Large Capacitor off Alternator

[Reefmagnet]

After a bit of thought on this, and the way the cap and wiring looks newish, and the fact it's a darn'd near battery sized polarised capacitor, I'm almost suspecting it's been installed to compensate for some alternator fault, possibly with the rectifier. I could be way off the mark, and it wouldn't be the first time, but it is just strikes me as totally the wrong component to choose for noise suppression.

[Wotname]

Originally Posted by Reefmagnet

After a bit of thought on this, and the way the cap and wiring looks newish, and the fact it's a darn'd near battery sized polarised capacitor, I'm almost suspecting it's been installed to compensate for some alternator fault, possibly with the rectifier. I could be way off the mark, and it wouldn't be the first time, but it is just strikes me as totally the wrong component to choose for noise suppression.

I think you are on the money!
As a general rule, one should eliminate electrical interference (of any sort) before trying to suppress it. Suppression is (or should be) the last resort.

For alternators, that means ensuring the brushes and slip ring are in good working order and the diode pack is not leaky!!!

IME, not many alternator repairers check the output for AC ripple, they simply do a DC check of the diodes only.

[Reefmagnet]

Originally Posted by Wotname

I think you are on the money!
As a general rule, one should eliminate electrical interference (of any sort) before trying to suppress it. Suppression is (or should be) the last resort.

For alternators, that means ensuring the brushes and slip ring are in good working order and the diode pack is not leaky!!!

IME, not many alternator repairers check the output for AC ripple, they simply do a DC check of the diodes only.

Yeah, I'm thinking it would boost the apparent DC voltage, under light loads at least, if the rectifier was on the blink.

[model 10]

As my own repairer, how do you check for AC ripple?

[Reefmagnet]

Originally Posted by Ecos

As my own repairer, how do you check for AC ripple?

Oscilloscope is the simple answer. And basic units are cheap enough in price these days to actually add one to the toolkit for occasional use.


The quick check answer is stick a DVM on the DC output but set to AC volts. I'd suspect the larger the ripple the higher the reading would be. THis may not be that accurate though.

[a64pilot]

[QUOTE=Ecos;2944764]

Originally Posted by Wotname

Re your Cessna, have the diode pack on the alternator checked, not just a simple DC go/no go check but for the AC ripple on the output. Or better still, just have them replaced.



Pretty sure he has a generator.

It was originally, but someone way back when, likely about the same time the wings were metallized, put a C-150 alternator on it under a field approval.
Of course the wiring was not changed to accept the higher current the alternator could provide.

[Bycrick]

The mechanical parts of an "alternator" will produce 3-phase AC voltage when the alternator is spinning. When each AC phase is passed through a pair of diodes, you will end up a signal that goes from zero to maximum voltage at twice the frequency of the original AC input. When you add the three rectified DC outputs together, you get mostly DC output with an AC ripple at 6 times the original AC frequency.

You can measure the ripple output by simply putting an AC voltmeter on the output of the alternator. The less ripple voltage the better.

Alternator ripple voltage has nothing to do with RF interference. It’s a relatively low frequency and with very slow rise and fall times.

But some electrical equipment is sensitive to ripple on the power input. Auto stereos, old ones, and some old VHF radios would power the audio amplifier directly from the power input wires, so any alternator ripple would be heard in the speakers.

Putting a large capacitor on the output of the alternator might help to reduce the ripple, but usually the battery itself tends to damp out the ripple. If you do this, you need to check the specs on the capacitor because they even if they’re the right capacitance and voltage, they can have different ratings for the amount of ripple current they’ll tolerate.

If the ripple is really a problem for some device, the previous suggestion of an inductive filter with a smaller capacitor is probably a better idea. These filters are installed at the device and therefore can be smaller, lighter and cheaper than one at the alternator output.

[Wotname]

Originally Posted by Ecos

As my own repairer, how do you check for AC ripple?

Originally Posted by Reefmagnet

Oscilloscope is the simple answer. And basic units are cheap enough in price these days to actually add one to the toolkit for occasional use.


The quick check answer is stick a DVM on the DC output but set to AC volts. I'd suspect the larger the ripple the higher the reading would be. THis may not be that accurate though.

