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Old 01-08-2019, 08:34   #1
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Large Capacitor off Alternator

I have a Motorola MA12N600 alternator that has a large 12000uF capacitor on the output terminal of the alternator. I'm assuming this is to smooth out the voltage coming out of the alternator, but it looks like it was installed after the fact. Has anyone seen anything like this before?
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Old 01-08-2019, 08:49   #2
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Re: Large Capacitor off Alternator

It reduces noise in other electronics like radio etc. It used to be commonly done but no so much anymore.
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Old 01-08-2019, 08:52   #3
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Re: Large Capacitor off Alternator

If I end up replacing the alternator should I leave the capacitor on there? I'm assuming it wouldn't hurt anything but I've been thinking about going to an internally regulated alternator as a replacement to what I have since I believe it's the original to the boat.
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Old 01-08-2019, 08:59   #4
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Re: Large Capacitor off Alternator

The physical installation plays a significant role in the effect of that capacitor.

The part should be close w/ short leads, not the zee folded long wires shown in the image.

A capacitor deteriorates with time, the effect accelerates with temperature. It is possible to measure the capacitor to determine if it is still good.
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Old 01-08-2019, 09:07   #5
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Re: Large Capacitor off Alternator

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Originally Posted by felizcortez View Post
If I end up replacing the alternator should I leave the capacitor on there? I'm assuming it wouldn't hurt anything but I've been thinking about going to an internally regulated alternator as a replacement to what I have since I believe it's the original to the boat.
Those Motorolas are great alternators. Keep it. Have it rebuilt if necessary. They are not that expensive, another option is to buy a spare just like it! it may go forever on the minimal use of boat hours. In trucks they would go 200,000-400,000 miles!

You can just disconnect that capacitor and see if you get a noisy radio etc, then decide. Someone may have put it on there if they had a Ham radio aboard or etc.
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Old 01-08-2019, 09:55   #6
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Re: Large Capacitor off Alternator

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Those Motorolas are great alternators. Keep it. Have it rebuilt if necessary. They are not that expensive, another option is to buy a spare just like it! it may go forever on the minimal use of boat hours. In trucks they would go 200,000-400,000 miles!

You can just disconnect that capacitor and see if you get a noisy radio etc, then decide. Someone may have put it on there if they had a Ham radio aboard or etc.
No ham radio on board but they did have a loran system . Got rid of that to free up some real estate on the nav station.
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Old 01-08-2019, 10:23   #7
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Large Capacitor off Alternator

This is what is usually used to suppress alternator noise.
Smaller than a C size battery by about half. Ignore price, itís an aircraft part.
They do suppress noise, but donít eliminate it. Alternator noise is almost always a whine, that increases and decreases frequency with engine RPM, that is how to be sure itís alternator noise, it changes with engine RPM
https://www.aircraftspruce.com/catal...atorfilter.php
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Old 01-08-2019, 10:29   #8
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Re: Large Capacitor off Alternator

Actually, if you want to reduce the rf noise from the alternator getting into the wiring, you need a low-pass "L" filter, which starts with an inductor from the source. The Z shaped wire sort of does this. A coil of wire or ferrite sleeve would be better, a real choke would be best.
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Old 02-08-2019, 10:04   #9
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Re: Large Capacitor off Alternator

Some things to consider:

1) The wires must be kept as short as possible as they act as antennas radiating RF noise.
2) Verify the specs on the capacitor state the self resonance frequency, including any wires you attach, is well above the highest frequency you are concerned with. No electrolytic or super capacitor can do this.
3) Remember since it is an alternator the current may be 100 Amps at 14 Volts (1,400 Watts) so if it is down 30 dB it is still a lot of RF power.
4) Also verify the power factor the capacitor is designed for.
5) A good low pass filter, or choke will help but remember wire lengths.
6) If you are using the forestay, backstay or any non circular polarized antenna remember the orientation of the wires is very important.
7) A good way to optimize these various considerations is to monitor the interference on the most sensitive amplitude modulated receiver you have, even an FM radio with received signal strength or very weak station be used in a pinch.

These should get you pointer in the right direction.
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Old 02-08-2019, 13:55   #10
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Re: Large Capacitor off Alternator

What I find worrisome is that the capacitor isn't fused. Presents a fire hazard as wired.
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Old 02-08-2019, 14:05   #11
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Re: Large Capacitor off Alternator

My real question would be why do i need noise suppression for the radio on the alternator on a sailboat auxiliary.
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Old 02-08-2019, 14:39   #12
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Re: Large Capacitor off Alternator

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My real question would be why do i need noise suppression for the radio on the alternator on a sailboat auxiliary.


Youíve never heard a whine in the VHF? Or maybe on the stereo?
My amplified VHF cockpit speaker picks up alternator noise, the harder the alternator is charging, the more noisy it is.
Itís probably picking it up on the power supply, just havenít bothered to fix it.
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Old 02-08-2019, 14:43   #13
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Re: Large Capacitor off Alternator

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Youíve never heard a whine in the VHF? Or maybe on the stereo?
My amplified VHF cockpit speaker picks up alternator noise, the harder the alternator is charging, the more noisy it is.
Itís probably picking it up on the power supply, just havenít bothered to fix it.

yes I have and it is so drying I do deal with ( k7wld )
however it is a sailboat the noise from the engine running is not a real concern as far as I see.
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Old 02-08-2019, 15:41   #14
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Re: Large Capacitor off Alternator

That's one heck of a capacitor for RF noise suppression.
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Old 02-08-2019, 15:43   #15
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Large Capacitor off Alternator

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yes I have and it is so drying I do deal with ( k7wld )

however it is a sailboat the noise from the engine running is not a real concern as far as I see.


You donít motor in the ICW? Maybe where you are there isnít much need for motoring, but thatís not true everywhere, in fact Iíd say many motor, never been there, but hear itís often called the motorterranian for example.
Average cruising boat itís my understanding puts 300 to 500 hours on their motors a year and I doubt thatís all at anchor charging batteries
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