iv the peak to peak ripple on the dc side capacitor voltage by KQ1YlacT

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									                         THE UNIVERSITY OF NOTTINGHAM


             SCHOOL OF ELECTRICAL AND ELECTRONIC ENGINEERING


                          POWER ELECTRONICS 2 (H5CPE2)


    EXAMPLES SHEET 3 - INVERTERS AND HIGH POWER FACTOR RECTIFIERS


1    A single phase inverter is fed from a 200V dc source. The inverter is operated with a
     quasi-square-wave switching scheme which achieves voltage control by adjusting the
     phase shift between the two bridge halves. The output voltage is set to maximum (ie
     each side switching in anti-phase). The inverter output frequency is 50Hz and the load
     consists of a 10 resistor in series with an inductor of 20mH.

     (i)     Calculate the rms fundamental, 3rd, 5th, 7th and 9th harmonic load voltages.

     (ii)    Calculate the rms fundamental, 3rd, 5th, 7th and 9th harmonic load currents.

     (iii)   Estimate the mean dc link current.

     Ans: (i) 180V, 60V, 36V, 25.7V, 20V (ii) 15.25A, 2.82A, 1.1A, 0.57A, 0.35A (iii)
     12.1A

2    A PWM single phase inverter is fed from a 300V dc supply and is operated with a
     modulation index of 0.8. The fundamental output frequency is 35Hz.

     (i)     Calculate the rms fundamental output voltage.

     The principal output voltage harmonics are the 13th and 17th whose amplitudes are 30%
     and 20% of the fundamental respectively. All other voltage harmonics may be
     considered to be negligible. The load current lags the voltage by 30o and is virtually
     sinusoidal with an rms value of 10A.

     (ii)    Determine the frequency components present in the dc link current spectrum and
             determine the rms value of each component.

     (iii)   Assuming that the dc link voltage is supplied from a rectifier whose output is
             smoothed by large inductor followed by a parallel capacitor, determine the rms
             current flowing in the capacitor.
     Ans:    (i) 170V (ii) dc (4.9A), 70Hz (4A), 420Hz (1.2A), 490Hz (1.2A), 560Hz (0.8A),
             630Hz (0.8A) (iii) 4.49A




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3     A single phase PWM rectifier is arranged using a bridge of 4 transistors with
      anti-parallel diodes together with a series inductor (value 5mH) on the ac side and a
      2000F capacitor on the dc side in parallel with a load which draws a smooth current.
      The ac supply is 240V, 50Hz and it has negligible reactance. The mean dc voltage is
      maintained at 500V by a control loop which also maintains unity displacement factor.
      The PWM switching frequency is high so that the effects of harmonics due to the
      switching may be neglected.

      (a)    If at a particular operating point the input current is 20A (rms) determine:

             (i)     the dc output power,

             (ii)    the PWM modulation index,

             (iii)   the phase shift between the supply voltage and the fundamental
                     component of the bridge input voltage,

             (iv) the peak to peak ripple on the dc side capacitor voltage.

      (b)    If the output power is maintained at the same value as in part (i) and the PWM
             modulation index is changed to 0.65 determine the new input displacement
             factor.

      Ans: (a) (i) 4800W (ii) 0.6846 (iii) 7.458o (iv) 15.4V (b) 0.931


Dr J C Clare Nov 1998




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