In one line
A three-phase induction motor has a stator with a distributed three-phase winding in a laminated slotted core, and a rotor that is either a squirrel cage (short-circuited bars) or a wound slip-ring type.
Explained simply
The stator is the stationary outer ring carrying three-phase windings that create the rotating field. Inside it, separated by a very small air gap, is the rotor. The cheapest rotor is literally a "cage" of bars, like a hamster wheel, joined at both ends. The other kind has a proper three-phase winding brought out through slip rings so you can add resistance from outside.
Technically, the stator core is made of laminated silicon-steel stampings with slots; the three windings are displaced by 120° electrical and may be star or delta connected. The air gap is kept very small (fractions of a millimetre in small motors) to reduce magnetising current and improve power factor.
How it works
| Feature | Squirrel-cage rotor | Slip-ring (wound) rotor |
|---|---|---|
| Rotor winding | aluminium or copper bars short-circuited by end rings | three-phase insulated winding, usually star-connected |
| Slip rings, brushes | none | three slip rings and brushes |
| External resistance | not possible | can be added in rotor circuit |
| Starting torque | low to moderate | high (with rotor resistance) |
| Starting current | high | lower (with rotor resistance) |
| Cost, maintenance | cheap, rugged, almost maintenance-free | costlier, brushes need maintenance |
| Applications | fans, pumps, compressors, lathes, drills, conveyors, blowers | cranes, hoists, lifts, heavy compressors, loads starting under heavy load |
- Cage rotor bars are usually skewed slightly to reduce humming noise and to prevent the rotor locking with the stator teeth (cogging).
- In a slip-ring rotor, the external resistance is used during starting and then cut out; the slip rings are short-circuited when running.
- Other parts: frame (cast iron or steel), shaft, bearings, cooling fan, terminal box.
Quick example
Most induction motors in industry are squirrel-cage type because of their ruggedness and low cost; a typical building water pump uses one. A factory overhead crane often uses a slip-ring motor because it must start while lifting a heavy load.
Common mistakes
- Saying a squirrel-cage rotor has slip rings.
- Saying the cage bars are insulated from the core; in cast aluminium cages they are usually not insulated because the induced voltage is small.
- Giving a large air gap as a design feature.
How MU asks it
Numericals are not expected in this module (as per the MU syllabus), so questions are theory. Typical: "Explain the construction of a three-phase induction motor" (5 marks) or "Compare squirrel-cage and slip-ring induction motors" (5 marks), or "State applications of three-phase induction motors". For full marks draw a sketch of stator and both rotor types, give a comparison table of at least five points, and list applications with reasons (starting torque, cost).