Test: Power Electronics

QUESTION: 1

A cycloconverter can be

A. step down
B. step up
C. step down or step up
D. none of the above

Solution:

The output frequency can be more or less than input frequency.

QUESTION: 2

In the below figure the average load current is 15 A. The rms value of transformer secondary current is

A. 15 A
B. 10.61 A
C. 7.5 A
D. 14.14 A

Solution:

RMS secondary current = = 15/1.414 = 10.608 A.

QUESTION: 3

In a 3 phase fully controlled converter the firing frequency is

A. 3 times the line frequency
B. 6 times the line frequency
C. 9 times the line frequency
D. 12 times the line frequency

Solution:

It is a 6 pulse converter.

QUESTION: 4

Two thyristor of same rating and same specifications

A. will have equal turn on and turn off periods
B. will have equal turn on but unequal turn off periods
C. may have equal or unequal turn on and turn off periods
D. will have unequal turn on and turn off periods

Solution:

Turn on and turn off times of similar SCRs may not be same.

QUESTION: 5

The amount of dielectric heating is inversely proportional to frequency.

A. True
B. False

Solution:

It is proportional to frequency.

QUESTION: 6

A semiconverter feeding an RLE load operates in

A. first quadrant only
B. first and second quadrants
C. first and fourth quadrants
D. first and third quadrants

Solution:

Semiconvertor operates only in first quadrant.

QUESTION: 7

The phenomenon of overlap in converter operation due to

A. source resistance
B. source inductance
C. both source resistance and source inductance
D. high value of firing angle

Solution:

Source inductance delays turn off and hence causes overlap.

QUESTION: 8

A thyristor has a maximum allowable junction temperature of 120ºC and the ambient temperature is 40ºC. If thermal resistance is 1.6º C/W, the maximum allowable internal power dissipation is

A.
20 W
B.
50 W
C.
92 W
D.
128 W

Solution:

QUESTION: 9

For a 12 pulse operation, the two 6 pulse units are fed by

A. Y - Y and Y - Y transformers
B. Δ - Δ and Δ - Δ transformers
C. Y - Y and Y - Δ transformers
D. Y - Δ and Y - Δ transformers

Solution:

30� phase difference is necessary.

QUESTION: 10

In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is

A. p - a
B. p
C. p + a
D. p - 2a

Solution:

Since conduction is from a to p + a, period of conduction is p in each half cycle.

QUESTION: 11

An SCR is triggered at 40ºin the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80º, the average anode current is likely to be

A.
50 A
B.
25 A
C.
less than 50 A but more than 25 A
D.
less than 25 A

Solution:

QUESTION: 12

The characteristics of a non-linear resistance is i = kv⁴. If i becomes 100 times, v becomes

A. about 100 times
B. about 10 times
C. about 3 times
D. about twice

Solution:

v ∝ (i)^1/4 ∝ (100)^1/4 ≈ 3.

QUESTION: 13

In a step down chopper using pulse width modulation, T_on = 3 x 10^-3 and T_off = 1 x 10^-3 s. The chopping frequency is

A. 333.33
B. 250
C. 500
D. 1000

Solution:

QUESTION: 14

Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.

A. True
B. False

Solution:

They are suitable for both dc and ac circuit breakers.

QUESTION: 15

Assertion (A): Inverters and choppers use fast switching thyristors

Reason (R): Fast switching SCR has low turn off time.

A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

Solution:

Low turn off time enables fast switching.

QUESTION: 16

The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by

A. forward characteristics of thyristors
B. reverse recovery characteristics of thyristors
C. both forward and reverse recovery characteristics of thyristors
D. none of the above

Solution:

Value of C is proportional to maximum difference between reverse charge of SCRs in series.

QUESTION: 17

A chopper

A. converts constant voltage dc into ac and then into variable voltage dc
B. converts constant voltage dc into variable voltage dc directly
C. converts ac of one frequency into ac of another frequency
D. converts ac to dc

Solution:

A chopper is a dc-ac converter. It converts a fixed dc voltage into a variable dc voltage.

QUESTION: 18

Assertion (A): If the duty cycle of chopper is low the current would not become discontinuous

Reason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous.

A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

Solution:

If duty cycle is low, current may become discontinuous.

QUESTION: 19

In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally

A. equal
B. different
C. equal or different
D. different but sometimes equal

Solution:

Firing angles are kept equal to get symmetrical output voltage.

QUESTION: 20

An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is

A. 16% maximum
B. 40% of maximum
C. 60% of maximum
D. 84% of maximum

Solution:

V_l = V a. Therefore V_l² - V² (a). Heating is proportional to a.

QUESTION: 21

A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is I_s and rms value of load current in I_rms. Then

A.
I_s = I_rms irrespective of the value of firing angle
B.
I_s = 0.5 I_rms irrespective of the value of firing angle
C.
I_s = I_rms if firing angle is less than 30º
D.
I_s = I_rms if firing angle is more than 30º

Solution:

QUESTION: 22

The terminals of a power BJT are called

A. emitter, base, collector
B. emitter, base, drain
C. source, base, drain
D. source, base, collector

Solution:

In a BJT terminals are emitter, base and collector irrespective of rating.

QUESTION: 23

In single phase half wave regulator, the average current over one full cycle

A. is always positive
B. may be positive or negative
C. may be negative
D. is always negative

Solution:

The input current during positive half cycle is less than that during negative half cycle.

QUESTION: 24

The dynamic equalising circuit consists of a series combination of capacitor C and resistor R_C across each thyristor. This resistance R_C (along with parallel connected diode)

A. limits charging current
B. limits discharging current
C. limits both charging and discharging currents
D. none of the above

Solution:

Discharge current flows through R_e .

QUESTION: 25

A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output current