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How to Calculate Motor Capacitor Size

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The power running through a motor depends on more than its voltage and current. Less power runs through it than the product of its voltage and current, and the conversion factor between the two values is known as the motor's power factor. A motor with too low a power factor runs with too little power for the voltage and current, wasting energy and money. Increasing the motor's load raises its power factor, but only marginally. Increasing the motor's capacitance can raise its power factor more significantly.

Multiply the voltage on the motor by the current that runs through it. If 100 volts act on the motor, producing a current of 5 amperes: 100 x 5 = 500.

Divide this product by 1,000: 500 / 1,000 = 0.5. This answer is kilovolt-amp value.

Multiply the original power factor by the kilovolt-amp value to find its power rating. With a power factor, for example, of 50 percent: 0.5 x 0.5 = 0.25 kilowatts.

Square the power rating, measured in kilowatts. With a power rating, for example, of 0.25 kilowatts: 0.25 x 0.25 = 0.0625.

Square the kilovolt-amps value: 0.5 x 0.5 = 0.25

Subtract the answer to Step 4 from the answer to Step 5: 0.25 - 0.0625 = 0.1875.

Find the square root of this answer: 0.1875 ^ 0.5 = 0.433. This answer is the system's kilovolt-ampere-reactance.

Repeat Steps 3 through 7 with the target power factor. A power factor of 75 percent, for example, produces a power rating of 0.375 kilowatts and a kilovolt-ampere-reactance of 0.33.

Subtract the new kilovolt-ampere-reactance from the original one: 0.433 - 0.33 = 0.103.

Add a capacitor with a reactance of 0.103 volt-ampere reactives.

About the Author

Ryan Menezes is a professional writer and blogger. He has a Bachelor of Science in journalism from Boston University and has written for the American Civil Liberties Union, the marketing firm InSegment and the project management service Assembla. He is also a member of Mensa and the American Parliamentary Debate Association.

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