Thyristor Ckt Mcqs
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THYRISTOR CKTS MCQ
3. Identify the below given circuit.
1. For the below given voltage commutation circuit, find the peak value of voltage across the freewheeling diode FD.
a) current commutated chopper b) voltage commutated chopper c) load commutated chopper d) load commutated inverter a) Vs b) 0 c) 2Vs d) Vs + Ldi/dt
View Answer
View Answer
Answer: a Explanation: Diode in anti-parallel with the main SCR. Hence (a).
Answer: c Explanation: The load current then flows through Vs-C-TA-Load. Hence, Vo = Vs + (capacitor voltage). As the capacitor is already charged to a voltage Vs. Vo = Voltage across FD = 2Vs.
2. A voltage commutated chopper feeds power to an electric car (battery-powered). The battery voltage is 60 V. The SCR turn-off time is 20 μs. Calculate the value of capacitor (C) required for the commutating circuit. a) 400 μF b) 40 nF c) 40 μF d) 0.4 mF View Answer Answer: c Explanation: Capacitor current = C dv/dt For constant load current Io, the above expression can be Io = C x (Vs/tc) or C = (tc x Io)/Vs
C = ( 40 x 60 x 10-6)/60 = 40 x 10-6 Farads.
4. A voltage commutated chopper feeds power to an electric car (battery-powered). The battery voltage is 60 V. Starting current is 60 A and the SCR turn-off time is 20 μs. Calculate the value of inductance (L) required for the commutating circuit. a) 400 μH b) 40 nH c) 40 μH d) 0.4 mH View Answer Answer: c Explanation: L = (Vs/Io)2 x C C = (tc x Io)/Vs
C = ( 40 x 60 x 10-6)/60 = 40 x 10-6 F L = (60/60)2 x 40 x 10-6 = 40 uH.
5. In the below given circuit, at the instant T1 the main SCR is
triggered, the voltage across the load terminal is
View Answer Answer: c Explanation: I = V √(C/L) = 230 x √(50/20) = 363.66 A.
8. A voltage commutated chopper has the following parameters: Vs = 220 V. Commutation circuit: L = 20 μH and C = 50 μF. Find the effective on period for a constant load current of 80 A and Ton = 800 μsecs. a) 1.075 μs b) 10.75 ms c) 1075 μs d) 1.075 sec a) Vs b) 0 c) 2Vs d) Vs + Ldi/dt
View Answer
View Answer
Answer: c Explanation: The effective on period Ton’ is given by Ton + 2Vs.C/Io.
Answer: a Explanation: When the main SCR is gated, the commutating circuit remains inactive. Hence the load is directly connected to Vs through the SCR. Considering SCR as an ideal switch Vo = Vs.
6. In current commutated copper circuit a) the capacitor cannot charge on no load b) the auxiliary thyristor is turned on automatically c) the auxiliary thyristor is naturally commutated d) the commutation is not reliable View Answer Answer: c Explanation: It is naturally commutated as its commutating current passes through zero value in the ringing circuit formed by L and C.
7. A current commutated chopper is fed from a source of 230 V. Its commutating components are L = 20 μH and C = 50 μF. Find the value of peak commutating current. a) 233 A b) 59 A c) 363 A d) 198 A
9. A voltage commutated chopper has the following parameters: Vs = 220 V. Commutation circuit: L = 20 μH and C = 50 μF. Find the turn-off time for the main SCR with a constant load current of 80 A and Ton = 800 μsecs. a) 137.5 μs b) 1075 μs c) 49.67 μs d) 200 μs View Answer Answer: a Explanation: C = (tc x Io)/Vs
tc = C.Vs/I = 137.5 μsecs. 10. The load commutated chopper circuit consists of a) two thyristors and one commutating capacitor b) four thyristors and one commutating capacitor c) two thyristors and two commutating capacitors d) four thyristors and two commutating capacitors View Answer
Answer: b Explanation: It consists of 4 SCRs and one capacitor. 4 SCRs forming 2 pairs of 2 SCRs conduct alternately. --------------------------------------------------------------------------------------------------------------------------1.
The process of commutating a SCR by applying a reverse voltage to an SCR through a previously charged capacitor is called as a) capacitor commutation b) forced commutation c) voltage commutation d) current commutation View Answer Answer: c Explanation: It is called as voltage commutation, which is a type of forced commutation.
