Power Electronics PYQ-2021

 B.E.U. 5th Semester PYQ 2021

 (a) The most suitable device for high frequency inversion in SMPS is

(i) BJT 

(ii) IGBT 

(iii) MOSFET 

(iv) GTO

The most suitable device for high-frequency inversion in SMPS is a MOSFET.

MOSFETs are preferred due to their fast switching speed, low switching losses, high voltage capability, compact size, and low on-resistance.

(b) As compared to power MOSFET, a BJT has

(i) lower switching losses but higher conduction loss 

(ii) higher switching losses and higher conduction loss 

(iii) higher switching losses but lower conduction loss 

(iv) lower switching losses and lower conduction loss

The correct answer is (i) higher switching losses and higher conduction loss.

Compared to power MOSFETs, BJTs (Bipolar Junction Transistors) generally have higher switching losses. This is because BJTs have a longer turn-on and turn-off time, leading to increased losses during the switching transitions.

Additionally, BJTs have higher conduction losses compared to power MOSFETs. BJTs have a higher on-resistance (Rdson) when fully turned on, resulting in increased power dissipation during conduction.

Therefore, the correct answer is (i) higher switching losses and higher conduction loss for BJTs as compared to power MOSFETs.

(c) In a thyristor-controlled rectifier, the firing angle of thyristor is to be controlled in the range of

(i) o° to 90° 

(ii) o° to 180° 

(iii) 90° to 180 ° 

(iv) 90° to 270°

The firing angle range in a thyristor-controlled rectifier is from 0° to 180°

because this allows control over the portion of the AC input waveform that is rectified. By varying the firing angle within this range, the timing of triggering the thyristor can be adjusted to regulate the output DC voltage.

This range covers the full cycle of the AC input waveform and provides flexibility in controlling the power flow to the load. Options (i) 0° to 90°, (iti) 90° to 180°, and (iv) 90° to 270° either restrict or extend the firing angle beyond the standard control range, which is why they are not valid choices.

(d) In a single-phase full converter, if load current is I and ripple free, then average and r.m.s. values of thyristor current are

(i) (1/2) l, (1/2) | 

(ii) (1/4)l, (0.5) | 

(iii) (1/3)l, (1/3)| 

(iv) l, (1/3)l

The correct answer is (ii) (1/4)l, (0.5)l.

In a single-phase full converter, when the load current is ripple-free, the average value of the thyristor current is (1/4) times the load current (l), and the root mean square (r.m.s.) value of the thyristor current is (0.5) times the load current (l).

The average value of the thyristor current is (1/4)| because, in a full converter, each thyristor conducts for one-quarter of the input cycle.

The r.m.s. value of the thyristor current is (0.5| because each thyristor conducts for half of the input cycle, and the r.m.s. value is proportional to the square root of the average value.

Therefore, the correct answer is (ji) (1/4)I, (0.5)I for the average and r.m.s. values of the thyristor current in a single-phase full converter with a ripple-free load current.

• Class A commutation is known as self-commutation because the device being commutated controls its own commutation.
• Class B commutation is known as current commutation because the current flowing through a device is used to commutate it.
• Class C commutation is known as complementary-impulse commutation because it involves the use of complementary devices to generate commutation impulses.
• Class D commutation is known as voltage commutation because the voltage across a device is used to commutate it.

Therefore, the correct answer is A-4, B-6, C-3, D-1.

(f) For a step-down d.c. chopper operating with discontinuous load current, what is the expression for the load voltage? (where K is duty ratio of chopper)

(i) Vo = Vd.c. x K 

(ii) Vo=Vd.c. / K 

(iii) Vo = Vd.c. / (l-K)

(iv) Vo = Vd.c. × (I-K)

The correct answer is indeed (i) Vo = Vd.c. x K.

In a step-down DC chopper operating with discontinuous load current, the load voltage (Vo) is proportional to the duty ratio (K). The duty ratio represents the fraction of the total switching period during which the chopper is ON. Therefore, the load voltage is given by multiplying the input

DC voltage (Vd.c.) by the duty ratio (K).

(g) In d.c. choppers, the waveforms for input and output voltages are respectively

(i) discontinuous and continuous 

(ii) both continuous 

(iii) both discontinuous 

(iv) continuous and discontinuous

In DC choppers, the waveform of the input voltage is continuous, meaning it remains constant without any interruptions. On the other hand, the waveform of the output voltage is discontinuous, consisting of pulses due to the chopping action.

Therefore, the correct answer is (iv) continuous and discontinuous for the waveforms of input and output voltages in DC choppers.

(h).A single-phase, voltage source, square wave inverter feeds a pure inductive load. The waveform of the current will be

(i) sinusoidal 

(ii) rectangular 

(iii) trapezoidal 

(iv) Triangular

When a single-phase, voltage source, square wave inverter feeds a pure inductive load, the waveform of the current is triangular. This is because the inductive load causes the current to ramp up and ramp down during each half-cycle of the square wave input voltage.

The inductance in the load resists changes in current, resulting in a triangular waveform where the current increases linearly during the ON period of the square wave and decreases linearly during the OFF period.

Therefore, the correct answer is (iv) Triangular for the waveform of the current in a single-phase, voltage source, square wave inverter feeding a pure inductive load.

(j) The most suitable solid-state converter for controlling the speed of the three- phase cage motor at 25 Hz is

(i) cycloconverter 

(ii) current source inverter 

(iii) voltage source inverter 

(iv) load commutated inverter

The most suitable solid-state converter for controlling the speed of a three-phase cage motor at 25 Hz is (i) cycloconverter.

A cycloconverter is a solid-state device used for frequency conversion in

AC systems. It can directly convert the frequency of the input AC power to a lower frequency. In this case, since the desired frequency is 25 Hz, a cycloconverter is the most suitable choice for controlling the speed of the three-phase cage motor at that frequency.

Therefore, the correct answer is (i) cycloconverter as the most suitable solid-state converter for controlling the speed of the three-phase cage sof6r at 25 Hz.