# Apparent Power in AC Circuits

#### By R.W. Hurst, Editor

Apparent Power Definition

Apparent power is a fundamental concept in electrical circuits, and it reflects the total power consumed or delivered by an AC circuit. Apparent power is different from real and reactive power because it considers the phase difference between the voltage and current. Understanding apparent power is essential for anyone working with AC power systems, as it allows for designing and operating more efficient and reliable electrical systems. Furthermore, by improving the power factor and reducing reactive power consumption, one can reduce energy costs and promote sustainable energy use.

Apparent power is measured in VA, the product of volts and amperes. In a purely resistive load, where the current and voltage are in phase, the apparent power is equal to the real power. However, in an inductive or capacitive load, where the current and voltage are not in phase, the apparent power is greater than the real power due to the presence of reactive power.

The formula for Apparent Power

The formula for calculating apparent power is:

S = VI

where S is the apparent power in VA, V is the voltage in volts, and I is the current in amperes.

The formula shows that the apparent power of a circuit is proportional to the product of its voltage and current, regardless of the nature of the load. Therefore, it is essential to know both the voltage and current to determine the apparent power of a circuit.

Difference between Real Power, Reactive Power, and Apparent Power

Real power, also known as active power, is the power that is used to do work. It is expressed in watts (W) and is the only type of power that can do useful work. Real power is the product of the voltage, current, and power factor, the ratio of real power to apparent power.

Reactive power, on the other hand, is the power that is required to maintain the electric and magnetic fields in the circuit. It is expressed in volt-amperes reactive (VAR) and is measured in units of reactive volt-amperes (VAr). Reactive power does not do any useful work, but it is necessary to support the functioning of inductive loads, such as motors and transformers.

Apparent power is the total power that is delivered to the circuit, and it is the sum of the real power and reactive power. It is measured in volt-amperes (VA) and considers the phase difference between the voltage and current.

Power Factor and Apparent Power

Power factor measures how much of a circuit's apparent power is converted into useful work (real power). It is the ratio of real power to apparent power and is expressed as a decimal or a percentage. The power factor ranges from 0 to 1, with a value of 1 indicating that all the power is used for useful work and no power is wasted.

A low power factor means that a significant amount of the apparent power is used for reactive power rather than performing useful work. In addition, a low power factor can cause problems in the distribution of electrical energy, such as voltage drops, overloading of transformers, and increased energy costs.

When power factor is low, the power system draws extra current and uses more VA. Therefore, the immediate benefit s of improving a circuit's power factor are lower current draw and reduced VA demand. This means that electricity and electrical assets can be used more effectively by converting more apparent power into useful work.

Importance of Apparent Power in AC Circuits

Apparent power is an essential concept in AC circuits because it reflects the maximum amount of power that is being delivered to the circuit.  In addition, it defines the required size of the utility and facility electrical infrastructure required to serve the circuit. Understanding the apparent power of a circuit is important because it allows us to design and operate electrical systems more efficiently and reliably.

In AC circuits, the impedance of the load affects the apparent power of the circuit. The impedance of a load can be purely resistive, inductive, or capacitive, or a combination of these. A purely resistive load has no reactive power, and the apparent power is equal to the real power. In contrast, an inductive or capacitive load has a reactive power component that increases the apparent power.

Power Triangle and Apparent Power

The relationship between real power, reactive power, and apparent power can be visualized using a power triangle. The power triangle is a right triangle with the hypotenuse representing the apparent power, the adjacent (horizontal) side representing the real power, and the opposite (vertical) side representing the reactive power.

The angle between the hypotenuse and the adjacent side is known as the power factor angle, and it indicates the phase difference between the voltage and current. The power factor angle is important because it affects the power factor and the efficiency of the circuit.

Measuring Apparent Power

Apparent power is measured using a wattmeter, which measures both the real power and the reactive power in the circuit. The wattmeter can be used to calculate the power factor and the apparent power of the circuit.

In three-phase power systems, the apparent power is the sum of the apparent powers of the individual phases. The total apparent power is calculated by adding the apparent powers of the three phases.

Power Factor Correction and Apparent Power

Power factor correction techniques can be applied to improve the power factor of a circuit and reduce reactive power consumption. These techniques involve using capacitors or inductors that provide reactive power to the circuit and thus reduce the amount of reactive power required from the power supply.

By reducing the reactive power consumption, power factor correction can also reduce the apparent power (kVA) and the total current that flows through the circuit. This can result in more effective use of electrical energy leading to a lower cost of energy and less demand on electrical assets.

What is the difference between real power and apparent power?

Real power is the power used to do work, while apparent power is the total power consumed or delivered by an AC circuit. Real power is expressed in watts (W), while apparent power is expressed in volt-amperes (VA).

What is the power factor, and how does it relate to apparent power?

Power factor is a measure of the amount of apparent power of a circuit that is being converted into useful work. It is the ratio of real power to apparent power and is expressed as a decimal or a percentage. The power factor is related to apparent power because it reflects the amount of both real and reactive power in the circuit. Power Factor indicates what percent of the apparent power is doing real work.

Power factor can also define the angle between the voltage and current waveforms because the cosine of this angle is the power factor. (Example Cosine 30° = 0.866 PF).

Low power factor indicates that more current is being consumed thank should be necessary for the given real power demand. This extra current increased the kVA demand on the power source and can increase circuit conductor and transformer losses.

Why is apparent power important in AC circuits?

Apparent power is important in AC circuits because it reflects the maximum amount of power that can being demanded by the circuit. It measures the total demand on a power source and defines how large the utility and facility electrical infrastructures must be to serve the loads. It is also used to calculate the power factor, which measures the percent of apparent power being converted into useful work, and therefore how effectively we are utilizing electrical assets.

What is the difference between real, reactive, and apparent power?

Real power is the power used to do work, while reactive power is the power required to maintain the electric and magnetic fields in the circuit. Finally, apparent power is the total power that is consumed or delivered by an AC circuit and is the sum of the real power and reactive power.

How does reactive power affect apparent power in electrical systems?

Reactive power affects apparent power in electrical systems by increasing the total power consumed or delivered to the circuit. Reactive power does not do any useful work, but it is necessary to support the functioning of inductive loads, such as motors and transformers. As a result, the apparent power is greater than the real power in circuits with inductive loads.

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