Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 08 min.
Q1. The field poles and armature core of a d.c. generator is laminated in order to reduce
(a) Hysteresis loss
(b) Eddy current loss
Q2. If number of poles in lap wound d.c. generator is doubled, the generated e.m.f. will be
(a) increased by a factor of 2
(b) decreased by a factor of 2
(c) increased by a factor of 4
Q3. Which one of the following methods gives voltage regulation higher than the actual value in an alternator?
(a) ZPF method
(b) MMF method
(c) EMF method
(d) ASA method
Q4. A circuit having power factor of 0.8 consumes 40 W. what will be the value of reactive power (in VAR) of the circuit?
Q5. The insulation thickness on the conductor in cables depends upon
(b) Power factor
(c) Reactive power
Q6. If the frequency of a transmission system is changed from 50 Hz to 100 Hz, the string efficiency will
(c) Remains unchanged
(d) Can’t say
Sol. We know that Resistance (R) = δ l/A
due to lamination, effective cross – section area (A) of each layer reduces and resistance increases and losses minimise.
To minimize the eddy current loss, we increase the resistance in the path of eddy current by laminating it,
Eddy current losses are directly proportional to the area of armature.
Sol. E = ZϕNP/60A
P = no. of poles
and A = no. of parallel path
for lap winding, A = P
So, by increasing pole number, parallel path will be equal to no. of poles and generated emf will remains unchanged.
EMF method or pessimistic method because gives VR higher than actual.
MMF method or optimistic method because VR lower than actual.
Sol. Given that, P = 40 W
pf = cos ϕ = 0.8
⇒ VA = 40/0.8=50 W
Q = VA sin ϕ
= 50 √(1-(0.8)^2 )
= 50 √0.36
= 50 × 0.6=30 VAR.
Sol. Higher the operating voltage of the cable, more is the insulation thickness required.
Sol. string efficiency is independent of frequency of transmission system.