POWER ELECTRONICS

PART –A                                                                 (14 Marks)

  1. a) What are the necessary conditions for turning ON a SCR?          [2M]    b) What is reactive power input of single phase full converter at α = 300?           [2M]
    1. Give the list of applications of three phase controlled converters.     [2M]
    2. What is the principle of operation of fly-back converter in CCM?     [3M]
    3. What are the difference between VSI and CSI?           [3M]
    4. Draw the waveforms of single phase ac voltage controller with R load with α = 600. [2M]
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PART –B                                                           (56 Marks)

  1. a) What are the types of switching characteristics of SCR? How do you known turn ON [7M] and turn OFF times from the switching characteristics? Explain.
    1. What are the requirements of good gate driver circuits of IGBT and MOSFET? [7M]

Explain.

 

  1. a) Explain the operation of single phase full-wave controlled rectifier using center tapped [7M] transformer with R-L load under continuous mode of operation. Draw the waveforms of output voltage, voltage across SCR and average load current for α = 450.
    1. A single phase fully controlled bridge converter is connected to R-L load with [7M] R = 10 Ω and L = 6 mH. The converter is supplied from 230 V, 50 Hz ac supply.  (i) Determine average and rms load current; (ii) if one SCR of the bridge configuration is open circuited due to fault, what will be the average and rms load current at this condition? 

 

  1. a) Draw the output voltage waveforms and derive the average and rms voltage [7M] expressions of three phase semi converter on discontinuous conduction mode.
    1. A three phase full converter is connected to a resistive load of 10 Ω. If the firing angle [7M] of SCR is α = 450 and it feeds 4 kW power to a resistive load determine the amplitude of maximum line input voltage.

 

  1. a) Explain the operation of boost converter in the CCM mode and obtain the expression [7M] for amplitude of ripple current.
    1. A buck-boost converter has the input voltage of 12 V and it operates at 20 kHz, when [7M]

the average output voltage is 24 V, the average load current is 1.2 A, it is having  L = 300 µH, C= 150 µF. Find duty cycle, ripple current through the inductor, ripple voltage and critical values of L and C.  

  1. a) Discuss various PWM techniques used in inverters. How sinusoidal PWM is useful in [7M] the harmonic elimination?
    1. A 50 Hz single – phase full bridge produces a square wave voltage across load when [7M]

operating from a 300 V DC supply, the AC load consists of a resistance of 30 Ω in series with inductance 15 mH. Determine the frequencies and r.m.s values of the lowest order harmonics in the AC load current. 

 

Explain the operation of a three – phase bidirectional AC voltage controlled feeding [14M] star connected with resistive load. Draw the output voltage waveform with α = 60o and  α = 120o

  1. a) Define holding current.          [2M]    b)         What are the drawbacks of source inductance on the performance of phase       [2M] controlled rectifiers?
  2. Compare continuous and discontinuous modes of operation of converters. [2M]
  3. List any two applications of choppers. [3M]
  4. List differences between CSI and VSI. [3M]
  5. Explain the principle of Integral Cycle control. [2M]

 

  1. a) Explain the various turn-on methods of SCRs.                    [7M]

Following are the specifications of a thyristor operating from a peak supply [7M] voltage of 500 V

 

       Repetitive peak current, Ip = 250 A;  di dva

(    )max  60A/s,(  )max 200V /s         dt  dt         .

Take a factor of safety as 2 for the specifications mentioned above. Design a suitable snubber circuit, if the minimum load resistance is 20 Ω. Take =0.65.

 

  1. a) Explain the effect of source inductance on the performance of a single-phase [7M] full converter with the help of voltage waveforms. Derive an expression for its output voltage in terms of supply voltage, source inductance and load current.
    1. A single phase semi converter is supplied from 230 V, 50 Hz source. The load [7M]

consists of R= 10 Ω and E = 100 V and a large inductor so as to maintain the load current constant. For a firing angle of 450, find i) average output voltage  ii) average output current iii) average and rms values of thyristor currents  iv) input power factor.

 

  1. a) Explain the working of three phase semi converter with relevant wave forms [7M] with highly inductive load for firing angle of 300.
    1. A three-phase, half-wave converter is supplying a load with a continuous [7M]

constant current of 40A over a firing angle range from 0 to 75 deg. What will be the power dissipated by the load at these limiting values of firing angle? The supply voltage is 415 V (line).

 

  1. a) Explain the working of boost converter with relevant waveforms in CCM mode   [10M] and also derive the expressions for critical values of L and C.
    1. The buck regulator has an input voltage of Vs = 15 V. The required average [4M]

output voltage is Va = 5 V at R = 400 Ω and the peak-to-peak output ripple voltage is 10 mV. The switching frequency is 20 kHz. If the peak-to-peak ripple current of inductor is limited to 0.6 A, determine: i) the duty cycle  ii) the filter inductance L, iii) the filter capacitor C.