The screen shoot is from Fluke document I found somewhere and the PDF ripple tutorial has some oscilloscope illustrations - they are both auto focused but an alternator is an alternator whether it is in a car or a boat.

[GordMay]

Originally Posted by Wotname

The screen shoot is from Fluke document I found somewhere and the PDF ripple tutorial has some oscilloscope illustrations - they are both auto focused but an alternator is an alternator whether it is in a car or a boat.

Checking Ripple Voltage
Ripple voltage or (AC voltage) can be measured by switching your DMM to AC and connecting the black lead to a good ground and the red lead to the "BAT" terminal on the back of the alternator, (not at the battery). A good alternator should measure less than .5 VAC with the engine running. A higher reading indicates damaged alternator diodes.

From:
"Beat the Book" ➥ http://assets.fluke.com/appnotes/aut...e/beatbook.pdf

[Nicholson58]

Originally Posted by felizcortez

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.

Do NOT go internal. Internal regulators are replaced, modified to extend battery life. This is an improvement to the original junk.

We use Balmar 624 on all alternators.

[MEC]

The capacitor is wired in series to the alternator output. This configuration will generate very large transient voltages when the alternator voltage changes, particularly when the alternator is shut off. This transient voltage will propagate into the alternator's bridge rectifier. Rectifiers have a maximum voltage withstand which, for a 12v rectifier is not a large voltage. Transient voltages generated by the cap will also be impressed onto the boat's DC system, which can damage any device which is energized when the transient occurs. For example, some NMEA 2000 based devices can be damaged if exposed to 30 volt transients. That cap is so large that the transient voltage can be hundreds of volts (depends on the rate of voltage change).


If an alternator filter is inductor based, it is wired in series. If an alternator filter is capacitor based, it is wired in shunt. The best filters have both inductors and capacitors.



VHF radios are sensitive to electrical noise. Selecting a marine rated alternator filter, like those manufactured by Newmar, and following OEM installation instructions, would be prudent.


I hope this information is useful.

[wingless]

Originally Posted by MEC

The capacitor is wired in series to the alternator output.

Welcome to the forum.

Sorry, but that is not correct.

The capacitor would instead need to be connected parallel to the output.

A capacitor "blocks" DC and "passes" AC, so it cannot get connected in series to the alternator output.

Originally Posted by MEC

If an alternator filter is inductor based, it is wired in series. If an alternator filter is capacitor based, it is wired in shunt. The best filters have both inductors and capacitors.

Yes, this is correct.

[Bycrick]

With most equipment today, and a properly working alternator with an acceptable level of ripple voltage, I’d think that the big capacitor at the alternator was unnecessary and just another point of failure. In the event that you have excessive noise on the DC wiring, which can happen with any charging device that outputs anything other than pure DC (which is almost all of them), I’d either replace the device itself (since it shouldn’t have a problem with some ripple on the power input), or put a smaller filter at the device itself.

[GordMay]

Originally Posted by MEC

... If an alternator [any] filter is inductor based, it is wired in series. If an alternator filter is capacitor based, it is wired in shunt. The best filters have both inductors and capacitors.
Indeed.
VHF radios are sensitive to electrical noise. Selecting a marine rated alternator filter, like those manufactured by Newmar, and following OEM installation instructions, would be prudent...

Indeed.

[kenbo]

Originally Posted by Nicholson58

Do NOT go internal. Internal regulators are replaced, modified to extend battery life. This is an improvement to the original junk.

We use Balmar 624 on all alternators.

This type of advice is a tad authoritarian. I have cruised with both internal and external regulators and have found both to be perfectly satisfactory if the systems are thoughtfully integrated. I also never use shore power unless I'm on the hard, I'd rather not join the galvanic party found in most marinas.

An external regulator is most often needed when someone installs a small case alternator with an over wound stator and then needs to derate it so it can survive it's inability to dissipate heat. There was a time where internal regulators had too low a Vset or overly protective temperature compensation but those days are past with a few exceptions (Hitachi come to mind).

I've had to derate IR small case alternators and it's not that difficult to insert a resistor in series with the field coil.

However, a more successful approach, if you feel compelled to over drive a small case alternator, is to remove the bridge rectifier from inside the alternator. Depending on the alternator that's at least 50% of the heat load.