Electric Motor Formula and Calculations

Electric motors are the most common drives used for pumps, fans etc in the refining, oil and gas and petrochem industries. A motor is an electrical machine that converts electrical energy into mechanical energy. It works by using the interaction between a magnetic field and an electric current. AC motors are the type most commonly used in industrial applications because they can handle large loads and operate at high speeds. The power supply available to the motor determines what is called the "motor characteristics" e.g. power, current consumption etc. The table below provides common formula that enables you to calculate the motor characteristics relative to the power supply.

TO FIND	DIRECT CURRENT	SINGLE PHASE	THREE PHASE
POWER	V x I x EFF	V x I x EFF x PF	1.732 x V x I x EFF x PF
HORSE POWER	V x I x EFF 746	V x I x EFF x PF 746	1.732 x V x I x EFF x PF 746
CURRENT	P V x EFF	P V x EFF x PF	P 1.732 x V x EFF x PF
EFFICIENCY	746 x HP V x I	746 x HP V x I x PF	746 x HP 1.732 x V x I x PF
POWER FACTOR	------	Input Watts V x I	Input Watts 1.732 x V x I
SHAFT SPEED	------	------	120 x F no. of poles

Where
V= Voltage (volts)	I= Current (amps)	P= Power (watts)
EFF= Efficiency	HP= Horsepower	F= Frequency (Hz)
PF= Power Factor. A measurement of the time phase difference between the voltage and current in an AC circuit. It is represented by the cosine of the angle of this phase difference. For an angle of 0 degrees, the power factor is 100% and the volt/amperes of the circuit are equal to the watts. (This is the ideal and an unrealistic situation.) Power factor is the ratio of Real Power-KW to total KVA or the ratio of actual power (watts) to apparent power (volt-amperes). For further information on power factor, see our Power Factor toolbox page.

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