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# Test: Power Electronics - 3

## 25 Questions MCQ Test Power Electronics | Test: Power Electronics - 3

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Attempt Test: Power Electronics - 3 | 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

### In a dual converter both converters work as rectifiers.

Solution:

One operators as rectifier and the other as inverter.

QUESTION: 2

### A single phase full wave regulator feeds R-L load. The best gating signal is

Solution:

To ensure proper triggering pulse train in necessary because load is R - L.

QUESTION: 3

### A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if

Solution:

If load torque is constant, the change in speed is very small.

QUESTION: 4

A cycloconverter cannot give a frequency higher than input frequency.

Solution:

Step up cycloconverter can give a frequency higher than input frequency.

QUESTION: 5

A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20ºC. If thermal resistance is 1.6º C/W, the junction temperature is

Solution:

.

QUESTION: 6

A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage is v = Vm sin ωt. The thyristor will conduct

Solution:

Thyristor can conduct only when it is forward biased. This occurs only when v > 50 V.

QUESTION: 7

The symbol in figure is for

Solution:
QUESTION: 8

In a 3 phase semiconverter, the firing frequency is

Solution:

Since the circuit has 3 thyristors, firing frequency is three times the line frequency.

QUESTION: 9

A thyristor has its gate current zero. To start conduction the forward voltage must exceed

Solution:

With zero gate current, conduction can be started only if forward voltage exceeds breakover voltage.

QUESTION: 10

On load tap changer using thyristors can have a maximum of two taps.

Solution:

Any number of taps are possible.

QUESTION: 11

Assertion (A): Snubber circuit has an RC series circuit in parallel with SCR

Reason (R): Voltage across a capacitor can not change instantaneously.

Solution:

Since voltage across capacitor cannot change instantaneously it protects SCR against high .

QUESTION: 12

In dielectric heating, the thyristor circuits consist of

Solution:

Input ac is rectified and then converted to high frequency ac.

QUESTION: 13

Assertion (A): A cycloconverter may be line commutated or forced commutated

Reason (R): A step up cycloconverter requires forced commutation.

Solution:
QUESTION: 14

In the below figure the rms voltage across each half of secondary is 200 V. The peak inverse voltage across each of thyristor is

Solution:

PIV = 2(2 x 200).

QUESTION: 15

Assertion (A): A GTO circuit is more compact than SCR circuit

Reason (R): A GTO can be turned off by negative gate pulse.

Solution:

Since commutation circuit is not required, GTO circuit is compact.

QUESTION: 16

In a single phase full bridge converter (B - 2 connection), the input voltage is v = Vm sin ωt. The output dc voltage for resistive load

Solution:

SCRs conduct from a to p in each half cycle.

QUESTION: 17

In a single phase full wave ac regulator, the firing angles in the two half cycles

Solution:
QUESTION: 18

As regards single phase dual converter

Solution:
QUESTION: 19

To protect an SCR against high di/dt we connect

Solution:

current through inductance cannot change instantaneously.

QUESTION: 20

In a parallel inverter

Solution:
QUESTION: 21

A silicon controlled switch has :

Solution:

Anode, cathode, anode gate and cathode gate.

QUESTION: 22

A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is

Solution:

.

QUESTION: 23

A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is

Solution:

During the period of overlap all the four thyristors conduct and load is effectively short-circuited. Output voltage is zero.

QUESTION: 24

In a semi converter feeding a resistive load, the firing angle is a. Each thyristor conducts for

Solution:

The conduction is from a to p.

QUESTION: 25

Between which points does UJT exhibit negative resistance characteristics

Solution:
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