109 + Mcqs in Power electronics and drives miscellaneous Page 1 McqOptions

1.	A single-phase voltage source inverter is controlled in a single pulse-width modulated mode with a pulse width of 150 in each half cycle. Total harmonic distortion is defined asTHD = Vrms - V1 100V1where V1 is the rms value of the fundamental component of the output voltage. The THD of output ac voltage wave form is
A.	65.65%
B.	48.42%
C.	31.83%
D.	30.49%
Answer» B. 48.42%

Discussion

2.	An UJT used for triggering an SCR has the supply voltage VBB = 25 V. The intrinsic stand off ratio, = 0.75. The UJT will conduct when the bias voltage Vz is
A.	25 V
B.	18.75 V
C.	33.3 V
D.	19.24 V
Answer» E.

Discussion

3.	For a sinusoidal input voltage, Vs = Vm sin t = 2 Vs sin t if the input voltage is connected to load for n cycles and is disconnected for m cycles, then rms output voltage is given by Vo = Vs K where duty cycle K is given by
A.	m + n
B.	m n
C.	mn
Answer» E.

Discussion

4.	If a positive voltage is applied to the gate of a reverse biased SCR, then
A.	this injects more electrons into junction J
B.	this increases reverse leakage current into anode
C.	hesting of junction is unaffected
D.	failure of junctions occurs due to thermal run away
E.	Both B and C
Answer» F.

Discussion

5.	MOS controlled thyristors have 1. Low forward voltage drop during conduction 2. Fast turn-on and turn-off time 3. Low switching losses 4. High reverse voltage blocking capability 5. Low gate input impedance Of these statements
A.	1, 2 and 3 are correct
B.	3, 4 and 5 are correct
C.	2, 3 and 4 are correct
D.	1, 3 and 5 are correct
Answer» B. 3, 4 and 5 are correct

Discussion

6.	Consider the following statements : The overlap angle of a phase-controlled converter would increase on increasing 1. supply voltage 2. supply frequency 3. load current 4. source inductanceOf these statements
A.	1, 2 and 3 are correct
B.	2, 3 and 4 are correct
C.	1, 2 and 4 are correct
D.	1, 3 and 4 are correct
Answer» D. 1, 3 and 4 are correct

Discussion

7.	A single-phase AC regulator with an inductance load has following details : Source voltage, 230 V, at a frequency of 50 Hz and L = 5 ohms. The control range of the firing angle is
A.	0 < <
Answer» C.

Discussion

8.	A fully controlled natural commutated 3-phase bridge rectifier is operating with a firing angle = 30 . The peak to peak voltage ripple expressed as a ratio of the peak output dc voltage at the output of the converter bridge is
A.	0.5
B.
C.	- 1
Answer» B.

Discussion

9.	A voltage commutated chopper has the following parameters : Vs = 200 V, Load circuit parameter 1 &ohm;, 2mH, 5V.Commutation circuit parameter : L = 25 H, C = 50 F. For constant load current at 100 A, the effective ON period and peak current through the main thyristor are respectively
A.	1000 s, 200 A
B.	700 s, 382.8 A
C.	700 s, 282.8 A
D.	1000 s, 382.8 A
Answer» C. 700 s, 282.8 A

Discussion

10.	In type A chopper source voltage is 100 V, d.c. ON period = 100 s, OFF-period = 150 s and load RLE consists of R = 2&ohm; , L = 5 mH, E = 10 V. For continuous conduction average output voltage and average output current for this chopper are respectively
A.	40 V, 5 A
B.	66.66 V 28.33 A
C.	60V, 25 A
D.	40V, 20 A
Answer» B. 66.66 V 28.33 A

Discussion

11.	A step down chopper is operated in the continuous conduction mode in steady state with a constant duty ratio D. If Vo is the magnitude of the dc output voltage and Vs is the magnitude of the dc input voltage, then ratio Vo / Vs is
A.	D
B.	1 D
Answer» B. 1 D

