Multiple Choice Questions on Electrical Machines - 4

Important Multiple Choice Questions on Electrical Machines

1.       Transformer excited from a sinusoidal voltage source will draw a no-load current which

A)     Is sinusoidal and of the same frequency as the voltage source

B)      Comprises the fundamental frequency (same as that of the voltage source) and the peaking third harmonic

C)      Comprises the fundamental frequency and the depressing third harmonic

D)     Is sinusoidal of frequency three times that of the voltage source

2.       A transformer on no-load is switched on to a source of voltage. It will draw a current which

A)     The same as the steady-state magnetizing current

B)      Several times the steady-state magnetizing current depending upon the initial state of the residual flux in the transformer core

C)      Several times the steady-state magnetizing current depending upon the initial state of the residual flux in the transformer core

D)     Twice the steady-state magnetizing current provided the core has no residual flux

3.       A 2/1 ratio, two winding transformer is connected as an auto transformer. Its KVA rating as an auto transformer compared to a two-winding transformer is

A)     Same

B)      1.5 times

C)      2 times

D)     3 times

4.       On the two sides of a star/delta transformer

A)     Voltages and currents are both in phase

B)      Voltages and currents both differ in phase by 30⁰

C)      Voltages differ in phase by 30⁰ but currents are in phase

D)     Currents differ in phase by 30⁰ but voltages are in phase

5.       The efficiency of transformer at full-load 0.85 pf lag is 95%. Its efficiency at full-load 0.85 pf lead will be

A)     Less than 95%

B)      More than 95%

C)      95%

D)     100%

6.       In a transformer the phase angle between primary and secondary terminal voltage is

A)     90⁰; primary voltage leading the secondary voltage

B)      90⁰; primary voltage lagging the secondary voltage

C)      A few degrees, primary voltage leading the secondary voltage

D)     A few degrees, primary voltage lagging the secondary voltage

7.       A 2500/250V, 25kVA transformer is connected as an auto transformer to give 2500/2750 V. Its rating would be

A)     250 kVA; 225 kVA is transformed conductively

B)      250 kVA; 225 kVA is transformed inductively

C)      275 kVA; 250 kVA is transformed inductively

D)     275 kVA; 250 kVA is transformed conductively

8.       A single phase transformer is to be switched to the supply to have minimum inrush current. The switch should be closed at

A)     Maximum supply voltage

B)      Zero supply voltage

C)      1/1.414 maximum supply voltage

D)     0.5  times maximum supply voltage

9.       Auto transformer is used in transmission and distribution

A)     When operator is not available

B)      When iron losses are to be reduced

C)      When efficiency considerations can be ignored

D)     When the transformer ratio is small

10.   Keeping in view the requirement of parallel operation, which of the 3-phase connections given below are possible?

A)     Delta-delta and delta-star

B)      Delta-delta and star-delta

C)      Star-star to delta-delta

D)     Delta-star to star-delta