How to Calculate 3 Phase Motor Power Consumption

Three-phase circuits distribute electrical power along multiple conductors in the same power line. The current waveform of each conductor is offset in time from the others. This allows more power to be transmitted on one line at a lower voltage, increasing efficiency without sacrificing safety. For three-phase motors, there is an added advantage: the three-phase power supply provides a smoother waveform (in effect, a more balanced supply of electricity).

Understand the difference in how power consumption is calculated for conventional and three-phase power circuits. In a conventional electric circuit, power (in watts) is equal to the voltage multiplied by the amperage. For a three-phase circuit, watts equal volts times amperes times the square root of 3.

Determine the voltage and amperage when the motor is running. Most three-phase motors are large devices and have their own readouts. However, you can measure the current (amperes) in some cases. An ammeter rated for three-phase applications will do the job and works much like an ordinary ammeter. You plug the motor into the ammeter and the ammeter into the power line. Be extremely careful in following manufacturer’s instructions, because three-phase applications generally draw a great deal of power.

Calculate three-phase motor power consumption by multiplying amps by volts by the square root of three (W = AV(sqrt 3). For example, if the motor is drawing 30 amps at 250 volts, you have 30 x 250 x sqrt 3 (about 1.73) = 12,975 watts).

Convert watts to kilowatts by dividing the number of watts by 1,000. Thus, a three-phase electric motor drawing 12,975 watts is consuming 12.975 kilowatts. For one hour, this equals 12.975 kilowatt/hours