Objective Type Questions On Power Electronics: Part-4

[1] In a SCR,
(a) Gate current is directly proportional to forward breakover voltage
(b) As gate current is raised, forward breakover voltage reduces
(c) Gate current has to be kept on continuously for conduction
(d) Forward breakover voltage is low in the forward blocking state.

Answer: B

[2] In a SCR, at switch on, there is a sudden increase of current, then
(a) The tolerance limit of di/dt varies from 3 to 30A/us
(b) di/dt upper limit is 3A/us
(c) di/dt rating can be increased by having an inductive load
(d) di/dt rating can be increased by a resistive load Which of the above statements are true?
(1) a, c
(2) c, d
(3) a, b, c, d
(4) a, b

Answer: 1

[3] The following PNPN device has a terminal for synchronising purpose
(a) SUS
(b) DIAC
(c) TRIAC
(d) Schmitt trigger

Answer: A

[4] In single phase full wave controlled rectifier, maximum output voltage is obtained at conduction angle….and minimum at conduction angle…
(a) 0°, 180°
(b) 180°, 0°
(c) 0°, 0°
(d) 180°, 180°

Answer: A

[5] A thyristor is triggered by a pulse train of 5kHz. The duty ratio is 0.4. If the allowable average power is 100W the maximum allowable gate drive power is
(a) 100√2 W
(b) 50 W
(c) 150 W
(d) 250 W

Answer: D

[6] The consequence of introducing inductance in the load circuit of SCR is
(a) Anode current through SCR rises slowly
(b) Anode current reached is more than without it
(c) SCR has to be derated for anode current
(d) SCR’s current rating can be raised
Which of the above statements are true?
(1) a, c
(2) a, b, c, d
(3) b, d
(4) a, d

Answer: 4

[7] When a SCR is in the forward blocking state,
(a) All the 3 junctions are reverse biased
(b) The anode and cathode junctions are forward biased but the gate junction is reverse biased
(c) The anode junction is forward biased but the cathode and gate junctions are reverse biased
(d) The anode and gate junctions are forward biased but the cathode junction is reverse biased

Answer: B

[8] When we need to drive a dc shunt motor at different speeds in both directions ( forward and reverse) and also to brake it in both the directions, which one of the following would you use?
(a) A half-controlled SCR bridge
(b) A full controlled thyristor bridge
(c) A dual converter
(d) A diode bridge

Answer: C

[9] The frequency of the ripple present in the output voltage of the 3 phase half controlled bridge rectifier depends on the
(a) Firing angle
(b) Load inductance
(c) Load resistance
(d) Supple frequency

Answer: D

[10] A fully controlled thyristor bridge drives a DC motor. The system is capable of
(a) Motoring and braking in both directions
(b) Only motoring in both directions, no braking
(c) Motoring in forward direction and braking in reverse direction
(d) Only motoring in forward direction, no braking

Answer: D

[11] Three-phase to three-phase
cycloconverter employing 18 SCRs and 36 SCRs have the same voltage and
current ratings for their component thyristors. The ratio of VA rating
of 36 SCR device to that of 18 SCR device is
(a) 1/2
(b) 1
(c) 2
(d) 4

Answer: C

[12] Three phase to three phase
cycloconverter employing 18 SCRs are 36 SCRs have the same voltage and
current ratings for their component thyristors. The ratio of power
handled by 36 SCR device to that handled by 18 SCR device is
(a) 4
(b) 2
(c) 1
(d) 1/2

Answer: A

[13] The number of SCRs required for
single phase to single phase cycloconverter of the mid point type and
for three phase to three phase type cycloconverter are respectively
(a) 4, 6
(b) 8, 18
(c) 4, 18
(d) 4, 36

Answer: C

[14] For dynamic equalizing circuit used for series connected SCRs, the choice of C is based on
(a) Reverse recovery characteristics
(b) Turn-on characteristics
(c) Turn-off characteristics
(d) Rise-time characteristics
OPTIONS:
1) A
2) B, C
3) D
4) A, B, C, D
5) None of the above options

Answer: 1

[15] The function of connecting zener diode in an UJT circuit, used for the triggering of SCRs, is to
(a) Expedite the generation of triggering pulses
(b) Delay the generation of triggering pulses
(c) Provide a constant voltage to UJT to prevent erratic firing
(d) Provide a variable voltage to UJT as the source voltage changes.
OPTIONS:
1) B, C
2) C
3) A
4) D
5) None of the above options

Answer: 2

Solution Hint:
[4] When the conduction angle is 0, it will not suppress any part of the input, whereas during 180 degree conduction angle, it will suppress entire input waveform.

[5] PMAX >= Pavg/(f.TON)
Duty cycle D =TON/T = f.TON
PMAX >= Pavg/D =100/0.4 =250W

[7] When a SCR is in the forward blocking state, outer junctions (j1 & j3) are forward biased and middle junction(j2) is reverse biased

[15] Remember that the use of zener diode in a circuit is to provide voltage regulation ( constant voltage)