2. In case of current commutation of SCR a) a diode is connected in series with the main SCR b) a diode is connected in parallel with the main SCR c) a diode is connected in anti-parallel with the main SCR d) none of the mentioned View Answer
a) current commutated chopper b) voltage commutated chopper c) load commutated chopper d) none of the mentioned View Answer Answer: b Explanation: It is a voltage commutated chopper or impulse-commutated chopper. If the diode was directly in parallel with the main SCR it would be a current commutated chopper circuit.
4. In the below shown voltage commutated chopper circuit, the chopper operation can only start when
Answer: c Explanation: The voltage drop across the anti parallel connected SCR reverse biases the main SCR.
3. Below given circuit is a
a) t1 is gated b) the capacitor is charged c) the diode is forward biased d) the capacitor is dis-charged
View Answer Answer: b Explanation: The chopper can only start (effectively) when the C in the commutating circuit is charged first.
5. In a voltage commutated chopper circuit, the capacitor C can be charged to Vs by a) Vs – T1 – Load – Vs b) S – C – T1 – Load – S c) C – T1 – L – D – C d) C – T1 – TA – C View Answer Answer: c Explanation: The commutation current is nothing but the capacitor current.
a) gating T1 b) gating TA c) closing the switch d) both gating TA and closing the switch
7. In the below given circuit, when the main SCR T1 is commutated at t2, the load voltage is
View Answer Answer: d Explanation: When switch is closed, the C gets charged through Vs, C, Switch. When TA is triggered C gets charged through Vs, C, TA and the load.
6. In the below given circuit, when the main thyristor T1 is triggered after charging the capacitor, the commutation current flows through the path
a) Vs b) 0 c) 2Vs d) Vs + Ldi/dt View Answer Answer: c Explanation: The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1. The load current then flows through Vs-C-TA-Load. Hence, Vo = Vs + (capacitor voltage).
As the capacitor is already charged to a voltage Vs. Vo = 2Vs. 8. In a voltage commutated SCR, the main SCR is commutated by a) triggering the auxiliary SCR b) sending a negative pulse to the main SCR c) removing the supply Vs d) none of the mentioned View Answer Answer: a Explanation: The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1.
1. Find the expression for peak capacitor voltage in case of a current commutated chopper circuit. a) Io √(L/C) b) Vs + √(L/C) c) Vs + Io √(L/C) d) Zero View Answer Answer: c Explanation: 1/2 CVc2 = 1/2 LIo2 Vc = Io √(L/C)
Vc (peak) = Vs + Io √(L/C). 2. Identify the below given chopper circuit.
9. Which type of commutation circuit does not work on no load? a) Voltage commutation b) Current commutation c) Both voltage and current commutation d) None of the mentioned View Answer Answer: a Explanation: This is because the capacitor won’t be able to charge on no load.
10. What is the expression for the value of the capacitor in the voltage commutation circuit? Let tc be the circuit turn off time and Io be the constant load current. a) Io x (dv/dt) b) Vs/Io c) (tc x Io)/Vs d) none of the mentioned View Answer Answer: c Explanation: Capacitor current = C dv/dt For constant load current Io, the above expression can be
Io = C x (Vs/tc) or C = (tc x Io)/Vs. ---------------------------------------------------------------------------------------------------------------------------
a) Type D chopper b) Type C chopper with commutating capacitor c) Load commutated step-down chopper d) Voltage commutated step-up chopper View Answer Answer: c Explanation: Load commutated chopper uses 4 SCRs and one commuting capacitor. As there is no L in the circuit, it is a step down chopper.
3. In the below given commutated chopper circuit, __ and __ SCRs act together in pairs.
a) T1-T2, T3-T4 b) T1-T3, T2-T4 c) T1-T4, T2-T4 d) T1, T2 View Answer Answer: a Explanation: T1-T2 and T3-T4 act together as one pair for conducting the load current alternately.
4. When T1 and T2 are conducting, which components act as the commutating components?
a) Vs b) 0 c) 2Vs d) Vs + Ldi/dt View Answer Answer: c Explanation: The main SCRs T1 and T2 are conducting, and the path is Vs –T1 – C – T2 – Load. Hence, Vo = Vs + (capacitor voltage). As the capacitor is already charged to a voltage Vs. Vo = 2Vs.
6. What is the expression for design of commutation capacitance in case of a load commutated chopper circuit? a) (I x Ton)/Vs b) Cannot be determined c) (I x Ton)/2Vs d) (I x T)/Vs View Answer
a) T3 and T4 b) All the SCRs c) T3, T4 and C d) T3, T4, C and FD View Answer Answer: c Explanation: Commutating components are the one which will help the main SCRs T1 and T2 in this case turn off safely. FD is not a commutating component, it only freewheels the current during inductive loads.
