Motor Applications: Applications of induction motors
1. Why Induction Motors Dominate Applications
Due to ruggedness, low cost, no brushes (squirrel cage), self-starting (3-phase), decent efficiency (85–95%), induction motors are first choice for most constant-speed applications. Choice between single-phase (fractional HP, home) vs three-phase (industrial, higher power) and squirrel cage vs wound rotor determines application.
2. Applications by Type (exam list — 6 with reason)
| Motor Type | Power | Typical Applications (reason) |
|---|---|---|
| Single-phase (capacitor-start) — Fractional HP (<1 HP) | 0.1–1 HP | Domestic: ceiling fans, washing machines, refrigerators, air-conditioners, mixers, water pumps (0.5 HP) — where single-phase supply only, need low cost, moderate torque |
| Single-phase (shaded pole) | Very low (10–50W) | Small fans, record players, hair dryers — cheap, low torque |
| Three-phase squirrel cage | 1 HP – 500 HP (standard), MW | Industrial workhorse: pumps, compressors, conveyors, lathes, mills, crushers, lifts, blowers — constant speed, high efficiency, low maintenance |
| Three-phase wound rotor (slip ring) | 5 HP – 1000 HP | Heavy starting loads needing high starting torque with external rotor resistance: cranes, hoists, elevators, rolling mills, traction — speed control via rotor |
| Inverter-fed induction | Variable | Variable speed drives (VFD) for energy saving: HVAC, paper machine, electric vehicle (now competing with BLDC) |
3. Selection Criteria (what exam may ask)
- Starting torque: wound rotor > squirrel cage > single-phase.
- Speed control: wound rotor with external , or VFD for both.
- Supply: single-phase where 3-phase not available (home), 3-phase where available (industry).
4. Worked Selection
Need to drive 10 HP pump at constant speed from 400V 3-phase: 3-phase squirrel cage 10 HP, 1440 rpm (4-pole) is standard, direct-on-line start.
Need to lift 5-ton crane load from rest: wound rotor 20 HP with rotor resistance starter gives high torque and smooth start.
Home ceiling fan 70W: single-phase capacitor-run (permanent capacitor for quiet, efficient).
5. Exam Points
- List induction motor applications: single-phase (home, fractional HP, fans/pumps/mixers) vs three-phase (industrial, pumps/compressors/conveyors) — 7 marks, very frequent (the most common applications question).
- Mention fractional HP vs industrial — 3-5 marks, frequent.
- Show squirrel cage (rugged, constant speed) vs wound rotor (high starting torque, speed control) choice — 5 marks, frequent.