Discussion

12.	Consider the following statements: The diodes in a voltage source inverter (Mc Murry inverter) should be able to 1. withstand a large voltage in the reverse direction. 2. carry commutating current excess of load current. 3. provide required reverse bias to the outgoing thyristor. 4. feedback reactive current to the source. Of these statements(a) 1, 2 and 3 are correct (b) 1, 3 and 4 are correct (c) 2, 3 and 4 are correct (d) 1, 2 and 4 are correct Consider the following statements: The diodes in a voltage source inverter (Mc Murry inverter) should be able to 1. withstand a large voltage in the reverse direction. 2. carry commutating current excess of load current. 3. provide required reverse bias to the outgoing thyristor. 4. feedback reactive current to the source. Of these statements
A.	1, 2 and 3 are correct
B.	1, 3 and 4 are correct
C.	2, 3 and 4 are correct
D.	1, 2 and 4 are correct
Answer» D. 1, 2 and 4 are correct

Discussion

13.	Consider the following statements regarding speed control of induction motors by means of external rotor resistors : 1. Reduction in speed is accompanised by reduced efficiency. 2. With a large resistance in the rotor circuit, the speed would vary considerably with variation in torque. 3. The method is very complicated. The disadvantage of such a method of speed control would include
A.	1 and 3
B.	2 and 3
C.	1 and 3
D.	1, 2 and 3
Answer» B. 2 and 3

Discussion

14.	The speed/torque regimes in a dc motor and the control methods suitable for the same are given respectively in Group II and Group I Group I Group II P. Field Control 1. Below base speed Q. Armature Control 2. Above base speed 3. Above base torque 4. Below base torqueThe match between the control method and the speed/torque regime is
A.	P-1; Q-3
B.	P-2; Q-1
C.	P-2; Q-3
D.	P-1; Q-4.
Answer» C. P-2; Q-3

Discussion

15.	An ac induction motor is used for a speed control application. It is driven from an inverter with a constant V/f control. The motor name-plate details are as follows : V = 415 V, Ph = 3, f = 50 Hz, N = 2850 rpm The motor is run with the inverter output frequency set at 40 Hz, and with half the rated slip. The running speed of the motor is
A.	2400 rpm
B.	2280 rpm
C.	2340 rpm
D.	2790 rpm
Answer» D. 2790 rpm

Discussion

16.	For an UJT employed for the triggering of an SCR, stand-off ratio h = 0.64 and dc source voltage VBB is 20 V. The UJT would trigger when the emitter voltage is ______V.
A.	4.1
B.	13.1
C.	51
D.	3.1
Answer» C. 51

Discussion

17.	A 3 phase, fully controlled, converter is feeding power into a d.c. load at a constant current of 150 A. The rms current through each thyristor of the converter is ______ A .
A.	150
B.
C.	None of the above
Answer» B.

Discussion

18.	A lossy capacitor Cx, rated operation at 5 kV, 50 Hz is represented by an equivalent circuit with an ideal capacitor Cp is parallel with a resistor Rp. The value of Cp is found to be 0.102 F and the value of Rp = 1.25 M&ohm;. Then the power loss and tan of the lossy capacitor operating at the rated voltage, respectively, are
A.	10 W and 0.0002
B.	10 W and 0.0025
C.	20 W and 0.025
D.	20 W and 0.04
Answer» D. 20 W and 0.04

Discussion

19.	Circuit turn-off time of an SCR is defined as the time (a) (b) (c) (d)
A.	taken by the SCR to turn off
B.	required for the SCR current to become zero
C.	for which the SCR is reverse biased by the commutation circuit
D.	for which the SCR is reverse biased to reduce its current below the holding current
Answer» D. for which the SCR is reverse biased to reduce its current below the holding current

Discussion

20.	A single phase fully controlled bridge converter supplies a load drawing constant and ripple free load current. If triggering angle is 30 , then input power factor will be
A.	0.65
B.	0.78
C.	0.85
D.	0.866
Answer» C. 0.85

Discussion

21.	A 220 V, 20 A, 1000 rpm, separately excited dc motor has an armature resistance of 2.5&ohm;. The motor is controlled by a step down chopper with a frequency of 1 kHz. The input dc voltage to the chopper is 250 V. The duty cycle of the chopper for the motor to operate at a speed of 600 rpm delivering the rated torque will be
A.	0.518
B.	0.608
C.	0.852
D.	0.902
Answer» C. 0.852