5. What is the voltage across the load when T1 and T2 are gated after charging the capacitor to Vs?
Answer: c Explanation: Total change in the voltage is 2xVs in time Ton. I = C x (2Vs/ Ton) C = (IxTon)/2Vs.
7. Which type of commutating circuit will have highest switching losses for same rating of all the components? a) current commutated chopper b) voltage commutated chopper c) load commutated chopper d) forced commutated chopper View Answer Answer: c Explanation: Load commutated choppers have 4 SCRs which require frequent turning on and
turning off which increases the switching losses.
8. The speed of a separately excited dc motor is controlled below base speed by using a type-A SCR based chopper. The supply voltage is 220 V dc and the armature resistance is 0.5 Ω. Calculate the minimum value of firing angle if the motor drives a constant torque load requiring an average armature current of 30 A. a) 1/2 b) 3/44 c) 59/46 d) 7/12
When D is active during Toff, consider inductor as a short circuit thus, Id = (VoxToff)/R = 80 x 0.2/10 = 1.6 A. 10. A chopper circuit is fed from an input voltage of 20 V delivers a power of 16 watts. If the chopper efficiency is 80 %, then find the value of input current. a) 0.64 A b) 1 A c) 0.8 A d) 1.25 A View Answer
View Answer
Answer: b Explanation: 0.8 = ouput/input Input power = 16/0.8 16/0.8 = V x I
Answer: b Explanation: For a motor, Vo = E + Ir The minimum possible speed = 0. This gives the motor counter emf E = 0. Therefore, Vo = αVs = Ir α x 220 = 30 x 0.5
V = 20 V. Hence, I = 1 A.
α = 15/220 = 3/44. 9. For the step down chopper shown in the figure below, Vs = 100 V and the duty cycle of the switch is 0.8. The load is sufficiently inductive so that the load current is ripple free. The average current through the diode (D) under steady state is
a) 2 A b) 1.6 A c) 8 A d) 6.4 A View Answer Answer: b Explanation: The diode is active only during Toff. If Ton = 0.8 then Toff = 0.2 Output voltage Vo = 0.8 x 100 = 80 V
3. Identify the below given circuit.
1. For the below given voltage commutation circuit, find the peak value of voltage across the freewheeling diode FD.
a) current commutated chopper b) voltage commutated chopper c) load commutated chopper d) load commutated inverter a) Vs b) 0 c) 2Vs d) Vs + Ldi/dt
View Answer
View Answer
Answer: a Explanation: Diode in anti-parallel with the main SCR. Hence (a).
Answer: c Explanation: The load current then flows through Vs-C-TA-Load. Hence, Vo = Vs + (capacitor voltage). As the capacitor is already charged to a voltage Vs. Vo = Voltage across FD = 2Vs.
2. A voltage commutated chopper feeds power to an electric car (battery-powered). The battery voltage is 60 V. The SCR turn-off time is 20 μs. Calculate the value of capacitor (C) required for the commutating circuit. a) 400 μF b) 40 nF c) 40 μF d) 0.4 mF View Answer Answer: c Explanation: Capacitor current = C dv/dt For constant load current Io, the above expression can be Io = C x (Vs/tc) or C = (tc x Io)/Vs
C = ( 40 x 60 x 10-6)/60 = 40 x 10-6 Farads.
4. A voltage commutated chopper feeds power to an electric car (battery-powered). The battery voltage is 60 V. Starting current is 60 A and the SCR turn-off time is 20 μs. Calculate the value of inductance (L) required for the commutating circuit. a) 400 μH b) 40 nH c) 40 μH d) 0.4 mH View Answer Answer: c Explanation: L = (Vs/Io)2 x C C = (tc x Io)/Vs
C = ( 40 x 60 x 10-6)/60 = 40 x 10-6 F L = (60/60)2 x 40 x 10-6 = 40 uH.
5. In the below given circuit, at the instant T1 the main SCR is
triggered, the voltage across the load terminal is
View Answer Answer: c Explanation: I = V √(C/L) = 230 x √(50/20) = 363.66 A.
8. A voltage commutated chopper has the following parameters: Vs = 220 V. Commutation circuit: L = 20 μH and C = 50 μF. Find the effective on period for a constant load current of 80 A and Ton = 800 μsecs. a) 1.075 μs b) 10.75 ms c) 1075 μs d) 1.075 sec a) Vs b) 0 c) 2Vs d) Vs + Ldi/dt
View Answer
View Answer
Answer: c Explanation: The effective on period Ton’ is given by Ton + 2Vs.C/Io.
