Boat Diesel – Alternators

These generate electric power and are driven by the engine, usually through a V-belt or multi V-belt. The alternator normally runs faster than the engine, the speed ratio being determined by the diameters of the respective pulleys.

The drive belt should be neither too tight nor too slack. It should be possible to move the belt at its longest part by plus or minus 12mm (l/2in). A belt that is too tight will shorten the life of the bearings in the alternator, while a slack belt will provide insufficient friction to transmit the power and the belt will slip, giving reduced speed. You can usually adjust the tension by rotating the alternator about its primary fixing point and locking it in position using the bolt in the slotted adjustment arm.

The alternator supplies alternating current (AC) which is converted into direct current (DC) by its own rectifiers and used primarily to charge the batteries. The batteries themselves, since they are accumulators, smooth the alternator output.

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