Question 1

A single phase half wave rectifier is feeding a resistive load. Input voltage v = Vm sin ωt. The output dc voltage is Vdc and output rms voltage is Vrms . If firing angle is 180°, Vdc and Vrms respectively are

Accepted Answer

Put a = 180° in Eqs Otherwise also a = 180° means no conduction. Hence zero output voltage.

Question 2

Assertion (A): A TRIAC is a bidirectional SCR. Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions.

Accepted Answer

Since a TRIAC can conduct in both directions it is bi directional SCR.

Question 3

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

Accepted Answer

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

Question 4

In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be

Accepted Answer

In bridge connection PIV is half of that in M-2 connection.

Question 5

If Vdc1 is the dc output voltage in half wave controlled rectifier circuit feeding resistive load and Vdc2 is the dc output voltage in full wave controlled rectifier circuit (M - 2 connection) feeding resistive load, then

Accepted Answer

and Vdc = (1 + cos a).

Question 6

The total number of leads in reverse conducting thyristor is

Accepted Answer

Question 7

Current derating may be necessary when thyristors are operating in

Accepted Answer

Since current does not divide equally, current derating is necessary.

Question 8

Assertion (A): A transistor requires a continuous base signal for conduction but a thyristor requires a gate pulse. Reason (R): Transistor find widespread application in power electronic circuits.

Accepted Answer

Transistors are used but not very widely.

Question 9

The phenomenon of overlap in converter operation due to

Accepted Answer

Source inductance delays turn off and hence causes overlap.

Question 10

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

Accepted Answer

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

Question 11

A semiconverter feeding an RLE load operates in

Accepted Answer

Semiconvertor operates only in first quadrant.

Question 12

Assertion (A): Inverters and choppers use fast switching thyristors Reason (R): Fast switching SCR has low turn off time.

Accepted Answer

Low turn off time enables fast switching.

Question 13

For a BJT a = 0.98, then β =

Accepted Answer

Question 14

The terminals of a power BJT are called

Accepted Answer

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

Question 15

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

Accepted Answer

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

Question 16

A semi converter operates in two quadrants.

Accepted Answer

It operates only in quadrant.

Question 17

Figure shows a chopper feeding RLE load, The free wheeling diode conducts when

Accepted Answer

FD is forward biased only when thyristor is off.

Question 18

A chopper

Accepted Answer

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

Question 19

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.

Accepted Answer

If duty cycle is low, current may become discontinuous.

Question 20

In a thyristor the gate current is increased, then

Accepted Answer

Gate current does not affect the magnitude of anode current.

Power Electronics Questions