Q.1. What is the function of bleeder resistor in a power supply?

Answer: Bleeder resistor not consume too much power while supply is on and voltage decay quickly to safe level when the supply is switched off. So, the function of bleeder resistor is to ensure a minimum current drain in the circuit.

Q.2. What are Mc Murray inverters?

Answers: Mc Murray inverters are force-cumulated inverters. In such inverters, the function of capacitor is to provide the energy required for cumulating the main thyristors. Inductance L is for limiting di/dt to a safe value in main and auxiliary thyristors.

Q.3. How can the third harmonic current be filtered in thyristor-controlled reactor?

Answer: In three-phase circuits, the thyristor controlled reactor is connected in delta so that triplex harmonics are confined to this closed delta and do not enter the a.c system.

Q.4. Why PWM switching is preferred in voltage source inverters?

Answer: Using PWM, output voltage can be controlled by varying pulse width and therefore, harmonic content in the output waveform. PWM switching can reduce low frequency harmonics hence reducing size of filter.

Q.5. Define firing angle?

Answer: Firing angle may be defined as –

• The angle between the instant thyristor would conduct if it were a diode and the instant it is triggered.
• The angle measured from the instant that gives the largest average output voltage to the instant it is triggered.

Q.6. What is the main use of Triac?

Answers: A Triac is a bidirectional thyristor with three terminals. It is used extensively for the control of power in ac circuits.

Q.7. What is forced commutation?

Answer: In forced commutation, the commutating components L and C do not carry load current continuously. In this commutation method, forward current of the thyristor is forced to zero by external circuitry called commutation circuit.

Q.8. What are the requirements for an SCR to be triggered by a gate pulse?

Answer: Requirements for an SCR to be triggered by a gate pulse are –

• SCR should be in forward biased mode.
• A positive gate voltage between gate and cathode.
• The gate pulse width should be chosen to ensure that anode current rises above the latching current.

Q.9. Why the feedback diodes are connected in antiparallel with thyristors in inverter circuits?

Answer: when in the inverter circuit, load is other than resistive, then load current is not in phase with load voltage. For such type of load, diodes are connected in anti parallel with thyristors to allow the current to flow when the main thyristors are turned off.

Q.10. What is the advantage of adding an inductor L in series to the load circuit of an SCR converter?

Answer: Current through an inductor L cannot change instantaneously, health and inductor L in series with the load circuit does not allow current to fall to zero and prevents discontinuous operation of the SCR converter. The load performance deteriorates if load current becomes discontinuous. Using inductance of very high value results into almost constant and ripple free load current.

Q.11. What are the different forced commutation methods used for improving the power factor in controlled AC- DC rectifiers?

• Resonant pulse commutation (or class B commutation).
• Complementary commutation (or Class C commutation).
• Impulse commutation (or class D commutation).

Q.12. How maximum string efficiency can be achieved?

Answer: String efficiency is a term that is used for measuring the degree of utilization of SCRs in a string. For obtaining highest possible string efficiency, the SCRs connected in series / parallel string must have identical IV characteristics. So the voltage distributed across SCR should be uniform.

Q.13. Why circulating current inductor is required in a dual converter?

Answer: Though average output voltages of the two converters in dual converter are equal, get their instantaneous voltages Vo1 and Vo2 are out of phase which results in a voltage difference and as a consequence, a large circulating current flow between the two converters but not throw the load. This circulating current is limited by inserting an inductor between the two converters.

Q.14. Why switched mode power supplies are preferred over the continuous types?

Answer: SMPS possesses higher efficiency because of its high frequency operation and suitable for low power application only.

Q.15. Which type of pulse can be used to reduce the turn on time of an SCR?

Answer:  A rectangular pulse of high amplitude and narrow width. The turn on time is reduced by using higher values of gate current. When Gate current is several times higher than the minimum Gate current required, thyristor is said to be hard fired or overdrive. Hard firing or over driving of a thyristor reduces its turn on time.

Q.16. How an SCR triggered by a current pulse applied to the gate cathode can be turned off?

Answer: If current pulse is applied to Gate cathode then the SCR can only turned off by operating it in Reverse blocking state or by reversing the polarity of the anode and cathode voltage.

