If you had 400Hz 3Φ in your shop it would be easy.
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If you had 400Hz 3Φ in your shop it would be easy.
Inside every little problem, is a big problem trying to get out.
I still say filter the alternator. Kill the noise at its source. why prune each leaf of a weed every time it grows a new one? Kill the root at its source. Still just a thought.
If the system is a 30 amp. the capacitor values could only need to be a few 1000 uf and possibly not need a filter choke.
Still just a thought.
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True lazyness is the real motivator to do it right the first time!
Doing it wrong, then doing it over is just misguided ambition!
I have tried "filtering" an alternator with just a large 47,000uF "computer grade" electrolytic capacitor, and it does NOT work. The effective series resistance of a large electrolytic capacitor is more than 10mOhms at 120Hz. I suspect that the ESR is much higher at kHz frequencies. The impedance being bypassed (30A loads, the battery) is much less than 10mOhms, so most of the ripple current still flows along the airframe to the loads and battery.Originally Posted by tcmtech
It would require putting a series choke of more than 100uH (that will not saturate at the max alternator current of 60A) between the alternator and the loads and battery.
Electrically that might help, but practically, it would never fly. It raises all kinds of FAA certification issues due to modifying an existing design, and introduces several failure modes. Those large electrolytics hate elevated temperatures (engine compartment is over 130degF), and they hate vibration. Fusing the path to the electrolytic wont work because the resistance of the fuse is a few hundred mOhms, which kills its effectiveness as a bypass for the ripple currents.
Ah. computer caps do have rather poor high frequency response.
Good point on the FAA regs though.
Chase the ground loops it is!
sure its just alternator? No feed back from the duel ignition circuits?
The intention of my suggestion (which apparently was not clear) was to use a separate external alternator and motor, not to drive the alternator in the aircraft.
It probably isn't necessary to disconnect the aircraft alternator when you connect up the external unit since the alternator is already looking at the battery voltage anyway.
Carl
Curmudgeon Elektroniker
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