7.2 Reactive Power - PSA Library

## Introduction When working with alternating current (AC) circuits, you might notice that not all the power supplied by the source is actually used to do work like lighting a bulb or running a motor. Some of the power seems to "go away" and then come back. This is because part of the power is stored temporarily in the circuit and then returned to the source. This stored and returned power is called reactive power. Understanding reactive power helps technicians diagnose and manage electrical systems more effectively, especially when dealing with inductors and capacitors. ## Key Concept Reactive power is the portion of AC power that oscillates between the source and reactive components (inductors and capacitors) in the circuit. It does not perform any real work but is essential for maintaining electric and magnetic fields in these components. Reactive power is measured in volt-amperes reactive (VAR) and is given by the formula: $ Q = I_{\text{RMS}}^2 X $ where: - $Q$ is the reactive power in VAR, - $I_{\text{RMS}}$ is the root mean square current in amperes, - $X$ is the reactance in ohms (either inductive reactance $X_L$ or capacitive reactance $X_C$). Reactive power exists because inductors and capacitors store energy temporarily in magnetic and electric fields, respectively, and then return it to the circuit during each AC cycle. ## How It Works - In an AC circuit, inductors resist changes in current by building a magnetic field. This magnetic field stores energy during part of the AC cycle. - Later in the cycle, the magnetic field collapses, releasing the stored energy back into the circuit. - Similarly, capacitors store energy in an electric field when they charge and return it when they discharge. - This back-and-forth energy exchange does not consume power but causes current and voltage to be out of phase. - Reactive power quantifies this energy storage and return process. - Unlike real power, which is consumed and converted into heat or mechanical work, reactive power is "wattless" power and is necessary for the operation of AC systems with inductive or capacitive loads. ## Real World Application Technicians encounter reactive power when working with motors, transformers, and fluorescent lighting. For example, an induction motor draws reactive power to create the magnetic field needed for operation. Although this power does not perform mechanical work, it affects the total current drawn from the supply and can cause increased losses in wiring and equipment. Understanding reactive power helps technicians size equipment correctly, improve power factor, and reduce energy costs by minimizing unnecessary current flow. ## Safety Notes Handling circuits with reactive power requires attention to proper grounding and insulation because the voltage and current can be out of phase, leading to unexpected current flow. NFPA 70E and OSHA guidelines emphasize the importance of de-energizing circuits before work and using appropriate personal protective equipment (PPE) to prevent electric shock or arc flash incidents. Reactive power can cause higher currents in conductors, so ensuring equipment is rated for the expected load is critical to avoid overheating and fire hazards. ## Summary Reactive power is the power that alternates between the source and reactive components in an AC circuit. It is measured in VAR and represents energy stored and returned by inductors and capacitors. While it does not perform useful work, reactive power affects the total current and system efficiency. Technicians must understand reactive power to properly analyze AC circuits, size equipment, and maintain safe and efficient electrical systems. ## References - NFPA 70E Standard for Electrical Safety in the Workplace - NETA ATS (Acceptance Testing Specifications) - IEEE Std 100 - IEEE Standard Dictionary of Electrical and Electronics Terms - Paul Scherz, Simon Monk, Practical Electronics for Inventors - Handbook for Electricity Metering, Edison Electric Institute > [!columns] > >[!info] Previous lesson > ⬅️ [[7.1 True Power]] > > >[!info] Next lesson > ➡️ [[7.3 Apparent Power]]