Q.17. What is GTO?

Answer:  GTO is pnpn device, can be turned on like an ordinary thyristor by a pulse of positive Gate current, but it can be turned off easily by a negative Gate pulse of appropriate amplitude.

Q.18. What are the advantages of a GTO as compared to conventional thyristor?

Answer:  The advantages of a GTO as compared to conventional thyristor are as follows –

• GTO has faster switching speed.
• Its surge current capability is comparable with an SCR.
• It has more di/dt rating at turn on.
• GTO circuit has lower size and weight as compared to SCR circuit unit.
• GTO has higher efficiency.
• GTO unit has reduced acoustical electromagnetic noise due to elimination of commutation chokes.

Q.19. What is dc chopper?

Answer:  A DC chopper is a static device that converts fixed DC input voltage to a variable DC output voltage directly.

Q.20. What is the difference between latching current and hold current for the thyristor?

Answer: Latching current- The minimum value of anode current which the thyristor must attain during turn on process to maintain conduction even after the removal of gate pulse is called latching current.

Holding current – It is defined as the minimum value of anode current below which it must fall for turning off the thyristors. In general latching current is greater than holding current.

Q.21. what happens then the cathode of a thyristor is made more positive than its anode?

Answer:  When Cathode is made more positive with respect to anode, thyristors operates in Reverse blocking mode, and the device behaves as if two diodes are connected in series with reverse voltage applied across them.

Q.22. How thyristors can be switched from a non conducting state to a conducting state?

Answer:  With anode positive with respect to cathode, thyristor can be turned on by any one of the following techniques –

• Forward voltage triggering.
• Gate triggering.
• dv/dt triggering.
• Light triggering.
• Temperature triggering.

Q.23. Why input power factor of a single phase half controlled bridge rectifier is higher than that for a fully controlled bridge rectifier supplying an RL load for the same firing angle?

Answer:  Input power factor of a single phase half bridge rectifier is higher than that for a fully controlled bridge rectifier of RL load for the same firing angle because some energy is returned by the inductor to source in a fully controlled bridge rectifier. So that power delivered to load will decrease. Where, Power factor of input supply = supplied power to load/   source volt ampere.

Q.24. What is a unijunction transistor?

Answer:   A unijunction transistor is made up of an n-type silicon base to which p-type emitter is embedded. It has three terminals the emitter, base one B1 and base two B2 between base B1 and B2, the unijunction behave like ordinary resistance.

Q.25. Why is a GTO thyristor preferred over SCR in chopper and inverter circuits?

Answer:  Due to its fastest switching speed and its elimination of forced commutation losses, mainly we use it in inverter and chopper circuits.

Q.26. Why efficiency of stepper motor is low?

Answer: Unlike Dc motors, stepper motor current consumption is independent of load. They draw the most current when they are doing no work at all. Because of this, they tend to run hot.

Q.27. What are the advantages of using freewheeling diodes?

Answer:  The advantages of using freewheeling diodes are: Input power factor is improved. Load current waveform is improved and as a result load performance is better. As energy stored in L is transferred to R during the freewheeling period, overall converter efficiency improves. Freewheeling diode prevents the load voltage V0 from becoming negative.

Q.28.How di/dt protection is achieved in a thyristor?

Answer:  If the rate of rise of anode current i.e. di/dt  is large as compared to the spread velocity of carriers local hotspots will be formed near the gate connection on account of higher current density. This localized heating may destroy the thyristor. Therefore, the value of di/dt can be maintained below acceptable limit by using a small inductor in series with the anode circuit.

Q.29. Give one example of uncontrolled turn on and off?

Q.30. When turn on of a thyristor takes place?

Answer:  When turn on of a thyristor is required, a positive Gate voltage between gate and Cathode is applied. With gate current established charges are injected into the inner player and voltage at which forward breakdown occurs is reduced. Higher the gate current, lower is the forward breakdown voltage.

Q.31. An SCR is in conducting state, a reverse voltage is applied between anode and cathode, but it fails to turn off. What could be the reason?

Answer: Holding current is the minimum value of anode current below which it must fall for turning off the thyristor.

Q.32. How SCR can be turned on?

