How you find power of ac?

Answer 1

With a DC, we only have to consider the resistance of the load when we calculate power. However, with AC, we must also consider reactance (inductive reactance and/or capacitive reactance) when we calculate power. In fact, we need to consider three 'types' of power: true power, reactive power, and apparent power:

• True power describes the rate at which energy is dissipated by the resistive component of the circuit. This energy cannot be restored. We measure true power in watts.
• Reactive power describes the rate at which energy is temporarily stored in the magnetic field (inductive component) or electric field (capacitive component), and returned to the source when the field collapses. We measure reactive power in reactive volt amperes.
• Apparent power is the net rate of transfer of energy, and is the vector sum of true and reactive power. We measure apparent power in volt amperes.

To find the true power of the circuit, you can use a wattmeter. To measure the apparent power of the circuit, it is simply the product of the supply voltage and load current. You will need to vectorially subtract the true power from the apparent power to determine the reactive power.

A very important ratio used in AC power calcuations is (true power)/(apparent power), and this is called power factor -it corresponds to the cosine of the angle (phi) by which the load current lags or leads the supply voltage.

Simple vector analysis will reveal that the true power = U I cos (phi)