 

  1. a) With necessary waveforms explain the working of single phase full bridge [7M] inverter with RL load and also derive the expression for RMS value of output voltage.
  1. Single phase half bridge inverter has a resistive load of R= 3 Ω and DC input [7M] voltage of 50 V. Calculate: i) RMS output voltage at fundamental frequency, ii) output power,   iii) Average and peak current of each thyristors.

 

  1. a) Describe the principle of phase control in single phase half wave ac voltage [7M] regulator. Derive the expressions for average and rms value of output voltage for this control.
    1. A single phase full wave ac voltage controller has a load of R= 5 Ω and input [7M] voltage is 230 V, 50 Hz. If the load power is 5 kW, find firing angle delay of SCR and input power factor.
  1. a) Why protection is required during the operation of SCR? [2M]  b) Calculate power delivered to a 1 kW heater which is connected to a single phase half [2M] wave controlled rectifier when α = 300, Supply voltage = 220V.
    1. Give the conduction periods of lower group thyristors in 6 pulse converters for 60 Hz [2M] frequency with α = 600.
    1. Write the advantages of buck-boost converter. [3M]
    2. Mention the advantages of pulse width modulation control strategy.    [3M]
    3. Draw the waveforms of single phase unidirectional ac voltage controller with RL load [2M] with α = 300.

    PART –B                                                           (56 Marks)

    1. a) Describe the switching characteristics of power MOSFET and what are the requirements [7M] of gate drive to get less turn OFF and turn ON times?
      1. Explain the design of snubber circuit used for a SCR, how it provides different voltage [7M] protections?  

     

    1. a) Explain the operation of single phase full-wave controlled rectifier using center tapped [7M] transformer having R-L load and freewheeling diode under discontinuous and continuous conduction modes with the help of wave forms.
      1. A single phase fully controlled bridge converter is supplied from 230 V, 50 Hz ac supply [7M]

    and it is fed to load consisting of R = 10 Ω and large inductance such that load current is constant. When the firing angle is 450, i) calculate average and rms current ii) if the source inductance of 1.5 mH is connected find the average voltage and overlap angle at the same firing angle.

     

    1. a) Derive the average and rms voltage expressions of three phase half-wave controlled [7M] rectifier having R load operated on discontinuous conduction mode.
      1. A three phase full converter is fed by 230 V, 50 Hz, three phase supply, the average load [7M] current is 25A and the load is highly inductive. For firing angle of 600 find average, rms and peak current through the SCR’s.

     

    1. a) Explain the operation of buck converter in the DCM mode and obtain the expression for [7M] amplitude of ripple current.
      1. A buck-boost converter has the input voltage of 24 V and it operates at 30 kHz, when the [7M] duty cycle is 0.25, L = 300 µH, C= 150 µF and the average load current is 1.5 A. Determine average output voltage, peak to peak ripple current through the inductor, peak to peak ripple voltage and critical values of L and C.

 

  1. a) Draw a neat circuit diagram for single phase full bridge inverter feeding inductive load. [7M] What is the function of feedback diodes?
    1. Explain the working of a current source inverter with a neat circuit diagram and [7M]

 

  1. a) A single phase voltage controller has input voltage of 230 V, 50 Hz; the load consists of [7M] a resistance 30 Ω in series with inductance 15 mH, for 6 cycles off and 4 cycles on. Determine the output voltage and input power factor.
    1. Explain the synchronous connection charge control of a Single–phase transformer [7M] connection charger.

 

  1. a) What is meant by reverse recovery time of a SCR? [2M]  b) Calculate power delivered to a 1 kW heater which is connected to a single phase full [2M] wave controlled bridge rectifier, when α = 900, Supply voltage = 220V? 
    1. Give the conduction periods of upper group thyristors in 6 pulse converters for 60 Hz [2M] frequency with α = 300. 
    2. What is meant by fly back mode of operation of buck-boost converter? [3M]
    3. Mention the advantages of sinusoidal pulse width modulation control.            [3M]
    4. Draw the static V – I characteristics of a TRIAC.     [2M]

PART –B                                                           (56 Marks)

  1. a) Describe the switching characteristics of power IGBT and what are the requirements [7M] of gate drive to get less turn OFF and turn ON times?
    1. Discuss about the different turn ON methods of a SCR. [7M]
  1. a) Explain the operation of single phase half-wave controlled rectifier having R-L load [7M] and freewheeling diode under discontinuous and continuous conduction modes with the help of wave forms.
    1. A single phase full converter is used to deliver a constant load current, it is operated [7M] such that overlap angle is 150 for a firing angle, α = 00. Determine the overlap angles for firing angles α = 300, α = 450 and α = 600.

