60% is the industry "rule of thumb" and allows some leeway but not much. Imagine
operating a motor in a continuous "brown out". For the sake of argument lets say you
have a refrigerator motor rated for 120 volts trying to operate with a 90 volt supply.
Soon it will burn out for numerous reasons. An induction motor used as an alternator
must have the proper sized capacitors for that motors hp. rating. When the rpm drops
below the lower gross deadband (the lowest rpm at which usable power can be
extracted, about 1820 for most asynchronous motors) The capacitors will loose their
saturation and fall off line. Resistors can be used to lower this target, but it can create
other problems as well. All of the AC motors that would appear in your circuit as a load
require at least 55 cps. (washer dryer etc.) to operate without burn out. When the rpm. of
your alternator drops so does the frequency. If you have a constant load applied (one
should maintain a minimum load of about half the hp. rating in watts).
You must always have more power available at the power source, (in this case water wheel) in order to maintain proper load and frequency requirements from the current provider (alternator, 5 hp. x 746 = 3730 watts max. out). When the contactor on a load carrying device such as a refrigerator closes (capacitor start motor always under load) a momentary load of up to 8 times the data plate rating is required just to start such a motor. If you have a 1/2 hp. motor on your refrigerator this condition (1/2 hp. 373 watts x 8 = 2984 watts-load) along with the simultaneous operation of several other small loads can exceed the capabilities of your 5 hp. alternator (3730 watts) for a period of up to 3-5 seconds on average. You need the extra power at the water wheel to "push" through these brief periods of brown out as well as other obvious reasons. Have you ever noticed the lights dim in your kitchen when you hear the fridge start up? This is why, "local circuit brown out", and this happens when you have an inexhaustible supply of highly regulated current. It's a precarious balance.
Offered by Jay.