Imagine ordering a meal at a new restaurant in town. You sit down and select something from the menu, and the waiter brings you a plate of food. If the meal is excellent and you eat everything, this is an efficient way of satisfying your hunger. However, it becomes less efficient if you do not like some of the food or if it is inedible. This part of the meal is still prepared, cooked, and served to you, but it is ultimately returned to the kitchen uneaten. You could calculate the proportion of the food that you ate compared to the size of the overall meal, which would tell you how efficient this restaurant is at meeting your needs.
More generally speaking, if you consider the proportion of any consumable that meets a need, compared to the proportion that is instead returned to the source, unused, you have a measure of how efficiently a system meets that need. When you apply this to an electrical circuit, where the consumable is electrical power, the efficiency measure is the power factor.
An efficient electric circuit is one in which the load uses most of the supplied power to do work. Whereas an inefficient electrical circuit is one that draws electrical power, but uses a part of it to do things that are not productive. During each alternating current cycle, the circuit draws that power from and returns it to the power source. The power factor is the ratio of the electrical power that the load uses compared to the overall amount of power supplied to the circuit. This calculation tells you how efficient the electrical circuit is at supplying power to do work.
Know your Powers
Active power
Active power is the name given to the power consumed by an electrical circuit doing something useful, like powering street lights or running a production line. The measurement of active power is watts (W), or multiples thereof, such as kilowatts (kW) and megawatts (MW). A device that runs on electrical power has a rating based on the active power that it consumes, such as a 60W light bulb or a 500kW arc furnace.
Reactive power
Reactive power is the term used to describe the power drawn by an electrical circuit to do things that are not productive like charging a capacitor or creating the magnetic field in an induction motor. Its unit of measurement is volt-ampere reactive (VAR). In an electrical circuit, devices producing or consuming reactive power, such as shunt reactors and capacitors, have a VAR rating.
Apparent power
Apparent power is the total power passing through an electrical circuit, both active and reactive. It is a significant value to consider when designing and rating an electrical circuit. The measurement of apparent power is volt-ampere (VA). You will often see devices used to generate or transform electrical power rated in kVA, or MVA. Examples may include a 50kVA transformer or a 1000kVA diesel generator.
How do you measure power factor?
You can connect monitoring devices to critical locations in an electrical circuit to measure voltage and current. These locations include generator outputs, transformer connections, and large, intermittent, or unpredictable loads. Such a monitoring device can use the measured voltage and current values to calculate the active, reactive, apparent power, and power factor. More advanced devices such as the PQube® 3 power analyzer, can record this data over time and generate regular reports.
Leading and lagging power factor
Returning to the restaurant, in a perfect world, the waiter will serve the main course and the side dishes simultaneously. This is the most efficient method of delivering your meal. If the side dishes arrive at your table very early or late, it will reduce how satisfied you are with the service. The earlier the side dishes lead, or the later they lag behind the main course, the less acceptable the situation becomes. In much the same way, the alternating current in an electrical circuit can lead, lag, or match the voltage, and this has an impact on the power factor.
In an ideal electrical circuit, where the impedance of the load is only resistive, the current and voltage pass through the load at the same time. When this occurs, the apparent power is equal to the active power and the power factor is one. This is also known as unity power factor. Power factor is also sometimes described as either leading or lagging. With a leading power factor, the current leads the voltage, and the circuit is generating reactive power. This is due to the impedance of the electrical circuit being mainly capacitive, such as with underground cables. With a lagging power factor, the opposite is the case, the current lags the voltage. This electrical circuit is consuming reactive power, caused by a predominantly inductive load, such as an induction motor.
In summary, power factor is simply expressed as the ratio of active power to apparent power. The closer this ratio is to one, the more of the apparent power is active power supplied to the load, and the more efficient the electrical circuit is. The more the power factor leads or lags, the larger the deviation from unity power factor, and the greater the power requirements for the electrical circuit.
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