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.
May you like these posts
Visit these relevant posts on Electrical Zindagi for deep dives and more practice:
(Click links to open the blog — these act as backlinks to related content on the site.)
Join us for regular updates
Subscribe to our channels for free notes, videos and quick revision tips.
Donate & Community
Support the community — your contributions help maintain free notes, host videos and keep content ad-free for many learners.
Thank you — every contribution helps students prepare for AE / JE exams.
Advertisement
Ads help us keep the notes free. If you find the content useful, consider donating or subscribing to our channels.
SEO Keywords
AC motor notes, induction motor, synchronous speed formula, slip calculation, AE JE motor questions, motor starting methods, star delta starter, VFD motor control, motor efficiency, motor protection, locked rotor torque, squirrel cage rotor, slip ring motor, motor nameplate, motor sizing, starting current, service factor motor, motor maintenance tips, motor selection checklist, power factor correction motors.
