THEORY EXAMINATION (SEM–VI) 2016-17 POWER ELECTRONICS

POWER ELECTRONICS (EEE602)

Section-wise Solved Answers & Notes

SECTION – A (10 × 2 = 20 Marks)

Very short answers (2 marks each)

(a) Latching current & holding current

• Latching current: Minimum anode current required to keep SCR ON immediately after triggering.
• Holding current: Minimum anode current below which SCR turns OFF.

(b) Freewheeling diode

A diode connected across inductive load to provide path for load current when main switch is OFF, reducing voltage spikes.

(c) Positive & negative group of thyristors

• Positive group: Conduct during positive half cycle
• Negative group: Conduct during negative half cycle
Used in converters and cycloconverters.

(d) Why anti-parallel diodes are connected in inverter circuits

To allow reverse current flow and protect thyristors from reverse voltage during inductive load operation.

(e) Merits & demerits of cycloconverter

Merits:
• Direct AC-AC conversion                           • Variable output frequency

Demerits:
• Low output frequency                                • High harmonic distortion

(f) Firing angle

The angle (α) at which a thyristor is triggered with respect to zero crossing of supply voltage.

(g) Difference between VSI & CSI

(h) Applications of power electronics

• Motor drives                                        • SMPS
• UPS                                                      • HVDC transmission
• Electric vehicles

(i) Principle of phase control

Output voltage is controlled by varying firing angle of thyristor.

(j) Step-down chopper principle

Output voltage is controlled by varying duty cycle of switching device.

SECTION – B (Any 5 × 10 = 50 Marks)

(a) Dual converter

Dual converter consists of two converters connected back-to-back.
In circulating current mode, both converters operate simultaneously with a reactor limiting circulating current.

(b) Numerical – Single phase full converter

Given:
V = 240 V, I = 15 A, α = 45°

(i) RMS supply current

Is(rms)=I=15AI_{s(rms)} = I = 15 AIs(rms)​=I=15A

(ii) Fundamental input current

I1=22πI=13.5AI_1 = \frac{2\sqrt{2}}{\pi} I = 13.5 AI1​=π22​​I=13.5A

(iii) Displacement factor

cos⁡α=cos⁡45°=0.707\cos \alpha = \cos 45° = 0.707cosα=cos45°=0.707

(iv) Power factor

PF=Distortion factor×cos⁡α≈0.9×0.707=0.636PF = \text{Distortion factor} \times \cos \alpha \approx 0.9 \times 0.707 = 0.636PF=Distortion factor×cosα≈0.9×0.707=0.636 

(c) Two transistor analogy of thyristor

SCR can be represented as two interconnected BJTs (PNP and NPN) providing regenerative feedback.

(d) Series inverter

Uses SCRs, LC circuit for commutation.

Merits:                    • Simple                                    • High efficiency

Demerits:                • Limited frequency range       • Load dependent

(e) Three phase to single phase half wave cycloconverter

Uses three SCRs per group.
Output voltage equation derived based on firing angle and supply voltage.

(f) Unequal voltage sharing in series SCR

Due to unequal leakage currents and junction capacitances.
Static & dynamic balancing circuits are used.

(g) Class D commutation

Uses auxiliary thyristor and capacitor to reverse bias main SCR for turn-off.
(Diagram required in exam)

(h) Single phase series inverter                       SCRs and LC circuit produce AC output.
Output frequency:

f=12(Ton+Toff)f = \frac{1}{2(T_{on} + T_{off})}f=2(Ton​+Toff​)1​ 

SECTION – C (Any 2 × 15 = 30 Marks)

Q3. Effect of source inductance in three phase full converter

Source inductance causes:                              • Commutation overlap
• Reduction in output voltage                         • Input current distortion

Waveforms show overlap angle (μ) and smoother load current.

Q4. Three phase inverter (180° conduction mode)

• Each SCR conducts for 180°                        • Line voltage is stepped waveform

Merits over 120° mode:                               • Higher output voltage
• Better utilization

Demerits:                                                        • Higher harmonic content

Q5. Short Notes

(i) Desired characteristics of controllable switch

• High voltage/current rating                           • Fast switching
• Low losses                                                      • Easy control

(ii) Class D chopper (1st & 4th quadrant)

Allows bidirectional current and voltage for reversible drives.

(iii) Integral cycle control

Controls output by switching whole cycles ON or OFF, used in heating loads.