Question 1

When the core of a saturable reactor is saturated, the inductance is high.

Accepted Answer

Since core is saturated, change of flux is small and inductance is low.

Question 2

Assertion (A): When an SCR is conducting, the voltage drop across is about 1 V. Reason (R): When an SCR is conducting, the junction temperature is about 200°C.

Accepted Answer

Junction temperature is about 120°C. Hence R is wrong.

Question 3

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

Accepted Answer

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

Question 4

A cycloconverter cannot give a frequency higher than input frequency.

Accepted Answer

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

Question 5

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

Accepted Answer

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

Question 6

Present day HVDC converters are all

Accepted Answer

To reduce ripple 12 pulse converters are used.

Question 7

A single phase electric heater uses phase controlled thyristors. The firing angle is 90°. The degree of heating is

Accepted Answer

Conduction occurs only during 50% of wave.

Question 8

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

Accepted Answer

SCRs conduct from a to p in each half cycle.

Question 9

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

Accepted Answer

Any number of taps are possible.

Question 10

In controlled heating circuits, thyristors act as

Accepted Answer

The output voltage is controlled ac.

Question 11

A single phase full converter gives maximum and minimum output voltage at firing angles of

Accepted Answer

Output voltage is maximum if a = 0 and is minimum if a = 180°.

Question 12

A Shockley diode can be used

Accepted Answer

It is a four layered PNPN device without a gate terminal. Its characteristics is similar to that of SCR with zero gate current. It is acting as a trigger switch for Thyristor.

Question 13

In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current

Accepted Answer

For resistive load output current is always proportional to output voltage.

Question 14

Assertion (A): Snubber circuit has an RC series circuit in parallel with SCR Reason (R): Voltage across a capacitor can not change instantaneously.

Accepted Answer

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

Question 15

Which of these commutation methods is used in AC-DC converters?

Accepted Answer

Class F is natural commutation AC = DC converter use natural (or line) commutation.

Question 16

A silicon controlled switch has :

Accepted Answer

Anode, cathode, anode gate and cathode gate.

Power Electronics Questions