Q1. The working principle of a DC motor is based on the fact that:
A. A magnetic field produces emf
B. A current carrying conductor placed in a magnetic field experiences force
C. Mechanical energy converts into heat
D. Induced emf depends on resistance
Correct Option: B
Explanation:
A DC motor works on Lorentz force law. When a current-carrying conductor is placed in a magnetic field, it experiences a force. The interaction of armature current and field flux produces torque.
Q2. The direction of rotation of a DC motor is determined by:
A. Fleming’s Right Hand Rule
B. Fleming’s Left Hand Rule
C. Lenz’s Law
D. Faraday’s Law
Correct Option: B
Explanation:
Fleming’s Left Hand Rule is used for motors to determine the direction of force and rotation. Fleming’s Right Hand Rule applies to generators.
Q3. In a DC motor, the torque developed is proportional to:
A. Speed only
B. Armature voltage
C. Flux × Armature current
D. Armature resistance
Correct Option: C
Explanation:
Electromagnetic torque in a DC motor is given by T ∝ ΦIa. Hence, torque increases with increase in flux or armature current.
Q4. If both the field current and armature current of a DC motor are reversed simultaneously, the motor will:
A. Reverse its direction
B. Stop immediately
C. Continue to rotate in the same direction
D. Produce zero torque
Correct Option: C
Explanation:
Torque depends on the product Φ × Ia. Reversing both flux and armature current causes two reversals that cancel each other, so the direction of rotation remains unchanged.
Q5. Which component of a DC motor is responsible for producing mechanical torque?
A. Field winding
B. Armature winding
C. Commutator
D. Brushes
Correct Option: B
Explanation:
Mechanical torque is produced in armature conductors because they carry current and are placed in the magnetic field.
Q6. At no-load condition, a DC motor draws minimum current because:
A. Back emf is maximum
B. Armature resistance is zero
C. Flux becomes zero
D. Torque is maximum
Correct Option: A
Explanation:
At no load, motor speed is high and back emf is maximum. Since Ia = (V − Eb)/Ra, the armature current becomes minimum.
Q7. In a DC motor under load, the magnetic neutral axis (MNA) shifts:
A. In direction of rotation
B. Opposite to direction of rotation
C. Remains fixed
D. By 90° electrical
Correct Option: A
Explanation:
Due to armature reaction, in a DC motor the magnetic neutral axis shifts in the direction of rotation.
Q8. Back emf in a DC motor is defined as the emf which:
A. Assists applied voltage
B. Opposes applied voltage
C. Is zero at no load
D. Is produced by field winding
Correct Option: B
Explanation:
Back emf is induced in the armature due to rotation and always opposes the applied voltage according to Lenz’s law.
Q9. The primary role of back emf in a DC motor is to:
A. Increase starting torque
B. Limit armature current
C. Reduce copper loss
D. Increase speed
Correct Option: B
Explanation:
Back emf acts as an automatic current regulator by limiting armature current during normal operation.
Q10. At the instant of starting a DC motor, back emf is:
A. Maximum
B. Zero
C. Equal to supply voltage
D. Negative
Correct Option: B
Explanation:
At starting, speed is zero, so no emf is induced in the armature. Hence, back emf is zero.
Q11. The voltage equation of a DC motor is:
A. V = Eb + IaRa
B. V = Eb − IaRa
C. Eb = V − IaRa
D. Eb = Ia / Ra
Correct Option: C
Explanation:
Applying KVL to the armature circuit gives Eb = V − IaRa, which is the fundamental voltage equation of a DC motor.
Q12. A DC motor draws very high current at starting because:
A. Armature resistance is high
B. Flux is weak
C. Back emf is zero
D. Speed is maximum
Correct Option: C
Explanation:
At start, back emf is zero and armature resistance is very small, so the starting current becomes very high.
Q13. A DC motor supplied with electrical energy behaves like:
A. Generator
B. Motor only
C. Both motor and generator
D. Transformer
Correct Option: B
Explanation:
When electrical energy is supplied, the machine converts it into mechanical energy and operates as a motor.
Q14. Back emf in a DC motor obeys which law?
A. Faraday’s law
B. Ohm’s law
C. Lenz’s law
D. Coulomb’s law
Correct Option: C
Explanation:
Back emf always opposes the cause producing it, which follows Lenz’s law.
Q15. DC motor and DC generator have same construction but differ in:
A. Armature
B. Field system
C. Energy conversion
D. Commutator
Correct Option: C
Explanation:
Construction is the same, but a DC motor converts electrical energy into mechanical energy, whereas a DC generator converts mechanical energy into electrical energy.
Q16. Loss of field in a DC motor causes:
A. Increase in torque
B. Normal operation
C. Zero torque
D. Dangerous overspeed
Correct Option: D
Explanation:
If flux becomes zero, torque becomes zero and motor speed increases dangerously, especially in shunt motors.
Q17. At no load, back emf of a DC motor is:
A. Zero
B. Minimum
C. Maximum
D. Negative
Correct Option: C
Explanation:
At no load, speed is maximum, hence back emf is maximum.
Q18. Back emf of a DC motor is directly proportional to:
A. Armature resistance
B. Speed
C. Load torque
D. Supply current
Correct Option: B
Explanation:
Back emf Eb ∝ ΦN. For constant flux, back emf is directly proportional to speed.
Q19. A DC motor supplied with 220 V has Ra = 0.5 Ω and Ia = 20 A. Back emf is:
A. 210 V
B. 200 V
C. 230 V
D. 220 V
Correct Option: A
Explanation:
Back emf Eb = V − IaRa = 220 − (20 × 0.5) = 210 V.
Q20. If back emf becomes equal to applied voltage, the motor will:
A. Draw infinite current
B. Stop running
C. Draw zero armature current
D. Run with maximum torque
Correct Option: C
Explanation:
When Eb = V, armature current becomes zero. Hence, no torque is produced by the motor.