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Power transformers have both voltage and current limitations so they are rated in volt-amperes instead of watts. Volt-ampere is not a unit of power. Some call it apparent power which is misleading. Electrical power is actually measured in watts which is the volt-amperes multiplied by the power factor.

Power factor is a measure of how closely the current and voltage wave-forms are in step with each other. The wave-forms alternate between positive and negative swings 120 times each second. If they are both at the crest of the positive swing at the same time then the power factor is 100% and volt-amperes equals wattage. If they are not, then the power factor is a number between 0 and 1 or a percentage between 0 and 100 depending on how far apart they are. Power factor exists in AC circuits because of capacitors and inductors. Capacitors are used on capacitor-start motors to help them start and by the utility at the top of power poles. Inductors are coils of any type, like on transformers, motor windings and florescent ballasts.

Capacitors tend to move the current wave-form ahead of the voltage. Inductors do the opposite. If the values of capacitance and inductance in a circuit are balanced, the wave-forms are in step as if no reactive components (capacitive and inductive) existed in the first place which makes the power factor 100%.

Marc

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