MCQ in Power Electronics Part 4 | ECE Board Exam

(Last Updated On: August 10, 2022)

This is the Multiple Choice Questions Part 4 of the Series in Power Electronics as one of the Electronics Engineering topic. In Preparation for the ECE Board Exam make sure to expose yourself and familiarize in each and every questions compiled here taken from various sources including but not limited to past Board Exam Questions in Electronics Engineering field, Electronics Books, Journals and other Electronics References.

Continue Practice Exam Test Questions Part 4 of the Series

⇐ MCQ in Power Electronics Part 3 | ECE Board Exam

Choose the letter of the best answer in each questions.

151. A static induction thyristor

A. is a majority carrier device

B. is a minority carrier device

C. either a majority carrier or minority carrier device

D. has a total number of 2 leads

152. A single phase step up cycloconverter changes 50 Hz to 100 Hz. Then, one half wave of input will give rise to

A. one half wave of output

B. one full wave of output

C. two full waves of output

D. either (b) or (c)

153. Which of these triggering methods for series connected SCRs use LASCR in the gate circuit?

A. Resistance triggering

B. Simultaneous triggering

C. Sequential triggering

D. Optical triggering

154. A 3 phase ac regulator uses

A. 3 thyristors

B. 6 thyristors

C. 9 thyristors

D. 12 thyristors

155. The average load current supplied by a thyristor depends on

A. firing angle

B. firing frequency

C. magnitude of gate current

D. all of the above

156. In a 3-phase full wave regulator feeding a star connected resistance load the input voltage is 400 V line to line. The firing angle is 160°. The line-to-line output voltage would be

A. 400 V

B. about 100 V

C. about 20 V

D. Zero

157. A thyristor is reverse biased. A positive gate pulse is applied. The thyristor

A. will be turned on

B. may or may not turn on

C. will not turn on

D. will turn on after some time

158. A single phase series converter is used for

A. high voltage output

B. high current output

C. high voltage and high current output

D. improving the circuit efficiency

159. A single phase half wave-controlled rectifier circuit has an R-L load. A freewheeling diode is also in the circuit. When freewheeling diode is conducting the SCR

A. is forward biased

B. is reverse biased

C. may be forward biased or reverse biased

D. forward biased initially but reverse biased afterwards

160. An SBS can conduct is one direction only.

A. True

B. False

C. none

D. all

161. The number of doped regions in a SCR is

A. 2

B. 3

C. 4

D. 5

162. All inverters use forced commutation.

A. True

B. False

C. none

D. all

163. In 180° mode of operation of a 3-phase bridge inverter, two thyristors conduct at one time.

A. True

B. False

C. none

D. all

164. A reactor is used in a dual converter operating in non-circulating current mode.

A. True

B. False

C. none

D. all

165. Which of these commutation methods uses an auxiliary SCR?

A. Class A

B. Class B

C. Class C

D. Class D

166. A thyristor needs protection against

A. high

B. high

C. both high and high

D. either high or high

167. In integral cycle control the period of on-off control should be more than mechanical time constant of load.

A. True

B. False

C. none

D. all

168. A single phase half wave converter is feeding a resistive load. Just before triggering, the voltage across thyristor is

A. Zero

B. about 1 V

C. same as input voltage

D. twice the input voltage

169. In a 3-phase full wave regulator feeding resistance load connected in star, the possible range of firing angle a is

A. 0 ≤ θ ≤ 180°

B. 0 ≤ θ ≤ 150°

C. 0 ≤ θ ≤ 120°

D. 0 ≤ θ ≤ 90°

170. A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of load current is

A. 40 A

B. 20 A

C. 10 A

D. 5 A

171. In an SCR, the anode current is controlled by

A. gate current only

B. external circuit only

C. both gate current and external circuit

D. none of the above

172. The rated current of a thyristor is 40 A. The holding current will be about:

A. 20 A

B. 1 A

C. 100 mA

D. 10 mA

173. The number of leads in a power semiconductor diode are

A. 2

B. 3

C. 4

D. 2 or 3

174. If ton and toff are the turn on and turn off times of a thyristor, then

A. t_on and t_off are almost equal

B. t_on >> t_off

C. t_off >> t_on

D. t_on is about twice t_off

175. UJT can be used for

A. firing an SCR

B. turning off an SCR

C. both firing and turning off an SCR

D. none of the above

176. In a full wave converter (M – 2 connection) feeding an RLE load, the peak thyristor current is 15 A. For fully controlled bridge circuit and same load current, the peak thyristor current will be

A. 30 A

B. 152 A

C. 15 A

D. none

177. During forward blocking state of SCR, the voltage and current respectively are

A. high and high

B. low and low

C. high and low

D. low and high

178. In a 3-phase full converter the average load current is 150 A. The average thyristor current is

A. 150 A

B. 75A

C. 50 A

D. 25 A

179. The two modes of operation of three phase bridge inverters are known as

A. 60° mode and 120° mode

B. 90° mode and 120° mode

C. 120° mode and 180° mode

D. 120° mode and 240° mode

180. In a 3 phase fully controlled converter the firing frequency is

A. 3 times the line frequency

B. 6 times the line frequency

C. 9 times the line frequency

D. 12 times the line frequency

181. Two thyristor of same rating and same specifications

A. will have equal turn on and turn off periods

B. will have equal turn on but unequal turn off periods

C. may have equal or unequal turn on and turn off periods

D. will have unequal turn on and turn off periods

182. The amount of dielectric heating is inversely proportional to frequency.

A. True

B. False

C. none

D. all

183. A semi converter feeding an RLE load operates in

A. first quadrant only

B. first and second quadrants

C. first and fourth quadrants

D. first and third quadrants

184. The phenomenon of overlap in converter operation due to

A. source resistance

B. source inductance

C. both source resistance and source inductance

D. high value of firing angle

185. 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

A. 50 A

B. 25 A

C. less than 50 A but more than 25 A

D. less than 25 A

186. In a single-phase full wave regulator, the firing angles in the positive and negative half cycles are generally

A. Equal

B. Different

C. equal or different

D. different but sometimes equal

187. Current derating may be necessary when thyristors are operating in

A. Series

B. Parallel

C. series or parallel

D. none of the above

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

A. 4

B. 5

C. 6

D. 8

189. The terminals of a power MOSFET are called

A. emitter, base, collector

B. source, gate, drain

C. source, base, drain

D. emitter, gate, drain

190. In a thyristor the gate current is increased, then

A. anode current will increase

B. anode current will decrease

C. anode current will remain constant

D. anode current may increase or decrease

191. Thyristors are not suitable for logic circuits.

A. True

B. False

C. none

D. all

192. The number of leads in an SCR are:

A. 2

B. 3

C. 4

D. 5

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

A. 6 μs

B. less than 6μs

C. more than 6 μs

D. either 6 μs or less

194. The total number of leads in reverse conducting thyristor is

A. 2

B. 3

C. 1

D. 4

195. 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.

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

196. A semi converter operates in two quadrants.

A. True

B. False

C. none

D. all

197. 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

A. 127 V

B. 120 V

C. 110 V

D. 100 V

198. 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)

199. 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)

200. The waveshape of output voltage of half bridge inverter is

A. sinusoidal

B. square

C. triangular

D. either (a) or (b)

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