71. The purpose of a heat sink in a power device is to
A) Reduce resistance
B) Increase efficiency
C) Maintain device temperature within safe limits
D) Reduce voltage
Answer: C)
72. The SMPS converts
A) AC to DC using transformer only
B) AC to DC with switching regulators
C) DC to AC using inverter
D) DC to DC using resistors
Answer: B)
73. The main advantage of SMPS over linear regulators is
A) Higher efficiency
B) Simpler design
C) Higher cost
D) Lower frequency
Answer: A)
74. A UPS is used to
A) Supply uninterrupted AC power
B) Step up DC voltage
C) Convert DC to DC
D) Control speed of motor
Answer: A)
75. The inverter section in a UPS converts
A) AC to DC
B) DC to AC
C) AC to AC
D) DC to DC
Answer: B)
76. PWM stands for
A) Power Wave Modulation
B) Pulse Width Modulation
C) Phase Wave Modulation
D) Pulse Wave Measurement
Answer: B)
77. PWM is mainly used to
A) Control output voltage and reduce harmonics
B) Increase current
C) Reduce conduction losses
D) Improve commutation
Answer: A)
78. In a three-phase inverter, number of pulses per cycle depends on
A) Number of switching devices
B) PWM technique
C) Supply voltage
D) Load impedance
Answer: B)
79. The function of LC filter in an inverter is to
A) Reduce harmonic content
B) Increase output frequency
C) Step up voltage
D) Step down voltage
Answer: A)
80. The feedback in SMPS is used to
A) Stabilize the output voltage
B) Increase switching speed
C) Reduce load current
D) Control frequency
Answer: A)
81. The efficiency of a typical SMPS is around
A) 40–50%
B) 60–70%
C) 80–90%
D) 30–40%
Answer: C)
82. The major loss in a switching regulator occurs during
A) On-state
B) Off-state
C) Transition period
D) Idle period
Answer: C)
83. The isolation between input and output in SMPS is achieved using
A) Transformer
B) Inductor
C) Capacitor
D) Diode
Answer: A)
84. The push-pull converter uses
A) One switch
B) Two switches
C) Four switches
D) Six switches
Answer: B)
85. In a half-bridge inverter, the number of active switches used are
A) 2
B) 3
C) 4
D) 6
Answer: A)
86. The main function of soft switching is to
A) Reduce switching losses
B) Increase switching frequency
C) Reduce output voltage
D) Improve commutation
Answer: A)
87. In zero-voltage switching, the device switches when
A) Voltage is zero
B) Current is zero
C) Both current and voltage are zero
D) Maximum voltage
Answer: A)
88. In zero-current switching, the device switches when
A) Voltage is zero
B) Current is zero
C) Both voltage and current are zero
D) Minimum voltage
Answer: B)
89. The H-bridge inverter can be used for
A) Unidirectional load
B) Bidirectional load
C) Only resistive load
D) AC to DC conversion
Answer: B)
90. The main function of an opto-isolator in power circuits is
A) Signal amplification
B) Electrical isolation between control and power circuit
C) Power conversion
D) Voltage regulation
Answer: B)
91. The commutation method used in voltage source inverter is
A) Natural commutation
B) Forced commutation
C) Load commutation
D) Line commutation
Answer: B)
92. A power transistor is turned ON by
A) Applying base current
B) Applying collector current
C) Applying voltage to emitter
D) Removing base current
Answer: A)
93. The primary limitation of a BJT in power circuits is
A) High input impedance
B) Low switching speed
C) High voltage gain
D) Low current gain
Answer: B)
94. The MOSFET has a
A) Negative temperature coefficient
B) Positive temperature coefficient
C) Zero temperature coefficient
D) Constant temperature
Answer: B)
95. In a controlled rectifier, for α = 90°, the average output voltage is
A) Maximum
B) Zero
C) Minimum
D) Negative
Answer: B)
96. The regenerative braking of DC motor is achieved by
A) Feeding energy back to supply
B) Increasing torque
C) Increasing speed
D) Increasing armature current
Answer: A)
97. The braking mode in which motor operates as generator is
A) Dynamic braking
B) Plugging
C) Regenerative braking
D) Counter current braking
Answer: C)
98. The function of a speed controller in a DC drive is to
A) Control supply voltage
B) Control motor speed
C) Increase torque only
D) Reduce field current
Answer: B)
99. In chopper-controlled DC drives, the speed is controlled by
A) Varying duty ratio
B) Varying frequency
C) Varying load
D) Changing resistance
Answer: A)
100. The torque-speed characteristic of a DC drive with chopper control is similar to
A) Constant torque
B) Constant power
C) Variable torque
D) Variable power
Answer: A)
101. The converter-fed DC drive offers
A) Regenerative braking
B) Plugging only
C) Dynamic braking only
D) No braking
Answer: A)
102. The main disadvantage of line-commutated converters in drives is
A) High cost
B) Poor power factor
C) High efficiency
D) Compact size
Answer: B)
103. The regenerative mode of a converter occurs when
A) Load current and voltage are in same direction
B) Load current reverses
C) Firing angle α > 90°
D) Firing angle α < 90°
Answer: C)
104. The dual converter provides
A) Two-quadrant operation
B) Four-quadrant operation
C) Single-quadrant operation
D) Three-quadrant operation
Answer: B)
105. The PWM inverter has
A) Variable DC input
B) Constant DC input
C) Sinusoidal output
D) Low harmonic output
Answer: D)
106. The main cause of harmonics in inverter output is
A) Switching
B) Inductance
C) Resistance
D) Capacitance
Answer: A)
107. The effect of harmonics in motor drives is
A) Smooth torque
B) Torque pulsation and heating
C) Decrease efficiency only
D) No effect
Answer: B)
108. The protection of power devices is necessary to prevent
A) Overvoltage and overcurrent
B) Undervoltage
C) Noise
D) Frequency variation
Answer: A)
109. The fuse used in power electronics should have
A) Slow speed
B) Fast speed
C) Medium speed
D) No delay
Answer: B)
110. The surge suppressor used across SCR protects against
A) dv/dt
B) di/dt
C) Reverse current
D) Temperature rise
Answer: A)
111. A metal-oxide varistor (MOV) is used for
A) Over-voltage protection
B) Current limiting
C) Power regulation
D) Frequency control
Answer: A)
112. The dv/dt protection is provided by
A) RC snubber
B) RL circuit
C) Fuse
D) Diode
Answer: A)
113. The di/dt protection in SCR is achieved by
A) Series inductor
B) Series resistor
C) Shunt capacitor
D) Diode
Answer: A)
114. In high power applications, which device is most preferred?
A) MOSFET
B) GTO
C) IGBT
D) BJT
Answer: B)
115. The device suitable for low-voltage, high-frequency switching is
A) MOSFET
B) SCR
C) GTO
D) IGBT
Answer: A)
116. The isolation between control and power circuit in PWM drives is achieved by
A) Optocoupler
B) Transformer
C) Inductor
D) Diode
Answer: A)
117. The term “hard switching” refers to
A) Switching at high current and voltage simultaneously
B) Switching at zero current
C) Switching at zero voltage
D) Soft switching
Answer: A)
118. The GTO can be turned off by
A) Negative gate current
B) Reverse bias
C) Open gate
D) Gate resistor
Answer: A)
119. The main function of a snubber circuit in MOSFET switching is
A) Limit voltage spikes
B) Increase gain
C) Reduce current
D) Reduce conduction loss
Answer: A)
120. The phase-controlled converter operates as an inverter when
A) Firing angle α < 90°
B) Firing angle α > 90°
C) α = 90°
D) α = 0°
Answer: B)
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