Concept Notes
1. DC Motor Basics
A DC motor converts electrical energy into mechanical energy using the principle:
When a current-carrying conductor is placed in a magnetic field, it experiences a force (Lorentz Force).
2. Types of DC Motors
(a) Shunt Motor
- Field winding in parallel with armature.
- Constant speed, good for lathes, fans.
- Torque is moderate.
(b) Series Motor
- Field winding in series with armature.
- Very high starting torque.
- Speed varies widely with load.
- Used in cranes, traction, elevators.
(c) Compound Motor
- Combination of series + shunt field.
- Cumulative compound: High starting torque + better speed regulation.
- Differential compound: Rarely used.
(d) Permanent Magnet DC Motor (PMDC)
- Uses permanent magnets for field.
- Compact, efficient, widely used in toys, EVs, robotics.
3. Torque in DC Motors
The force on a conductor:
Torque:
For a motor with 
conductors, 
poles, flux per pole 
, and armature current 
:
Where:
= number of parallel paths- = flux per pole
- = armature current
Simplified:
4. Speed Equation of DC Motor
Back emf:
Voltage equation:
So,
- Shunt motor → Speed almost constant (since is constant).
- Series motor → Speed varies widely with load.
⚙️ Formulas
- Force on conductor:
- Torque:
- General torque eqn:
- Proportional torque:
- Back emf:
- Motor equation:
- Speed:
🔟 10 MCQs
Q1. The basic principle of DC motor operation is:
a) Lenz’s law
b) Faraday’s law
c) Lorentz force law
d) Ampere’s circuital law
Q2. Which DC motor has nearly constant speed under varying load?
a) Series
b) Shunt
c) Compound
d) PMDC
Q3. A series motor is preferred for:
a) Fans
b) Lathes
c) Electric traction
d) Measuring instruments
Q4. The torque of a DC motor is directly proportional to:
a) 
b) 
c) 
d) None
Q5. The speed of a DC motor is given by:
a) 
b)
c) 
d) 
Q6. In a DC shunt motor, if flux reduces by 10% (constant V), speed will:
a) Remain same
b) Increase
c) Decrease
d) Zero
Q7. For maximum torque in a DC motor:
a) 
b) 
c) Both high
d) Both low
Q8. Which motor is best suited for elevators?
a) Shunt
b) Series
c) Compound
d) PMDC
Q9. If back emf equals supply voltage in a DC motor:
a) Motor runs normally
b) Motor stops
c) Armature current becomes zero
d) Both b and c
Q10. The starting torque of a DC series motor is:
a) Low
b) Medium
c) High
d) Negligible
✅ Answer Key
| Q.No | Answer |
|---|---|
| 1 | c |
| 2 | b |
| 3 | c |
| 4 | b |
| 5 | b |
| 6 | b |
| 7 | c |
| 8 | b |
| 9 | c |
| 10 | c |
🧠 Explanations
- Q1: Motor works on Lorentz force principle → (c).
- Q2: Shunt motor → nearly constant speed → (b).
- Q3: Series motor → high starting torque → traction → (c).
- Q4: From torque eqn → → (b).
- Q5: → (b).
- Q6: If flux decreases → speed increases → (b).
- Q7: Torque is maximum when both and are large → (c).
- Q8: Elevators need high starting torque → series motor → (b).
- Q9: If
, then → motor stops → (c). - Q10: Series motor → very high starting torque → (c).
🎯 Motivation / Why Practice Matters
DC motors form the heart of many electrical machines. Questions on torque equation, speed control, and motor types are guaranteed in ECET 2026. Practicing ensures you can quickly distinguish motor characteristics and solve numerical torque/speed problems confidently.
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