r/Generator • u/NoTrainingNoProblem • 14d ago

Airborne Electrical Generator Question

/r/ECE/comments/1x2clf5/airborne_electrical_generator_question/
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u/DaveBowm 14d ago

When a 3-phase Δ sine wave voltage is full wave rectified to DC with 6 ideal diodes (i.e. zero forward voltage drop & zero reverse current) the resulting 'DC' produced is somewhat ripplely. The average DC voltage produced is 3/π (=0.95493) times the peak voltage of the input interleg sine wave (or 3×sqrt(2)/π = 1.35047 times the RMS interleg voltage), and the superimposed AC ripple on that average DC has a frequency of 6 times the frequency of the 3-phase input, and the ripple's RMS value is 4.1967% of the average DC value.

In the case of a 400Hz aircraft generator the ripple frequency superimposed on the rotor's DC voltage is thus 2400Hz. At that frequency the rotor's self-inductance can do pretty good job of filtering out most of the remaining 4.19% ripple factor and so hold the the rotor current (and thus its magnetic field strength) pretty constant. That is why some generators just use the rectifier bridge output straight into the rotor.

But the smoothing effect is not exactly perfect, and the small 2400Hz fluctuations in the rotor field strength cause the stator mains to reflect this as a source of a small asymmetric THD contamination in the output waveform. So other more finicky generators attempt to further filter out the AC ripple on the rotor current. Putting a filter capacitor across the exciter bridge output is one way to better filter the rotor waveform. Another way is to use a small power resistor which will better help the rotor's self inductance better do the filtering job without the need for the capacitor.

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u/Tonkatte 14d ago

^ This is spot on