 

  1. a) Describe the operation of three phase full converter feeding an R load and draw the [7M] wave forms for any firing angle which is more than the 900.
    1. A three-phase three pulse controlled rectifier with freewheeling diode DF is fed from [7M]

three phase, 400 V, 50 Hz ac supply and it is connected with a constant current load of 90 A at firing angle of α = 450. Calculate dc output voltage, rms output voltage, average and rms current of free-wheeling diode. 

 

  1. a) Explain the working of buck converter in continuous conduction mode. Derive the [7M] expression for output voltage.
    1. A boost converter has the input voltage of 24 V and it operates at 20 kHz, when the [7M]

average output voltage is 12 V, the average load current is 1.2 A, it is having  L = 300 µH, C= 150 µF. Find duty cycle, ripple current through the inductor, ripple voltage and critical values of L and C ? 

 

  1. a) Explain the working of a three phase inverter with 1200 mode of conduction with three             [7M] phase delta connected resistive load. 
    1. A 50 Hz PWM inverter employs sinusoidal pulse width modulation based on [7M]

sine-triangle comparison.  If the triangular carrier frequency is 15 kHz, what will be the number of pulses per half period in the output waveform and with 80% modulation index what will be the width of the longest pulse? 

 

  1. a) Describe the operation of single phase full wave ac regulator feeding resistive load. [7M] Derive the expression for output voltage.
    1. A single phase ac voltage controller is employed for controlling the power flow from [7M] 230 V, 50 Hz source in to a load circuit consisting of R = 4 Ω and ωL = 3 Ω. Calculate: i) the control range of Firing angle; ii) the maximum power delivered to load and power factor; iii) the maximum values of average and rms SCR currents.

 

 

 

  1. a) Define Latching Current. [2M]  b) Why power factor of semi converter is better than full converter? [2M]
    1. Draw the circuit diagram of three phase M-3 controlled converter. [2M]
    2. List the applications of step up choppers. [3M]
    3. What is shoot through fault? Explain. [3M]
    4. A 1-phase voltage controller has input voltage of 230 V, 50 Hz for 6 cycles on and 4 [2M] cycles off. Determine RMS output voltage.  
  1. a) Explain the operation of snubber circuit and also design the parameters of snubber [7M] circuit.
    1. Draw the two-transistor analogy of a SCR? Explain SCR operation with this [7M] analogy.

 

  1. a) Explain the operation of single phase two pulse midpoint converter with relevant [7M] voltage and current waveforms and also derive the expression for average output voltage.
    1. A single phase semi converter is delivering power to RLE load with R= 5Ω, [7M] L= 10 mH and E = 80 V. The ac source voltage is 230 V, 50 Hz. For continuous conduction, find the average value of output current for a firing angle of 500. If one of the SCR is damaged and open circuited find the new value of average output current on the assumption to continuous conduction. Also sketch the output voltage and current waveforms?

 

  1. a) Explain the working of three-phase half wave uncontrolled rectifier with relevant [7M] wave forms for ‘R’ load.
    1. A Three phase fully controlled bridge converter is connected to a supply voltage of [7M] 230 V per phase and frequency of 50 Hz. The source inductance is 3 mH. The load current on dc side is constant at 15 A. If load consists of a dc source voltage of 400 V having an internal resistance of 1 Ω. Find the firing angle and overlap angle.

 

 

  1. a) Explain the principle of operation and working of buck converter with relevant [10M] waveforms in CCM mode.
    1. A boost regulator has an input voltage of Vs = 5 V. The average output voltage      [4M]

Va = 15 V and the average load current Ia = 0.5 A. The switching frequency is  

25 kHz. If L = 150 µH and C = 220 µF, determine (i) the duty cycle, (ii) the ripple current of inductor ∆I, (iii) the critical values of L and C.

 

  1. a) With necessary waveforms explain the working of single phase half bridge inverter [7M] with RL load and also derive the expression for RMS value of output voltage.
    1. Single phase full bridge inverter has a resistive load of R= 2.4 ohms and DC input [7M] voltage of 48 volts. Calculate: i) RMS output voltage at fundamental frequency, ii) output power,  iii) Average and peak current of each thyristors.

 

  1. a) Explain the operation of a single phase AC voltage controller with a neat circuit [7M] diagram and output wave forms with respect to source voltage waveforms at α = 60 degrees for R-load.
    1. Explain the principle of integral cycle control with relevant waveforms and also [7M] derive the expression for rms value of output voltage, power delivered to load and input power factor.