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UPRVUNL’21 EE: Daily Practices Quiz 15-July-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 08 min.

Q1. Which part of DC machines converts the alternating current induced in armature conductors into unidirectional current in the external load circuit?
(a) Pole coils
(b) Commutator
(c) Armature windings
(d) nArmature core

Q2. Power transformers are generally designed to have maximum efficiency around
(a) No load
(b) Half load
(c) Near full load
(d) None of the above

Q3. A circuit is disconnected by isolators, when:
(a) Circuit breaker is not open
(b) Line is energized
(c) Line is on full load
(d) There is no current in the line

Q4. Conductors, which connects the consumer’s terminal to distribution line is known as…
(a) Feeders
(b) Service-mains
(c) Distributors
(d) General cables

Q5. In PMMC instruments, damping torque is provided by:
(a) Eddy currents
(b) Air friction
(c) Fluid friction
(d) Thermal effect

Q6. In 3-phase induction motor the mechanical power developed in terms of air-gap power(P_ag) is:
(a) (1-s) P_ag
(b) (s-1) P_ag
(c) P_ag/((1-s))
(d) P_ag/s

SOLUTIONS
S1. Ans.(b)
Sol. Commutator will convert AC to DC in DC generators and vice versa in DC motors.
In generator the main function of commutator is to facilitate collection of current from the armature conductors and to convert the alternating emf induced (AC) in the armature conductors to unidirectional emf (DC) in the external load circuit.
In motor the main function of commutator is to supply current to armature from the external DC circuit and to convert this unidirectional emf (DC) of the external DC circuit to alternating emf in armature.

S2. Ans.(c)
Sol. Power transformers are generally designed to have maximum efficiency near full load, whereas distribution transformers have maximum efficiency at 50% of full load.

S3. Ans.(d)
Sol. Isolators are used to open a circuit under no load.
Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line. Isolators are generally used on both ends of the breaker so that repair or replacement of circuit breaker can be done without any danger.

S4. Ans.(b)
Sol. The service mains conductors form connecting links between distributors and metering points at the consumer terminal.

S5. Ans.(a)
Sol. In PMMC instruments, damping torque is provided by eddy currents.

S6. Ans.(a)
Sol. For 3-phase induction motor: P_mech=(1-s) P_ag

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