McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Electrical Engineering , Electronics and Communication Engineering .
| 201. |
The gate current required to turn on an SCR is |
| A. | few amperes |
| B. | a few mA |
| C. | almost equal to anode current |
| D. | about 50% of anode current |
| Answer» B. a few mA | |
| 202. |
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 = Vm sin ωt. The thyristor will conduct |
| A. | from 0 to 180° |
| B. | when v > 50 V |
| C. | from 0 to 90° |
| D. | When v < 50 V |
| Answer» B. when v > 50 V | |
| 203. |
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 |
| Answer» B. 3 times the line frequency | |
| 204. |
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 |
| Answer» B. forward breakover voltage | |
| 205. |
On load tap changer using thyristors can have a maximum of two taps. |
| A. | True |
| B. | False |
| C. | none |
| D. | all |
| Answer» B. False | |
| 206. |
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 |
| Answer» A. are always equal | |
| 207. |
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 |
| Answer» B. the change in operation from one quadrant to the other is faster in circulating current mode | |
| 208. |
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 |
| Answer» D. an inductance in series with SCR | |
| 209. |
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 |
| Answer» A. the commutating capacitor C is across the primary of transformer and the load is across the secondary | |
| 210. |
A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is |
| A. | Vm |
| B. | 0.5 Vm |
| C. | zero |
| D. | 0.5 Vm - voltage drop across inductance |
| Answer» C. zero | |
| 211. |
Which of these commutation methods is used in AC-DC converters? |
| A. | Class A |
| B. | Class C |
| C. | Class D |
| D. | Class F |
| Answer» D. Class F | |
| 212. |
When thyristors are connected in series and parallel, it may be necessary to have : |
| A. | current derating |
| B. | voltage derating |
| C. | both current and voltage derating |
| D. | none of the above |
| Answer» C. both current and voltage derating | |
| 213. |
A single phase half wave regulator uses |
| A. | 2 diodes and 2 thyristors |
| B. | 2 thyristors |
| C. | 1 diode and 1 thyristor |
| D. | 2 diodes |
| Answer» C. 1 diode and 1 thyristor | |
| 214. |
A high frequency series inverter consists of |
| A. | a number of series inverters in series |
| B. | a number of series inverters in parallel |
| C. | a number of parallel inverters in parallel |
| D. | a number of parallel inverters in series |
| Answer» B. a number of series inverters in parallel | |
| 215. |
In an SCR, the anode current consists of |
| A. | only holes |
| B. | only electrons |
| C. | both electrons and holes |
| D. | either electrons or holes |
| Answer» C. both electrons and holes | |
| 216. |
A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load |
| A. | rectification efficiency will be more than 0.6 and TUF will be more than 0.35 |
| B. | rectification efficiency will be more than 0.6 but TUF will be less than 0.35 |
| C. | rectification efficiency will be less than 0.6 and TUF will be less than 0.35 |
| D. | rectification efficiency will be less than 0.6 but TUF will be more than 0.35 |
| Answer» A. rectification efficiency will be more than 0.6 and TUF will be more than 0.35 | |
| 217. |
In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current |
| A. | are always similar |
| B. | can never be similar |
| C. | may be similar or dissimilar |
| D. | are similar only if = 0 |
| Answer» A. are always similar | |
| 218. |
Natural commutation can be used in : |
| A. | DC circuits only |
| B. | AC circuits only |
| C. | both AC and DC circuits |
| D. | none of the above |
| Answer» B. AC circuits only | |
| 219. |
In a thyristor the gate lead is connected to the n layer near the p layer to which anode is connected. |
| A. | True |
| B. | False |
