Q1. Commutation in a DC generator is the process of:
A. Reversal of current in armature conductors
B. Generation of emf
C. Reduction of armature current
D. Strengthening of main field
Correct Option: A
Explanation:
Commutation is the controlled reversal of current in an armature coil as it passes under successive poles. This ensures unidirectional current at the generator terminals and occurs when the coil is short-circuited by the brush.
Q2. Poor commutation in a DC generator results in:
A. Reduced copper loss
B. Sparking at brushes
C. Increased efficiency
D. Constant terminal voltage
Correct Option: B
Explanation:
If current reversal is incomplete during commutation, a high induced voltage appears across commutator segments, causing sparking at the brushes. This leads to heating and excessive brush wear.
Q3. The emf induced in a coil undergoing commutation is called:
A. Back emf
B. Generated emf
C. Reactance voltage
D. Residual emf
Correct Option: C
Explanation:
Due to the self-inductance of the armature coil, an emf is induced that opposes the change in current during commutation. This induced emf is known as reactance voltage.
Q4. Reactance voltage during commutation is proportional to:
A. Armature resistance
B. Rate of change of current
C. Field current
D. Speed only
Correct Option: B
Explanation:
Reactance voltage is given by Er = L (di/dt). Hence, it is directly proportional to the rate of change of current during commutation.
Q5. Which factor mainly causes sparking at brushes?
A. High terminal voltage
B. Reactance voltage
C. Armature resistance
D. Field weakening
Correct Option: B
Explanation:
Reactance voltage opposes current reversal. If the armature coil leaves the brush contact before current reversal is complete, sparking occurs at the brushes.
Q6. The time of commutation depends on:
A. Number of poles
B. Width of brush and speed
C. Flux per pole
D. Field resistance
Correct Option: B
Explanation:
Time of commutation depends on the duration for which the coil remains under the brush. This is governed by brush width and the peripheral speed of the commutator.
Q7. In resistance commutation, sparking is reduced by:
A. Increasing brush resistance
B. Increasing field current
C. Decreasing armature speed
D. Using compensating winding
Correct Option: A
Explanation:
High-resistance carbon brushes introduce a voltage drop that assists gradual current reversal, thereby reducing the effect of reactance voltage.
Q8. Which brush material is preferred to improve commutation?
A. Copper
B. Aluminium
C. Carbon
D. Steel
Correct Option: C
Explanation:
Carbon brushes are preferred due to their higher contact resistance, good lubrication properties, and reduced wear, all of which improve commutation.
Q9. Interpoles are also known as:
A. Pole shoes
B. Compoles
C. Dummy poles
D. Shading poles
Correct Option: B
Explanation:
Interpoles are auxiliary poles specifically used to improve commutation and are therefore called commutating poles or compoles.
Q10. Interpoles are connected in:
A. Parallel with shunt field
B. Series with armature
C. Parallel with armature
D. Series with shunt field
Correct Option: B
Explanation:
Interpoles are connected in series with the armature so that their flux is proportional to armature current, ensuring effective commutation at all load conditions.
Q11. The polarity of interpoles in a DC generator is:
A. Same as main pole ahead
B. Same as main pole behind
C. Opposite to main pole ahead
D. Independent of main poles
Correct Option: C
Explanation:
In a DC generator, interpoles have polarity opposite to the next main pole in the direction of rotation so that the induced emf aids current reversal.
Q12. Main function of interpoles is to:
A. Reduce armature reaction
B. Improve commutation
C. Increase generated emf
D. Reduce iron losses
Correct Option: B
Explanation:
Interpoles neutralize reactance voltage and ensure sparkless commutation. Reduction of armature reaction is achieved by compensating windings.
Q13. Interpoles produce flux proportional to:
A. Field current
B. Speed
C. Armature current
D. Load voltage
Correct Option: C
Explanation:
Since interpole windings are connected in series with the armature, the flux produced by interpoles is directly proportional to armature current.
Q14. Which winding neutralizes reactance voltage?
A. Field winding
B. Compensating winding
C. Interpole winding
D. Armature winding
Correct Option: C
Explanation:
Interpole windings induce an emf in the commutating coil that directly cancels the reactance voltage, enabling smooth current reversal.
Q15. In absence of interpoles, sparkless commutation can be obtained by:
A. Increasing speed
B. Shifting brushes to MNA
C. Increasing armature current
D. Reducing flux
Correct Option: B
Explanation:
Without interpoles, brushes are shifted to the magnetic neutral axis to minimize induced emf in the commutating coil, although this method is load dependent.
Q16. If armature current changes from +20 A to –20 A in 0.002 s and coil inductance is 0.02 H, the reactance voltage is:
A. 200 V
B. 400 V
C. 800 V
D. 1600 V
Correct Option: B
Explanation:
Change in current = 40 A. Using Er = L (di/dt) = 0.02 × (40 / 0.002) = 400 V.
Q17. Which method gives best commutation in modern DC machines?
A. Resistance commutation
B. Brush shifting
C. Interpole commutation
D. High resistance brushes
Correct Option: C
Explanation:
Interpole commutation is automatic, load independent, and highly effective, making it the most widely used method in modern DC machines.
Q18. Interpoles are placed:
A. Under main poles
B. Between main poles
C. On armature core
D. On pole shoes
Correct Option: B
Explanation:
Interpoles are installed midway between main poles along the geometrical neutral axis to assist commutation.
Q19. Compensating winding differs from interpoles because it:
A. Is connected in parallel
B. Neutralizes cross-magnetising effect
C. Improves voltage regulation
D. Reduces copper loss
Correct Option: B
Explanation:
Compensating winding neutralizes the cross-magnetising effect of armature reaction under pole faces, whereas interpoles are used to improve commutation.
Q20. Reactance voltage becomes zero when:
A. Current reversal is instantaneous
B. Coil inductance is zero
C. Brush width is zero
D. Speed is zero
Correct Option: B
Explanation:
From Er = L (di/dt), if the inductance L is zero, the reactance voltage becomes zero, eliminating sparking during commutation.