The Stovebolt Site Forums The Shop Area The Electrical Bay RPM's and Alternator

I have a single wire self exciting alternator on my truck as part of a 12 volt conversion.
I have the idle RPM set at 500 and the Amp Meter is slightly negative. If I pick up the RPM's the meter swings positive.
Do the RPM's need to be set at a higher idle speed for a self exciting alternator to work correctly?

Jim

Not really, just rev the motor after starting to bring the RPMs up and the alternator will start charging.

Beltfed's advise should work but if you rev it a bit and it starts charging but it drops back to a discharge position when it returns to idle try a smaller pulley on the alternator.

Hi Tiny,

When I bought it I had the smallest pulley I could get put on.
I picked this alternator because it's the one all the local farmers used to convert their tractors to 12 volts.

Rev it to over 1,500 rpm and see if it then charges at lower rpm. My one-wire alternator "excited" at 900 rpm but other people have reported higher "excitation" rpms. What rpm have you revved it up to?

At 600 rpm the ammeter centers, at 650 it goes slightly positive. At 700 rpm with the high beams on and the blower motor on high speed it still stays positive. When I drive it the meter will go way positive until I stop and when it goes back to idle speed it drops back to slightly negative. I don't know how high I've revved it up to but I've driven up to 45 mph. At idle it is only slighly negative unless I turn on the lights or heater.

You have a couple of things to consider on these alternators. I am assuming your using an old style 10si Delco with built in self exciting regulator. These regulators are an aftermarket product and there are several companies making different versions of them with different cut in settings. The self exciting regulator is offered in a low rpm cut in and it sounds as if yours does not have that particular regulator. This regulator will cut in when rpms hit around 300-400. Once this happens you should be good to go. If after that your still dipping down into the discharge then it sounds as if your drawing more amps than your alternator will put out at idle. This should not be and you could possible have internal issues with the alternator. On a low rpm application a smaller pulley will increase your cut in time as it will increase the rpms of the alternator, but its not necessary if the alternator is built correctly. I use a regulator that excites instantly, as soon as the alternator starts turning regardless of the rpms and I always use an alternator that is 55 amps plus, personal preference. If your going into discharge at idle with lights etc. on then something is wrong, you could get better performance than that out of the original generator.
Have the alternator tested at a local shop and they should be able to tell you if you have diodes etc. that may be bad and what the under load amperage output is.