Mystery of the Alternator

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Frank Hague of Jersey City, New Jersey, asks:

"I repowered my boat last year with a new Westerbeke engine that came with a 100-amp alternator with an external regulator. I also have a solar array and an Air Breeze wind turbine feeding juice to my house batteries. When the house batteries are full and I am feeding them power from the alternator, my tach needle will suddenly go flat, as will the alternator output. If I don’t promptly switch the alternator feed to just my engine cranking battery, the engine will sometimes falter, with rpms dropping for no apparent reason.

This never happened with my old engine and alternator. I’m coping with the situation now by watching my battery condition and alternator output closely."

Nigel Calder replies:

Many engine tachometers work by sensing the alternating current (AC ) waveform from the alternator before it is rectified to DC. Thus, you will find there is a “tach” connection on the side of most alternators. The frequency of the AC sine wave is directly proportional to engine speed, so can be translated into RPMs.

To answer one part of your question: when the batteries come up to charge, I suspect the external voltage regulator is shutting down the alternator (it does this by shutting down something known as the “field current”), which causes the simultaneous loss of charging current and the tach read-out. However, it shouldn’t do this. It should maintain enough residual output to keep the tach going, which is effectively what happens when you switch the cranking battery into the circuit. (It places a small load on the charging system.) You best consult the alternator manufacturer about this problem.

Assuming this is a mechanically governed engine, what I cannot see is any connection between this and the engine faltering, which is, of course, the key thing you want to sort out! There may well be no connection. Normally when an engine falters like this there is a problem with its fuel supply. Be sure your fuel is clean and is flowing freely to the engine before you look for other causes to the problem.

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