Answer: a Explanation: When the main SCR is gated, the commutating circuit remains inactive. Hence the load is directly connected to Vs through the SCR. Considering SCR as an ideal switch Vo = Vs.
6. In current commutated copper circuit a) the capacitor cannot charge on no load b) the auxiliary thyristor is turned on automatically c) the auxiliary thyristor is naturally commutated d) the commutation is not reliable View Answer Answer: c Explanation: It is naturally commutated as its commutating current passes through zero value in the ringing circuit formed by L and C.
7. A current commutated chopper is fed from a source of 230 V. Its commutating components are L = 20 μH and C = 50 μF. Find the value of peak commutating current. a) 233 A b) 59 A c) 363 A d) 198 A
9. A voltage commutated chopper has the following parameters: Vs = 220 V. Commutation circuit: L = 20 μH and C = 50 μF. Find the turn-off time for the main SCR with a constant load current of 80 A and Ton = 800 μsecs. a) 137.5 μs b) 1075 μs c) 49.67 μs d) 200 μs View Answer Answer: a Explanation: C = (tc x Io)/Vs
tc = C.Vs/I = 137.5 μsecs. 10. The load commutated chopper circuit consists of a) two thyristors and one commutating capacitor b) four thyristors and one commutating capacitor c) two thyristors and two commutating capacitors d) four thyristors and two commutating capacitors View Answer
Answer: b Explanation: It consists of 4 SCRs and one capacitor. 4 SCRs forming 2 pairs of 2 SCRs conduct alternately. --------------------------------------------------------------------------------------------------------------------------1.
The process of commutating a SCR by applying a reverse voltage to an SCR through a previously charged capacitor is called as a) capacitor commutation b) forced commutation c) voltage commutation d) current commutation View Answer Answer: c Explanation: It is called as voltage commutation, which is a type of forced commutation.
2. In case of current commutation of SCR a) a diode is connected in series with the main SCR b) a diode is connected in parallel with the main SCR c) a diode is connected in anti-parallel with the main SCR d) none of the mentioned View Answer
a) current commutated chopper b) voltage commutated chopper c) load commutated chopper d) none of the mentioned View Answer Answer: b Explanation: It is a voltage commutated chopper or impulse-commutated chopper. If the diode was directly in parallel with the main SCR it would be a current commutated chopper circuit.
4. In the below shown voltage commutated chopper circuit, the chopper operation can only start when
Answer: c Explanation: The voltage drop across the anti parallel connected SCR reverse biases the main SCR.
3. Below given circuit is a
a) t1 is gated b) the capacitor is charged c) the diode is forward biased d) the capacitor is dis-charged
View Answer Answer: b Explanation: The chopper can only start (effectively) when the C in the commutating circuit is charged first.
5. In a voltage commutated chopper circuit, the capacitor C can be charged to Vs by a) Vs – T1 – Load – Vs b) S – C – T1 – Load – S c) C – T1 – L – D – C d) C – T1 – TA – C View Answer Answer: c Explanation: The commutation current is nothing but the capacitor current.
a) gating T1 b) gating TA c) closing the switch d) both gating TA and closing the switch
7. In the below given circuit, when the main SCR T1 is commutated at t2, the load voltage is
View Answer Answer: d Explanation: When switch is closed, the C gets charged through Vs, C, Switch. When TA is triggered C gets charged through Vs, C, TA and the load.
6. In the below given circuit, when the main thyristor T1 is triggered after charging the capacitor, the commutation current flows through the path
a) Vs b) 0 c) 2Vs d) Vs + Ldi/dt View Answer Answer: c Explanation: The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1. The load current then flows through Vs-C-TA-Load. Hence, Vo = Vs + (capacitor voltage).
As the capacitor is already charged to a voltage Vs. Vo = 2Vs. 8. In a voltage commutated SCR, the main SCR is commutated by a) triggering the auxiliary SCR b) sending a negative pulse to the main SCR c) removing the supply Vs d) none of the mentioned View Answer Answer: a Explanation: The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1.
1. Find the expression for peak capacitor voltage in case of a current commutated chopper circuit. a) Io √(L/C) b) Vs + √(L/C) c) Vs + Io √(L/C) d) Zero View Answer Answer: c Explanation: 1/2 CVc2 = 1/2 LIo2 Vc = Io √(L/C)
Vc (peak) = Vs + Io √(L/C). 2. Identify the below given chopper circuit.
