Basics of Electrical Power Principles in AC Motors – Formulas and Conversion Factors ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Basics of Electrical Power Principles in AC Motors – Formulas and Conversion Factors

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The Electric power in single phase or three phase AC motors can be seen in two ways:
(a) Electrical power consumed by the AC motors
(b) Mechanical power output often referred to as Shaft power, delivered by the motor

When electrical power is fed into an electric motor using alternating current, a magnetic field is created in the stator which in turn induces voltage in the rotor creating mechanical rotational motion. The rotational mechanical power produced called shaft power is what is used to drive loads like pumps. Below is a schematic showing the relationship between electrical input power (P1) and mechanical or shaft power (P2):


In Europe, shaft power is usually measured in kilowatts. In the USA, however, shaft power is measured in terms of horsepower (HP).

To convert HP to KW, we use:
KW = 0.746 * HP     since 1HP = 0.746KW

Conversely to convert KW to HP, we use:
HP = 1.341 * KW

Electrical Power Consumed by Motor
The electrical input power to the electric motor is also the power consumed by the motor:
For single phase AC motors, the power input or consumed (P1)  is given by:








Where:
P1 = power consumed in KW
I = current in Amps
V = Voltage in volts

For three phase AC motors, the power input or consumed (P1) is given by:









Where:P1 = power consumed in KW
VL = Line voltage in Volts
IL = Line current in Amps


AC Motor Shaft Power
Shaft Power (P2) is given by:
For single phase electric motors, we have:








For three phase electric motors, we have: