Multiple choice question for engineering
Set 1
1. During the transition time or turn-on time
a) The forward anode voltage decreases from 90 % to 10 % & the anode current also decreases from 90 to 10 % of the initial value
b) The forward anode voltage increases from 10 % to 90 % & the anode current also increases from 10 % to 90 % of the initial value
c) The forward anode voltage decreases from 90 % to 10 % & the anode current increases from 10 % to 90 % of the initial value
d) The forward anode voltage increases from 10 % to 90 % & the anode current decreases from 90% to 10% of the initial value
Answer
Answer: c [Reason:] During the turn on time, the voltage across the SCR is going down and the current through it is slowly rising as it is going into the conduction mode.
2. For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and
iii) Spread time
During the delay time the
a) anode current flows only near the gate
b) anode current rises from zero to very high value
c) losses are maximum
d) anode to cathode voltage is zero
Answer
Answer: a [Reason:] Initially for a fraction of a microsecond (delay time) after the gate signal is applied the anode current only flows near the gate terminal where the gate current density is maximum, as the gate current takes some time to spread all over the cross section of the device.
3. The minimum value of anode current below which it must fall to completely turn-off the device is called as the
a) holding current value
b) latching current value
c) switching current value
d) peak anode current value
Answer
Answer: a [Reason:] The device will remain in the conducting state unless the anode current falls below the holding current value.
4. For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and
iii) Spread time
During the rise time the
a) anode current flows only near the gate
b) anode current rises from zero to very high value
c) losses are maximum
d) anode to cathode voltage is zero
Answer
Answer: c [Reason:] The losses are maximum during the rise time because both Ia & Va are high.
5. The latching current is _________ than the holding current
a) lower
b) higher
c) same as
d) negative of
Answer
Answer: b [Reason:] The latching current is the value of current on which the device will remain in the on state even after removal of the gate signal. Whereas, the holding current is the threshold above which the device will work.
6. For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and the
iii) Spread time
The spread time interval depends upon
a) the value of gate current
b) junction temperature
c) area of the cathode
d) area of the anode
Answer
Answer: c [Reason:] During the spread time the conduction starts spreading all over the SCR cathode cross-section structure, which depends upon the structure of the gate & cathode. Higher the cathode area more is the time required for the charges to spread all over.
7. For effective turning off of the SCR after the anode current has reached zero value, ______________
a) chargers are injected by applying reverse anode-cathode voltage
b) chargers are removed by applying reverse anode-cathode voltage
c) chargers are injected by applying gate signal
d) chargers are removed by applying gate signal
Answer
Answer: b [Reason:] To enable the device to regain its reverse blocking capabilities, the stored charges in the junctions of the SCR must be removed.
8. To avoid commutation failure
a) circuit turn-off time must be greater than the thyristor turn-off time
b) circuit turn-off time must be lesser than the thyristor turn-off time
c) circuit turn-off time must be equal to the thyristor turn-off time
d) none of the above mentioned
Answer
Answer: a [Reason:] If the thyristor turn off time is more than the circuit turn off time, the circuit will be turned off and the thyristor will keep conducting, which is not at all desirable.
9. The gate characteristics of thyristor is a plot of
a) Vg on the X-axis & Ig on the Y-axis
b) Ig on the X-axis & Vg on the Y-axis
c) Va on the X-axis & Ig on the Y-axis
d) Ig on the X-axis & Va on the Y-axis
Answer
Answer: b [Reason:] It is the gate current versus the gate voltage plot and gives the minimum and maximum values of gate parameters.
10. The area under the curve of the gate characteristics of thyristor gives the
a) total average gate current
b) total average gate voltage
c) total average gate impedance
d) total average gate power dissipation
Answer
Answer: d [Reason:] As the gate characteristics is a plot of Ig vs Vg consisting of two curves one for the maximum values & other for the minimum the area between them gives the total average gate power dissipation. (A very important parameter in designing of the triggering circuits).
Set 2
1. Name the below given circuit.
a) Type D chopper
b) Type B chopper
c) Two-quadrant type A chopper
d) Two-quadrant type B chopper
Answer
Answer: c [Reason:] It is a type C chopper or Two quadrant type A chopper. It operates in the first and second quadrant.
