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The three phase electric motor is usually a large piece of equipment that uses a "polyphase" circuit to draw heavy power loads at relatively low voltages. This improves power line efficiency and provides the smooth power flow required by many such motors. The cost of electricity for electric motor three phase operation is based on the kilowatt hours used, just like any other electrical device. However, figuring power usage is more complicated because the conventional equation for power usage must be modified to apply to three phase motors.

Find the voltage and amperage used by the three phase electric motor. The line voltage will be given by the manufacturer's specifications. Most such motors have readouts for the amperage. If that's not the case, use an ammeter designed to handle three phase currents to measure the amperage. Follow the ammeter manufacturer's instructions for hooking the ammeter into the power line to measure amperes.

Calculate the power the motor consumes while in operation. The equation is W = AV(sqrt 3) where A is amperes, V is volts, and sqrt 3 is the square root of 3 (about 1.73). W is the power consumption in watts. For example, if the electric motor uses 50 amps at 240 volts, the wattage is 50 x 240 x 1.73, or 20,760 W. Electricity costs are based on kilowatts (kW), so divide watts by 1000 to convert to kilowatts (20,760 W/1000 = 20.76 kW).

Record the time the motor is in operation. For example, in a manufacturing plant, a three phase electric motor might run 8 hours a day, 5 days a week. This works out to an average of 173.3 hours per month.

Multiply the power consumption by the hours of operation to find kilowatt hours. A 3 phase electric motor drawing 20.76 kW for 173.3 hours per month will use 3771.7 kWh (kilowatt hours) of electricity per month.

Multiply the total kilowatt hours used by the rate per kilowatt hour charged by the power company to find the cost. For instance, the cost of electricity for three phase motor consuming 3771.7 kWh per month at a rate of £0.10 / kWh would equal £377.17.

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