Power Factor Explained | Power Factor
A brief explanation of the Power Factor
Hello Everyone. Today I am going to share the power factor explained. I will try to start off really easily with some simple analogies that will help you to understand the basics and then I will try to advance it into electrical engineering terms with some worked examples as well as looking at what is power factor, what is a good and bad power factor, what causes bad power factor and how to fix bad power factor. Let’s start.
What is the power factor?
Power factor is a unit list number using alternating current circuits. It can be used to refer to a single piece of equipment such as an induction motor or the electricity consumption of an entire building.
Both cases represent the ratio between true power and apparent power. The formula is:
So what does that mean?
To understand the meaning, I used an analogy to explain, named the beer analogy. We pay for beer by the glass, but inside the glass, there is both beer and foam. As seen in the picture.
The more beer in the glass means the foam will be less there. So we get very good value for money. If there is a lot of foam, then there’s not a lot of beer and so we’re not getting very good value for money.
The beer represents our true power or Kilowatts, which is the useful stuff we need and want. This is what does the work. The foam represents our reactive power or kVAr means Kilovolt Amps Reactive.
This is the use of stuff. There will always be some. We have to pay for it, we can’t use it, so we don’t really want much of it. It does actually have a use and a purpose, but we’ll know why later in the post.
The combination of this KW and kVAr means the Apparent power or KVA, which means the Kilovolt Amps.
Power factor is therefore the ratio of useful power or true power in kilowatts or kW divided by what we’re charged for in KVA, kilovolt amps. So it’s telling us how much value for money we’re getting for the power we consume.
If we briefly touch on electrical engineering and I will keep this part brief, then we might see this expression as a power triangle.
In this case, I’ll draw it as a leading power factor as is easier to visualize. The true power which is beer is an adjacent line. Then the foam which is reactive power on the opposite. Then the hypotenuse side which is the apparent power is the largest side.
This is at an angle from the true power. The angle is known as theta. You see as the reactive power or the foam increases and so does the apparent power on the KVA. Then we could use trigonometry to calculate this triangle. I won’t in this post as I’m just covering the basics.
Engineering Formulas Trigonometry:
So we’ll just see the formulas you need, but we’ll do some calculations and work examples later in this post.
If we look at a typical residential electricity bill, then we’ll typically see just a fee for the number of kilowatt hours used because the power factor and the electricity consumption will be very low.
So electricity companies tend to not worry about this. However, on commercial and industrial electricity invoices, especially building through smart or interval electricity meters, then we’ll likely see charges and information for the number of kilowatts.
Kilowatt-hours, kilovolt amps, and kilovolt amps reactive Use large buildings, in particular, will often see reactive power charges there, but this depends on the electricity supplier and the agreement they have with the consumer.
The reason they charge a penalty for this. Because when large consumers have bad power factors, they are increasing the current flow through the electricity network and occurred voltage drops, which affect the supplier’s distribution, reduce distribution capacity, and have a knock-on effect on the other customers.
To handle a certain amount of current flowing through the cables is rated. So if a lot of large consumers connect with bad power factors, then the cables could overload.
They could also struggle to meet the demand and the capacity agreements, and then no new customers will be able to connect until they either replace the cables or install additional cables.
When the power factor of a building falls below a certain level, reactive power charges occur. The level is specified by the electricity supplier that typically starts around 0.95 and below. A perfect power factor would be one.
However, in reality, this is almost impossible to achieve. I’ll discuss this part later in the post.
Rule of Thumb for Commercial Buildings
In large commercial buildings, the overall power factor is likely to sit in the following categories good power factor is generally between one and 0.95.
Poor power factor is anything from 0.85 and 0.95. But a bad power factor is any value below 0.85.
The power factor in commercial office buildings is usually somewhere between 0.98 and 0.92. An industrial building’s power factor could be as low as 0.7.
We’ll look at what causes this shortly.
Let’s first have a look at an example. If we compare two induction motors that both have an output of 10 kW and are connected to a three-phase 415V/50Hz supply. One motor has a power factor of 0.87 and the other one with a power factor of 0.92.
Both motors will deliver 10kW, but the first motor has a lower power factor compared to the second one, meaning we’re not getting as much value for money. The first motor will need to draw 11.5 KVA from the electricity grid to provide the ten kW of power.
The second motor will need to draw just 10.9 KVA from the electricity grid to provide the same 10kW power which means the first motor has 5.7 kVAr and the second motor has just 4.3 kVAr.
Remember, our kilowatts are the beer and that’s the useful stuff. The kVAr is the foam that’s the not-so-useful stuff. The KVA is what we’re going to pay for and that’s the kilowatts and the KVR combined.
How do I calculate that?
So what causes poor power factor?
In most cases, inductive loads affect the power factor. If we have a purely resistive load such as an electrically resistive heater, then the voltage and the current waveforms would be in sync or very close.
They would both pass through their maximum and minimum point and then pass through the zero axis at the same time.
The power factor, in this case, is one that is perfect. If we drew a phasor diagram, then the voltage and current would be parallel which means all the energy drawn from the electricity supply goes to doing work, and nothing will be wasted.
In this case, creating heat. But if we took an inductive load such as an induction motor, then the coil’s magnetic field holds back the current and results in a phase shift where the voltage and the current waveforms fall out of sync with each other.
So the current passage through the zero point after the voltage. This value is referred to as a lagging power factor. Earlier in the post, I said the foam or the cave are useless. That’s not exactly true.
To create and maintain the magnetic field, we actually need some reactive power because it rotates the motor.
In this sense, the reactive power is wasted and we get no work from it. But we still have to pay for it, although we do need it to be able to do the work in the first place.
If we drew a phaser diagram for a purely inductive load, then the current would be at an angle below the voltage line meaning, that not all the electricity consumed is doing work. If we took a purely capacitive load, then the opposite happens to the inductive load.
The voltage and current are out of phase, except this time the voltage is held back. This causes a leading power factor again, this will mean that not all the electricity being used is being used to do work, but we still have to pay for it regardless.
If we drew a phaser diagram for a purely capacitive load, then the current line would be at an angle above the voltage line as it’s leading.
Correcting poor power Factor
What can we do to correct poor power factors and reactive power charges?
In most cases, we come across a lagging power factor caused by inductive loads to correct the poor power Factor we can add Capacitors or Inductors to the circuit which will realign the current back into phase and bring the power factor closer to one.
If we have a Lagging power factor caused by high inductive loads in the circuit, then we add Capacitors.
If we have a leading power factor caused by high Capacitive loads, then we add an Inductive load to the circuit. These need to be calculated and we’ll see some example calculations of this at the end of the post.
Why should we fix the poor power factor?
Poor power factor means you need to draw more power from the electricity network to do the same amount of work.
Therefore, the cables need to be larger so the installation is going to cost more money. If the power factor becomes too low, then the electricity supplier might charge you a penalty fee or a reactive power charge.
Because poor power factors lead to gains high heat and also it can cause losses in equipment like transformers.
Capacitor Calculations for Power Factor Correction
Let’s look at the simplified example of calculating the size of a Capacitor to improve the power factor of a load.
