Reactive power (Q) is the oscillating energy exchange in AC circuits due to inductors and capacitors, which does not contribute to real power (P).
When the circuit is a DC circuit, we can quickly multiply volts by amps to get watts of power used. This is true to purely resistive AC circuits as well.
But in reactive components like inductors or capacitors, things get interesting. The volt- amp product (VA) can change with frequency, affecting the circuit’s reactive power.
Apparent Power
In AC circuits, VA (volt-amperes) or kVA (kilo volt-amperes) represents the product of voltage and current, also known as apparent power (S).
This applies to purely resistive circuits, for example heaters or light bulbs where resistance dominates, and voltage and current are “in-phase.”
Here, multiplying voltage with current (root mean square values) gives us the equivalent DC power or heating effect.
Out-of-Phase and Power Loss
But reactive components cause the voltage and current waveforms to be “out-of-phase,” meaning their peaks dont line up.
At maximum 90-degree phase angle, the volt-amp product constantly flips between positive and negative values.
In other words the reactive circuit gives back as much power as it takes, resulting in zero average power consumption.
Energy just flows back and forth between the source and the load.
Active power depends on the phase angle
Since voltage and current exist but no power is dissipated, the simple P = VI (rms) formula doesn’t hold true. The volt-amp product alone isn’t enough.
To find “real power” used (called active power, symbol P), we need to consider both the VA and the phase angle difference between voltage and current. This can be obtained by the equation: P = VI cosΦ.
Apparent vs. Real
This equation relates apparent power (S) to active power (P) by considering the phase angle (Φ), as shown below:
- Active Power = Apparent Power (S) * Power Factor (pF)
- Power Factor (pF) = Active Power (P) in Watts / Apparent Power (S) in volt-amps
Power Factor and Reactive Power in AC Circuits
Power factor (PF) is the ratio of active power (P) in watts to apparent power (S) in volt-amperes which indicates how efficiently power is used. In purely resistive circuits (no inductors or capacitors), active and apparent power are equal (P/S = 1), causing a unity power factor.
But, AC circuits often have a phase angle due to reactive components. This introduces reactive power (Q) which is sometimes called imaginary power, measured in volt-amperes reactive (VAR) and calculated as VI sinΦ. Reactive power doesn’t perform useful work but affects efficiency.
Reactive power (VAR) isn’t true power, rather it arises from the interaction of voltage and current that are out of sync (out-of-phase).
It acts as a crucial but invisible force maintaining the electric and magnetic fields needed by AC equipment.
The amount of VAR present depends on the phase angle, the difference in timing between voltage and current. Like active power, VAR can be positive (delivered) or negative (absorbed).
Most equipment relying on magnetic fields, like motors, transformers, and even transmission lines (to compensate for losses), require VAR for proper working.
The three types of power in an AC circuit – active (watts), apparent (VA), and reactive (VAR) – can be pictured as the sides of a right angled triangle. This diagram is called a power triangle, like the one shown here:
The power triangle shows us that, AC circuits deal with two main power types: active power (watts) and reactive power (VAR).
Active power does the real work (watts used by appliances), and it’s always positive.
Reactive power however, can be positive or negative, and it doesn’t do direct work.
Ideally we would want to minimize reactive power because it reduces overall system efficiency.
The main benefit of AC is the ability to adjust voltage levels using transformers. But, transformers and motors in appliances (AC units, industrial equipment) consume reactive power.
This extra current flow takes up space on power lines, requiring larger conductors and transformers which ultimately increases costs.
A Transport Truck Analogy
Let’s think the AC circuit like a delivery truck. Active power (watts) is like the actual goods delivered, used by appliances for their work. You pay for the amount of goods delivered.
Reactive power (VAR) however, is like passengers in the truck. They add weight and require the truck to work harder, but they don’t contribute directly to the delivery.
Although it is not a cost itself, reactive power increases the overall load on the system requiring thicker wires and larger transformers, which can translate to higher costs in the long run.
Reactive Power in Industry
In industrial electricals, reactive power (VAR) plays a surprising role.
Even though active power (watts) does the real work (powering motors, heating, etc.), reactive power helps maintain voltage levels.
This ensures that efficient power transfer across the grid and transmission lines.
Reducing reactive power generally improves efficiency (power factor). However a minimum amount is crucial for voltage regulation and transmission losses.
Without enough reactive power, there may be voltage drops, inhibiting active power delivery.
It is important finding the balance. Too much reactive power creates excessive heat (I2*R losses) and voltage drops along the lines, leading to power loss.
Improving Efficiency: Power Factor Correction
Reactive power can lead to additional charges for industrial customers.
To avoid these charges, power factor correction capacitors can be installed which help to optimize the ratio of active power (doing real work) to apparent power (total delivered power).
Unlike residential customers who are typically billed only for active power usage (kWh), industrial users with three-phase power supplies can have varying power factors.
Electricity companies may penalize them for power factors below a certain level.
This is because, low power factor requires larger equipment (conductors, transformers) to handle the increased current, which causes higher costs for the utility company.
In simpler words, power factor correction helps industries and factories to avoid extra charges, by ensuring they’re billed only for the “real” power used, not the reactive power that doesn’t do actual work.
Power Factor: Maximizing Your Power Utilization
The power factor tells us, how efficiently a circuit uses power. Here’s a clear breakdown:
- Power Factor Below 0.95: This indicates that the circuit needs more reactive power. Although it doesn’t do real work, we need it for efficient operation.
- Power Factor Above 0.95: This is considered good, as it indicates that the circuit uses power effectively, minimizing wasted reactive power.
- Power Factor of 1.0 (Unity): This is ideal as it indicates that the circuit uses only real (active) power and doesn’t require any reactive power.
Remember, the apparent power is the total delivered power (a combination of active and reactive power).
Active power comes from resistive components (like heaters), while reactive power comes from inductors and capacitors. Most AC circuits have a mix of these components (R, L, and C).
Since reactive power doesn’t do actual work, it’s important that we consider it in AC systems.
We need to ensure that the power source can supply enough apparent power (VA) to meet the load’s demands, considering both active and reactive power. This is crucial, for a stable and efficient AC power system.
References: Reactive Power in AC
https://www.electricityforum.com/iep/power-quality/reactive-power-formula
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