Chopper Mcqs

CHOPPER This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Choppers-1″. 1. In the ___________ type of chopper, two stage conversions takes place. a) AC-DC b) AC link c) DC link d) None of the mentioned Answer: In AC link chopper, DC is converter to AC than stepped up/down than again AC to DC conversation takes place. 2. Choppers converter a) AC to DC b) DC to AC c) DC to DC d) AC to AC Answer: c Explanation: Choppers are used to step up or step down DC voltage/current levels. Hence, they are DC to DC converters. 3. A chopper may be thought as a a) Inverter with DC input b) DC equivalent of an AC transformer c) Diode rectifier d) DC equivalent of an induction motor Answer: b Explanation: It is a DC equivalent of an AC transformer because it behaves in the similar manner i.e. converting fixed DC to variable DC. 4. Which device can be used in a chopper circuit? a) BJT b) MOSFET c) GTO d) All of the mentioned Answer: d Explanation: All of the devices which can be used as a switch can be used in a chopper. 5. A chopper is a a) Time ratio controller b) AC to DC converter c) DC transformer d) High speed semiconductor switch Answer: d Explanation: It is a high speed on/off semiconductor switch. Note that it behaves like a DC transformer, does not mean it is a DC transformer. There is no DC transformer. 6. What is the duty cycle of a chopper ? a) Ton/Toff b) Ton/T

c) T/Ton d) Toff x Ton Answer: b Explanation: It is the time during which the chopper is on (Ton) relative to the whole period (T = Ton+Toff). 7. The load voltage of a chopper can be controlled by varying the a) duty cycle b) firing angle c) reactor position d) extinction angle Answer: a Explanation: The output voltage can be changed by changing the duty cycle (Ton/T). 8. The values of duty cycle (α) lies between a) 0<α<1 b) 0>α>-1 c) 0<=α<=1 d) 1<α<100 Answer: c Explanation: The duty cycle is between 0 and 1. It can be 0 if the chopper switch is never on and it can be 1 when the chopper switch is always on 9. If T is the time period for a chopper circuit and α is its duty cycle, then the chopping frequency is a) Ton/α b) Toff/α c) α/Toff d) α/Ton Answer: d Explanation: α = Ton/T T = Ton/ α f = 1/T = α/Ton. 10. Find the output voltage expression for a step down chopper with Vs as the input voltage and α as the duty cycle. a) Vo = Vs/α b) Vo = Vs x α c) Vo = Vs2/α d) Vo = 2Vs/απ Answer: b Explanation: The chopper output voltage is Duty cycle x the input voltage (ideal condition). ------------------------------1. The below given figure is that of a _________ (IGBT is used as a chopper switch)

Explanation: When the switch is off, the current freewheels through L-R-FD-L and voltage is zero because the FD short circuits the load.

a) step-up/step-down chopper b) step-down chopper c) step-up chopper d) none of the mentioned Answer: b Explanation: When the switch is closed current flows through the load and also charges the inductor L, when the switch is off, the current freewheels through L-R-FD-L and voltage is zero because the FD short circuits the load.

4. In a step down chopper, if Vs = 100 V and the chopper is operated at a duty cycle of 75 %. Find the output voltage. a) 100 V b) 75 V c) 25 V d) none of the mentioned Answer: b Explanation: Vo = Duty cycle x Vs = 0.75 x 100 = 75 V. 5. If Vo is the output voltage and Vs is the input DC voltage, then for the below given circuit.

2. In the below given circuit, when switch (SW) is on

a) voltage is non-zero and current is rising b) voltage is non-zero and current is decaying c) voltage is zero and current is rising d) voltage is zero and current is decaying Answer: a Explanation: When the switch is closed current flows through the load and also charges the inductor L, hence voltage is Vo and current is rising.

a) Vo = Vs b) Vo < Vs c) Vo > Vs d) Insufficient information Answer: c Explanation: The given circuit is that of a step-up chopper. For a step up chopper the output voltage is always (even if duty cycle is very very small) greater than the input voltage. 6. In the below given circuit, when switch (SW) is on

