power electronics Mcqs
101. Very large values of modulation index (greater than 3.24) lead to___________?
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A. Sine AC output voltage

B. Square AC output voltage

C. Trapezoidal AC output voltage

D. Triangular AC output voltage

102. ON state voltage drop across SCR lie between the range_____________?
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A. 1 1.5 V.

B. 0.5 1 V.

C. 1.5 2 V.

D. 0 0.5 V.

103. In a 3 phase half wave rectifier, when firing angle is less than 90 degree, then the average dc output voltage becomes__________________?
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A. Zero

B. Negative

C. Positive

D. None of these

104. Snubber circuit is used with SCR_________________?
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A. either series or parallel

B. in series

C. in parallel

D. anti parallel

105. For swept frequency measurements, the input impedance of the mismatched transmission line would vary with frequency as the electrical length of the transmission line would
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A. Increase with frequency

B. Decrease with frequency

C. Either A. or B.

D. Remains same with change in frequency

106. In BJT, switching losses occurs___________?
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A. Only at turn on

B. Only at turn off

C. Both at turn on and off

D. None of these

107. Silicon controlled rectifier can be turned on____________?
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A. By applying a gate pulse and turned off only when current becomes zero

B. By making current non zero and turned off by making current zero

C. And turned off by applying gate pulse

D. By applying a gate pulse and turned off by removing the gate pulse

108. If a shunt motor is started with its field winding open then___________?
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A. None of these

B. It will rotate at dangerously high speed

C. It will rotate at less speed as that with its field winding closed

D. It will rotate at the same speed as that with its field winding closed

109. Advantages of HVDC transmission over AC system is / are____________?
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A. Reversal of power can be controlled by firing angle

B. They can link two AC system operating unsynchronized

C. All of these

D. Very good dynamic behavior

110. Thyristor can be protected from over voltages by using
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A. 23 mA.

B. 60 mA.

C. 40 mA.

D. 10 mA.
111. Let of a thyristor Vc1, Vc2, Vc3 are forward break over voltage for gate current Ig1, Ig2, Ig3 respectively. Then________________?
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A. Vc1 > Vc2 > Vc3 when Ig1 < Ig2 < Ig3.

B. Vc1 = Vc2 = Vc3 any value of Ig.

C. Vc1 > Vc2 > Vc3 when Ig1 > Ig2 > Ig3.

D. Vc1 > Vc2 > Vc3 when Ig1 aYEN Ig2 &Atil

112. Compared to transistor, _________ have lower on state conduction losses and higher power handling capability?
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A. MOSFETs

B. Thyristor

C. Semi conductor diodes

D. TRIACs

113. The power MOSFET device is a___________?
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A. Current controlled bipolar device

B. Voltage controlled unipolar device

C. Voltage controlled bipolar device

D. Current controlled unipolar device

114. Choppers is a_______________?
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A. DC AC converters

B. DC DC converters

C. AC DC converters

D. AC AC converters

115. During which time maximum conduction spreading take place in the thyristor during turn ON?
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A. Rise time

B. Delay time

C. Spread time

D. Same for every case

116. An SCR can be used___________?
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A. All of these

B. as static conductor

C. for speed control of dc shunt motor

D. for power control

117. The switching function of semiconductor devices can be characterized with___________?
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A. Duty ratio only

B. Duty ratio and frequency

C. Frequency only

D. Duty ratio, frequency and time delay
118. A power MOSFET has three terminals called___________?
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A. Collector, emitter and gate

B. Drain, source and base

C. Drain, source and gate

D. Collector, emitter and base

119. The opposite of susceptibility is___________?
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A. Interference

B. Electromagnetic compatibility

C. Emission

D. Immunity
120. A capacitive load in voltage source inverters generates ________________?
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A. Small current spikes and can be increased by using an inductive filter

B. Small current spikes and can be reduced by using an inductive filter

C. Large current spikes and can be increased by using an inductive filter

D. Large current spikes and can be reduced by using an inductive filter
121. The ac voltage controller can be used for ___________?
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A. On line transformer tap changing

B. Lighting and heating control

C. All of these

D. Soft starting

122. Thyristor can be protected from over voltages by using____________?
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A. fuse.

B. heat sink.

C. voltage clamping device.

D. snubber circuit.

123. Form factor of a rectifier is the ratio of ______________?
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A. Peak value of current and voltage to its root mean square value

B. Average value of current and voltage to its root mean square value

C. Root mean square value of voltage and current to its peak value

D. Root mean square value of voltage and current to its average value
124. If holding current of a thyristor is 2 mA then latching current should be_____?
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A. 0.009 A.

B. 0.002 A.

C. 0.004 A.

D. 0.01 A.

125. In reverse blocking mode of a thyristor______?
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A. junction J3 is in forward bias and J1, J2 in reverse bias.