| C. | none |
| D. | all |
| Answer» B. False | |
| 220. |
McMurray Bedford full bridge inverter uses |
| A. | natural commutation |
| B. | auxiliary commutation |
| C. | complementary commutation |
| D. | any of the above |
| Answer» C. complementary commutation | |
| 221. |
Which of these commutation methods uses an external pulse for commutation? |
| A. | Class A |
| B. | Class C |
| C. | Class E |
| D. | Class F |
| Answer» C. Class E | |
| 222. |
A relaxation oscillator uses |
| A. | tunnel diode |
| B. | UJT |
| C. | both tunnel diode and UJT |
| D. | PIN diode |
| Answer» B. UJT | |
| 223. |
In a 3 phase bridge inverter with 180° mode of operation the numbers of thyristors conducting at one time are |
| A. | 1 |
| B. | 2 |
| C. | 3 |
| D. | 4 |
| Answer» C. 3 | |
| 224. |
A single phase full converter can operate in |
| A. | 4 quadrants |
| B. | 3 quadrants |
| C. | 2 quadrants |
| D. | 1 quadrant |
| Answer» C. 2 quadrants | |
| 225. |
In a 3 phase semiconverter, the freewheeling diode comes into operation only if firing angle |
| A. | is zero |
| B. | is 60° |
| C. | is more than 60° |
| D. | is more than 90° |
| Answer» C. is more than 60° | |
| 226. |
The two-transistor model of a thyristor consists of two transistors : |
| A. | one n-p-n and other p-n-p |
| B. | both p-n-p |
| C. | both n-p-n |
| D. | one n-p-n and other UJT |
| Answer» A. one n-p-n and other p-n-p | |
| 227. |
In a thyristor, the forward breakover voltage |
| A. | is constant |
| B. | may be constant or may depend on gate current |
| C. | decreases as gate current is increased |
| D. | increases as gate current is increased |
| Answer» C. decreases as gate current is increased | |
| 228. |
The gate lead, in a thyristor is welded to : |
| A. | p layer to which anode is connected |
| B. | n layer nearest to the anode terminal |
| C. | p layer nearest to the cathode terminal |
| D. | outside n layer |
| Answer» B. n layer nearest to the anode terminal | |
| 229. |
In a 3 phase full converter, the firing angle is less than 60°. The instantaneous output voltage |
| A. | will have positive part only |
| B. | will have both positive and negative parts |
| C. | may have both positive and negative parts |
| D. | will have negative part if load is inductive |
| Answer» A. will have positive part only | |
| 230. |
In a stud mounted SCR, the stud is |
| A. | in thermal contact with heat sink |
| B. | insulated from heat sink |
| C. | in electrical and thermal contact with heat sink |
| D. | in either electrical or thermal contact with heat sink |
| Answer» C. in electrical and thermal contact with heat sink | |
| 231. |
In a full bridge inverter, the waveshape of load current depends on |
| A. | relative values of load parameters R, L and C |
| B. | relative values of load parameters R and L |
| C. | relative values of load parameters L and C |
| D. | gating frequency |
| Answer» A. relative values of load parameters R, L and C | |
| 232. |
The characteristics of which device has peak point and valley point |
| A. | SCR |
| B. | PUT |
| C. | SCS |
| D. | Shockley diode |
| Answer» B. PUT | |
| 233. |
In an ordinary thyristor |
| A. | turn on and turn off times are equal |
| B. | turn on time is more than turn off time |
| C. | turn off time is more than turn on time |
| D. | turn on time may be equal, less than or more than turn off time |
| Answer» C. turn off time is more than turn on time | |
| 234. |
Thyristors connected in series need : |
| A. | static equalizing circuit |
| B. | dynamic equalizing circuit |
| C. | both static and dynamic equalizing circuits |
| D. | none of the above |
| Answer» C. both static and dynamic equalizing circuits | |
| 235. |
If 4 quadrant operation is required we need |
| A. | dual converter |
| B. | full converter |
| C. | semi converter |
| D. | none of the above |
| Answer» A. dual converter | |
| 236. |
Which of the following devices exhibit negative resistance characteristics? |
