What is the formula for calculating the input and output power of a DC motor
The calculation formula of electric power is the voltage multiplied by the current. This formula is the definition of electric power. It is always correct and applicable to any situation.
For purely resistive circuits, such as resistance wires, lamps, etc., it can be calculated by the formula of "square of current times resistance" and "square of voltage divided by resistance", which is derived from Ohm's law.
But for non-pure resistance circuits, such as electric motors, the formula "voltage multiplied by current" can only be used, because for motors, Ohm's law does not apply, that is, voltage and current are not proportional. This is because the motor generates "back-EMF" 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. At this time, the current is (8-6) / 2 = 1 (A) instead of 4 A. So the power is 8 × 1 = 8 (Watts).
In addition, the Joule's law is the formula of resistance heating. The heating power is "the square of the current times the resistance", which is always correct.
Taking the above example, the power generated by the motor is 1 × 1 × 2 = 2 (W), that is, the total power of the motor is 8 W, the heating power is 2 W, and the remaining 6 W is used for Mechanical work.
P = IU, commonly known as the universal formula, that is, it can be used at any time;
P = I ^ 2 * R, used when the current in the circuit is equal or when calculating electric heating;
P = U ^ 2 / R, used when the voltages in the circuit are equal.