Capacitor Noise Filter

Electrical and electronic topics for small boats
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jimp
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Capacitor Noise Filter

Postby jimp » Mon Dec 18, 2017 2:10 pm

On 1989 Wilbur 34 with CAT 3208 TA:

A little confused by this one. Was told the capacitor (attached photo) was a noise filter for the alternator. One side was connected to the negative on the alternator, the other end had a broken connection that was stuffed in a wire run (not connected to anything, old broken connector seen in photo)). It had a blown 10 amp fuse on the + side too. Was it purposely disabled at some time in the past?

Is that what this is? What is the correct wiring for it? Should the "+" side of the capacitor be connected to the "+ on the alternator?

I don't know and Internet searches aren't providing clear wiring or guidance.

Thanks.

Jim

P.S. I did check on my Squall this weekend--still drying.
Attachments
20171216 126.JPG
Sprague capacitor
20171216 126.JPG (232.06 KiB) Viewed 5871 times

jimh
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Re: Capacitor Noise Filter

Postby jimh » Mon Dec 18, 2017 3:37 pm

From the photograph: I read the values of the ELECTROLYTIC capacitor as:

Capacitance = 20,000-uFD
Maximum working voltage = 25-Volts DC
Temperature rating = 95-degrees-C

From the narrative: the positive terminal of the capacitor was wired to something--probably the 12-Volt battery positive--via a fuse of 10-Amperes. The negative terminal of the capacitor was wired to the 12-Volt negative common bus.

Here is what I can tell you:

This capacitor was being used as a power filtering capacitor, probably connected across the 12-Volt DC bus. The value of the capacitor is very high, which means it can store a considerable electrical charge. A capacitor like this will tend to smooth out variations in the voltage on the 12-Volt bus created by the charging currents from the alternator.

The typical alternator on an inboard diesel engine is driven by a belt and is a field-current alternator, not a permanent magnet alternator. It outputs a pulsating DC voltage, usually created by three sets of coils, producing a frequency at three times the shaft rotation rate. So if the alternator shaft is spinning about 1000-RPM, the voltage output would have a fundamental frequency of 3000-pulses per minute. Since we usually talk about audio frequencies in pulses per second, that means the alternator frequency would be 3000/60 = 50-pulses per second, or 50-Hz.

The alternator shaft is usually geared up by the belt pulley diameter. If the the alternator pulley diameter is different than the crankshaft drive pulley, then it will be speeded up or down in its rotation by the ratio of the circumferences of the two pulleys. Let's assume the crankshaft pulley diameter is 1 and the alternator pulley diameter is 0.5. Each revolution of the crankshaft moves the pulley belt a distance of π. The alternator pulley is rotated that same distance. But since the pulley diameter is only 0.5, the shaft has to turn 1/0.5 = 2 revolutions to travel that same distance. So the alternator rotation speed is in proportion to the ratio of the pulleys. This means if the engine runs at 1,500-RPM the alternator is turning 3,000-RPM.

When the alternator turns 3,000-RPM and generates three pulses per rotation, fundamental output frequency is 9000-pulse-per-minute. That is 150-Hz. This is the frequency of the "alternator whine" that you can often hear on radio transmissions where the supply voltage is being modulated by the alternator.

Normally the storage battery does a good job of filtering out any AC ripple voltages coming off the alternator, but perhaps on this boat someone added the 20,000-uFD filter capacitor to add more filtering.

Now to figure out why the 10-Ampere fuse has blown: the capacitor probably failed and developed a short or a very high leakage current. The capacitor started drawing a lot of current through the capacitor, and the excess current blew the fuse.

Usually once a capacitor is charged up, it stops drawing current. The way it acts as a filter is to supply current to the load when the alternator voltage is down, and to store current on the capacitor when the alternator voltage is higher. In this way the capacitor smooths out voltage variations. If the load is light, a big capacitor can supply almost all the load current in the part of the cycle when the alternator voltage is lower than peak. The effect is to reduce the swing of voltage variation on the output--called ripple voltage.

