POWER ELECTRONICS
Very Short Answer type question ( about 30-40 words)
1. Define SCR or Thyristor and draw its symbol.
Ans1: SCR stands for Silicon Controlled Rectifier. It is a four layer (p-n-p-n) semiconductor and three terminals (anode,cathode and gate) devices.
Symbol:
2. Define Latching current.
Ans2: Latching Current (IL): It is the minimum anode current value to keep the thyristor in conducting state after the gate pulse is removed.
3. Define holding current.
Ans3: Holding Current (IH): It is the minimum anode current at which thyristor continue to conduct. If the anode current is less than IH , the thyristor will be turned off.
4. What is thyristor commutation?
Ans4: It is the process to turn – off the thyristor. There are three ways to turn- off the thyristor i.e. natural commutation, forced commutation and gate turn-off process (by applying negative gate current).
5. Define UJT.
Ans5: UJT stands for Uni- Junction Transistor. It is a two layer (PN) and three terminal solid state device. The pulse obtained by UJT relaxation oscillator is used to trigger the thyristor.
6. Why a heat sink is required with a thyristor?
Ans6: To keep the junction temperature constant, thyristor requires a heat sink to transfer the heat energy from thyristor to atmosphere.
7. What is Triac?
Ans7: Triac is a bidirectional device that conduct in both direction. It can be used for speed control of small single phase series & induction motors.
8. SCR can be used as unidirectional switch.
9. What is dv/dt triggering of SCR?
Ans9: dv/dt triggering:- If the rate of increase of voltage dv/dt is made more than the value of critical rate of increase of voltage, then the SCR starts conducting. This method is known as dv/dt triggering, but it is not desirable because a high charging current may damage the thyristor.
10. What do you mean by ‘firing’ of SCR?
Ans10: Firing means the turning – ON of an SCR. The output voltage of SCR is controlled by controlling firing angle (α) of SCR.
11. What do you mean by gate triggering method of SCR?
Ans11: When the SCR is turned – ON by applying the positive signal to the gate of a forward biased SCR . This type of triggering is known as Gate Triggering. As the gate current is increased, the forward blocking voltage is reduced.
12. What is forced commutation of thyristor?
And12: When the SCR has to be turned – off by a special commutation circuit using some extra components. This technique is known as forced commutation.
13. Define GTO.
Ans13: GTO stands for Gate turn OFF thyristor. The GTO can br turned – ON by a positive gate signal and can also be turned off by a negative gate signal.
Symbol of GTO:
14. What are controlled rectifier?
Ans14: When the rectifier are built with SCRs or the combination of SCRs and Diodes is known as controlled rectifier which convert a.c. input voltage in to a controllable dc output voltage by controlling firing angle of SCR.
15. What are the effects of firing angle on an converter?
Ans15: When the firing angle is in between o˚ to 90˚, then a converter is acts as rectifier and when firing angle is greater than 90˚ but less than 180˚, then converter acts as an Inverter.
16. Why 3- phase controlled rectifiers are preferred over single phase rectifier? OR what is the main advantages of 3- phase controlled rectifier than single phase rectifier?
Ans16: The main advantage of 3- phase controlled rectifier is that they produce less ripple voltage than a single phase controlled rectifier.
17. What is inverter?
Ans17: Inverter is an electronic circuit that converts direct current (D.C) to alternating current (A.C).
18. Define chopper?
Ans18: Chopper is an electronics circuit that converts a fixed DC voltage to a variable DC voltage, it is also known as DC-DC converter.
19. What is current source inverter(CSI) & voltage source inverter(VSI)?
Ans19: Current Source Inverter(CSI): When the inverter is fed from a DC source voltage with large inductance in series so that the current remains constant.
Voltage Source Inverter(VSI):- When the inverter is fed from a DC source voltage with negligible internal resistance , then it is called Voltage Source Inverter.
20. Define Duty Cycle.
Ans20: It is the ratio of Turn on time of chopper (Ton) to the sum of turn on & turn off (Toff) of chopper.It is denoted by α
α = Ton ___
Ton+Toff
21. What are cycloconverter?
Ans21: It converts AC at one frequency to AC at another frequency. It consists of positive group & negative group. The positive group provides positive half cycles and negative group provides negative half cycles of output.
22. Define Dual Converter?
Ans23: When two full converter in antiparallel connected to the same load, such arrangement is known as Dual Converter. It is generally used in the application where reversible DC is required.
23. A class – D chopper operates in 1st and 4th quadrant.
24. Writes the application of inverter?
Ans24: i) Inverter can be designed to provide 115VAC from 12VDC source provided in automobile.
ii) It is used in UPS.
iii) It is used in variable frequency drives and electric vehicle drives.
iv) It is used in HVDC power transmission.
25. What is electric drive?
Ans25: An electric motor together with its control equipment and energy-transmitting mechanism is called “electric drive”.For example, a celling – fan motor with its speed regulator is called electric drive.
26. Name two methods of speed control of D.C. motor.
Ans26:
Chopper Drive
Chopper Drive
Armature voltage control method and field control method
27. What are the advantages of A.C. drves over D.C. drives?
Ans27: Advantages of AC drives:- 1. A.C. motor requires less maintenance than D.C. motors.
2. For the same power rating , ac motors are lighter in weights than dc motors.
3. Cost of a.c. motors is less as compared to equivalent d.c.
4. A.C motors can be used in hazardous environments like chemicals & petrochemicles etc. While d.c motors are not suitable for such environments because of sparking.
28. Name the methods of speed control of 3- phase induction motor?
Ans28: i) Stator Voltage Control method
ii) Variable – Frequency control method
iii) Cycloconverter controlled A.C. Drive
29. What is VFD?
Ans29; Variable -
Frequency Drive(VFD):-
Frequency Drive(VFD):-
30. What is UPS?
Ans30: UPS stands for Uninterrupted Power Supply. The system which provide an interruption free supply of a.c is called UPS.
31. What are the types of UPS?
Ans31: UPS are of three types; OFF-Line UPS, ON-Line UPS and Line interactive UPS.
32. What is SMPS?
33. SMPS satands for Switched Mode Power Supply. In low to medium power requirement (more than 50W), a dc powr supply is required which contains negligible ac ripples. SMPS is a multistage dc supply and is used for higher ratings.
34. Write the application of UPS?
Ans34; UPS are used in : computers
Data processors, data transmitters,
Microwave relay station,
Nuclear reactor control and communication etc.
35. What are the types of storage devices of an UPS?
Ans35: Battery are the main storage devices of an UPS. Battery may be of wet type and dry type, such as Lead Acid Battery(wet type), Nickel – Cadmium battery(dry type) and Sealed Maintenance Free battery.
36. In a dual converter, one converter operates as rectifier and other as Inverter.
37. What is parallel inverter?
Ans37: In parallel inverter, commutating components i.e. capacitor is connected parallel across the load through transformer.
38. What is series inverter?
Ans38: In Series inverter, commutating components i.e. inductors and capacitors are connected in series with the load, forming the under damped circuit.
