Question 1

Assertion (A): An ideal rectifier should have unity harmonic factor Reason (R): An ideal rectifier should have unity displacement factor.

Accepted Answer

Ideal rectifier should have zero harmonic factor and unity displacement factor.

Question 2

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

Accepted Answer

.

Question 3

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

Accepted Answer

v ∝ (i)1/4 ∝ (100)1/4 ≈ 3.

Question 4

The number of leads in an SCR are :

Accepted Answer

Anode, cathode and gate.

Question 5

Thyristors are not suitable for logic circuits.

Accepted Answer

Thyristors can be used for logic circuits.

Question 6

A cycloconverter can be

Accepted Answer

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

Question 7

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

Accepted Answer

30° phase difference is necessary.

Question 8

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

Accepted Answer

.

Question 9

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

Accepted Answer

.

Question 10

Assertion (A): Power transistor has lower switching time than SCR Reason (R): Power transistor has greater switching time than SCR

Accepted Answer

Power transistor does not have high rating.

Question 11

In a single phase full wave converter (M-2 connection) feeding on R - L load, the input voltage is v = Vm sin ωt. The expression for dc output voltage is

Accepted Answer

Due to inductive load the period of conduction is a to p + a in each half cycle.

Question 12

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

Accepted Answer

.

Question 13

Assertion (A): The surge current which an SCR can withstand is much higher than rms on state current. Reason (R): The duration of surge current is very small.

Accepted Answer

Since duration of surge current is small, the associated energy is small.

Question 14

A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is a combination of R and L. The firing angle is a. The period of conduction of SCR and free wheeling diode respectively are

Accepted Answer

SCR conducts from a to p and free wheeling diode from p to (2p + a).

Question 15

A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be

Accepted Answer

Inductive load increases the turn on time.

Question 16

In a multiphase chopper, all choppers operate together.

Accepted Answer

They operate in a sequence.

Question 17

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

Accepted Answer

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

Question 18

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

Accepted Answer

Vl = V a. Therefore Vl2 - V2 (a). Heating is proportional to a.

Question 19

A single phase half wave rectifier is feeding a resistive load. When a = 0, dc output voltage is 127 V. When a = 30°, the dc output voltage will be about

Accepted Answer

Using Eq = 0.933. Hence Vdc for 30° firing angle = 0.933 x 127 = ≈ 120 V.

Question 20

A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is

Accepted Answer

.

Power Electronics Questions