3. In the below given circuit, when switch (SW) is off

a) voltage is non-zero and current is rising b) voltage is non-zero and current is decaying c) voltage is zero and current is rising d) voltage is zero and current is decaying Answer: d

a) voltage is non-zero and current is rising b) voltage is zero and current is decaying c) voltage and current both are non-zero d) voltage and current both are zero Answer: c Explanation: When the switch is on, the current flows from Vs-L-SW-Vs and charges the inductor (L), hence voltage and current both are zero. 7. In the below given circuit, when switch (SW) is first switched closed and then opened the

Wout = (Vo – Vs) x Toff x (I1+I2)/2 Considering ideal condition, Win = Wout Vo = Vs x (T/Toff) α = Ton/T Vo = T/T-Ton = Vs/1-α.

a) voltage is non-zero and current is rising b) voltage is zero and current is decaying c) voltage is non-zero and current is decaying d) voltage is zero and current is rising Answer: c Explanation: When the switch is off, the inductor is has stored energy from the earlier chopper on operation. The current starts to flow from Vs-L-D-Load-Vs, hence current is decaying as the inductor is discharging and the voltage is Vs + Vl (inductor voltage) (apply KVL). 8. What is the voltage across the load when the SW is first closed and then opened?

a) Vs b) duty cycle x Vs c) Vs + L (di/dt) d) zero Answer: c Explanation: When the switch is off, the inductor is has stored energy from the earlier chopper on operation. The current starts to flow from Vs-L-D-Load-Vs, hence current is decaying as the inductor is discharging and the load voltage is Vs + Vl (inductor voltage) Vl = L(di/dt) Vo = Vs + L(di/dt). 9. Find the expression for output voltage for a step-up chopper, assume linear variation of load current and α as the duty cycle. a) Vs b) Vs/α c) Vs/(1-α) d) Vs/√2 Answer: c Explanation: During Ton, the energy stored in the inductor is Win = Vs x Ton x (I1+I2)/2 During Toff, the energy released by the inductor is

10. Find the output voltage for a step-up chopper when it is operated at a duty cycle of 50 % and Vs = 240 V. a) 240 V b) 480 V c) 560 V d) 120 V Answer: b Explanation: Vo = Vs/1-α. ------------------------------------------------------

1. If a step up chopper’s switch is always kept off then (ideally) a) Vo = 0 b) Vo = ∞ c) Vo = Vs d) Vo > Vs Answer: c Explanation; If it is said the a chopper is always kept off, that means the switch is always open. As such, Ton = 0 Duty cycle = 0 Vo = Vs/1-duty cycle . . . (for a step-up chopper). 2. If a step up chopper’s switch is always kept open then (ideally) a) Vo = 0 b) Vo = ∞ c) Vo = Vs d) Vo > Vs Answer: b Explanation: If it is always in then, Ton = T. Duty cycle = 1. Vo = Vs/1-duty cycle = Vs/0 Therefore, Vo = ∞. 3. Find the average value of output voltage of a basic step-down chopper, with duty cycle = α and load = R Ω.

a) I = Vs x α b) I = (Vs x α)/R c) I = 0

d) I = Vs/R Answer: b Explanation: Vo = Vs x α I = Vo/R.

c) 0 d) R/α Answer: d Explanation: Effective input resistance of the chopper circuit = (dc source voltage)/(average source current) = (Vs.R)/(Vs.α) = R/α.

4. For the below given circuit, find the output current at the instant of commutation.

8. A step-up chopper has input voltage of 220 V and output voltage of 660 V. If the conducting time of the IGBT based chopper is 100 μs, compute Toff width of the output voltage pulse. a) 100 μs b) 150 μs c) 50 μs d) Insufficient data Answer: c Explanation: 660 = 220/1-α α = 2/3 = Ton/T Ton = 2T/3 = 100μs. This gives chopping period T = 100 x 3/2 = 150 μs Toff = T – Ton = 150 – 100 = 50 μs.

