--- title: "Synchronous motor features, applications and excitation" subject: "Electric Machines II" chapter: 2 key: "semester_5/electric_machines_ii:motor-features-applications-excitation" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-features-applications-excitation site: BCT Engineering --- # Synchronous motor features, applications and excitation Electric Machines II · Chapter 2: Three Phase Synchronous Motor - Asked in: 2076 Chaitra, 8 marks - Video: [What is Synchronous Motor ? - Construction & Working Principle | Animated](https://www.youtube.com/watch?v=JC73hjt6Plw) — Electrical Deck - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-features-applications-excitation - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2 ## Question State the characteristic features and application of synchronous motor. Explain the effect of excitation on armature current and power factor on synchronous motor with diagram. ## Answer A synchronous motor is an AC motor whose rotor, excited by DC, locks with the stator rotating field and runs at exactly synchronous speed $N_s = 120f/P$. #### Characteristic features 1. Runs only at **synchronous speed**; speed does not change with load (zero speed regulation) as long as supply frequency is constant. 2. **Not self-starting**; needs damper winding, pony motor or variable frequency supply to start. 3. Power factor can be controlled by field current: **lagging, unity or leading**. 4. When over-excited it supplies reactive power, so it can work as a **synchronous condenser**. 5. If load torque exceeds the pull-out torque, it falls out of step and stops. 6. Needs a DC source for excitation, so it is costlier and more complex than an induction motor. 7. Efficiency is high, especially in large and low-speed ratings. 8. May **hunt** (oscillate about $N_s$) under sudden load changes; damper windings reduce this. #### Applications - Power factor correction in industries and substations (synchronous condenser). - Constant speed drives: compressors, large fans, blowers, centrifugal pumps, rolling mills, cement mills. - Low-speed high-power drives: reciprocating compressors, ball mills. - Motor-generator sets, frequency changers; small versions (reluctance, hysteresis) in clocks and recorders. - Voltage regulation at the end of long transmission lines. #### Effect of excitation on armature current and pf At constant load the input $3VI_a\cos\phi$ is fixed, so the active component $I_a\cos\phi$ is constant. Changing the field current changes $E_f$ and only the reactive component of $I_a$. ```text Ia (A) |\ / | \ lagging / leading | \ / | \___________/ <- min Ia, upf +----------------------- If under normal over-excited ``` - **Under-excitation** ($E_f < V$): resultant $E_r = V - E_f$ makes $I_a$ lag $V$; large current, **lagging pf**. - **Normal excitation** ($E_f$ roughly equal to $V$): $I_a$ in phase with $V$; current **minimum**, **unity pf**. - **Over-excitation** ($E_f > V$): $I_a$ leads $V$; current rises again, **leading pf**. ```text Ia(lead) \ Ia(upf) \ | -----------\------+------> V \ | Ia(lag) | tip of Ia moves on vertical line ``` The plot of $I_a$ against $I_f$ is the **V-curve**; joining the minimum points gives the unity pf compounding curve. Curves for higher load lie above, and the pf curve has an inverted-V shape. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does speed of a synchronous motor vary with load? A. Falls linearly B. It stays at exactly Ns (zero speed regulation) while frequency is constant C. Rises with load D. Varies with excitation Answer: B. Rotor locks to the rotating field, so speed is constant at 120f/P regardless of load. (Key point: Speed characteristic) 2. Which statement about starting is correct? A. It is self-starting B. It is not self-starting and needs damper winding, pony motor or variable frequency C. It starts only on DC D. It needs no starting means if field is excited Answer: B. No net starting torque exists at standstill. (Key point: Starting) 3. At constant load, which component of armature current changes with field current? A. The active component Ia cos(phi) B. The reactive component only C. Neither D. Both equally Answer: B. Input 3VIa cos(phi) is fixed, so the active component is constant; Ef changes only the reactive component. (Key point: Excitation effect) 4. Which application uses the over-excited motor's leading current? A. Hunting reduction B. Power-factor correction as a synchronous condenser C. Starting torque increase D. Speed control Answer: B. Over-excitation supplies reactive power, correcting plant pf and regulating voltage at the ends of long lines. 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