AC MOTOR
Key one-liners & tricks (exam-focused)
Each line is a compact fact or mnemonic that appeared frequently in AE/JE papers — quick, memorable, and exam‑friendly.
1. AC motor definition
An AC motor converts alternating current electrical power into mechanical torque using rotating magnetic fields — remember: "Alternating creates rotation".
2. Synchronous speed (Ns)
Ns (rpm) = 120 × f / P. Quick trick: double frequency or halve poles doubles speed — used often in speed selection problems.
3. Slip (s)
Slip = (Ns − N)/Ns. For induction motors, small slip (2–8%) is common; sheet problems ask to compute torque at given slip.
4. Rotor types
Squirrel-cage: rugged, low maintenance. Slip-ring (wound): high starting torque via external resistors — exam favourite distinction.
5. Starting methods
Direct-on-line (DOL), Star-Delta, Auto-transformer, Soft-starter. Match starting torque and supply constraints when asked in AE tests.
6. Torque-speed curve
Torque rises to Tmax at a certain slip then falls — memorize shape: peak near moderate slip, zero at synchronous speed for induction motors.
7. Maximum torque (Tmax)
Tmax ∝ rotor resistance at that slip — increasing Rr shifts peak torque to higher slip (useful for starting torque design).
8. Power flow
Air-gap power = power transferred across magnetic field; mechanical power = air-gap power × (1 − s). Common in efficiency questions.
9. Efficiency calculation
η = Pout / Pin. Remember to subtract stator copper, rotor copper, iron, and mechanical losses — AE problems often give these as percentages.
10. Rotor copper loss location
Rotor copper loss is part of air-gap power; mechanical power developed = air-gap power − rotor copper loss.
11. Starting current
Induction motors draw 5–8× rated current at DOL start. Star‑Delta reduces starting current to about 1/3 but also reduces starting torque.
12. Power factor
Induction motors operate at lagging PF; PF improves with load — a common topic in electrical machines questions.
13. Motor rating
Nameplate gives kW (or HP), voltage, current, speed, and power factor — always use nameplate data for calculations in practical problems.
14. Locked rotor torque (LRT)
Torque at zero speed (start) — slip = 1. Important for starting heavy loads; specify LRT in selection problems.
15. No-load test
Used to determine core loss and magnetizing currents (like open-circuit test for transformers) — often applied in equivalent circuit derivations.
16. Blocked-rotor test
Analogous to short-circuit test: used to find combined series resistance and leakage reactance referred to stator side.
17. Equivalent circuit
Remember the per-phase equivalent with referred rotor parameters — many exam problems require working in per‑phase values.
18. Referred values
Use turns ratio to refer rotor resistance/reactance to stator side (multiply by (Ns/Nr)^2 or simpler ratio). Helps to avoid mistakes in power calculations.
19. Single phase starting
Single-phase induction motors need starting torque — use auxiliary winding with phase shift (capacitor) or split-phase methods.
20. Capacitor-run motors
Provide improved torque and PF. Remember capacitor sizing questions appear as quick calculation problems in AE/JE exams.
21. Star-Delta connection trick
Line voltage in delta = phase voltage in star × √3. For starters, remember current reduces to 1/3 in star compared to delta for same line voltage.
22. Thermal protection
Overload relays and thermal sensors protect windings — exam may ask what protects motors from continuous overload (answer: thermal/O/L relays).
23. Cooling & service factors
Motors rated with service factor (SF). SF 1.15 means motor can run at 15% overload briefly — useful when sizing motors for occasional loads.
24. Direction reversal
Swap two supply phases in three-phase induction motors to reverse rotation — a very common practical question.
25. Harmonics & motors
Non-sinusoidal supply increases heating and torque pulsations — exam may ask mitigation: filters, proper drive selection (VFD with PWM filtering).
26. Variable Frequency Drive (VFD)
VFD controls speed by changing frequency & voltage—question often contrasts V/Hz control vs scalar/vector control briefly.
27. Synchronous motors
Run at synchronous speed; used when constant speed is required. Can operate at leading PF when over-excited (useful for power factor correction).
28. Field weakening
Used to increase speed above synchronous for special motors (not common in induction) — know concept for higher-level questions.
29. Motor selection checklist
Consider starting torque, duty cycle, ambient temperature, supply type, and protection devices — often asked as checklist-style exam item.
30. Practical tip: Sizing fuse
Fuse sizing often 125–150% of full load current for motors; remember this range for protection-related questions.
31. Quick mnemonic for poles
"More poles — slower wheels" — more poles reduce synchronous speed; helpful to answer speed‑based MCQs quickly.
32. Common exam trap
Watch units: kW vs kVA, rpm vs rad/s. Convert carefully — many students lose marks on unit conversion only.
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