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Let's look at why you can have an overvoltage condition
existing with an alternator. The cause of that is an excessive field current,
along with high rpm. An alternator has an output which is V = rad/sec x field
current. What the typical overvoltage protection device does is put a short on
the field circuit breaker, causing it to pop, removing the source of field
current. If your problem stems from a VR inside the alternator which puts max
current on the field, then you will have to isolate that somehow. Otherwise you
have to look at an external VR having a problem or a short from the
alternator output to the field, which is one of the more unlikely events. If you
keep the rpm low then you may not be able to produce an OV even with max field
current. But at high rpm, the alternator can put out in excess of 100V; there
used to be a device that you put on your car to get over 100V DC to run
series-wound saws and drills. This either gave max field and you regulated
the output voltage by varying the rpm, or you set the rpm high and you had a VR
which regulated it for 110V-120V.
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