In an electrical circuit, the active power P is the real power transmitted to loads such as motors, lamps, heaters, computers … The electrical active power is converted into mechanical power, heat or light.
In a circuit where the applied r.m.s. voltage is Vrms and the circulating r.m.s. current is Irms, the apparent power S is the product Vrms x Irms.
The apparent power is then the basis for electrical equipment rating. One piece of equipment (transformer, cable, switch …) must be designed in relation to the r.m.s. values of voltage and current.
But what is reactive power?
Reactive power Q is present when voltage and current are not in phase. In this situation, the current can be split up into two components:
– one component which is in phase with the voltage, called active current and which is the sole responsible for transmission of active power,
– one component which is in quadrature, called reactive current and which is commonly considered as the generator of flux in ferromagnetic circuits such as transformers, motors, ballasts.
The problem with the reactive power is that it is impossible to manage without it, and it is resulting in an increase of equipment rating.
It can be compared to the foam in a glass of beer: it is not the real stuff, but there is no way to avoid it. The glass must not be sized only on active power unless you’ll have overflow.