2. In the below given circuit output voltage will be 0 when
a) SW1 is on
b) FD is not conducting
c) D2 is conducting
d) SW2 is on
Answer
Answer: d [Reason:] Output voltage will be zero if SW 2 is on, because it will short circuit the load circuit and output voltage will be zero.
3. In the below given circuit
a) current can never be zero
b) current can flow in both the directions of the load
c) voltage can never be zero
d) none of the mentioned
Answer
Answer: b [Reason:] In type C chopper, current can flow in both the directions. It would be negative if SW2 in on or D2 conducts.
4. In the below given type C chopper circuit, _______ and _________ together operate like type A chopper.
a) CH1, FD
b) CH2, FD
c) CH1, D2
d) CH2, D2
Answer
Answer: a [Reason:] In the above shown type C chopper circuit, when CH1 is on and current flows VS-CH1-L-E-Vs. When CH1 is switched off, current freewheels through the FD. Hence, current and voltage both are positive giving type A chopper operation.
5. In the below given type C chopper circuit, ________ and ___________ together operate like type B chopper.
a) CH1, FD
b) CH2, FD
c) CH1, D2
d) CH2, D2
Answer
Answer: d [Reason:] In the above shown type C chopper circuit, when CH2 is on current flows in the negative direction E-L-CH2-E and hence voltage is zero. When D2 operates E and L deliver power to the supply Vs hence, giving a type B chopper operation.
6. Which type of chopper is used in the regenerative braking of DC motors?
a) type A
b) type B
c) type C
d) type D
Answer
Answer: c [Reason:] In regenerative braking of DC motors, power needs to flow from load to source as well as source to load depending on whether the machine is in the motoring mode or generative braking.
7. Type C chopper consists of
a) two diodes and two switches
b) one diode and one switch
c) one diode and three switches
d) three diodes and two switches
Answer
Answer: a [Reason:] Type C consists of, two diodes and two switches. One diode switch pair is used for operation in a particular quadrant.
8. In a type C chopper, if only one switch is operated
a) only one quadrant operation will be obtained
b) two quadrant operation can be obtained
c) the chopper won’t work
d) none of the mentioned
Answer
Answer: a [Reason:] Only one quadrant operation will be operated if only one switch is used. It can be either 1st and 2nd quadrant depending on which switch is operated.
9. A type C chopper is operated with only positive load current. Choose the correct statement.
a) CH1 would remain idle
b) D2 would remain idle
c) CH2 would remain idle
d) Both D2 and CH2 would remain idle
Answer
Answer: d [Reason:] It is a type C chopper or two quadrant type A chopper. It operates in the first and second quadrant. CH1 and FD in the first quadrant and CH2 and D2 in second quadrant.
10. A type C chopper is operated with only negative load current. Choose the correct statement.
a) CH1 would remain idle
b) FD would remain idle
c) CH2 would remain idle
d) Both FD and CH1 would remain idle
Answer
Answer: d [Reason:] It is a type C chopper or two quadrant type A chopper. It operates in the first and second quadrant. CH1 and FD in the first quadrant and CH2 and D2 in second quadrant.
Set 3
1. A type D chopper is a
a) two quadrant type-B chopper
b) two quadrant type-A chopper
c) two quadrant type-C chopper
d) none of the mentioned
Answer
Answer: a [Reason:] It is a two quadrant (I and IV) type B chopper as load current is always positive but the voltage can be positive or negative.
2. In a type-D chopper
a) current can flow in both the directions of the load
b) current cannot flow in both the directions of the load
c) voltage can only be positive
d) voltage can only be negative
Answer
Answer: b [Reason:] It is a two quadrant (I and IV) type B chopper. In quadrant I and IV current is only positive.
3. What is the load voltage value when both the chopper switches are on?
a) 0
b) Vs
c) 2Vs
d) Vs/2
Answer
Answer: b [Reason:] When both the switches are on, load is directly in parallel with Vs (source voltage).
4. For a type D chopper, the average value of output voltage will be positive when
a) Ton = Toff
b) Ton < Toff
c) Toff = 0
d) Ton > Toff
Answer
Answer: d [Reason:] When the chopper on time (SW1 and SW2) is more than the chopper off time than the average voltage will be positive. When the chopper switches are off, both the diodes are operating and the voltage is negative.
