I was surprised when I found many basic questions about power factor. So, I decided to answer 8 important questions about power factor.
What Is The Unit Of Power Factor?
There is no measure unit for the power factor, which is the ratio of active (KW) to apparent power (KVA).
That is clear on motors nameplate, The power factor is only mentioned as a number (like, PF = 0.85).
Its formula is presented in two forms;
- Power Factor = KW/KVA =Active Power/Apparent Power
- Power Factor = Cos (Theta)
Also, because it is the value of a certain degree, there will be no unit.
The second question I have in this article is about power factor correction.
Is power Factor correction Required?
Yes, power factor correction is required for large industrial and commercial applications.But it is not required for your home. In fact power factor correction at home is useless as most appliances are unity power factor.
Improving the power factor leads in less current, which results in cheaper energy costs, less heat, and longer electrical system lifespan.
Countless electricity providers charge both a base load (kW) rate and a maximum demand tariff. If the maximum demand tariff is evaluated in kVA, increasing the power factor lowers the kVA of the loads, lowering the maximum demand tariff and, as a result, the power bills.
It is a network requirement that users maintain a particular minimum power factor depending on your location. When a customer’s power factor is less than a specified level, utility companies may add a penalty on top of consumption charges.
For more details about power factor correction importance you can read my article here.
Talking about power factor correction leads us to the important fact that, to improve power factor we use capacitor bank! Why capacitors?
Why Do We Use Capacitors In Power Factor Correction?
Capacitors are used in electric power distribution to rectify power factor. The goal is to counterbalance inductive loading from devices such as induction motors, and transformers, making the load look primarily resistive.
Capacitors supply Lagging VAR required by the appliances, thereby reducing the Reactive power drawn from the Mains supply. This in turn will reduce the power factor of the entire system. Also, the system efficiency is improved.
Most electrical loads power factor is lagging in nature.Its clear that most industrial loads are large induction motors and transformers. Inductance is the main reason for lagging power factor in almost all the loads.
Also loads like fans, AC air compressors and also refrigerator and air conditioners, they all have inductive power that makes them low power factor sources.
When these appliances are in use, all make use of lagging VAR. If the appliances consume more VAR, then the efficiency of the machine is poor and losses occur.
Capacitors ideally only draw Reactive Power. Capacitors don’t absorb energy consistently. They always return bounce electrical energy back to the source.
It is important to mention that a capacitor has a leading current at 90 degrees to the voltage. Therefore, always, capacitor draws zero active power. They either supply Lagging VARs or Consume Leading Vars.
You can find more details about apparent and reactive power in my articles, “What is Reactive Power?” And , ” What is Apparent Power?“
Is Power Factor Always Positive?
No, Power factor can sometimes be negative especially when the active power is negative.
Power factor is described as the ratio of the actual power flowing to the load, to the apparent power in the circuit, and is a number representing efficiency of the electrical system, but not measured on a scale of physical units and it is within a boundary of -1 to 1.
When power flows back towards the source in an electrical system, there will be a negative power factor. This usually considered as a generator.
For example, if an induction motor is used to run an electric railway locomotive, the power factor will be trailing by less than 90 degrees while the train climbs a hill. If the motor is nearly loaded, the power factor can be roughly 0.85. If the track is level at the top of the hill, the motor’s load will be lowered and the power factor could drop to 0.65.
As the locomotive descended further, the power factor declined much more and may have even turned negative as the engine started to behave as a generator, returning braking power to the source.
However, the fact that PF is negative indicates that active power is negative also. When you are evaluating a load, this indicates that the load sends energy to the system, therefore it operates as a generator.
If you would examine a generator, the negative active power indicates it acts as a load, therefore it drains energy from the system. This may happen in reality in one and only situation, when the generator’s turbine failed and rotation of the rotor is caused by rotating magnetic field of the system (the generator functions as a motor and it is turning the turbine) (the generator works as a motor and it is rotating the turbine).
Why Does DC Not Have Power Factor?
A DC circuit has no reactive components. Because current and voltage are always in phase, their phase difference is 0 degrees, and the cosine of zero is one i.e unity power factor.
I know it’s tough to remember, but the power factor in a DC circuit is always one, only one, and never anything other than one. Power factor only applies to alternating current supply.
It is important to note that electrical components are classified into three types especially when power is being applied to them, all of which are passive: resistive, capacitive, and inductive. Let’s go a bit deeper.
To begin, what exactly do I mean by resistive components? Resistive components limit the passage of electricity and generate voltage drops.
Second, inductive, an inductor is any conducting element with a coil that mutually causes the passage of current.
What occurs when an alternating current source is passed via an inductor? The inductor retains the current and pushes it across the opposite terminal, causing a delay, but the voltage is unaffected.
As a consequence, currents lag behind voltage, which is determined by XL = L = 2fL, where f is the frequency of the supplied voltage and L is the inductance of the inductor.
In addition, when a direct current (DC) supply is applied across an inductor, the inductor initially acts as a short circuit to the DC source, depending on the conditions. As a consequence, either the power supply or the inductor will be destroyed.
Third, when an electrical current is applied to a capacitor, it is said to be capacitive.
AC power supply: Instead of storing current like an inductor, the capacitor performs the opposite. It stores and delays voltage while current simply flows through it; the reactance is given as Xc = 1 / (2*pi*f*C).
Also, when Direct Current (DC) is applied across a capacitor, it works as an open circuit at first, and then when its capacity is reduced, i.e. the energy stored is at its maximum, it behaves as a short circuit.
It is clear that power factor applies solely to alternating current supply. Because capacitor acts as an open circuit and inductor acts as a short circuit. Both open and short circuit has no power factor.
Which is Better Leading Or Lagging Power Factor?
Both are circuitry defects that must be corrected, but the leading power factor is preferable.
The primary disadvantage of the leading power factor is that it generates a high voltage in the circuit, which may impair both the load and the power supply circuit.
A leading power factor implies a capacitive load current, while a lagging power factor provides an inductive load current.
The primary distinction between leading and lagging power factor is that leading power factor is attained when the load current leads the supply voltage.
While lagging power factor denotes that current lag behind voltage by a specific phase angle.
Increasing inductive loads may correct a leading power factor, but increasing capacitive loads may correct a lagging power factor.
Why Current Decrease After Power Factor Correction?
The whole purpose of power factor correction is to decrease current. A low power factor causes a higher current and voltage drop. Improving the power factor reduces the load current and voltage drop.
If the load is reactive, the current will be out of phase with the voltage by a certain amount. The voltage and the true current are in phase. The reactive current is 90 degrees – plus or minus – out of phase. Real current = total current multiplied by cos. Total current x sin = reactive current.
Vector addition is used to add the reactive current to the actual current. The increased current has no effect on power, but it does have an effect on line loss and transformer loss.
Can I Measure Power Factor?
Yes, Power factor is measured by one of two ways.
- Using a wattmeter since it is specified as W/VA. The wattmeter measures genuine power and generally provides a direct reading of volt-amperes as well as power factor.
- Another advanced way to measure the power factor is to use a power quality analyzer, This advanced device measures both active and apparent power (kW and kVA), and to calculate the ratio of kW/kVA which in fact is the power factor.
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