# Relationship of real and apparent power

When the impedance is a pure resistance, the apparent power is the same as the true power. But when reactance exists, the apparent power is greater than the. Reactive Power. We know that reactive loads such as inductors and capacitors dissipate zero power, yet the fact that they drop voltage and draw current gives. 1 real power p alternative words used for real power actual power true power watt full power useful power real power and active power in a dc circuit power.

As the voltage reaches the stored level in the capacitor, the rectifier diode switches on, forcing the current to flow for a shorter time interval than the voltage.

While the load current is drawn from the capacitor continuously, or at the high switching frequency of the converter, the capacitor is recharged only during the interval when the input rectifiers conduct. No current flows into the capacitor from any point along the voltage waveform where its amplitude falls below the capacitor's d-c voltage.

Current only flows when it again rises above the d-c value during the next mains half cycle. Low power factor results when the load current is drawn over only a part of each mains cycle. This is a common result in off-line rectifiers where the input diode does not conduct until the peak of the rectified mains waveform exceeds the d-c level across the input capacitors. Conversely, the period during which current does flow into the capacitor is the rectifier's conduction angle.

## Relationship between Real Power, Apparent Power and Power Factor

The ratio of these angles depends upon the filter's capacitance and how much energy is being withdrawn by the power converter which is the capacitor's load. This, in turn, depends on the amount of power demanded by the output load on the converter. With a light load, the conduction angle may be just a few degrees. At full rated load, the conduction angle will be larger, but even with heavy loads, conduction is not continuous.

The current has the form of relatively large, short-duration pulses. Because the a-c mains exhibit a non-zero source impedance, the high current peaks cause some clipping distortion on the peaks of the voltage sinusoid. Fourier analysis would show that this lowers the power factor significantly.

### True, Reactive, and Apparent Power | Power Factor | Electronics Textbook

Since power factor represents the ratio of real to apparent power, the high apparent power that yields a low power factor translates into a higher current than the load actually needs to satisfy its real power requirement. The difference between the current that produces the real power consumed by the load and the current measured on an ammeter is known as the circulating current. It is so called because even though it does no real work, it continuously flows back and forth between the mains and the load.

FIGURE 16 - Waveforms illustrating the peak flattening effect that the narrow current pulses impose on the mains voltage A switching converter with 80 percent efficiency and an uncorrected power factor of 0. Equipping this power supply with power factor correction, despite lower conversion efficiency, allows it to use the full 12 amperes to produce real power for its load. With an overall efficiency of The discontinuous current pulses created by the charging action of a power supply's input circuit is another form of EMI.

As such it can affect the operation of sensitive equipment operated in close proximity to the a-c mains. This interference takes two forms. First, the high amplitude of the current pulses generate electromagnetic fields strong enough to be detected by sensitive amplifiers.

Second, as the current pulses occur around the peaks of the voltage waveform, the IR drop in the wiring flattens the voltage waveform producing harmonic distortion. This may adversely affect instruments that depend upon the presence of a normal a-c sinusoid.

**KVA and KW, Active Reactive and Apparent Power difference**

When more than one power supply operates from such distorted mains, the problem is compounded as each power supply tries to charge its input capacitor from the same peak of the a-c voltage. EN The European electrical system distributes power at volts. This means that the current is half what it would be in the USA for an equivalent load. Because of this, European distribution systems use smaller gauge wire and lower amperage fuses.

As a result, they are more sensitive to circulating current than their USA counterparts. With the goal of minimizing circulating current, the International Electrotechnical Committee IEC took a look at the discontinuous currents produced by switch mode power converters and other electrical equipment.

## True, Reactive, and Apparent Power

Any discontinuous waveform consists of a pure sine wave at the fundamental frequency plus sine waves of various amplitudes occurring at each of the fundamental's harmonic frequencies. To distinguish reactive power from active power, it is measured in units of " volt-amperes reactive ", or var. These units can simplify to Watts, but are left as var to denote that they represent no actual work output.

Energy stored in capacitive or inductive elements of the network give rise to reactive power flow. Reactive power flow strongly influences the voltage levels across the network. Voltage levels and reactive power flow must be carefully controlled to allow a power system to be operated within acceptable limits. A technique known as reactive compensation is used to reduce apparent power flow to a load by reducing reactive power supplied from transmission lines and providing it locally.

For example, to compensate an inductive load, a shunt capacitor is installed close to the load itself. This allows all reactive power needed by the load to be supplied by the capacitor and not have to be transferred over the transmission lines. This practice saves energy because it reduces the amount of energy that is required to be produced by the utility to do the same amount of work. Additionally, it allows for more efficient transmission line designs using smaller conductors or fewer bundled conductors and optimizing the design of transmission towers.

A capacitor is an AC device that stores energy in the form of an electric field.

As current is driven through the capacitor, charge build-up causes an opposing voltage to develop across the capacitor. This voltage increases until some maximum dictated by the capacitor structure. In an AC network, the voltage across a capacitor is constantly changing. The capacitor opposes this change, causing the current to lead the voltage in phase. Capacitors are said to "source" reactive power, and thus to cause a leading power factor. Induction machines are some of the most common types of loads in the electric power system today.

These machines use inductorsor large coils of wire to store energy in the form of a magnetic field. When a voltage is initially placed across the coil, the inductor strongly resists this change in current and magnetic field, which causes a time delay for the current to reach its maximum value.