127 + Interview Questions in Power Electronics in Electronics and Communication Engineering Page 1 InterviewSolution

1.	In a dual converter both converters work as rectifiers.
Answer» One operators as rectifier and the other as inverter.

Discussion

2.	A single phase full wave regulator feeds R-L load. The best gating signal is
Answer» To ensure proper triggering pulse train in necessary because load is R - L.

Discussion

3.	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
Answer» If load torque is constant, the change in speed is very small.

Discussion

4.	A cycloconverter cannot give a frequency higher than input frequency.
Answer» Step up cycloconverter can give a frequency higher than input frequency.

Discussion

5.	A dc circuit breaker must use
Answer» Natural commutation is possible only in ac circuits.

Discussion

6.	In a single phase full wave regulator feeding R-L load, the minimum firing angle can be zero.
Answer» Range of control of a is 0 ≤ a ≤ p.

Discussion

7.	An RC snubber circuit is used to protect a thyristor against :
Answer» Especially high during switching.

Discussion

8.	UJT can be used for
Answer» UJT relaxation oscillator is used to provide gate pulses to SCR for triggering.

Discussion

9.	In a series inverter supplying a load resistance R, the commutating elements L and C should be such that
Answer» Load should be under damped.

Discussion

10.	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
Answer» In both the connections the output voltage and peak thyristor current are the same. Both are full wave rectifiers.

Discussion

11.	Induction heating systems require low frequency inverters.
Answer» It uses high frequency inverter.

Discussion

12.	A single phase half wave controlled rectifier circuit is feeding on load. The circuit has a freewheeling diode. The input voltage is = V sin ω. If I is the load current during the conduction of freewheeling diode, the KVL equation during the conduction of free wheeling diode is
Answer» Load is short-circuited by free wheeling diode.

Discussion

13.	HVDC transmission systems generally use
Answer» To reduce ripple, 12 pulse converters are used in HVDC.

Discussion

14.	A single phase semiconverter is feeding a highly inductive load. The circuit has a freewheeling diode across the load and firing angle is . During one cycle of input voltage, the total duration of conduction of freewheeling diode is
Answer» Freewheeling diode conducts for a° in each half cycle.

Discussion

15.	During forward blocking state of SCR, the voltage and current respectively are
Answer» Voltage is high but current is nearly zero.

Discussion

16.	In a 3 phase full converter the average load current is 150 A. The average thyristor current is
Answer» Each thyristor conducts for one-third of one cycle.

Discussion

17.	The two modes of operation of three phase bridge inverters are known as
Answer» In 180° mode each thyristor conducts for 180° and in 120° mode each thyristor conducts for 120°.

Discussion

18.	A dc separately excited motor has a constant field current. The armature is fed from a single phase supply through a semi-converter. When = 0, speed is 600 rpm. If = 90°, the speed is likely to be
Answer» Average output voltage is reduced to 50% if a = 90°. Hence speed is also reduced to 50%.

Discussion

19.	A static induction thyristor
Answer» It can be turned on by positive gate pulse and turned off by negative gate pulse. It is minority carrier device. It can be manufactured in high current and high voltage ratings and its on state voltage drops is low.

Discussion

20.	A single phase step up cycloconverter changes 50 Hz to 100 Hz. Then one half wave of input will give rise to
Answer» Since output frequency is 2f, one half wave of input must equal one full wave of output.

Discussion

21.	The rated current of a thyristor is 40 A. The holding current will be about :
Answer» The holding current is much less then 1% of rated current.

Discussion

22.	A cycloconverter is
Answer» It changes ac at frequency f into ac at a frequency different from f.

Discussion

23.	The number of leads in a power semiconductor diode are
Answer» A diode has 2 leads irrespective of its rating.

Discussion

24.	If and are the turn on and turn off times of a thyristor, then
Answer» Turn on time is less than 3 μs while turn off time is between 10-300 μs.

Discussion

25.	Which of these triggering methods for series connected SCRs use LASCR in the gate circuit?
Answer» Optical triggering uses LASCR.

Discussion

26.	In a 3 phase full wave regulator feeding resistance load connected in star, the possible range of firing angle is
Answer» Since output voltage is zero if a ≥ 150°, the possible range of firing angles is 0 ≤ a ≤ 150°.

Discussion

27.	Current chopping leads to
Answer» e = L . In current chopping is high. Hence e is high.

Discussion

28.	A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 &ohm; per phase. The peak value of load current is
Answer» Two resistances are in series at any time.

Discussion

29.	In an SCR, the anode current is controlled by
Answer» Once SCR has been triggered the gate has no control. Only external resistance controls the anode current.

Discussion

30.	Which of these commutation methods uses an auxiliary SCR?
Answer» Class C commutation uses an auxiliary SCR and it is also called auxiliary commutation.

