Test: Power Electronics

QUESTION: 1

In a dual converter both converters work as rectifiers.

A. True
B. False

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

A. short duration pulses
B. long duration pulses
C. pulse train
D. either (a) or (b)

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

A. load torque is constant
B. load torque increases with increase in speed
C. load torque decreases with increase in speed
D. either (a) or (b)

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.

A. True
B. False

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

A.
114ºC
B.
164ºC
C.
94º C
D.
84º C

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 = V_m sin ωt. The thyristor will conduct

A.
from 0 to 180º
B.
when v > 50 V
C.
from 0 to 90º
D.
When v < 50 V

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

A. DIAC
B. P-N junction diode
C. Schottky diode
D. None of the above

Solution:

QUESTION: 8

In a 3 phase semiconverter, the firing frequency is

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

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

A. about 0.5 V
B. forward breakover voltage
C. on state voltage
D. peak working off state forward voltage

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.

A. True
B. False

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.

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:

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

QUESTION: 12

In dielectric heating, the thyristor circuits consist of

A. rectifier-chopper combination
B. controlled rectifier
C. ac regulator
D. rectifier-inverter combination

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.

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:

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

A. 200 V
B. 2(200) V
C. 2(400) V
D. 800 V

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.

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:

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 = V_m 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

A. are always equal
B. are sometimes equal
C. are never equal
D. may be equal or unequal

Solution:

QUESTION: 18

As regards single phase dual converter

A. the change in operation from one quadrant to the other is faster in non-circulating current mode
B. the change in operation from one quadrant to the other is faster in circulating current mode
C. the time response to change in operation from one quadrant to other is the same in both the modes
D. the time response to change in operation from one quadrant to the other may be faster in any of the two modes

Solution:

QUESTION: 19

To protect an SCR against high di/dt we connect

A. a capacitance in series with SCR
B. a capacitance in parallel with SCR
C. an inductance in parallel with SCR
D. an inductance in series with SCR

Solution:

current through inductance cannot change instantaneously.

QUESTION: 20

In a parallel inverter

A. the commutating capacitor C is across the primary of transformer and the load is across the secondary
B. the commutating capacitor and load are connected across the secondary of transformer
C. the commutating capacitor C and load are connected in series and the combination is across the secondary of transformer
D. none of the above

Solution:

QUESTION: 21

A silicon controlled switch has :

A. 2 leads
B. 3 leads
C. 4 leads
D. 5 leads

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