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A motor is a rotating machine that converts electrical energy to mechanical
energy. A motor delivering 1 hp -- 746 watts -- is actually consuming about
1,000 watts from the power line. The difference of 254 watts is lost as
heat in the motor, friction in the bearings, the power that it takes to
run the motor even when it’s idling, and other factors. It’s important
to remember this discrepancy between watts delivered and watts consumed
when selecting wire for installing a motor and when considering operating
costs.
The amperage drawn from the power line depends on the horsepower delivered
by the motor -- whether it’s overloaded or underloaded. The watts are not
in proportion to the amperes, because in motors the power factor must
be considered. Your cost for the power is based on watts consumed, but you
must provide wire size in proportion to the amperes. When the motor is first
turned on it momentarily consumes several times its rated current. After
it comes up to speed but is permitted to idle, delivering no load, it consumes
about half its rated current. Rated current is consumed when the motor is
delivering its rated horsepower, and more current is consumed if it’s overloaded. |