| A. | UJT |
| B. | PUT |
| C. | Both UJT and PUT |
| D. | UJT, PUT and SBS |
| Answer» C. Both UJT and PUT | |
| 237. |
As compared to BJT, MOSFET has |
| A. | lower switching losses and higher conduction losses |
| B. | higher switching losses and higher conduction losses |
| C. | higher switching losses and lower conduction losses |
| D. | lower switching losses and lower conduction losses |
| Answer» A. lower switching losses and higher conduction losses | |
| 238. |
A single phase bridge type cycloconverter uses eight thyristors. |
| A. | True |
| B. | False |
| C. | none |
| D. | all |
| Answer» A. True | |
| 239. |
In the basic series inverter circuit, the source supplies load |
| A. | only in positive half cycle |
| B. | both in positive and negative half cycles |
| C. | only in part of positive half cycle |
| D. | in positive half cycle and part of negative half cycle |
| Answer» A. only in positive half cycle | |
| 240. |
In a parallel inverter |
| A. | each thyristor is turned on twice during each cycle |
| B. | each thyristor is turned on once during each cycle |
| C. | each thyristor is turned on either once or twice in each load cycle |
| D. | each thyristor may be turned on upto four times in each load cycle |
| Answer» A. each thyristor is turned on twice during each cycle | |
| 241. |
The specified maximum junction temperature of thyristors is about |
| A. | 50°C |
| B. | 80°C |
| C. | 120°C |
| D. | 200°C |
| Answer» C. 120°C | |
| 242. |
A half bridge inverter |
| A. | needs a 3 wire dc supply |
| B. | may use a two wire or 3 wire dc supply |
| C. | has higher output voltage if fed from 3 wire dc supply |
| D. | has higher efficiency if fed from 3 wire dc supply |
| Answer» A. needs a 3 wire dc supply | |
| 243. |
A 3 phase series inverter has |
| A. | 12 thyristors |
| B. | 6 thyristors |
| C. | 3 thyristors |
| D. | 3 or 6 thyristors |
| Answer» B. 6 thyristors | |
| 244. |
Surge current rating of an SCR is |
| A. | the maximum repetitive surge current in which the SCR can withstand |
| B. | the maximum non-repetitive surge current in which the SCR can withstand |
| C. | the maximum repetitive or non repetitive surge current in which the SCR can withstand |
| D. | the maximum surge current in which the SCR can withstand if both surge current and steady current exist together |
| Answer» B. the maximum non-repetitive surge current in which the SCR can withstand | |
| 245. |
Which of the following devices has metal-silicon junction? |
| A. | General purpose power diode |
| B. | Schottky diode |
| C. | SCR |
| D. | MOSFET |
| Answer» B. Schottky diode | |
| 246. |
In a series inverter |
| A. | the commutating elements L and C are in parallel |
| B. | the commutating elements L and C are in series with load |
| C. | the commutating L and C may be in series or parallel with the load |
| D. | none of the above |
| Answer» B. the commutating elements L and C are in series with load | |
| 247. |
In thermal ohm's law, temperature and power loss are analogous to |
| A. | V and I respectively of ohm's law |
| B. | I and V respectively of ohm's law |
| C. | R and V respectively of ohm's law |
| D. | I and R respectively of ohm's law |
| Answer» B. I and V respectively of ohm's law | |
| 248. |
A string of series connected thyristors needs equalising circuit to |
| A. | ensure proper firing |
| B. | ensure proper current division |
| C. | ensure proper voltage division |
| D. | none of the above |
| Answer» C. ensure proper voltage division | |
| 249. |
The minimum gate current which can turn on SCR is called : |
| A. | trigger current |
| B. | holding current |
| C. | junction current |
| D. | breakover current |
| Answer» A. trigger current | |
| 250. |
Which of these devices can conduct in both directions? |
| A. | SCR |
| B. | SUS |
| C. | SBS |
| D. | UJT |
| Answer» C. SBS | |
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