127 + Interview Questions in Power Electronics in Electronics and Communication Engineering Page 2 InterviewSolution

51.	A single phase semiconverter has a freewheeling diode. If the firing angle is and the load is purely resistive, the periods of conduction and freewheeling respectively are
Answer» Freewheeling diode conducts only when load is inductive. If load is purely resistive free wheeling diode does not come into operation.

Discussion

52.	In a 3 phase semiconverter, the dc output voltage for a certain firing angle is 50% of the maximum possible value. The firing angle is
Answer» Vdc ∝ (1 + cos a). Maximum possible output voltage occurs when a = 0.

Discussion

53.	A high frequency series inverter consists of
Answer» A high frequency series inverter has many series inverters in parallel. Only one inverter operates at one time and they operate in fixed sequence.

Discussion

54.	In an SCR, the anode current consists of
Answer» Both electrons and holes contribute to current.

Discussion

55.	In the below figure, the average value of load current is 15 A. The load is highly inductive. The peak value of thyristor current is
Answer» Because of highly inductive load, output current is nearly constant.

Discussion

56.	When the core of a saturable reactor is saturated, the inductance is high.
Answer» Since core is saturated, change of flux is small and inductance is low.

Discussion

57.	In controlled heating circuits, thyristors act as
Answer» The output voltage is controlled ac.

Discussion

58.	In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current
Answer» For resistive load output current is always proportional to output voltage.

Discussion

59.	In a single phase dual converter, , and are firing angles of the two converters. Then
Answer» Since one converter is rectifying and the other is inverting Vdc1 = -Vdc2 or a1 + a2 = p.

Discussion

60.	A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/ is constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be
Answer» In this case synchronous speed at 50 Hz is 1500 rpm. Therefore synchronous speed at 25 Hz is 750 rpm. Since slip is about 3 - 5%, speed is about 720 rpm.

Discussion

61.	The gate current required to turn on an SCR is
Answer» Only a very small gate current is required to trigger SCR into conduction.

Discussion

62.	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 sin ω. The thyristor will conduct
Answer» Thyristor can conduct only when it is forward biased. This occurs only when v > 50 V.

Discussion

63.	In a 3 phase semiconverter, the firing frequency is
Answer» Since the circuit has 3 thyristors, firing frequency is three times the line frequency.

Discussion

64.	A thyristor has its gate current zero. To start conduction the forward voltage must exceed
Answer» With zero gate current, conduction can be started only if forward voltage exceeds breakover voltage.

Discussion

65.	In dielectric heating, the thyristor circuits consist of
Answer» Input ac is rectified and then converted to high frequency ac.

Discussion

66.	On load tap changer using thyristors can have a maximum of two taps.
Answer» Any number of taps are possible.

Discussion

67.	In the below figure the rms voltage across each half of secondary is 200 V. The peak inverse voltage across each of thyristor is
Answer» PIV = 2(2 x 200).

Discussion

68.	In a single phase full bridge converter (B - 2 connection), the input voltage is = V sin ω. The output dc voltage for resistive load
Answer» SCRs conduct from a to p in each half cycle.

Discussion

69.	To protect an SCR against high / we connect
Answer» current through inductance cannot change instantaneously.

Discussion

70.	A silicon controlled switch has :
Answer» Anode, cathode, anode gate and cathode gate.

Discussion

71.	A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is
Answer» .

Discussion

72.	A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is
Answer» During the period of overlap all the four thyristors conduct and load is effectively short-circuited. Output voltage is zero.

Discussion

73.	When thyristors are connected in series and parallel, it may be necessary to have :
Answer» Thyristors in series do not share the voltage equally and require voltage derating. Thyristors in parallel need current derating.

Discussion

74.	A single phase half wave regulator uses
Answer» Regulation is possible only in positive half cycle.

Discussion

75.	A Shockley diode can be used
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.

Discussion

76.	An SCR has a turn on time of 4 μs. The gate pulse should have a duration of about
Answer» Duration of gate pulse should be slightly more than turn time of SCR.

Discussion

77.	In a multiphase chopper, the different choppers operate in
Answer» Only one chopper operates at one time.

Discussion

78.	In a semi converter feeding a resistive load, the firing angle is . Each thyristor conducts for
Answer» The conduction is from a to p.

Discussion

79.	A thyristor has maximum allowable junction temperature of 110° C and the ambient temperature is 40°C, If the maximum internal power dissipation is 50 W, the thermal resistance is
Answer» .

Discussion

80.	A single phase electric heater uses phase controlled thyristors. The firing angle is 90°. The degree of heating is
Answer» Conduction occurs only during 50% of wave.

Discussion

81.	Which of these commutation methods is used in AC-DC converters?
Answer» Class F is natural commutation AC = DC converter use natural (or line) commutation.

Discussion

82.	Present day HVDC converters are all
Answer» To reduce ripple 12 pulse converters are used.

Discussion

83.	A single phase full converter gives maximum and minimum output voltage at firing angles of
Answer» Output voltage is maximum if a = 0 and is minimum if a = 180°.