a) Vo/R b) Vs/R c) 0 d) α/2 Answer: b Explanation: The circuit is that of a step-down chopper. The output current is commutated by the switch(IGBT) at the instant t = Ton. Therefore, output current at the instant of commutation is Vs/R. 5. For a step-down chopper, find the rms value of output voltage. Let α be the duty cycle and Vs be the input voltage. a) α x Vs b) Vs/α c) √α x Vs d) Vs/2 Answer: c Explanation: [(Ton/T).Vs2]1/2 = √α x Vs. 6. A step down chopper is operated at 240V at duty cycle of 75%. Find the value of RMS switch (IGBT/MOSFET) current. Take R = 10 Ω. a) 2.07 A b) 200 mA c) 1.58 A d) 2.4 A Answer: a Explanation: The switch could be anything, a IGBT, a SCR, a GTO etc. The current through the switch is the same as the current through the load for a step down chopper. Vo = Vs/1-α α = 0.75 I = √α x Vs/R. 7. Find the expression for effective input resistance of a step down chopper. With R load and duty cycle = α. a) R x α b) R/2

9. For a step-up chopper, when the duty cycle is increased the average value of the output voltage a) increases b) decreases c) remains the same d) none of the mentioned Answer: a Explanation: Vo = Vs/1-duty cycle Hence, as duty cycle increases the output voltage increases. 10. For a step-down chopper, when the duty cycle is increased the average value of the output voltage a) increases b) decreases c) remains the same d) none of the mentioned Answer: a Explanation; Vo = Duty cycle x Vs. Hence, output voltage is directly proportional to the duty cycle. -----------------------------------------------------1. For a step-down chopper shown in the figure below, dc source voltage = 230 V and the voltage drop across the chopper = 2 V. Find the value of average output voltage for a duty cycle of 0.4.

a) 230 V b) 92 V

c) 90 V d) 91.2 V Answer: d Explanation: The voltage drop across the switch occurs only when it is on. Vo = (Vs – 2) x Duty cycle = 91.2 V. 2. For the step-down chopper shown in the figure below, dc source voltage = 230 V and the voltage drop across the chopper = 2 V. Find the value of rms output voltage for a duty cycle of 0.4.

a) 2300 W b) 2097 W c) 2560 W d) 5290 W Answer: b Explanation: Power input to the chopper is Vs . Io Vs = 230 V Vo = (Vs – 2) x Duty cycle = 91.2 V Io = Vo/R = 91.2/10 Pi = Vs.Io = 2097.6 W.

a) 91.2 V b) 230 V c) 144.2 V d) 145 V Answer: c Explanation: The voltage drop across the switch will occurs only when it is on. Vo (rms) = √0.4 x (230 – 2) = 144.2 V.

5. The below given chopper circuit is that of a

3. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the power delivered to the load of R = 10 Ω. Duty cycle = 40 %. Take voltage drop at the switch to be 2 V.

a) step-up chopper b) step-down chopper c) step-up/step-down chopper d) none of the mentioned Answer: c Explanation: It is a step-up/step-down chopper. By controlling the duty cycle it can be made to behave like a step-up or a step down chopper.

a) 1 kW b) 2.08 kW c) 569 W d) 207 W Answer: b Explanation: Vo (rms) = √0.4 x (230 – 2) = 144.2 V P = 144.22/10 = 2079.3 W = 2.08 kW. 4. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the power input to the chopper If load of R = 10 Ω is connected. Duty cycle = 40 %.

6. The expression for a step-up/step-down chopper with α as the duty cycle and Vs as the dc input voltage is a) Vs/1 – α b) α x Vs c) Vs (α/1-α) d) Vs (α/1+α) Answer: c Explanation: A step-up, step-down chopper can behave as a step up chopper for α > 0.5 and step –down chopper for α < 0.5. 7. For a step-up/step-down chopper, if the duty cycle > 0.5 then a) Vo = Vs b) Vo < Vs c) Vo > Vs d) None of the mentioned Answer: c Explanation: Vo = Vs (α/1-α)

For α is greater than 0.5, the chopper behaves as a step-up chopper. Hence, Vo > Vs 8. A step down chopper has Vs = 230 V and R = 10 Ω. For a duty cycle of 0.4, the power taken by the chopper is 2097 Watts. Find the chopper efficiency. Take the voltage drop across the chopper switch as 2 V. a) 98 % b) 89.96 % c) 99.14 % d) 96.54 % Answer: c Explanation: Vo (rms) = √0.4 x (230 – 2) = 144.2 V Po = 144.22/10 = 2079.3 Watts Pi = 2097 Watts η = Po/Pi = 99.14 %. 9. A step down chopper has input dc voltage of 220 V and R = 10 Ω in series with L = 65 mH. If the load current varies linearly between 11 A and 17 A, then find the duty cycle α. a) 1 b) 0.4 c) 0.6 d) 0.7 Answer: d Explanation: Average load current = 11+17 / 2 = 14A Average load voltage = IxR = 14×10 = 140V But, Vo = α x Vs Therefore, α x 220 = 140, α = 0.7. 10. For a step-up/step-down chopper, if α (duty cycle) = 0.5 then a) Vo = Vs b) Vo < Vs c) Vo > Vs d) none of the mentioned Answer: a Explanation: Vo = Vs (α/1-α) For α = 0.5 Vo = Vs x (0.5/0.5). -----------------------------------------------------1. In the figure shown below, the capacitor C is used to

