1. Define controlled rectifierIt converts fixed AC to variable DC and it uses thyristor as the switching device

2. What are the different types of converter circuits?1. single phase 2. three phaseEach can be further subdivided into half controlled, semi controlled and fully controlled converter circuits.

3. Draw the waveform for 2-pulse converter with resistive load

4. Draw the waveform for 2-pulse converter with RL load

5. Draw the circuit diagram for single phase fully controlled converter bridge circuit.

6. Define inverter operation or inversion modeIn a bridge controlled converter circuit with RL-load, when the firing angle is greater than 90 degree, the average output voltage is negative and it acts as an inverter. i.e. output voltage is positive and current is negative and the converter is operated in the second quadrant.

7. Draw the circuit diagram for three phase fully controlled converter bridge circuit.

8. What are the advantages of three phase converter circuits?1. Distortion on the AC side is reduced. 2. Inductance required in series is reduced 3. Power handling capacity is high9. What are the uses of phase controlled rectifier circuits?Speed control of DC motor drives

10. What are the effects of source inductance?It reduces the average output voltage at the load side and reduces the displacement factor in the source side.

11. Define overlap period or commutation periodDuring overlap period, both incoming and outgoing thyristors are conducting together and the resultant output voltage is zero in single phase converter and it decrease the output voltage in three phase converter.

12. Define overlap angle or commutation angleThe angle during which both incoming and outgoing thyristors are conducting is known as overlap angle.

13. Define AC voltage controllerIt converts fixed AC to variable AC without change in frequency and it uses SCR or TRIAC as a switching device.

14. What are the different control techniques for AC regulator?For power transfer, two types of control are normally used: 1. Phase angle control 2. On-off control. In phase control, thyristor switches connect the load to the AC source for a portion of each cycle of input voltage. In on-off control, thyristor switches connect the load to the AC source for a few cycles of input voltage and then disconnect it for another few cycles.

15. What are the applications of AC regulator circuits?1. Industrial heating 2. on-load transformer tap changing 3. light control 4. speed control of poly phase induction motor

16. Define pulse numberIt is the number of discrete output pulses produced during one cycle of the AC supply waveform.

17. What are the functions of freewheeling diode?It prevents reversal of load voltageIt transfers the load current away from the main rectifier thereby allowing all of its thyristors to regain their blocking states.

18. What are the advantages of freewheeling diode? Or what are the effects of freewheeling diode in converters?

Input power factor is improved Load current waveform is improved. As a result, load performance is better.

19. Define input displacement angle It is defined as the angular displacement between the fundamental component of the AC input current and the input voltage.

20. Define Input displacement factorIt is the cosine of the input displacement angle

21. Define input power factorIt is the ratio of the total mean power to the total RMS input volt ampere.Output power = VrmsI1cos Φ1

Input VA = Vrms Irms

PF = Output power / Input VA

22. Define input current distortion factorThe distortion factor is defined as the ratio of the RMS amplitude of the fundamental component to the total RMS amplitude.

Distortion factor

23. Define input Harmonic factorIt is the ratio of the total harmonic content to the fundamental component.

24. Define ripple factor Voltage ripple factor is defined as the ratio of the net harmonic content of the output voltage to the average output voltage.

25. What is the effect of source inductance on the output voltage in a three phase bridge rectifier?The effect of source inductance reduces the average DC output voltage.Voltage drop due to overlap is 3LsIo/

26. What is the effect of inductive load in the performance of a three phase bridge rectifier?Load current is continuousAverage output voltage is reduced

27. What are the parameters that are evaluated in rectifiers?

1. Displacement factor 2. Input power factor 3. Distortion factor 4. Total harmonic factor 5. Ripple factor

28. Write output voltage relation of single phase full converter. Also draw its variation with firing angle.Vdc =Vo = 2Vm cos/For waveform refer notes

29. How rectification and inversion are possible in phase controlled thyristor converter?In fully controlled bridge converter with RL-Load, when firing angle is less than 90 degree, the average output voltage is positive and the converter is operated in rectification mode. When firing angle is greater than 90 degree, the average output voltage is negative and the converter is operated in inversion mode.

UNIT - II

1. What is meant by phase controlled rectifier?It converts fixed ac voltage into variable dc voltage.

2. Mention some of the applications of controlled rectifier.a. Steel rolling mills, printing press, textile mills and paper mills employing dc motor drives.b. DC tractionc. Electro chemical and electro-metallurgical processd. Portable hand tool drivese. Magnet power suppliesf. HVDC transmission system

3. What is the function of freewheeling diodes in controlled rectifier?It serves two process.

a. It prevents the output voltage from becoming negative.b. The load current is transferred from the main thyristors to the

freewheeling diode, thereby allowing all of its thyristors to regain their blocking states.

4. What are the advantages of freewheeling diodes in a controlled in a controlled rectifier?

a. Input power factor is improved.b. Load current waveform is improved and thus the load

performance is better.

5. What is meant by delay angle?The delay angle is defined as the angle between the zero crossing of the input voltage and the instant the thyristor is fired.

6. What are the advantages of single phase bridge converter over single phase mid-point converter?

a. SCRs are subjected to a peak-inverse voltage of 2Vm in a fully controlled bridge rectifier. Hence for same voltage and currnt ratings of SCRs, power handled by midpoint configuration is about

b. In mid-point converter, each secondary winding should be able to supply the load power. As such, the transformer rating in mid-point converter is double the load rating.

7. What is commutation angle or overlap angle?The commutation period when outgoing and incoming

thyristors are conducting is known as overlap period. The angular period, when both devices share conduction is known as the commutation angle or overlap angle.

8. What are the different methods of firing circuits for line commutated converter?

a. UJT firing circuit.b. The cosine wave crossing pulse timing control.c. Digital firing schemes.

9. Give an expression for average voltage of single phase semiconverters.

10. What is meant by input power factor in controlled rectifier?The input power factor is defined as the ratio of the total mean input power to the total RMS input volt-amperes.

where V1 = phase voltage, I1 = fundamental component of the supply current, ϕ1 = input displacement angle, Irms = supply rms current.

11. What are the advantages of six pulse converter?a. Commutation is made simple.b. Distortion on the ac side is reduced due to the reduction in

lower order harmonics.c. Inductance reduced in series is considerably reduced.

12. What is meant by commutation?It is the process of changing the direction of current flow in a particular path of the circuit. This process is used in thyristors for turning it off.

13. What are the types of commutation?a. Natural commutationb. Forced commutation

14. What is meant by natural commutation?Here the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off.

15. What is meant by forced commutation?In this commutation, the current flowing through the thyristor is forced to become zero by external circuitry.