Discussion

22.	A single-phase, 230 V, 50 Hz ac mains fed step down transformer (4 : 1) is supplying power to a half-wave uncontrolled ac-dc converter used for charging a battery (12 V dc) with the series current limiting resistor being 19.04 &ohm;. The charging current is
A.	2.43 A
B.	1.65 A
C.	1.22 A
D.	1.0 A
Answer» B. 1.65 A

Discussion

23.	A step-up chopper is used to feed a load at 400 V dc fr om a 250 V dc source. The inductor current is continuous. If the off time of the switch is 20 s, the switching frequency of the chopper in kHz is _____.
A.	32.5
B.	21.5
C.	31.0 to 31.5
D.	31.0
Answer» D. 31.0

Discussion

24.	An inverter capable of supplying a balanced threephase variable voltage, variable frequency output is feeding a three-phase induction motor rated for 50 Hz and 440 V. The stator winding resistances of the motor are negligibly small. During starting, the current in such can be avoided without sacrificing the starting torque by suitably applying
A.	low voltage at rated frequency
B.	low voltage keeping the V/I ratio constant
C.	rated voltage at low frequency
D.	rated voltage at rated frequency
Answer» E.

Discussion

25.	When a line commutated converter operates in the inverter mode
A.	it draws both real and reactive power from the A.C. supply
B.	it delivers both real and reactive power to the A.C. supply
C.	it delivers real power to the A.C. supply
D.	it draws reactive power from the A.C. supply
Answer» D. it draws reactive power from the A.C. supply

Discussion

26.	A voltage source inverter is normally employed when
A.	source inductance is large and load inductance is small
B.	source inductance is small and load inductance is large
C.	both source and load inductances are small
D.	both source and load inductances are large
Answer» C. both source and load inductances are small

Discussion

27.	Consider the following statements: The diodes in a voltage source inverter (Mc Murry inverter) should be able to
A.	1, 2 and 3 are correct
B.	1, 3 and 4 are correct
C.	2, 3 and 4 are correct
D.	1, 2 and 4 are correct
Answer» D. 1, 2 and 4 are correct

Discussion

28.	A 3-phase fully controlled bridge converter with free wheeling diode is fed from 400 V, 50 Hz AC source and is operating at a firing angle of 60 . The load current is assumed constant at 10 A due to high load inductance. The input displacement factor (IDF) and the input power factor (IPF) of the converter will be
A.	IDF = 0.867IPF = 0.828
B.	IDF = 0.867IPF = 0.552
C.	IDF = 0.5 IPF = 0.478
D.	IDF = 0.5 IPF = 0.318
Answer» D. IDF = 0.5 IPF = 0.318

Discussion

29.	A single - phase fully controlled thyristor bridge ac - dc converter is operating at a firing angle of 25 and an overlap angle 10 with constant dc output current of 20 A. The fundamental power factor (displacement factor) at input ac mains is
A.	0.78
B.	0.827
C.	0.866
D.	0.9
Answer» B. 0.827

Discussion

30.	An SCR is considered to be a semi-controlled device because
A.	it can be turned OFF but not ON with a gate pulse
B.	it conducts only during one half-cycle of an alternating current wave
C.	it can be turned ON but not OFF with a gate pulse
D.	it can be turned ON only during one half-cycle of an alternating voltage wave
Answer» D. it can be turned ON only during one half-cycle of an alternating voltage wave

Discussion

31.	A three phase fully controlled bridge converter is feeding a load drawing a constant and ripple free load current of 10 A at a firing angle of 30 . The approximate Total Harmonic Distortion (%THD) and the rms value of fundamental component of the input current will respectively be
A.	31% and 6.8 A
B.	31% and 7.8 A
C.	66% and 6.8 A
D.	66% and 7.8 A
Answer» C. 66% and 6.8 A

