Induction Motors: Overview of single-phase and three-phase induction motors
1. What an Induction Motor Is
Induction motor = AC rotating machine where rotor currents are induced by relative motion between rotating magnetic field (RMF) from stator and rotor conductors (Faraday), and torque produced by Lorentz force on those induced currents. No brushes/commutator (unlike DC), rugged, cheap, self-starting (3-phase) — workhorse of industry (>80% of motors).
2. Why "Induction"
Stator is primary (like transformer primary) directly connected to AC, rotor is secondary with short-circuited conductors (squirrel cage) where currents are induced (like transformer secondary shorted) ⇒ electro-magnetic induction gives rotor current without external connection (hence "induction").
3. Overview — Single-Phase vs Three-Phase
| Feature | Single-Phase Induction Motor | Three-Phase Induction Motor |
|---|---|---|
| Supply | 1-phase AC (230V) | 3-phase AC (400V line) |
| RMF creation | Not self-starting with single winding (pulsating field, no rotation at start) — needs auxiliary winding + capacitor (capacitor-start) or shaded pole to create phase split ⇒ elliptical RMF to start | Self-starting: 3 windings spaced 120° with 3-phase currents naturally produce circular RMF at synchronous speed |
(P poles) |
| Construction | Stator with main + auxiliary winding, squirrel cage rotor (same) | Stator with 3-phase distributed winding, rotor squirrel cage (bars + end rings) or wound rotor (slip ring) with external |
| Torque/speed | Lower starting torque, need capacitor, efficiency slightly lower | Higher torque, high efficiency, smooth |
| Applications | Fractional HP (<1 HP): fans, washing machines, refrigerators, pumps, mixers (home) | Industrial: pumps, compressors, conveyors, mills, traction (few HP to MW) |
4. Basic Parts (both)
- Stator: laminated core with slots, 1-/3-phase winding, creates RMF.
- Rotor: Squirrel cage (copper/aluminium bars shorted by end rings, skewed) or wound rotor (3-phase winding via slip rings to external for starting).
- Air gap small (0.5 mm).
- Frame, bearings.
5. Worked Checks
(a) 4-pole, f=50Hz: Ns=120·50/4=1500 rpm; slip at N=1440 rpm ⇒ s=(1500-1440)/1500=0.04=4% ✓
(b) Single-phase 230V fan motor: capacitor-start 3µF creates phase shift ⇒ RMF to start, then centrifugal switch cuts auxiliary.
(c) Why squirrel cage no brushes? Rotor currents induced, no external connection needed ⇒ maintenance-free.
6. Exam Points
- Define induction motor as RMF + induced rotor currents (induction) — 5 marks, very frequent (almost certain to ask principle in one line).
- Distinguish single-phase (not self-starting, needs capacitor/shaded pole) vs three-phase (self-starting, RMF) — 7 marks, very frequent (the most common overview question).
- List applications: single-phase (home, fractional HP), three-phase (industrial) — 5 marks, frequent.
- Mention — 3-5 marks, frequent.