# Reactive Power Formula in AC Circuits

#### By R.W. Hurst, Editor Reactive power is a critical component of AC power systems, and it plays a crucial role in sustaining the magnetic and electric fields of inductors and capacitors. The reactive power formula is Q = V × I × sin(φ), where Q is the reactive power, V is the voltage, I is the current, and φ is the phase angle between the voltage and the current.

The power factor is crucial in the reactive power formula, as it affects the amount of reactive power required to sustain the circuit. Reactive power compensation is used to balance the reactive power in the circuit, while power factor correction is used to improve the power factor of AC circuits, particularly induction motors. Therefore, understanding the reactive power formula is essential for electrical engineers and technicians to design, maintain, and operate AC power systems.

In an AC power system, reactive power is crucial in sustaining the magnetic and electric fields of inductors and capacitors. However, reactive power is an alternating current component that does not contribute to the useful work performed by the circuit, such as heating, lighting, or mechanical motion. Instead, reactive power oscillates back and forth between the source and the load, causing current and voltage phase shifts.

Reactive power is measured in Volt-Amperes Reactive (VAR) or Kilovolt-Amperes Reactive (KVAR). Therefore, understanding the formula is essential for designing, maintaining, and analyzing AC power systems. This article will explain the concept of reactive power, the formula, how to calculate it in AC circuits, and the significance of power factor in reactive power.

Reactive Power

In an AC power system, reactive power is the power that flows back and forth between the source and the load without being consumed or dissipated. Reactive power is essential for sustaining the magnetic and electric fields of inductors and capacitors. It is not used for performing useful work but is necessary for operating electrical equipment, including motors, transformers, and generators. Reactive power is denoted by Q and is measured in VAR or KVAR. In an ideal AC circuit, the voltage and current are in phase, and there is no reactive power flow. However, the voltage and current are not always in phase, and the reactive power flow can be inductive or capacitive.

Power Triangle

The power triangle visually represents the relationship between active, reactive, and apparent power in an AC circuit. Active power is the component of the power converted into useful work, measured in watts (W) or kilowatts (kW). Reactive power is the component that is not converted into useful work and is measured in VAR or KVAR. Apparent power is the total power delivered to the circuit, which is the vector sum of active and reactive power. The power triangle is a right triangle where the hypotenuse represents the apparent power, the adjacent side represents the active power, and the opposite side represents the reactive power. The Greek letter phi (φ) represents the power factor angle between the apparent and active power. The power factor is defined as the ratio of active power to apparent power.

Reactive Power Formula

The formula for reactive power is Q = V × I × sin(φ), where Q is the reactive power, V is the voltage, I is the current, and φ is the phase angle between the voltage and the current. The phase angle is the angular difference between the voltage and the current waveforms and is measured in degrees or radians. If the current waveform leads the voltage waveform, the phase angle is negative, and if the current waveform lags the voltage waveform, the phase angle is positive. The  formula can calculate the reactive power in an AC circuit if the voltage, current, and power factor are known.

Reactive Power Calculation

We need to know the voltage, current, and phase angle to calculate reactive power. In a single-phase AC circuit, we can use a voltmeter and an ammeter to measure the voltage and current. The phase angle can be calculated using a phase meter or a power analyzer. In a three-phase AC circuit, we need to measure the voltage and current in each phase and calculate the average power factor and phase angle. If the voltage, current, and power factor are known, we can use the formula to calculate the reactive power in an AC circuit.

The formula for 3-phase reactive power is Q = sqrt(3) x V x I x sin(φ), where Q is the reactive power, V is the line voltage, I is the line current, and φ is the phase angle between the voltage and the current. The factor sqrt(3) is the square root of three, which accounts for the three phases in a three-phase AC circuit.

KVAR Calculation

Sometimes, we may need to calculate the KVAR in an AC circuit. KVAR is simply the reactive power in Kilovolt-Amperes Reactive. To convert VAR to KVAR, we need to divide the VAR value by 1000. Therefore, the formula for calculating KVAR is Q (KVAR) = Q (VAR) / 1000.

Power Factor

The power factor measures the efficiency of an AC circuit, and it is defined as the ratio of active power to apparent power. A value represents the power factor between 0 and 1 or a percentage between 0% and 100%. A high power factor means the circuit is efficient, and most of the power is used for useful work. In contrast, a low power factor means that the circuit is inefficient and a significant portion of the power is wasted. Power factor correction can improve the power factor, which involves adding capacitors to the circuit to reduce the reactive power.

Induction Motors

Reactive power is an important factor in the operation of induction motors. Induction motors require both active power and reactive power to operate. The ratio of reactive power to active power is known as the motor's power factor. A low power factor in an induction motor can cause problems such as reduced efficiency, overheating, and equipment damage. Therefore, power factor correction is used to improve induction motors' power factor and optimize their operation.

Reactive Power Compensation

Excessive reactive power in an AC circuit can cause problems such as voltage drops, power losses, and equipment damage. To address this issue, reactive power compensation is used to balance the reactive power in the circuit. Reactive power compensation is achieved using devices such as capacitors, inductors, and static VAR compensators (SVCs).

• 1
• 2
• 3
• 4
• 5
• 6
• 7
• 8
• 9
• 10
• 11
• 12
• 13
• 14
• 15
• 16
• 17
• 18
• 19
• 20
• 21
• 22
• 23
• 24
• 25
• 26
• 27
• 28
• 29
• 30
• 31

Top