Discussion

32.	A voltage source inverter is used to control the speed of a three-phase, 50 Hz, squirrel cage induction motor. Its slip for rated torque is 4%. The flux is maintained at rated value. If the stator resistance and rotational losses are neglected, then the frequency of the impressed voltage to obtain twice the rated torque at starting should be
A.	10 Hz
B.	5 Hz
C.	4 Hz
D.	2 Hz
Answer» D. 2 Hz

Discussion

33.	A single-phase voltage source inverter is controlled in a single pulse-width modulated mode with a pulse width of 150 in each half cycle. Total harmonic distortion is defined as
A.	65.65%
B.	48.42%
C.	31.83%
D.	30.49%
Answer» B. 48.42%

Discussion

34.	Six MOSFETs connected in a bridge configuration (having no other power device) MUST be operated as a Voltage Source Inverter (VSI) . This statement is
A.	True, because being majority carier devices, MOSFETs are voltage driven
B.	True, because MOSFETs have inherently anti - parallel diodes
C.	False, because it can be operated both as Current Source Inverter (CSI) or a VSI
D.	False, because MOSFETs can be operated as excellent constant current sources in the saturation region
Answer» E.

Discussion

35.	A three - phase, fully controlled thyristor bridge converter is used as line commutated inverter to feed 50 kW power at 420 V dc to a three - phase, 415 V (line), 50 Hz ac mains. Consider dc link current to be constant. The rms current of the thyristor is
A.	119.05 A
B.	79.37 A
C.	68.73 A
D.	39.68 A
Answer» E.

Discussion

36.	A single phase full-wave half-controlled bridge converter feeds an inductive load. The two SCRs in the converter are connected to a common DC bus. The converter has to have a freewheeling diode
A.	because the converter inherently does not provide for free-wheeling
B.	because the converter does not provide for freewheeling for high values of triggering angles
C.	or else the free-wheeling action of the converter will cause shorting of the AC supply
D.	or else if a gate pulse to one of the SCRs is missed, it will subsequently cause a high load current in the other SCR
Answer» C. or else the free-wheeling action of the converter will cause shorting of the AC supply

Discussion

37.	The speed of a 3 phase, 440 V, 50 Hz induction motor is to be controlled over a wide range from zero speed to 1.5 time the rated speed using a 3-phase voltage source inverter. It is desired to keep the flux in the machine constant in the constant torque region by controlling the terminal voltage as the frequency changes. The inverter out put voltage vs frequency characteristic should be
A.	<img src="http://images.interviewmania.com/wp-content/uploads/2019/12/as-622.jpg">
B.	<img src="http://images.interviewmania.com/wp-content/uploads/2019/12/as-623.jpg">
C.	<img src="http://images.interviewmania.com/wp-content/uploads/2019/12/as-624.jpg">
D.	<img src="http://images.interviewmania.com/wp-content/uploads/2019/12/as-625.jpg">
Answer» B. <img src="http://images.interviewmania.com/wp-content/uploads/2019/12/as-623.jpg">

Discussion

38.	A three-phase, 440 V, 50 Hz ac mains fed thyristor bridge is feeding a 440 V dc, 15 kW. 1500 rpm separately excited dc motor with a ripple free continuous current in the dc link under all operating conditions. Neglecting the losses, the power factor of the ac mains at half the rated speed, is
A.	0.354
B.	0.372
C.	0.90
D.	0.955
Answer» B. 0.372

Discussion

39.	A 220 V 1400 rpm, 40 A separately excited dc motor has an armature resistance of 0.4&ohm;. The motor is fed from a single phase circulating current dual converter with an input ac line voltage of 220 V (rms). The approximate firing angles of the dual converter for motoring operation at 50% of rated torque and 1000 rpm will be
A.	43 , 137
B.	43 , 47
C.	39 , 141
D.	39 , 51
Answer» D. 39 , 51

Discussion

40.	An SCR can withstand a maximum junction temperature of 120 C with an ambient temperature of 75 C. If the SCR has thermal resistance from junction to ambient as 1.5 C / W, the maximum internal power dissipation allowed is ______W .
A.	1
B.	6
C.	15
D.	30
Answer» E.

