--- title: "Hunting; loaded operation of synchronous motor" subject: "Electrical Machines" chapter: 6 key: "semester_4/electrical_machines:sync-motor-hunting-loaded-operation" source: https://www.bctengineer.com/q/semester_4/electrical_machines/sync-motor-hunting-loaded-operation site: BCT Engineering --- # Hunting; loaded operation of synchronous motor Electrical Machines · Chapter 6: Three Phase Synchronous Machines - Asked in: 2078 Poush, 8 marks - Video: [hunting in synchronous motor | hunting in synchronous machine | Industrial automation | machines](https://www.youtube.com/watch?v=u70AgTulUjk) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/sync-motor-hunting-loaded-operation - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-6 ## Question What is hunting in synchronous motor? Explain the loaded operation of three phase synchronous motor. ## Short answer Hunting is rotor oscillation about synchronous position due to sudden load change, damped by amortisseur windings. Under load, rotor falls back by angle δ; back emf shifts, armature current increases, developing torque to match load. ## Answer #### Hunting Hunting is the **oscillation of the rotor of a synchronous machine about its steady-state synchronous position** (the load angle $\delta$ swings above and below its mean value) following a sudden change in load or supply. **Cause:** when the load changes suddenly, the rotor does not take up its new load angle immediately. Because of its inertia it overshoots, and the restoring (synchronising) torque pulls it back, so it oscillates about the new position. The oscillations die out slowly. **Effects:** variation in current, power and voltage, possible loss of synchronism, mechanical stress and noise. **Prevention:** damper (amortisseur) windings on the pole faces damp the oscillations by developing a torque that opposes relative motion. A flywheel (to increase inertia) and proper design of the governor (prime mover control, for generators) also help. #### Loaded operation At no load, the rotor poles lie almost in line with the stator rotating-field poles. The back emf $E_b$ is almost equal to $V$ and nearly in phase with it ($\delta \approx 0$), so the net voltage $E_r = V - E_b$ across the armature is tiny and the current is small. When mechanical load is applied: 1. The rotor momentarily slows down and falls back by the load angle $\delta$; it then continues at synchronous speed, with only its position shifted. 2. $E_b$ shifts by angle $\delta$ with respect to $V$, so the resultant voltage $E_r = V - E_b$ increases. 3. The armature current $I_a = \frac{E_r}{Z_s}$ increases and lags $E_r$ by $\theta = \tan^{-1}(X_s/R_a)$. 4. The motor torque rises until it equals the load torque. 5. The speed remains constant at $N_s$ for all loads up to the pull-out torque. $$ P = \frac{3VE_b}{X_s}\sin\delta, \qquad T = \frac{P}{\omega_s} $$ Maximum power occurs at $\delta = 90^\circ$ (pull-out). If the load exceeds it, the motor loses synchronism and stops. ```text Phasors (motor): O------------------------> V `-._ delta | `-._ | `-._ Eb | Er = V - Eb `-._ | `-._ | `-._* Ia = Er/Zs lags Er by theta (close to 90 deg) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is hunting? A. Continuous high speed B. Oscillation of the rotor about its synchronous position after a sudden change C. Loss of field D. Overheating Answer: B. The rotor overshoots its new load angle because of inertia and oscillates. (Key point: Hunting) 2. Which helps prevent hunting? A. Damper windings B. Open field C. Higher voltage D. Higher frequency Answer: A. Damper windings develop a torque opposing relative motion and damp the swings. (Key point: Prevention) 3. When mechanical load is applied, what happens to the speed? A. It falls in proportion B. It remains Ns; the rotor only falls back by load angle delta C. It doubles D. It stops Answer: B. Speed is constant at Ns; load changes only delta. (Key point: Load on motor) 4. Where is maximum power developed? A. delta = 0 B. delta = 45 deg C. delta = 90 deg (pull-out) D. delta = 180 deg Answer: C. P = (3 V Eb/Xs) sin(delta) is maximum at 90 deg; beyond it the motor loses synchronism. (Key point: Maximum power) ## Related in Electrical Machines - [No-load and loaded operation, constant flux](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aideal-transformer-no-load-loaded-flux) — Electrical Machines, chapter 2, asked 3× - [Effect of excitation on synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-excitation-effect-pf) — Electrical Machines, chapter 6, asked 5× - [Shunt motor loaded speed at 50 A](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adcm-shunt-1-25kw-loaded-speed) — Electrical Machines, chapter 4 - [3.3 kV synchronous motor back emf](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-3-3kv-back-emf) — Electrical Machines, chapter 6, asked 2× - [Synchronous motor: pf when input rises to 1500 kW](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-pf-input-1500kw) — Electrical Machines, chapter 6 - [Synchronous motor not self-starting; starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-not-self-starting-methods) — Electrical Machines, chapter 6, asked 2× ## Related in other subjects - [Hunting in synchronous motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ahunting-synchronous-motor) — Electric Machines II, chapter 2, asked 2× - [Synchronous motor features and lagging/leading operation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-lagging-leading) — Electric Machines II, chapter 2 - [Output equation of three phase induction motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainduction-motor-output-equation) — Electric Machine Design, chapter 4, asked 6× - [Construction and operation of synchronous generator](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asynchronous-generator-construction-principle) — Electric Machines II, chapter 1