Discussion

84.	A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20°C. If thermal resistance is 1.6° C/W, the junction temperature is
Answer» .

Discussion

85.	A semiconverter feeding an RLE load operates in
Answer» Semiconvertor operates only in first quadrant.

Discussion

86.	The phenomenon of overlap in converter operation due to
Answer» Source inductance delays turn off and hence causes overlap.

Discussion

87.	A thyristor has a maximum allowable junction temperature of 120°C and the ambient temperature is 40°C. If thermal resistance is 1.6° C/W, the maximum allowable internal power dissipation is
Answer» .

Discussion

88.	For a 12 pulse operation, the two 6 pulse units are fed by
Answer» 30° phase difference is necessary.

Discussion

89.	In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is
Answer» Since conduction is from a to p + a, period of conduction is p in each half cycle.

Discussion

90.	A cycloconverter can be
Answer» The output frequency can be more or less than input frequency.

Discussion

91.	In the below figure the average load current is 15 A. The rms value of transformer secondary current is
Answer» RMS secondary current = = 15/1.414 = 10.608 A.

Discussion

92.	In a 3 phase fully controlled converter the firing frequency is
Answer» It is a 6 pulse converter.

Discussion

93.	Two thyristor of same rating and same specifications
Answer» Turn on and turn off times of similar SCRs may not be same.

Discussion

94.	The amount of dielectric heating is inversely proportional to frequency.
Answer» It is proportional to frequency.

Discussion

95.	The number of leads in an SCR are :
Answer» Anode, cathode and gate.

Discussion

96.	In a single phase full wave converter (M-2 connection) feeding on R - L load, the input voltage is = V sin ω. The expression for dc output voltage is
Answer» Due to inductive load the period of conduction is a to p + a in each half cycle.

Discussion

97.	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
Answer» .

Discussion

98.	The characteristics of a non-linear resistance is = . If becomes 100 times, becomes
Answer» v ∝ (i)1/4 ∝ (100)1/4 ≈ 3.

Discussion

99.	In a step down chopper using pulse width modulation, T = 3 x 10 and T = 1 x 10 s. The chopping frequency is
Answer» .

Discussion

100.	Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.
Answer» They are suitable for both dc and ac circuit breakers.

Discussion

Explore topic-wise InterviewSolutions in .

A single phase semiconverter has a freewheeling diode. If the firing angle is and the load is purely resistive, the periods of conduction and freewheeling respectively are

In a 3 phase semiconverter, the dc output voltage for a certain firing angle is 50% of the maximum possible value. The firing angle is

A high frequency series inverter consists of

In an SCR, the anode current consists of

In the below figure, the average value of load current is 15 A. The load is highly inductive. The peak value of thyristor current is

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

In controlled heating circuits, thyristors act as

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

In a single phase dual converter, , and are firing angles of the two converters. Then

A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/ is constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be

The gate current required to turn on an SCR is

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 sin ω. The thyristor will conduct

In a 3 phase semiconverter, the firing frequency is

A thyristor has its gate current zero. To start conduction the forward voltage must exceed

In dielectric heating, the thyristor circuits consist of

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

In the below figure the rms voltage across each half of secondary is 200 V. The peak inverse voltage across each of thyristor is

In a single phase full bridge converter (B - 2 connection), the input voltage is = V sin ω. The output dc voltage for resistive load

To protect an SCR against high / we connect

A silicon controlled switch has :

A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is

A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is

When thyristors are connected in series and parallel, it may be necessary to have :

A single phase half wave regulator uses

A Shockley diode can be used

An SCR has a turn on time of 4 μs. The gate pulse should have a duration of about

In a multiphase chopper, the different choppers operate in

In a semi converter feeding a resistive load, the firing angle is . Each thyristor conducts for

A thyristor has maximum allowable junction temperature of 110° C and the ambient temperature is 40°C, If the maximum internal power dissipation is 50 W, the thermal resistance is

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

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

Present day HVDC converters are all

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

A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20°C. If thermal resistance is 1.6° C/W, the junction temperature is

A semiconverter feeding an RLE load operates in

The phenomenon of overlap in converter operation due to

A thyristor has a maximum allowable junction temperature of 120°C and the ambient temperature is 40°C. If thermal resistance is 1.6° C/W, the maximum allowable internal power dissipation is

For a 12 pulse operation, the two 6 pulse units are fed by

In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is

A cycloconverter can be

In the below figure the average load current is 15 A. The rms value of transformer secondary current is

In a 3 phase fully controlled converter the firing frequency is

Two thyristor of same rating and same specifications

The amount of dielectric heating is inversely proportional to frequency.

The number of leads in an SCR are :

In a single phase full wave converter (M-2 connection) feeding on R - L load, the input voltage is = V sin ω. The expression for dc output voltage is

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

The characteristics of a non-linear resistance is = . If becomes 100 times, becomes

In a step down chopper using pulse width modulation, T = 3 x 10 and T = 1 x 10 s. The chopping frequency is

Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.