Discussion

41.	When the firing angle of a single phase, fully controlled rectifier feeding constant d.c. current into a load is 30 , the displacement power factor of the rectifier is ______ .
A.	0.5
B.	1.5
C.	5
D.	15
Answer» B. 1.5

Discussion

42.	A 3-phase full converter has an average output voltage of 200 V for zero degree firing angle and for resistive load. For a firing angle of 90 , the output voltage would be ______V
A.	0
B.	1
C.	2
D.	3
Answer» B. 1

Discussion

43.	A six pulse thyristor rectifier bridge is connected t o a balanced 50 Hz, three phase ac source. Assuming that the dc output current of the rectifier is constant, the lowest frequency harmonic component in the ac source line current is ______ Hz
A.	150
B.	250
C.	450
D.	550
Answer» C. 450

Discussion

44.	The speed/torque regimes in a dc motor and the control methods suitable for the same are given respectively in Group II and Group I
A.	P-1; Q-3
B.	P-2; Q-1
C.	P-2; Q-3
D.	P-1; Q-4.
Answer» C. P-2; Q-3

Discussion

45.	For an UJT employed for the triggering of an SCR, stand-off ratio h = 0.64 and dc source voltage V
A.	4.1
B.	13.1
C.	51
D.	3.1
Answer» C. 51

Discussion

46.	A SCR has an anode supply of sine voltage 200 V rms., 50 Hz applied through a 100&ohm; resistor and fired at an angle of 60 . Assuming no voltage drop, the rms value of the output voltage is ______V .
A.	326.7
B.	126.7
C.	101.7
D.	All of the above
Answer» C. 101.7

Discussion

47.	A thyristor is triggered by a pulse train of 5 kHz. The duty ratio is 0.4. If allowable average power is 100 W, then maximum allowable gate drive power is ______W .
A.	250
B.	50
C.	150
D.	450
Answer» B. 50

Discussion

48.	The latching current of SCR is 18 mA. Its holding current will be ______ mA .
A.	3
B.	6
C.	7
D.	21
Answer» C. 7

Discussion

49.	The average on-state current for an SCR is 20 A for a conduction angle of 120 . The average on state current for 60 conduction angle will be ______A .
A.	10
B.	20
C.	30
D.	40
Answer» C. 30

Discussion

50.	If in a 3-phase, half-wave rectifier, if per phase input voltage is 200 V, then average output voltage is ______ V.
A.	233.91
B.	33.91
C.	333.91
D.	None of the above
Answer» B. 33.91

Discussion

Explore topic-wise MCQs in Power Electronics and Drives.

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A voltage source inverter is normally employed when

Consider the following statements: The diodes in a voltage source inverter (Mc Murry inverter) should be able to

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An SCR is considered to be a semi-controlled device because

A three phase fully controlled bridge converter is feeding a load drawing a constant and ripple free load current of 10 A at a firing angle of 30 . The approximate Total Harmonic Distortion (%THD) and the rms value of fundamental component of the input current will respectively be

A single-phase voltage source inverter is controlled in a single pulse-width modulated mode with a pulse width of 150 in each half cycle. Total harmonic distortion is defined as

Six MOSFETs connected in a bridge configuration (having no other power device) MUST be operated as a Voltage Source Inverter (VSI) . This statement is

A three - phase, fully controlled thyristor bridge converter is used as line commutated inverter to feed 50 kW power at 420 V dc to a three - phase, 415 V (line), 50 Hz ac mains. Consider dc link current to be constant. The rms current of the thyristor is

A single phase full-wave half-controlled bridge converter feeds an inductive load. The two SCRs in the converter are connected to a common DC bus. The converter has to have a freewheeling diode

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A six pulse thyristor rectifier bridge is connected t o a balanced 50 Hz, three phase ac source. Assuming that the dc output current of the rectifier is constant, the lowest frequency harmonic component in the ac source line current is ______ Hz

The speed/torque regimes in a dc motor and the control methods suitable for the same are given respectively in Group II and Group I

For an UJT employed for the triggering of an SCR, stand-off ratio h = 0.64 and dc source voltage V

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