If they were sine waves you'd be right (even given lots of alternator
poles), but when you whack the top off a sine wave you get a kind of square
wave that includes lots of frequencies. They'll radiate over any appropriate
antenna. The more sensitive the electronics you add to a car, the bigger the
problem can be. Usually it's solved where the problem occurs--on the new
electronics, but it's just as easy and often more effective to solve it at
the source. The other big advantage is if you have a capacitor on the output
and your battery gets marginal the alternator output will be smoothed and
there's not possibility of unfortunate phasing between the alternator and
the points--sometimes a source of misses.
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Randall Young
Sent: Friday, August 18, 2006 4:20 PM
To: fot@autox.team.net
Subject: RE: [FOT] alternators
> Therefore, the waveform going into the battery and electrical system
> is made up of 6 half sine waves at double the frequency of the
> alternator, i.e. if the alternator is turning 6000 RPM, 100 revs per
> second or 100 HZ, the frequency of the half sine waves is 200 HZ.
Tim, you're assuming the alternator only has two poles. Real units have
more than two poles, typically 10 or 14.
> I'm not
> aware of any automotive system that used capacitors in the alternator
> circuit for smoothing the sine waves or for noise suppression.
I have had to add capacitors to auto stereos, to eliminate "alternator
whine". Recently helped someone else, whose "rearward looking radar"
wouldn't work right without some power line filtering.
It's also worth noting that an open (or shorted) output diode will cause a
big increase in the output noise.
> But on the cars they were used on there were little if any electronics
> to be affected.
Unless of course the owner tries to add some, like a CB radio ...
Randall
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