AC Motors: Power Consumption

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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.

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