Power electronics Viva voce or interview questions part-7

 Q-151 What is the maximum firing angle of R-triggering circuit and why?

Ans The maximum firing angle is 90°. This is because the source voltage reaches maximum value of 90° point and the gate current has to reach Ig(min) some where between 0-90°. This limitation means that load voltage waveform can only be varied from α = 0° to α = 90°.

Q-152 What are the disadvantages of R triggering?

Ans Trigger angle a is greatly dependent on the SCR’s Ig(min) and this value varies between SCR’s and it is also temperature dependent. · Maximum triggering angle achievable is 90°.

Q-153 In R-triggering circuit why is Rmin is connected in series with variable resistor?

Ans The limiting resistor Rmin is placed between anode and gate so that the peak gate current of the thyristor Igm is not exceeded.

Q-154 What is the maximum firing angle of RC-triggering and why?

Ans Maximum firing angle is 180°. This is because capacitor voltage and AC line voltage differ in phase. By adjusting the value of R it is possible to vary the delay in turning on the SCR from 0 to 10 msec and hence vary the firing angle from 0° to 180°.

Q-155 What is an UJT and draw its equivalent circuit?

Ans UJT-uni junction transistor. It has only one type of charge carriers. It has three terminals emitter, base 1 and base 2. (‘Duo base’ as it has 2 bases)

Q-156 Why is an UJT used in SCR firing circuit?

Ans The voltage at base 1 of UJT is smaller than the voltage needed to trigger the Scrim the voltage is high, then it will trigger the SCR as soon as the ac supply is on.

Q-157 Why is the isolation needed between Thyristor and firing circuit?

Ans The trigger circuit operates at low power levels (5-20 volts) whereas thyristors operate at high voltage levels (250 volts). Hence if the Thyristor acts as a short the entire 250volts get applied across the firing circuit causing damage. Hence isolation is needed.

Q-158 How is a pulse transformer different from other transformer?

Ans A pulse transformer is one in which the input at the primary is current which is

transformed into a pulse at the secondary. Thus it does not step-up and step-

down as other transformers.

Q-159 What are the features of pulse transformer?

Ans The primary magnetizing inductance is high, coupling efficiency is high, and

interwinding capacitance is low and has greater insulation.

Q-160 What are the advantages of using pulse transformer?

Ans *Multiple secondary windings allow simultaneous gating signals to series and

parallelconnected thyristors.

*Control circuit and power circuit can be isolated.

Q-161 Why is UJT used in SCR firing circuit?

Ans As the UJT works in a mode called as a relaxation oscillator i.e. UJT turns on or

off depending on the charging and discharging of the capacitor. Time constant

can be varied with Chance delay angle can be varied .The UJT thus gives a firing

angle range of 0- 180.Vz is supply to UJT, the discharging current when passed

through the pulse transformer triggers SCR with pulses.

Q-162 Why is the sneer diode used?

Ans The sneer diode provides a constant supply voltage for UJT. It enables

synchronization with zero crossings. Sneer diode acts as a regulator. The sneer

clamps the rectified voltage to vs. to prevent erratic firing. This sneer voltage

acts as a supply for UJT relaxation oscillator.

Q-163 What is meant by ramp control, open loop control or manual control with

respect to UJT firing circuit?

Ans Ramp control-The graph of time period in milliseconds with the firing angle in

degrees is a ramp. The ramp slope can be controlled by the potentiometer.

Manual control-The potentiometer in the kit can be used to get various firing

angles. This is manual control.

Q-164 What is a firing circuit?

Ans It is a circuit, which is used to trigger a device at various instants of time.

Q-165 Why a bridge rectifier is used?

Ans The bridge rectifier gives a full wave rectified output, which is high in efficiency

and least ripple factor.

Q-166 What is the load used?

Ans Load is high power dissipation resistor.

Q-167 What is time constant of a circuit?

Ans Time constant of a circuit=RC where R=resistance C=capacitance It gives the

time of charging and discharging of a capacitor.

Q-168 What are the merits of UJT firing circuit over RC triggering circuit?

Ans * Firing angle remains stable.

*Advantages of pulse transformer.

Q-169 What are the advantages of UJT pulse trigger circuit?

Ans The resistors, capacitors depend heavily on the trigger characteristics of the

Thyristor used. The power dissipation is high due to prolonged pulse. But the

pulse triggering can accommodate wide tolerances in triggering characteristics

by instantaneously overdriving the gate. The power level in such circuits is lower

as the triggering energy can be stored slowly and discharged rapidly when the

triggering is required.

Q-170 Why is UJT used as relaxation oscillator?

Ans The UJT is used as a relaxation oscillator to obtain sharp, repetitive pulses with

good rise time. Also it has good frequency stability against variation in the

supply voltage and temperature.

Q-171 What are the applications of UJT trigger circuits?

Ans *Used to trigger SCR’s in single-phase converters, single-phase ac regulators.

*Used in oscillators

 *Used in timing circuits

Q-172 What is valley voltage?

Ans It is the voltage at which the UJT turns off and the capacitor starts charging

again.

Q-173 What is the discharging path if the capacitor?

Ans The capacitor discharges through emitter, base and primary of the pulse

transformer.

Q-174 What is relaxation oscillator?

Ans When the capacitor discharges to a valley voltage, the UJT turns off and

capacitor starts charging again. This mode of working of UJT is called relaxation

oscillator.

Q-175 What is negative resistance?

Ans After the capacitor charges to Vp it starts discharging. During this period the

voltage V decreases with increase in current, hence this portion of V-I

characteristics is called negative resistance.