12 volt dc motor factory tells you the formula for calculating the output of the DC motor and the input power.
DC motor output and input power calculation formula, electric power calculation formula, multiply voltage by current, this formula is the definition of electric power, always correct, suitable for any situation.
For purely resistive circuits, such as resistance wires, bulbs, etc., you can use the formula of "square of current multiplied by resistance" and "square of voltage divided by resistance", which is derived from Ohm's law.
However, for non-pure resistor circuits, such as motors, only the formula of "voltage multiplied by current" can be used, because Ohm's law does not apply to motors, etc., that is, voltage and current are not proportional. This is because the motor generates a "back electromotive force" when it is running. For example, the external voltage is 8 volts, the resistance is 2 ohms, and the back electromotive force is 6 volts. The current at this time is (8-6)/2 = 1 (ampere) instead of 4 amps. Therefore the power is 8 x 1 = 8 (watts).
In addition, a Joule law is the formula for resistance heating. The heating power is “current squared by resistance”, which is always correct.
Also take the above example, the power of the motor heating is 1 × 1 × 2 = 2 (watts), that is, the total power of the motor is 8 watts, the heating power is 2 watts, and the remaining 6 watts are used for Mechanical work.
P=IU, commonly known as the universal formula, that is, when it can be used;
P=I^2*R, used when the currents in the circuit are equal or when calculating electric heat;
P=U^2/R, used when the voltages in the circuit are equal.
