Calculation of electric motor power

The calculation of electric motor power is done according to the laws of physics. You have to know the current type, voltage value, current value, efficiency, and power factor.


Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points (i.e., voltage) in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points.

In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule (of work) per 1 coulomb (of charge).

The official SI definition for volt uses power and current, where 1 volt = 1 watt (of power) per 1 ampere (of current). This definition is equivalent to the more commonly used ‘joules per coulomb’. Voltage or electric potential difference is denoted symbolically by ∆V, but more often simply as V, for instance in the context of Ohm’s or Kirchhoff’s circuit laws.


Electric power, like mechanical power, is the rate of doing work, measured in watts, and represented by the letter P. The term wattage is used colloquially to mean “electric power in watts.” The electric power in watts produced by an electric current I consisting of a charge of Q coulombs every t seconds passing through an electric potential (voltage) difference of V is:


  • Q is electric charge in coulombs
  • t is time in seconds
  • I is electric current in amperes
  • V is electric potential or voltage in volts

Power factor

The power factor of an AC electrical power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit

The power factor is defined as the ratio of real power to apparent power. As power is transferred along a transmission line, it does not consist purely of real power that can do work once transferred to the load, but rather consists of a combination of real and reactive power, called apparent power. The power factor describes the amount of real power transmitted along a transmission line relative to the total apparent power flowing in the line.


An electric current is the rate of flow of electric charge past a point or region. An electric current is said to exist when there is a net flow of electric charge through a region. In electric circuits this charge is often carried by electrons moving through a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionized gas (plasma).

One ampere is the flow of electric charge across a surface at the rate of one coulomb per second.  Electric current is measured using a device called an ammeter.

The ampere symbol: A, often shortened to “amp”, is the base unit of electric current in the International System of Units (SI). It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

Efficiency and losses

  1. Electrical Power losses include the resistance of the primary rotor and secondary stator windings, generally referred to as the Copper Loss, and calculated as I2R.
  2. Magnetic Power losses include energy dissipated as HEAT in the iron core of the motor, and field strength losses as the magnetic field crosses the air-gap between the stator and the rotor.
  3. Mechanical Power losses include bearing Friction, air resistance as the rotor turns, brush friction against the commutator, and more air resistance if the motor tums its own cooling fan.
  4. Stray losses include harmonic distortion of the power flow through the motor under load, flux crowding in the core material, and other magnetic leakages.
  5. The fewer losses you have the higher efficiency you get.
  6. Efficiency is usually a percent between 0 and 100%. The perfect motor is the one that has 100% efficiency and it exists only in theory.

The calculation of electric motor power is very important when you have to calculate the wire size and motor protection.

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