a) maintain the load voltage constant b) protect the load

c) protect the chopper switch (SW) d) maintain the load current constant Answer: a Explanation: It is a step-up/step-down chopper, the C maintains the load voltage constant. 2. In the below shown step-up/step-down chopper circuit, when the switch (SW) is on q4.png”>

a) current through inductor rises and the load voltage is zero b) current through inductor rises and the load voltage is Vs c) current through inductor decays and the load voltage is zero d) current through inductor decays and the load voltage is Vs Answer: b Explanation: During Ton, current flows from Vs-CH-L-Vs. Current through L rises from I1 to I2 and Vs appears across the inductor and load as they are in parallel. Voltage drop at the diode has to be considered as zero. 3. For a step-up/step-down chopper, if α (duty cycle) < 0.5 then a) Vo = Vs b) Vo < Vs c) Vo > Vs d) none of the mentioned Answer: b Explanation: Vo = Vs (α/1-α) For duty cycle is less than 0.5, the circuit behaves like a step down chopper. Hence, Vo < Vs. 4. In the below shown step-up/step-down chopper circuit, when the chopper switch (SW) if first on and then off, then the current flows from path q4.png”>

a) Vs-CH-L-Vs b) L-Load-D-L c) L-D-Load-L

d) no current flows Answer: b Explanation: When the chopper switch is off, polarity of emf induced in the inductor L gets reversed biased. Emf induced in L forward biases the diode D and L starts to discharge from inductor-load-diode-inductor.

5. For a step-up/step-down chopper, if current increases from I1 to I2 linearly during Ton, then find the energy stored in the inductor during Ton a) zero b) Vs x (I1 + I2) c) Vs x [ (I1 + I2)/2 ] x Ton d) Vs x [ (I1 + I2)/2 ] x T Answer: c Explanation: Iavg = [ (I1 + I2)/2 ] Energy stored = Win = Vs x [ (I1 + I2)/2 ] x Ton. 6. A step-down chopper is also called as a a) first-quadrant chopper b) second-quadrant chopper c) third-quadrant chopper d) fourth-quadrant chopper Answer: a Explanation: It is called as a first quadrant chopper as the current always flows from source to load and the current and voltage both are always positive.

a) during Toff, Vo = (E + Ldi/dt) and D is conducting b) during Toff, Vo = (E + Ldi/dt) and D is not-conducting c) during Toff, Vo = 0 d) during Toff, Vo = Vs Answer: a Explanation: When switch Is off, Vo = E + Ldi/dt exceeds source voltage Vs and D (diode) is forward biased and begins to conduct. 9. The below given chopper is that of a

7. For the type-B chopper shown below a) step-up chopper b) step-down chopper c) step-up/step-down chopper d) depends on the duty cycle Answer: a Explanation: The load voltage Vo = E + L(di/dt) is greater than Vs, hence it is a step-up chopper.

a) during Ton, Vo = 0 and L stores energy b) during Ton, Vo is not 0 and L stores energy c) during Ton, Vo = 0 and L releases energy d) during Ton, Vo is not 0 and L releases energy Answer: a Explanation: During Ton, SW is on and vo = 0 as the switch S.C’s the load. E drives current through L and charges it. 8. For the type-B chopper shown below

10. The type-C chopper or two quadrant type-A chopper has a) type-A and type-B choppers in series b) type-A and type-B choppers in parallel c) two type-A choppers in series d) two type-A choppers in parallel Answer: b Explanation: The type-C chopper is a two quadrant chopper, it operates in the 1st and the 2nd quadrant. It has type-A and type-B choppers connected in parallel.