Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 6
Time: 6 min.
Q1. A dc series motor develops a torque of 10 N-m when the armature current is 20 A. What will be the torque developed when armature current is increased to 40 A and if the field circuit is unsaturated?
(a) 10 N-m
(b) 5 N-m
(c) 20 N-m
(d) 40 N-m
Q2. In measuring instruments loading effect is caused by which feature of the instrument?
(a) High resistance
(b) High sensitivity
(c) low sensitivity
(d) High range
Q3. What is the effect of increase in speed of a dc motor on the back emf and the line current?
(a) Back emf decreases and line current increases
(b) Back emf increases and line current decreases
(c) Both back emf and line current increase
(d) Both back emf and line current decrease
Q4. Which is the most essential condition for parallel operations of two single phase transformers?
(a) Should have the same percentage impedance.
(b) Should have the same kVA rating.
(c) Should have the same polarity.
(d) Should have the same voltage ratio.
Q5. ………. increases the steady state accuracy
(c) Phase lead compensator
(d) Phase lag lead compensator
Q6. The machine in which energy transfer happens both through conduction and induction, is called
(a) Three phase induction motor
(b) Single phase induction motor
(c) Auto transformer
(d) Two winding transformers
Sol. For DC series motor, if the field circuit is unsaturated, then T α I_a^2.
∴T_2/T_1 =(I_2/I_1 )^2
⇒T_2=T_1×(I_2/I_1 )^2=10×(40/20)^2=40 Nm.
Sol. The maximum loading effect is caused by an instrument having the lowest sensitivity.
Sol. For DC motor, E_b α N & V_T α I
As E_b α N, so if speed increases, back emf will also increase.
so, if speed increases, back emf will also increase and V_T will decrease. Hence, line current will decrease.
Sol. Should have the same polarity.
Sol. Integrator increases the steady state accuracy.
• Phase lag compensator is an integrator and it reduces steady state error and also reduces bandwidth.
Sol. In auto-transformer, energy transfer happens both through conduction and induction.