5. For a type D chopper, if duty cycle = 0.5 then the
a) average voltage is positive
b) average voltage is negative
c) average voltage is zero
d) chopper cannot be operated with duty cycle = 0.5
Answer
Answer: c [Reason:] When α = 0.5. Ton = Toff hence the positive and negative value of voltages become equal. Making the average voltage = 0 V.
6. In the below given circuit, if only SW1 is operated then
a) the power flows from load to source
b) the power flows from source to load
c) the average voltage is positive
d) none of the mentioned
Answer
Answer: d [Reason:] If only one switch is operated, circuit is not completed and the current does not flow.
7. For a type D chopper, if duty cycle α < 0.5 then the
a) average voltage is positive
b) average voltage is negative
c) average voltage is zero
d) none of the mentioned
Answer
Answer: b [Reason:] α = Ton/T
If α < 0.5, Ton < Toff
The average value of voltage will be negative, as the switches will be off most of the times.
8. In the below given circuit, if Vo = -Vs then
a) both the chopper switches are on
b) only SW2 and D1 is active
c) both the chopper switches are off and both the diodes D1 and D2 are active
d) both the chopper switches are on and both the diodes D1 and D2 are reversed biased
Answer
Answer: c [Reason:] When diodes are operating, power flows from load to source and the inductor (L) in the load reverses its polarity.
9. Find the expression for average output voltage in a type D chopper on RL load.
a) Vs x Duty cycle (α)
b) Vs x T
c) Vs x (T
d) Vs2/2
Answer
Answer: c [Reason:]
During Ton = Vs x Ton/T
During Toff = – Vs x Toff/T
Vo = Vs (Ton – Toff)/T.
10. Find the average output voltage for a type D chopper if it is given a dc supply of 220 V. Chopper frequency is 2 kHz and both the chopper switches are operated for 0.3 ms.
a) 220 V
b) 108 V
c) 98 V
d) 44 V
Answer
Answer: d [Reason:] T = 1/f = 0.5 ms
Ton = 0.3 ms
Toff = 0.2 ms
Vo = Vs (Ton – Toff)/T = 220 x (1/5) = 44 V.
Set 4
1. Identify the given circuit
a) type D chopper
b) two quadrant type C chopper
c) type E chopper
d) type F chopper
Answer
Answer: c [Reason:] It is a type E chopper. Type C consists of 4 diodes and 4 IGBT switches.
2. A type C chopper consists of __________ diodes and _________ switches in anti-parallel.
a) 2, 2
b) 3, 3
c) 4, 4
d) 3, 4
Answer
Answer: c [Reason:] Type C consists of 4 diodes and 4 switches arranged in anti-parallel. The switches and diodes are always numbered occurring to the quadrant in which they operate.
3. A type C chopper can operate in
a) Ist and IInd quadrants
b) IInd and IIIrd quadrants
c) Ist, IInd and IIIrd quadrants
d) all the four quadrants
Answer
Answer: d [Reason:] Type C can operate in all the four quadrants by controlling appropriate switches and reversing the emf in the load circuit.
4. For the first quadrant operation of type E chopper
a) Only CH1 is on
b) CH1 and CH2 is on
c) CH1 and CH3 is on
d) CH1 and CH4 is on
Answer
Answer: d [Reason:] With CH1 and CH4 on, load voltage = Vs and load current flows. Both V and I are positive giving Ist quadrant operation.
5. For the second quadrant operation of type E chopper
a) Only CH2 is on
b) CH1 and CH2 is on
c) CH2 and CH3 is on
d) CH2 and CH4 is on
Answer
Answer: d [Reason:] With CH2 on, negative current flows through L, CH2, CH3 and CH4. When CH2 is turned off, current is fed back to the diodes by D1 and D4. Load voltage is positive and current is negative, hence second current operation is operated.
6. For a type E chopper, when CH2 is first on and then switched off then the
a) power flows from source to load
b) power flows from load to source
c) load is short circuited
d) none of the mentioned
Answer
Answer: b [Reason:] With CH2 on, negative current flows through L, CH2, CH3 and CH4. When CH2 is turned off, current is fed back to the diodes by D1 and D4. Load voltage is positive and current is negative, hence second current operation is operated.
7. What is the expression for load voltage when the chopper is operated in the second quadrant?
a) Vs
b) E
c) 0
d) E + Ldi/dt
Answer
Answer: d [Reason:] In the second quadrant, the chopper acts as a step up chopper and current flows from the load to source. With CH2 on, negative current flows through L, CH2, CH3 and CH4.