Answer:  The SCR can be turned on by following ways: dv/dt Triggering, i.e. applying anode voltage at a sufficiently fast rate. When anode voltage is greater than Vbo i.e., forward break over voltage SCR can be turned on. Applying sufficiently large Gate current will turn on SCR. But increasing the temperature of SCR to sufficiently large value will damage the device.

Q.33. Which one is the fastest switching device among JFET, BJT, MOSFET and triode?

Answer:  MOSFET is the fastest switching device, because it is a majority carrier device it does not have minority carriers, which take long time to settle down.

Q.34. Which snubber circuit is used to shape the turn on switching trajectory of thyristor and / or to Limit di/dt during turn on?

Answer: L-R snubber polarized circuit is used to shape the turn on switching trajectory of thyristor and / or to limit di/dt during turn on.

Q.35. To turn off a GTO what is required at the gate?

Answer:   A negative Gate current of 20 to 25% of anode current is required to turn off GTO.

Q.36. Why the use of multiphase rectifier is preferred in place of single phase rectifier?

Answer:  Multiphase rectifiers are preferred because:

• Higher DC voltage.
• Better Transformer utilization factor.
• Better input power factor.
• Less ripple content in output current.
• Lower size of filter circuit parameters because of higher ripple frequency.

Q.37.Define input power factor of the converter circuit?

Answer:  This is defined as the ratio of the total mean input power to the total RMS Apparent power input to the converter.

Q.38. What is the waveform of the current flowing through the diode in a buck boost converter?

Q.39.What is the relation between the output power of a single phase full bridge inverter and a single phase half bridge inverter?

Answer:  Output voltage of a single phase full bridge inverter is two times of output voltage of single phase half bridge inverter.

Q.40. What is the effect of blanking time on output voltage in PWM inverter?

Answer:   High order time harmonics in output voltage.

Q.41. What is the reason of using a tertiary winding in a forward converter?

Answer: In forward converter a tertiary winding is added to the Transformer so that the magnetizing current can return to the input source vs. when the transistor turns off. The Transformer magnetizing current must be reset to zero at the end of each cycle. Otherwise, the Transformer can be driven into saturation, which can cause damage to the switching device.

Q.42. In a single phase to single phase cycloconverter, the magnitudes of harmonic components are quite large. How they can be reduced?

Answer:  In a single phase to single phase cycloconverter, the magnitude of harmonics is quite large, it can be reduced by using a 3 phase input supply.

Q.43. Which device is used for switching in a switched mode power supply?

Answer: The device used for switching in a switched mode power supply is MOSFET because it is used in high frequency operations. In SMPS we require high frequency operating device like MOSFET.

Q.44. What is the use of inter group reactor in a single phase cycloconverter circuit?

Answer: Though the output voltages of the two converters have the same average value, there output voltage waveforms as a function of time are however different and as a result, there will be a net potential difference across the two converters which results in a circulating current. This circulating current can be avoided by inserting intergroup reactor. Therefore, the intergroup reactors are used to limit circulating current.

Q.45. How many switches are used to construct a three phase cycloconverter?

Answer: Three phases of three phase cycloconverter use three sets of three phase half wave circuits and each circuit requires 6 thyristors. So, total 18 thyristors are required.

Q.46. Why the performance of a phase controlled converter is degraded for large values of firing angle α?

Answer:  As the output voltage of phase controlled converter depends on firing angle (decreases with increase in Alpha), the performance of converter degraded for large value of α.

Q.47. Which converter can operate both in 3 pulse and 6 pulse modes?

Answer:  A 3 phase semi converter has the unique feature of working as a 6 pulse converter α less than 60 degree and a three pulse converter for α greater than or equal to 60 degree.

Q.48. What do you mean by slewing in stepper motor?

Answer: When a stepper motor operates at high speed, then it is termed as slewing.

Q.49. For what firing angles the thyristor controlled inductor will work as a variable inductor or as fixed inductor?

Answer:  A thyristor controlled inductor work as a fixed inductor when (α≤ 90o) and as a variable inductor when (α > 90o).

Q.50. What do you understand by string efficiency related to thyristors?

Answer:  String efficiency is a term that is used for measuring the degree of utilization of SCR in a string.