Discussion

31.	A thyristor needs protection against
Answer» Both high and high can destroy SCR.

Discussion

32.	Two semiconverters are connected in series to form a single phase series converter. The input = V sin ω and = are the firing angles of the two converters. If = = 0, the dc output voltage is
Answer» Output voltage is for each.

Discussion

33.	In integral cycle control the period of on-off control should be more than mechanical time constant of load.
Answer» It should be less than mechanical time constant.

Discussion

34.	A single phase half wave converter is feeding a resistive load. Just before triggering, the voltage across thyristor is
Answer» Before triggering the whole of applied voltage is across SCR and load current is zero.

Discussion

35.	The number of doped regions in a SCR is
Answer» p = n = p = n.

Discussion

36.	All inverters use forced commutation.
Answer» Line commutated inverters are also used especially in HVDC.

Discussion

37.	In 180° mode of operation of a 3 phase bridge inverter, two thyristors conduct at one time.
Answer» In 180° mode three thyristors conduct simultaneously.

Discussion

38.	In a single phase full wave ac regulator the rms value of load current is 20 A load is purely resistive. Then
Answer» Each thyristor conducts for 50% of time.

Discussion

39.	A reactor is used in a dual converter operating in non-circulating current mode.
Answer» Reactor is used only for circulating current mode.

Discussion

40.	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
Answer» Free wheeling diode is forward biased only when SCR is reverse biased.

Discussion

41.	An SBS can conduct is one direction only.
Answer» It is a bilateral switch.

Discussion

42.	A 750 A circuit uses thyristor of current rating 175 A. If derating is 25%, the number of thyristors in parallel is
Answer» .

Discussion

43.	In a single phase full wave converter (M - 2 connection), the rms transformer voltage is 300 V across each half of secondary. The PIV for each thyristor is
Answer» PIV is twice the peak voltage across each half of secondary.

Discussion

44.	The average load current supplied by a thyristor depends on
Answer» The firing angle controls the period of conduction and hence average load current.

Discussion

45.	A dc chopper is feeding an RLE load. The maximum steady state ripple is
Answer» (Imax - Imin)max = .

Discussion

46.	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
Answer» In a three-phase full wave regulator, output voltage is zero if a ≥ 150°.

Discussion

47.	A thyristor is reverse biased. A positive gate pulse is applied. The thyristor
Answer» Thyristor can turn on only when it is forward biased.

Discussion

48.	A single phase series converter is used for
Answer» Series connection increases the voltage.

Discussion

49.	A 3 phase ac regulator uses
Answer» Two thyristors for each phase.

Discussion

50.	A DIAC has two terminals, anode and cathode.
Answer» They are designated as T1 and T2 because diac can conduct in both directions.

Discussion

Explore topic-wise InterviewSolutions in .

In a dual converter both converters work as rectifiers.

A single phase full wave regulator feeds R-L load. The best gating signal is

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 cycloconverter cannot give a frequency higher than input frequency.

A dc circuit breaker must use

In a single phase full wave regulator feeding R-L load, the minimum firing angle can be zero.

An RC snubber circuit is used to protect a thyristor against :

UJT can be used for

In a series inverter supplying a load resistance R, the commutating elements L and C should be such that

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

Induction heating systems require low frequency inverters.

A single phase half wave controlled rectifier circuit is feeding on load. The circuit has a freewheeling diode. The input voltage is = V sin ω. If I is the load current during the conduction of freewheeling diode, the KVL equation during the conduction of free wheeling diode is

HVDC transmission systems generally use

A single phase semiconverter is feeding a highly inductive load. The circuit has a freewheeling diode across the load and firing angle is . During one cycle of input voltage, the total duration of conduction of freewheeling diode is

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

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

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

A dc separately excited motor has a constant field current. The armature is fed from a single phase supply through a semi-converter. When = 0, speed is 600 rpm. If = 90°, the speed is likely to be

A static induction thyristor

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

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

A cycloconverter is

The number of leads in a power semiconductor diode are

If and are the turn on and turn off times of a thyristor, then

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

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

Current chopping leads to

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

In an SCR, the anode current is controlled by

Which of these commutation methods uses an auxiliary SCR?

A thyristor needs protection against

Two semiconverters are connected in series to form a single phase series converter. The input = V sin ω and = are the firing angles of the two converters. If = = 0, the dc output voltage is

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

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

The number of doped regions in a SCR is

All inverters use forced commutation.

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

In a single phase full wave ac regulator the rms value of load current is 20 A load is purely resistive. Then

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

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

An SBS can conduct is one direction only.

A 750 A circuit uses thyristor of current rating 175 A. If derating is 25%, the number of thyristors in parallel is

In a single phase full wave converter (M - 2 connection), the rms transformer voltage is 300 V across each half of secondary. The PIV for each thyristor is

The average load current supplied by a thyristor depends on

A dc chopper is feeding an RLE load. The maximum steady state ripple is

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

A single phase series converter is used for

A 3 phase ac regulator uses

A DIAC has two terminals, anode and cathode.