# Power Electronics MCQ Electrical Engineering

50 Questions 30 Mins

### About Power Electronics

Following are some of the multiple choice questions on the Power Electronics with answers that will help the students in developing their knowledge.

## Power Electronics MCQ

### 1. Which device can be used in a chopper circuit?

• BJT
• MOSFET
• GTO
• ALL THE ABOVE

### 2. Which of the following statements is correct in connection with the inverters?

• VSI and CSI both require feedback diodes
• only CSI require feedback diodes
• GTO is used in current source inverter
• only VSI requires feedback diodes

### 3. The major advantage of using dual converters is that

• it is cheaply available
• no mechanical switch is required to change the mode of operation
• it has better pf
• its operating frequency is very high

### 4. In AC voltage controllers the

• variable ac with fixed frequency is obtained
• variable ac with variable frequency is obtained
• variable dc with fixed frequency is obtained
• variable dc with variable frequency is obtained

### 5. The shape of the output voltage waveform in a single PWM is

• quasi-square wave
• sine wave
• triangular wave
• square wave

### 6. In an inverter, if the fundamental frequency is 50 Hz, if third harmonic is eliminated, then frequencies of other components in the output voltage wave, in Hz, would be

• 250, 350, 500, higher frequencies
• 50, 250, 350, 500
• 50, 250, 350, 550
• none of these

### 7. In case of current commutation of SCR

• a diode is connected in parallel with the main SCR
• a diode is connected in anti-parallel with the main SCR
• a diode is connected in series with the main SCR
• none of the mentioned

• 1<α<100
• 0<=α<=1
• 0>α>-1
• 0<α<1

• firing angle
• duty cycle

### 10. A single phase half wave controlled converter always operates in the ________ conduction mode.

• continuous
• discontinuous
• both (a) and (b)
• none of the above

### 11. When the temperature increases, the inter-base resistance (RBB) of a UJT ___________.

• Increases
• Decreases
• Remains the same
• None of the above

### 12. A voltage source inverter is normally employed when

• source inductance is large and load inductance is small
• source inductance is small and load inductance is large
• both source and load inductance are large
• both source and load inductance are small

### 13. In the ___________ type of chopper, two stage conversions takes place.

• AC-DC
• None of the mentioned

### 14. Which of the following is not a characteristic of UJT?

• Intrinsic stand off ratio
• Negative resistance
• Peak-point voltage
• Bilateral conduction

### 15. Dual converters provide

• two quadrant operation
• three quadrant operation
• four quadrant operation
• none of the mentioned

### 16. In voltage source inverters

• load voltage Vo depends on load impedance but load current Io does not depend on load impedance Z
• both Vo and Io depends on Z
• Io depends on Z but Vo does not depend
• none of these

### 17. In the three-phase bridge inverter, each step consists of

• 30°
• 60°
• 90°
• will depend on firing angle

### 18. The load voltage of a chopper can be controlled by varying the..................

• duty cycle
• firing angle
• reactor position
• extinction angle

### 19. All the modern AC-DC converters are using GTOs instead of SCRs because

• emperature is an issue in SCRs
• Switching losses are high in SCRs
• GTOs have reliable commutation
• GTOs are cheaply available today

### 20. What are rheostats used for?

• Control large amounts of heat
• Vary large amounts of current
• Control of a circuit's power rating
• Vary a circuit's tolerance

• α
• π
• α+π
• π-α

### 22. In the continuous conduction mode if the firing angle of the converter is increased beyond _________ degrees the converter operates in the _______ mode.

