# Test: Power Electronics - 1

## 25 Questions MCQ Test Power Electronics | Test: Power Electronics - 1

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Attempt Test: Power Electronics - 1 | 25 questions in 25 minutes | Mock test for Electrical Engineering (EE) preparation | Free important questions MCQ to study Power Electronics for Electrical Engineering (EE) Exam | Download free PDF with solutions
QUESTION: 1

### A cycloconverter can be

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

Solution:

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

QUESTION: 3

### In a 3 phase fully controlled converter the firing frequency is

Solution:

It is a 6 pulse converter.

QUESTION: 4

Two thyristor of same rating and same specifications

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.

Solution:

It is proportional to frequency.

QUESTION: 6

A semiconverter feeding an RLE load operates in

Solution:

Semiconvertor operates only in first quadrant.

QUESTION: 7

The phenomenon of overlap in converter operation due to

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

Solution:

.

QUESTION: 9

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

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

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

Solution:

.

QUESTION: 12

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

Solution:

v ∝ (i)1/4 ∝ (100)1/4 ≈ 3.

QUESTION: 13

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

Solution:

.

QUESTION: 14

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

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.

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

Solution:

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

QUESTION: 17

A chopper

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.

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

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

Solution:

Vl = V a. Therefore Vl2 - 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 Is and rms value of load current in Irms. Then

Solution:

.

QUESTION: 22

The terminals of a power BJT are called

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

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 RC across each thyristor. This resistance RC (along with parallel connected diode)

Solution:

Discharge current flows through Re .

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

Solution:

When freewheeling diode conducts output current exists but output voltage is zero.

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