Power that is not delivered to the load but instead results in losses due to heating and radiation is called reactive power. It is equal to the product of the root mean square (RMS) values of current and voltage multiplied by the sine of the phase angle between them.
Reactive power is the electrical power exchanged between a generator and a load during the creation and collapse of alternating magnetic and electric fields. Reactive power does not directly affect the load (unlike active power). It is responsible for the generation of a field when inductive and capacitive elements are present in the circuit and is determined by the reactive component of the current. Reactive power is a component of apparent power and is characterized by reactance. Reactive power is positive if the voltage leads the current in phase. Due to various losses, reactive power can damage coils or capacitors through which the reactive current flows.
In direct current (DC) circuits, the instantaneous and average power over a given period are the same, and the concept of reactive power does not exist. In alternating current (AC) circuits, this occurs only when the load is purely resistive, such as an electric heater or an incandescent lamp. With such a load, the voltage and current phases coincide, and all the power is transferred to the load (active power). In radio communication, this operating mode is achieved in an untuned feeder when working into a matched load. The transmission line operates in traveling wave mode.
If the load is inductive (transformers, electric motors), the current lags the voltage; if the load is capacitive (various electronic devices), the current leads the voltage. Since current and voltage are out of phase (reactive load), only part of the total power that could be delivered to the load if the phase shift were zero (purely resistive load) is actually transferred. Thus, reactive power is a quantity that characterizes the load.
In power networks, reactive power causes additional active losses (requiring more energy generation at power plants) and voltage drops (worsening voltage regulation conditions). In some electrical installations, reactive power can be much greater than active power, leading to large reactive currents and overloading current sources. To eliminate overloads and improve the power factor of electrical installations, reactive power compensation is implemented.
For RF electromagnetic signals in transmission lines, the equivalent of reactive power is the power reflected from the load. In amateur radio jargon, it is referred to as forward or reflected power, measured with an SWR meter.
The unit of reactive power is the volt-ampere reactive (var). In practice, the concept of the power factor (cos x) is often encountered as a measure of the efficiency of an electrical installation in terms of energy savings. The higher the cosine of the phase angle x, the greater the proportion of energy from the source that reaches the load.
Related articles: Active Power, SWR, TWR.