• 90, rectifier
• 180, inverter
• 90, inverter
• 180, rectifier

### 23. When a UJT is turned ON, the resistance between emitter terminal and lower base terminal ___________

• Remains the same
• Is decreased
• Is increased
• None of the above

### 24. The series-inverter control method is a/an..........

• external frequency control method
• external voltage control method
• internal voltage control method
• none of the mentioned

### 25. When the temperature increases, the intrinsic stand off ratio ___________.

• Increases
• Decreases
• Essentially remains the same
• None of the above

• Ton/Toff
• Ton/T
• T/Ton
• Toff x Ton

### 27. In a half wave bridge inverter circuit, the power delivered to the load by each source is given by

• 2(Vs x Is)
• Vs x Is
• (Vs x Is)/2
• None of the mentioned

• π+α
• 2π+α
• π-α
• α

### 29. In a UJT, the p-type emitter is ___________ doped

• Lightly
• Heavily
• Moderately
• None of the above

### 30. After peak point, the UJT operates in the ___________ region.

• Cut-off
• Saturation
• Negative resistance
• None of the above

### 31. The TRIAC is most sensitive in the _________ quadrants

• 1st & 3rd with positive gate current
• 1st with positive gate current & 3rd with negative gate current
• 3rd with positive gate current & 1rd with negative gate current
• 1st & 3rd with negative gate current

### 32. A diac has ___________ semiconductor layers

• Three
• Two
• Four
• None of the above

### 33. What is the purpose of a resistor?

• control current flow
• allow current flow
• prevent current flow
• allow heat to build up

### 34. Cycloconverter converts

• ac voltage to dc voltage
• dc voltage to dc voltage
• ac voltage to ac voltage at same frequency
• ac voltage at supply frequency to ac voltage at load frequency

### 35. A PWM switching scheme is used with a three phase inverter to

• reduce THD with modest filtering
• minimize the load on DC side
• increase the life of the batteries
• reduce low order harmonics and increase high order harmonics

### 36. A UJT has ___________.

• Two pn junctions
• One pn junction
• Three pn junctions
• None of the above

### 37. When the emitter terminal of a UJT is open, the resistance between the base terminal is generally ___________.

• High
• Low
• Extremely low
• None of the above

### 38. In case of circulating current type dual converters, the reactor is inserted between

• no reactor is used in case of circulating type dual converter
• across the load
• supply and converter
• between the converters

### 39. The ____________ region in a power diode increases its reverse voltage blocking capacity.

• pn junction
• drift
• potential barrier
• depletion layer

### 40. Single phase half controlled converters are most suitable for loads requiring ___________________ voltage and current.

• bidirectional
• unidirectional
• unidirectional as well as bidirectional
• none of the above

### 41. The UJT may be used as ___________.

• Am amplifier
• A sawtooth generator
• A rectifier
• None of the above

### 42. The Gate-Drain transfer capacitance of a MOSFET has large value in the ________________ region and small value in the ________________ region.

• saturation, ohmic
• ohmic, saturation
• triode, linear
• ohmic, active

• Triac
• FET
• SCR
• Diac

### 44. The four quadrant operation of dual converters can be obtained by

• moving the mechanical lever
• changing the firing angle value
• adding inductance to the circuit
• none of the mentioned

### 45. A single phase full bridge inverter can operate in load commutation mode if load consists of

• RLC overdamped
• RLC critically damped
• RLC underdamped
• RLC undamped

### 46. A single full converter alone can given a

• two quadrant operation
• three quadrant operation
• four quadrant operation,
• none of the mentioned

### 47. A UJT is sometimes called ___________ diode.

• Low resistance
• High resistance
• Single-base
• Double-base

### 48. 1. In case of three-phase dual converter, one of the converter circuits is fired at an angle of 60°. For both the converter circuits to have equal average output voltage, what is the value of the firing angle for the other converter circuit?

• 60°
• 120°
• 100°
• Insufficient data

### 49. The process of commutating a SCR by applying a reverse voltage to an SCR through a previously charged capacitor is called as

• capacitor commutation
• forced commutation
• voltage commutation
• current commutation

### 50. How does resistance affect current?

• less resistance - less current
• more resistance - less current
• more resistance - more current
• resistance does not affect current