Testing A Dynamo
Tractor dynamos convert external power via a belt to electrical power. They are relatively efficient, possibly between 80 and 90%. A dynamo outputting 14 volts and 30 amps is producing 420 watts. At 90% efficiency this would require 420/.9 or 467 watts from the engine… a little over 0.6 hp.
When a belt squeals it is slipping because it cannot transmit this power. A cold engine just started has discharged the battery and the dynamo at this point is asked to provide its maximum output, often with a damp belt and pulleys.
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