B. junction J1, J3 is in reverse bias and J2 is in forward bias.

C. junction J1 and J2 is in forward bias and J3 is in reverse bias.

D. junction J2 is in reverse bias and J1, J3 is in forward bias.

126. For bidirectional operation of converters____________?
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A. A series combination of controllable switch and a diode is used

B. A parallel combination of controllable switch and capacitor is used

C. A series combination of controllable switch and a capacitor is used

D. A parallel combination of controllable switch and a diode is used
127. For power output higher than 15 kW, the suitable rectifier is________________?
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A. Poly phase

B. 3 phase

C. Single phase

D. Only (b) and (c)
128. The IGBT resulted in higher switching speed and lower energy losses. It can be used for___________?
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A. Induction heating system

B. All of these

C. Constant voltage and frequency power supplies

D. Uninterruptible power supplies

129. In bipolar modulation, the carrier is symmetric about zero with amplitude equal to Cm, the PWM output
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A. Switches between 1 and + 1

B. Switches between 1 / 2 and + 1 / 2

C. Switches between 0 and + 1

D. Zero

130. The typical value of SCR for modern alternator is____________?
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A. 1.2.

B. 1.0.

C. 1.5.

D. 0.5.
131. Bidirectional semiconductor device is___________?
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A. BJT

B. SCR

C. TRIAC

D. Diode

132. The man made noise sources is / are_____________?
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A. Computers, digital electronics

B. All of these

C. Switches, radio interferences

D. Motors

133. Due to non sinusoidal waveform of the input current, the power factor of the rectifier is____________?
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A. Negatively affected by firing angle

B. Negatively affected by distortion of the input current

C. Positively affected by both firing angle and distortion of the input current

D. Both (a) and (b)
134. Voltage commutation circuit can be converted into a current commutation by interchanging the positions of____________?
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A. Capacitor and SCR

B. Diode and capacitor

C. Capacitor and load

D. Inductor and capacitor

135. To meet high current demand we use SCRs in______________?
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A. series connection.

B. parallel connection.

C. anti parallel connection.

D. both B and C.

136. Switching frequency of SITH is____________?
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A. 5 KHz

B. 60 KHz

C. 100 KHz

D. 10 KHz

137. The switching function of semiconductor devices can be characterized with____________?
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A. Duty ratio, frequency and time delay

B. Duty ratio only

C. Duty ratio and frequency

D. Frequency only

138. The power demand can be estimated approximately by___________?
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A. Statistical method

B. Mathematical method

C. Load survey method

D. Economic parameters

139. A modern power semiconductor device that combines the characteristic of BJT and MOSFET is_____________?
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A. GTO

B. IGBT

C. MCT

D. FCT

140. In the SPWM, the modulating signal is___________?
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A. Saw tooth

B. Sinusoidal

C. Square

D. Triangular

141. The effects of EMI can be reduced by_____________?
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A. Reducing the efficiency of the coupling path

B. Suppressing emissions

C. Reducing the susceptibility of the receptor

D. All of these
142. The ac output voltage waveform of VSI and AC output current waveform of CSI respectively is composed of___________?
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A. High dv / dt, high di / dt

B. High dv / dt, low di / dt

C. Low dv / dt, low di / dt

D. Low dv / dt, high di / dt

143. For commutation in three phase PAC, normally balanced three phase voltages VR, VY and VB are connected to the three legs of the converter via______________?
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A. Three resistance

B. Three inductances

C. Three transistors

D. Three capacitances

144. A single phase full bridge inverter can operated in load commutation mode in case load consist of______?
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A. RLC overdamped.

B. RL.

C. RLC underdamped.

D. RLC critically damped

145. The transfer function of PWM is generally developed in______________?
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A. None of these

B. Frequency domain

C. Either A. or B.

D. Time domain

146. Ripple factor is the ratio of______________________?
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A. Rms value of the ac component of load voltage to the dc voltage

B. Average value of the ac component of load voltage to the peak value of voltage

C. Peak value of the dc component of load voltage to the ac voltage

D. Average value of the dc component of load voltage to the ac voltage

147. A thyristor can termed as__________?
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A. AC switch

B. DC switch

C. Both a and B

D. Square wave switch

148. Double fourier series analysis of PWM is_____________?
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A. Two dimensional functions

B. Three dimensional functions

C. All of these

D. One dimensional functions

149. The capacitance of reversed bised junction J2 in a thyristor is CJ2 = 20 pF and can be assumed to be independant of the off state voltage. The limiting value of the charging current to turn on the thyristor is 16 mA. What is the critical value of dv/dt?
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A. 600 V/s

B. 1000 V/s

C. 800 V/s

D. 1200 V/s

150. Why resistor is used in Snubber circuit_________________ ?
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A. To minimize the discharging current

B. All of these

C. To minimize the loss

D. To minimize the charging current