase | Self-starting. High power and efficiency. | Industrial conveyors, compressors, large pumps, elevators, electric trains. |
| Single-Phase | Requires a capacitor to start. | Household appliances (fans, refrigerators, washing machines), small pumps. |