Harmonic performance of Randomized PWM, EPWM & RSPWM

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Cecube technology consulting

Three Phase OPWM

Three Phase Harmonic Elimination PWM for AC Drives

but the Cecube alternative for IGBT converter technology - EPWM (Entropy PWM)

EPWM Video Demonstration across 2.5kHz Spectrum (288KB)

EPWM Video Download from DC to 80Hz Spectrum (253KB)

For a detailed explanation of how this type of randomized PWM works see Paper 1. The benefits are:

  1. EPWM can be operated with no continuous fixed frequency PWM harmonics. This energy is converted into noise energy throughout the spectrum.
  2. Switching frequency can adapted on line to suit conditions without changing the spectral characteristics.
  3. Delays and non-linearities within the converter electronics become insignificant when enclosed by the feedback mechanisms of EPWM.
  4. Self monitoring of EPWM output waveforms can identify a malfunction.
  5. Uses instantaneous demands and control to provide better accuracy of output waveform than is possible by pre-calculated or lookup techniques.
  6. Absence of large continuous PWM harmonics reduces risk of line resonance on overhead line equipment supply systems.
  7. Harmonic performance is not compromised by converter outage, unlike conventional schemes which depend on interlacing for signalling compatibility and reduced EMC emissions.
  8. Less distinct audio noise from converter and main transformer than with RSPWM.
  9. Potentially very small DC component under all modes of converter operation.
  10. Peak clipping mode of operation to allow for designed operation up to the over-current trip level without nuisance tripping. Estimated to be worth 15% of power converter rating due to margin reduction. This also helps the operational reliability of the equipment.
  11. Converter system operation more robust to partial failure, such as loss of part of link filter.
  12. Spectrum spreading of load (or inverter) intermodulation components due to randomized PWM.

RSPWM Randomized PWM

The mains 50Hz component is the same in both spectra for examples of regular sampling (RSPWM) and EPWM, however the PWM harmonic footprints are quite different. So how is the switching pattern changed to achieve this harmonic transformation? This can be seen, by careful examination, in the next diagram of the supply current for both cases. To "SPOT THE DIFFERENCE" closely compare the switching angles.

reduced EMC emissions
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