The capacitor is rated for 25-VDC working voltage. You'd think that would be safe for a 12-Volt battery, but maybe not completely true. An alternator can put out a lot of voltage if its regulator gets a bit out of whack or if the attached battery is in poor condition and does not act to regulate the terminal voltage.

Also, all electrolytic capacitors have a fluid inside. Over time the fluid tends to escape, particularly if the voltage stress on the capacitor is high or if the operating temperature is high.

I notice that the pictured capacitor is rated for 95-degree-C. That is a very good rating.

Take the capacitor out and try to measure its resistance with a DVM. If the capacitor is still good, you should see the resistance reading rise over time as the voltage from the DVM charges up the capacitor. If you see any sort of low resistance, lower than several thousand Ohms, the capacitor dielectric has probably been bridged.

For replacement, consider getting a capacitor with higher voltage rating, maybe 40-Volts DC. But keep the temperature rating high if the capacitor is in the engine room where temperatures get warm.

jimh
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Re: Replacment Part

Postby jimh » Mon Dec 18, 2017 3:59 pm

Here is a possible replacement part:

https://www.digikey.com/product-detail/ ... ND/1595939

Note only rated to 85-degrees-C. Electrolytic capacitors rated to 95-degree-C were not usually items in stock at big distributors and would need special ordering.

jimh
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Re: Capacitor Noise Filter

Postby jimh » Mon Dec 18, 2017 4:17 pm

Check the engine wiring instructions to see if the capacitor is shown as part of the OEM recommended installation of the engine and alternator. Normally I would not expect to see a filter capacitor in the engine area. I'd expect it to be at an electrical distribution panel on the load side of the panel wiring. Being in the engine area subjects the capacitor to heat and vibration, which could reduce its service life.

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jimp
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Re: Capacitor Noise Filter

Postby jimp » Mon Dec 18, 2017 5:12 pm

jimh -

Thanks. That's a lot to digest!

I've also queried the previous owner to see if he can shed some light. With the dust/grim, etc. on the wires I'd guess that it had been disconnected for a while.

I'll get to your recommendations on checking things out... likely in a week or two. Daughter #2 called in mid-November and told us that instead of getting married next summer, they wanted to get married Dec 23rd! Oh yeah, and Christmas comes after that. Going to busy for a bit.

Merry Christmas to you and Chris!

JimP

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jimp
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Re: Capacitor Noise Filter

Postby jimp » Mon Dec 18, 2017 6:15 pm

jimh -

Just heard back.
The capacitor near the alternator was to reduce any interference it could cause with vhf radios. This is what was told to me when we acquired the boat. I think the radios of today don’t have that problem anymore…


Since it wasn't operating, since the radios work fine... and possibly not needed, it's coming off.

JimP

jimh
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Re: Capacitor Noise Filter

Postby jimh » Mon Dec 18, 2017 11:37 pm

Radios "of today" might have better power input filtering than radios of yesteryear, but I still hear plenty of recreational boat radios with very poor modulation caused by alternator whine. If you are not getting any reports of alternator whine on your boat radio tranmissions, then I concur: no need to reconnect the capacitor that is not in the circuit right now.

If you do get a report of alternator whine, I'd use a more effective filter in the power wiring to the radio. Instead of a just a shunt capacitor, I'd get an LC filter with a series inductor.

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jimp
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Location: Juneau, Alaska

Re: Capacitor Noise Filter

Postby jimp » Tue Dec 19, 2017 10:46 am

jimh -

Thanks.

The previous owner bought the boat from his Dad 20-years ago, so that capacitor has been there over 20-years. How long it's been disabled is unknown. Regardless, the two VHFs (Standard Horizon Matrix GX3000S and iCom) are fairly new. Never had any complaints about them.

Will keep my ears open next Spring.

JimP