Alternator questions

[Ric]

I'm building a propshaft alternator for my boat out of an old alternator from a citroen CX bought from a scrappy. I'd be grateful for help on the following questions:

1) The alternator has an arrow to show which direction it should turn. Am I right in thinking this is just so that the centrifugal cooling fan works efficiently? Presumably the alternator will not suffer damage from brief periods of running in reverse when reversing the boat?

2) The alternator has three electrical connections. There is one big positive, one big negative (presumably to connect to battery) and one smaller positive. Am I right in guessing that the smaller "plus" connection is the priming coil? How do I connect this?

3) Would it be advisable to include a relay so that the propalternator is open-circuit when the motor is working?

4) What wave form is the rectified output? Is it simple half-inverted sine wave? Or is the output smoother and closer to pure DC?

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[halcyon]

1 ) the fan will be sucking in reverse, should be ok for short times.
2 ) the small one will be the earth for a warning light, i.e. ignition type light, it powers the rotor to start charge, will back feed the bulb to turn it of and show it's working.
3 ) I assume you are using under sail, with engine running the rectifier diodes will block any current, so you should not need a relay.
4 ) the output is three phase ( star or delta ) probably full wave rectified, depends on rpm you achieve, but should be fairly smooth with the battery acting as a smoothing capacitor.

Brian

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[Rick]

>2) The alternator has three electrical connections. <snip>
It sounds like a 3 wire internally regulated alternator, in which case the big positive and negative terminals are to connect directly to the battery, and the other wire is used to both provide power to start, and to monitor battery voltage for the regulator.

You can "cheat" the regulator a little by putting one or more diodes into this line drop the battery voltage, and therefore cause the regulator to provide higher output voltage - I use 2 inline (via a switch) on my engine driven alternator to allow me keep pumping into the batteries once the regulator cuts back the charging voltage. It's a problem with, and nature of automotive style regulators - they're designed primarily to run accessories, not charge batteries.

I'd be wary of the ability to generate much power via the prop shaft - the alternator probably needs to spin at more than 3,000 rpm to get any real output, and 6,000 to 7,000 rpm would be better. If it's doing less than 1,000 rpm or so, you may be draining, not charging your supply.


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[andyball]

A nice low "cut-in" speed for an alternator would be 600rpm....800 up to 1500 is more common (varies between models/voltages)

Modern types should start producing real output not too much higher, but it's worth checking first that your shaft driven arrangement will actually achieve these speeds when sailing in real conditions.

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[William_H]

Hello Ric Those who have already posted have much wisdom ie this is not an easy idea to make work well. You will need a large pulley on the prop shaft to get some reasonable alternator speed. This could be too much physically for the alternator to cope with when motoring.
The back of the alternator should have a plate held down by two screws. The plate may in fact be a plastic box. If it is a box this is a regulator. If it is a plate then no regulator the plate removed will hold two brushes and likely the small terminal if you are only able to generate small amont of power then you won't need a regulator but can run the alternator at full excitation power however the connections required will depond on the connections in the alternator. Keep asking questions and maybe borrow a book on the Citroen CX wiring. regards will

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