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Old 04-09-2010
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Alternator RPMs after replacing an engine driven Refrigeration system

I've replaced an engine-driven refrigeration system with a 12 volt system and am quite happy with the new system. I also removed a number of the components for the engine driven system from the engine compartment and freed up a lot of space to get at the engine itself. The last step will be to remove the compressor itself.

I am concerned about driving the alternator in the right RPM range after removing the compressor. Right now the engine drives the compressor, which has a double pulley, and a belt from the compressor drives the alternator. Is there a method of figuring out if I need to re-size the pulley on the alternator?
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Old 04-09-2010
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Does the alternator have a standard sized looking pulley on it? If it does, frankly, I don't think it's that critical. The voltage regulator will regulate the voltage at varying rpms and the alternator is only rated at a particular amperage anyway. The alternator actually works less for a given amount of power when running at a faster speed, so unless it has a grossly oversized or undersized pulley I think you'll be fine.



edit: I just drew out two pulley senarios on paper to see the end result while using an intermediate pulley, and I don't see a change in the end result. The pulley in the middle doesn't matter, it's the size of the crankshaft pulley and the alternator pulley at the end that determines how fast your alternator spins.
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Old 04-09-2010
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The size of the pulleys obviously will determine the speed of any driven component. Alternators like to run relatively high not only from an output perspective but also to run the cooling fans and given that your diesel probably runs at a quite a low RPM, the ratio needs to be at least 2 (crankshaft) to 1 (alt).

If the driven and driving pulleys on the compressor were the same size then like ERPS said, the pulleys you already have are good to go. The ratios will be the same as they were. Whatever goes into the system to tension the belt will have no effect on the speeds.
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Thanks to both of you for your replies. The crankshaft pulley diameter is about 5.1 inches and it drives the compressor, which has a pulley diameter of about 4.9". This in turn drives the alternator, which has a pulley diameter of about 2.9 inches. It may be that taking the 4.9" pulley out of the picture and driving the 2.9" pulley with the crankshaft's 5.1" pulley will work fine. Any further opinions?
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