JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (May/June - 2019 - R13)

Draw static V-I characteristics of Silicon Controlled Rectifier. How the device can be controlled as a switch? What are the conditions required for turn on?

A resonant commutation circuit supply voltage is 200 V. Load current is 10 A and the device turn off time is 20μs. The ratio of peak resonant current to load current is 1.5. Determine the value of L and C of the commutation circuit.

Draw the circuit required to turn on thyristor using UJT. What are significant points to be noted in designing UJT firing circuit? Explain.

A thyristor is connected in series with a source of 100 V (dc) and a load of 20 ohms resistance and 0.5 H inductance. The thyristor is fired with a gate pulse of 50 micro sec. Will the SCR turned on reliably? If not why? The latching current of SCR is 50mA.

Derive an expression for average output voltage of single phase half controlled rectifier with common cathode.

A three phase fully controlled rectifier is supplied at 150 V/phase, 50 Hz, the source inductance being 1.2 mH per phase. Assuming a thyristor voltage drop of 1.5 V and continuous load current of 30 A. determine average load voltage at firing angles (i) 30 deg (ii) 60 deg

Derive an expression for source current of single phase fully controlled rectifier feeding power to R-L load. Assume load current to be continuous.

A 3 phase fully controlled bridge rectifier is operating from a 400 V, 50 Hz supply. There is a FWD across the load. Find the average output voltage for a firing angle of (i) 45 deg (ii) 60 deg (iii) 90 deg

Draw the circuit of load commutated dc-dc converter. Explain its working. Sketch the output voltage waveform and voltage across thyristor for a typical duty ratio.

Two loads having 10 ohm resistance each are to be serviced from a dc supply. The supply voltage is 24 V. Design commutation circuit component when turn off time specification of thyristor(s) is 20 micro sec. The chopper operating frequency is 1 kHz. Choose the duty ratio as per requirement.

Draw the circuit of Jone’s chopper. Explain its operation highlighting method of commutation of thyristor.

Explain the operation of single phase mid-point cyclo-converter. Derive an expression for rms value of output voltage.

A resistance heating load is controlled from a single phase supply using a TRIAC in the phase angle control mode. Determine firing angle when the output power is at (i) 80 % of its maximum (ii) 30% of its maximum.

Explain how the output frequency can be controlled in single phase cyclo converters. What are measures adopted to reduce THD in output voltage?

A single phase load of 9 ohms and 0.03 H inductance, is controlled by inverse-parallel-connected thyristors using phase angle delay. The ac supply voltage is 230 V, 50 Hz. Determine the load current waveform and load power at delay angle of 30 deg.

Explain the operation of 3-phase bridge inverter in 120 degrees conduction mode.

A 400 Hz, single phase, center-tapped inverter is required to generate an output close to sine wave shape from 72 V dc source into 10 ohms load via 1/1 transformer. Determine the required values of capacitance and inductance. Neglect losses.

Describe how sine-pulse width modulation is implemented in inverters. How THD reduction can be accomplished with PWM? Explain.

A three-phase sine-PWM inverter operates from a dc link voltage of 600 volts. For modulation index = 1.0. Compute the rms value of line voltage of fundamental frequency.