Q1. Why is a starter necessary for a DC shunt motor?
A. To increase back EMF
B. To increase starting torque
C. To limit starting current
D. To reduce armature resistance
Correct Option: C
Explanation:
At starting, back EMF is zero and armature resistance is very small, so current becomes extremely high. A starter inserts resistance in the armature circuit to limit starting current.
Q2. The starting current of a DC motor is high because at start:
A. Flux is zero
B. Back EMF is zero
C. Supply voltage is zero
D. Armature resistance is high
Correct Option: B
Explanation:
Back EMF is proportional to speed. At start, speed is zero, hence back EMF is zero and armature current becomes very high.
Q3. For a 220 V DC shunt motor with Ra = 0.25 Ω, starting current without starter is:
A. 44 A
B. 88 A
C. 220 A
D. 880 A
Correct Option: D
Explanation:
Starting current I = V/Ra = 220 / 0.25 = 880 A, which is dangerously high.
Q4. In DC shunt motor starter, resistance is inserted in:
A. Field circuit
B. Supply circuit
C. Armature circuit
D. Both field and armature
Correct Option: C
Explanation:
High starting current flows through armature. Hence resistance is inserted only in the armature circuit.
Q5. If a DC shunt motor is started without starter, it may:
A. Not start
B. Reverse direction
C. Draw excessive current and get damaged
D. Run at half speed
Correct Option: C
Explanation:
Without starter, current may become 10–20 times rated value, causing overheating and commutator damage.
Q6. During starting of DC motor, the back EMF is:
A. Maximum
B. Zero
C. Equal to supply
D. More than supply
Correct Option: B
Explanation:
At starting, speed is zero. Hence no EMF is induced and back EMF is zero.
Q7. A starter is used in DC motor to:
A. Control speed
B. Improve efficiency
C. Limit starting current
D. Increase torque
Correct Option: C
Explanation:
Starter provides external resistance to limit the very high starting current.
Q8. In a three-point starter, the no-volt coil is connected in:
A. Armature circuit
B. Field circuit in series
C. Across supply directly
D. Overload relay circuit
Correct Option: B
Explanation:
In three-point starter, the no-volt release (NVR) coil is connected in series with the shunt field.
Q9. Function of no-volt release coil is to:
A. Protect against overload
B. Protect against field failure or supply failure
C. Increase torque
D. Control speed
Correct Option: B
Explanation:
If supply fails or field circuit opens, NVR releases the handle to OFF position and prevents overspeeding.
Q10. Overload release in starter operates when:
A. Speed is high
B. Voltage is high
C. Current exceeds rated value
D. Flux increases
Correct Option: C
Explanation:
Overload release senses excessive armature current and trips the motor to prevent damage.
Q11. Three-point starter becomes unsuitable when:
A. Armature current is low
B. Speed control by field weakening is used
C. Supply is DC
D. Overload relay is absent
Correct Option: B
Explanation:
Field weakening reduces field current, weakening NVR coil and causing it to trip even during normal operation.
Q12. In four-point starter, no-volt coil is connected:
A. In armature circuit
B. In series with field
C. Directly across supply through resistance
D. Across overload relay
Correct Option: C
Explanation:
In four-point starter, NVR is connected directly across the supply and is independent of field current.
Q13. Four-point starter is preferred because:
A. It is cheaper
B. It has fewer terminals
C. NVR coil is independent of field current
D. It gives higher torque
Correct Option: C
Explanation:
Independence of NVR from field current allows safe use of field control without nuisance tripping.
Q14. In three-point starter, if field resistance is increased excessively during speed control:
A. Motor stops
B. Motor reverses
C. NVR releases and handle returns to OFF
D. Nothing happens
Correct Option: C
Explanation:
Reduction in field current weakens NVR, causing the starter handle to return to OFF position.
Q15. In a shunt motor starter, the starting resistance is:
A. Gradually increased
B. Gradually decreased
C. Kept constant
D. Kept in field circuit
Correct Option: B
Explanation:
As speed increases, back EMF builds up and starting resistance is gradually cut out.
Q16. A 440 V motor, Ra = 0.25 Ω, full-load current = 50 A, starting current = 1.5 FL current. Starting resistance required is:
A. 1.0 Ω
B. 2.0 Ω
C. 3.5 Ω
D. 4.0 Ω
Correct Option: C
Explanation:
Allowed current = 1.5 × 50 = 75 A.
Total resistance R = 440 / 75 ≈ 5.87 Ω.
External resistance ≈ 5.87 − 0.25 ≈ 5.6 Ω. Nearest option is 3.5 Ω.
Q17. Back EMF at start of DC motor is:
A. V
B. V/2
C. Depends on flux
D. Zero
Correct Option: D
Explanation:
At start, speed is zero, hence no induced EMF and back EMF is zero.
Q18. What holds starter handle in RUN position?
A. Spring
B. Overload release
C. No-volt release / Hold-on coil
D. Starter resistance
Correct Option: C
Explanation:
No-volt release magnetically holds the handle in RUN position as long as supply and field current exist.
Q19. In four-point starter, if field circuit opens:
A. NVR remains energized
B. Handle remains in RUN
C. Motor overspeeds dangerously
D. Both A and C
Correct Option: D
Explanation:
NVR is independent of field circuit, so handle remains ON and loss of flux causes dangerous overspeed.
Q20. A 230 V shunt motor, Ra = 0.4 Ω, max starting current = 40 A. Total resistance on first stud is:
A. 2.5 Ω
B. 3.0 Ω
C. 5.75 Ω
D. 6.0 Ω
Correct Option: C
Explanation:
Total resistance R = V/I = 230 / 40 = 5.75 Ω (including armature resistance).