8. For a type E chopper operating in the first quadrant, find the expression for average output voltage.
a) Vs
b) E – Ldi/dt
c) 0
d) E + Ldi/dt
Answer
Answer: a [Reason:] With CH1 and CH4 on, load voltage = Vs and load current flows.
9. The load emf E must be reversed for
a) first and second quadrant operation
b) third quadrant operation
c) fourth quadrant operation
d) both third and fourth quadrant operation
Answer
Answer: d [Reason:] The load E must be reversed for both third and fourth quadrant operation for proper current flow.
10. For the third quadrant operation of type E chopper
a) CH1 is on
b) CH2 and CH1 are on
c) CH3 is on
d) None of the mentioned
Answer
Answer: d [Reason:] For third quadrant operation of type E chopper, CH2 is kept on and CH3 is operated. First both CH2 and CH3 are on, and then CH3 is switched off thus the current freewheels through CH2 and D4.
Set 5
1. The external control of ac output voltage can be achieved in an inverter by
a) connecting a cyclo-converter
b) connecting an ac voltage controller between the output of the inverter and the load
c) connecting an ac voltage controller between the dc source and inverter
d) connecting an ac voltage controller between the load and the dc source
Answer
Answer: b [Reason:] By connecting a AC voltage controller, the ac output from the inverter can be varied and then fed to the load.
2. The series-inverter control method is a/an
a) internal voltage control method
b) external frequency control method
c) external voltage control method
d) none of the mentioned
Answer
Answer: c [Reason:] It is a external voltage control method where the outputs of the two inverters are connected to the transformers where the secondary of the transformer sums up the two input voltages.
3. In the series-inverter control method
a) two inverters are connected back-to-back
b) the output from the inverter is taken serially
c) output voltages of two inverters are summed up with the help of a transformer
d) output voltages of two inverters are summed up with the help of a third inverter
Answer
Answer: c [Reason:] It is a external voltage control method where the outputs of the two inverters are connected to the transformers where the secondary of the transformer sums up the two input voltages.
4. In case of the series inverter control, if two inverters are connected in series through a transformer, and two secondary voltages are V1 and V2, then the resultant output is given by
a) V1 + V2
b) √(V12 + V22)
c) [V12 + V22 + 2.V1.V2.cosθ]1/2
d) [V12 + V22 + 2.V1.V2.sinθ]1/2
Answer
Answer: c [Reason:] The resultant output will be the phasor sum of V1 and V2.
5. External control of dc input voltage can be obtained by the use of a
a) transformer
b) chopper
c) inverter
d) converter
Answer
Answer: b [Reason:] A chopper is used to control the dc input voltage to the inverter, which then converters the variable dc to variable ac.
6. In the external control of dc input voltage
a) a chopper is placed just after the inverter block
b) a chopper is placed just after the filter block
c) a chopper is placed before the filter and the inverter block
d) none of the mentioned
Answer
Answer: c [Reason:] Constant AC – Rectifier – Chopper – Filter – Inverter.
7. __________ method is an internal method for controlling the inverter output voltage.
a) series connection of inverters
b) chopper method
c) commutating capacitor
d) pulse width modulation
Answer
Answer: d [Reason:] The PWM method, (pulse width modulation method) is an internal controlling method.
8. In the PWM method
a) external commutating capacitors are required
b) more average output voltage can be obtained
c) lower order harmonics are minimized
d) higher order harmonics are minimized
Answer
Answer: c [Reason:] In all the PWM methods, only odd harmonics are present. The lower order harmonics are eliminated along with its output voltage control.
9. Which of the following is not a PWM technique?
a) Single-pulse width modulation
b) Multiple-pulse width modulation
c) Triangular-pulse width modulation
d) Sinusoidal-pulse width modulation
Answer
Answer: c [Reason:] There is no such “Triangular” PWM.
10. In pulse width modulation
a) the output voltage is modulated
b) the input voltage is modulated
c) the gating pulses are modulated
d) none of the mentioned
Answer
Answer: c [Reason:] In PWM, the gating pluses are modulated, i.e. the gating pulses or firing pulses are made to go on of rapidly which changes the output voltage values accordingly.
