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Old 11-10-2009, 15:28   #1
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Alternator Rating

The altetrnators on my 44ft MY are rated to produce output at 1000 RPM. I have heard people say that you must run the motor over 1000RPM to generate any current. It seems to me that the rating would be for generator speed, which is overdriven on my motors. One characteristic of an alternator over a generator is that they put out more at lower RPM, or that is what I have heard in the past.

Does anyone know what this rating means.

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Old 11-10-2009, 16:57   #2
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Mine kicks in at a specific rpm, BUT once it kicks in, I can rev down to any rpm and it keeps on delivering quite some amps. Sure, if rev'd up I get next to full output.

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Old 13-10-2009, 14:54   #3
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Alternators deliver more current at lower RPM than generators (dynamos).
That said, they both drop off at low RPM.
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Old 13-10-2009, 15:48   #4
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Also, there are 'self-excited' alternators where charging doesn't kick in initially until a threshold RPM level is reached. It will then charge at lower RPMs, once it's in this state.

I'm presently running one of these (a Delco single wire alternator, it's supposed to be the spare). When I goose the throttle after starting the engine, the load that suddenly comes on the engine from the alternator is obvious.
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Old 13-10-2009, 16:36   #5
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See the earlier (very short) discussion at
cruisersforum.com/forums/f14/alternator-specification-basics-2532.html
Alternator Specification Basics
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Old 13-10-2009, 16:38   #6
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You need to find the output curve for your alternator. They all increase with rpm to a certain extent. Some are almost flat after a certain rpm, look at the output for the red line Balmar alternator in the second link as an example.

(Edit: Gord beat me to it with his own link)
Alternator Specification Basics

Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 5
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