A lite explanation of why this topic is important for ECET
Synchronous motors are widely used in industries for constant speed applications and power factor correction. The V-curves represent the relationship between armature current and field excitation. Understanding these curves is crucial for solving ECET questions on power factor improvement, efficiency, and synchronous motor operation. Many competitive exams, including ECET 2026, often test concepts of V-curves directly through numericals and theoretical MCQs.
📘 Concept Notes
1. Basics of Synchronous Motor
- A synchronous motor runs at constant synchronous speed given by:
where
is supply frequency, 
is number of poles. - It is not self-starting and requires auxiliary means to start.
2. Excitation in Synchronous Motor
- Under-excited motor: Acts like inductive load → lagging power factor.
- Over-excited motor: Acts like capacitive load → leading power factor.
- Normal excitation: Unity power factor.
3. V-Curves of Synchronous Motor
- Plot of armature current (I_a) vs. field excitation (E_f) at constant load.
- The curve shape resembles the letter “V”.
- At minimum current point → unity power factor.
- To the left (less excitation) → lagging pf.
- To the right (over-excitation) → leading pf.
4. Inverted V-Curves
- Plot of power factor vs. field excitation.
- Shows lagging → unity → leading as excitation increases.
5. Practical Uses
- Used for power factor correction in industries.
- Provides voltage regulation support in power systems.
- ECET questions often focus on finding excitation, armature current, or pf condition.
⚙️ Formulas
- Synchronous speed:
- Power factor angle:
- Armature current (approximate):
- Active power:
- Reactive power:
🔟 10 MCQs
Q1. The shape of V-curves in synchronous motors represents the relation between:
a) Speed vs Excitation
b) Armature current vs Excitation
c) Torque vs Excitation
d) Voltage vs Excitation
Q2. At the bottom of a V-curve, the motor operates at:
a) Lagging pf
b) Unity pf
c) Leading pf
d) Zero pf
Q3. A synchronous motor at under-excitation behaves like:
a) Capacitor
b) Inductor
c) Resistor
d) Generator
Q4. A 6-pole synchronous motor runs at 50Hz. Find synchronous speed.
a) 1000 rpm
b) 1200 rpm
c) 1500 rpm
d) 3000 rpm
Q5. If excitation is increased beyond unity pf point, the motor will draw:
a) Lagging current
b) Leading current
c) Zero current
d) Same current
Q6. Which of the following is NOT a property of V-curves?
a) They are symmetrical.
b) Minimum current occurs at unity pf.
c) They show excitation vs torque.
d) They shift with change in load.
Q7. In synchronous motors, reactive power control is achieved by:
a) Load torque
b) Armature resistance
c) Field excitation
d) Speed variation
Q8. A synchronous motor has 
, 
. Find 
.
a) 1200 rpm
b) 1500 rpm
c) 1800 rpm
d) 3600 rpm
Q9. Inverted V-curves are used to study:
a) Armature current vs Excitation
b) Power factor vs Excitation
c) Torque vs Load
d) Speed vs Excitation
Q10. A synchronous motor draws minimum current when operating at:
a) Leading pf
b) Unity pf
c) Lagging pf
d) Zero pf
✅ Answer Key
| Q.No | Answer |
|---|---|
| 1 | b |
| 2 | b |
| 3 | b |
| 4 | a |
| 5 | b |
| 6 | c |
| 7 | c |
| 8 | c |
| 9 | b |
| 10 | b |
🧠 Explanations
- Q1: V-curves show armature current vs excitation → (b).
- Q2: At bottom, current is minimum → unity pf → (b).
- Q3: Under-excited draws lagging current like inductor → (b).
- Q4:
→ (a). - Q5: Beyond unity pf, current becomes leading → (b).
- Q6: V-curves are excitation vs armature current, not torque → (c).
- Q7: Reactive power controlled by excitation → (c).
- Q8:
→ (c). - Q9: Inverted V-curves represent pf vs excitation → (b).
- Q10: Minimum current occurs at unity pf → (b).
🎯 Motivation / Why Practice Matters
Synchronous motor V-curves are a hot spot for ECET 2026 because they test concept clarity + numerical speed. Questions can be tricky, mixing excitation, pf, and current. Practicing these ensures you can answer in 30–40 seconds, giving you a clear edge over others. Remember: mastering V-curves helps not only in ECET but also in GATE and PSU exams later.
📲 CTA
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