9. Which type of commutation circuit does not work on no load? a) Voltage commutation b) Current commutation c) Both voltage and current commutation d) None of the mentioned View Answer Answer: a Explanation: This is because the capacitor won’t be able to charge on no load.
10. What is the expression for the value of the capacitor in the voltage commutation circuit? Let tc be the circuit turn off time and Io be the constant load current. a) Io x (dv/dt) b) Vs/Io c) (tc x Io)/Vs d) none of the mentioned View Answer Answer: c Explanation: Capacitor current = C dv/dt For constant load current Io, the above expression can be
Io = C x (Vs/tc) or C = (tc x Io)/Vs. ---------------------------------------------------------------------------------------------------------------------------
a) Type D chopper b) Type C chopper with commutating capacitor c) Load commutated step-down chopper d) Voltage commutated step-up chopper View Answer Answer: c Explanation: Load commutated chopper uses 4 SCRs and one commuting capacitor. As there is no L in the circuit, it is a step down chopper.
3. In the below given commutated chopper circuit, __ and __ SCRs act together in pairs.
a) T1-T2, T3-T4 b) T1-T3, T2-T4 c) T1-T4, T2-T4 d) T1, T2 View Answer Answer: a Explanation: T1-T2 and T3-T4 act together as one pair for conducting the load current alternately.
4. When T1 and T2 are conducting, which components act as the commutating components?
a) Vs b) 0 c) 2Vs d) Vs + Ldi/dt View Answer Answer: c Explanation: The main SCRs T1 and T2 are conducting, and the path is Vs –T1 – C – T2 – Load. Hence, Vo = Vs + (capacitor voltage). As the capacitor is already charged to a voltage Vs. Vo = 2Vs.
6. What is the expression for design of commutation capacitance in case of a load commutated chopper circuit? a) (I x Ton)/Vs b) Cannot be determined c) (I x Ton)/2Vs d) (I x T)/Vs View Answer
a) T3 and T4 b) All the SCRs c) T3, T4 and C d) T3, T4, C and FD View Answer Answer: c Explanation: Commutating components are the one which will help the main SCRs T1 and T2 in this case turn off safely. FD is not a commutating component, it only freewheels the current during inductive loads.
5. What is the voltage across the load when T1 and T2 are gated after charging the capacitor to Vs?
Answer: c Explanation: Total change in the voltage is 2xVs in time Ton. I = C x (2Vs/ Ton) C = (IxTon)/2Vs.
7. Which type of commutating circuit will have highest switching losses for same rating of all the components? a) current commutated chopper b) voltage commutated chopper c) load commutated chopper d) forced commutated chopper View Answer Answer: c Explanation: Load commutated choppers have 4 SCRs which require frequent turning on and
turning off which increases the switching losses.
8. The speed of a separately excited dc motor is controlled below base speed by using a type-A SCR based chopper. The supply voltage is 220 V dc and the armature resistance is 0.5 Ω. Calculate the minimum value of firing angle if the motor drives a constant torque load requiring an average armature current of 30 A. a) 1/2 b) 3/44 c) 59/46 d) 7/12
When D is active during Toff, consider inductor as a short circuit thus, Id = (VoxToff)/R = 80 x 0.2/10 = 1.6 A. 10. A chopper circuit is fed from an input voltage of 20 V delivers a power of 16 watts. If the chopper efficiency is 80 %, then find the value of input current. a) 0.64 A b) 1 A c) 0.8 A d) 1.25 A View Answer
View Answer
Answer: b Explanation: 0.8 = ouput/input Input power = 16/0.8 16/0.8 = V x I
Answer: b Explanation: For a motor, Vo = E + Ir The minimum possible speed = 0. This gives the motor counter emf E = 0. Therefore, Vo = αVs = Ir α x 220 = 30 x 0.5
V = 20 V. Hence, I = 1 A.
α = 15/220 = 3/44. 9. For the step down chopper shown in the figure below, Vs = 100 V and the duty cycle of the switch is 0.8. The load is sufficiently inductive so that the load current is ripple free. The average current through the diode (D) under steady state is
a) 2 A b) 1.6 A c) 8 A d) 6.4 A View Answer Answer: b Explanation: The diode is active only during Toff. If Ton = 0.8 then Toff = 0.2 Output voltage Vo = 0.8 x 100 = 80 V
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