What is Power Factor (pf) for Electrical Students
Power factor (pf) plays a significant role in electrical inductive systems. By maintaining pf you can reduce current and reduce the voltage drop in the transmission line and electrical distribution system. Reduced current minimizes heating of electrical equipment such as transformers, cables, etc.
In this topic, we will see how it affects the electrical system and its advantages of it.
How does it create?
There are three types of loads in AC (alternating current) circuits such as resistive load, inductive load, and capacitive load.
As you know that in DC (direct current)circuit there is no angle between voltage and current. It is also called resistive load. In DC circuit PF is unity.
In AC (alternating current)circuit there is some angle between voltage and current. The angle between voltage and current varies. The cosine angle between voltage and current is the power factor (PF). The angle can be changed, so by variation of angle, PF can be changed.
Also Read:- basic electrical definitions and laws
Power factor is defined in many ways:-
1. It is the cosine of the angle between voltage and current
2. It is the ratio of active (KW) and apparent power (KVA).
3. The ratio of resistance and impedance i.e. R/Z, where Z=√(R 2+ X 2) Also X=2πf L, where L is the inductance of the coil and f is the frequency.
It is represented as Cosθ, where θ is the angle between voltage and current or the angle between active P (KW) and apparent S (KVA). You can use any other angle.
Consider P, Q, and S as active, reactive, and apparent power respectively.
PF (Cosθ) =KW/KVA=KW/√3 VI, for 3 phase
KW (P):- It is also active, useful, Working, or real power. It is the combination of voltage, current, and cosine angle (Cosθ).
KVA(S):- It is multiplying voltage (V) and current (I).
KVA=√ (KW) ²+ (KVAR) ², where KVAR (P) is the reactive power
S=√P²+Q², where S, P, and Q are apparent, active, and reactive power.
So from the above formula of KVA, it’s clear that among KVA, KW, and KVAR, the value of KVA is highest among that of KW and KVAR.
KVAR ( Q ):- It also is called nonworking power. It is used for creating and maintaining a magnetic field in an inductive circuit (or inductive load). Inductive loads require reactive power for creating a magnetic field. You can not diminish it. Only you can minimize it.
In the single Phase, P=VI Cosθ, KVAR (Q) =VI Sinθ,
In three Phase, P= √3 VI Cosθ, KVAR ( Q) =√3 VI Sinθ
Some important points:-
- KWH=KW × H, where H is the hours in time (1 kW=1000 watts,1 MW =106 watts)
KVA=KVA × H
So p.f= KWH/KVAH
- current (I)= MW/√3VCos θ =MVA/√3 V, Where V is voltage ( for 3 phase )
OR, Current (I) = KVA/√3 V
Hence you can find out the value of the current by knowing the associated values.
How would you maintain pf?
In an inductive load system, you must maintain pf. As you know the cosine angle (Cosθ) between voltage and current is pf.
Consider, if you increase angle θ, then reactive power increases and Cosθ decreases. Thus pf deceases.
Now, if you decrease angle θ, then reactive power decreases Cosθ increases. Thus pf increases.
To maintain good pf, you should minimize reactive power.
Types of pf
As aforementioned, it’s clear that with the changes of an angle you could control PF. So it is lagging, leading, and unity
Lagging pf- If the current waveform reaches its peak value after the voltage waveform, it is called the lagging power factor. In this case, the current is lagging the voltage. It is called a purely inductive circuit.
Leading pf- If the current waveform reaches its peak value earlier than the voltage waveform, it is leading PF. In this case, the current is leading the voltage. It is called a purely capacitive circuit.
Unity pf – If we minimize reactive power as zero then there is no angle between voltage and current waveform, So PF is unity. In other words, if the voltage and current waveform are in the same phases is called the unity power factor. From the above voltage-current waveform current is in phase with the voltage. It is called a purely resistive circuit.
Causes of low Power factor
In industries, most of the machines used are inductive in nature such as Induction motors, transformers, Generators, Induction heating furnaces, welding machines, and Fluorescent tubes used for lighting. All these are Inductive loads.
In inductive loads, Current lags the Voltage from 90 degrees apart, and these large differences between voltage and current cause a low power factor.
All these are generally run or operate at low load or no load which causes a low power factor.
Advantages of high pf
1. If pf is high then the current flowing in Transmission lines, Cables, Motors, Generators, and Transformers decreases. So decrease copper loss, and iron loss and thus decreases the heating loss of Electrical equipment.
2. As the current decreases so voltage drop decreases and the inductive load could utilize proper voltage.
3. With the increase of pf, iron loss, and copper loss decrease so the efficiency of Motors, Generator, and Transformer increases.
4. As the pf is high, so reactive is low thus you can utilize maximum active or useful or working power for inductive load.
Disadvantage of low pf
1. For a given power supply, the current is increased due to low p.f which causes increased copper losses and thereby decreased the efficiency of both types of equipment and power systems.
2. Generators, Transformers, Rotating machines, and Transmission lines with low p.f become overloaded.
3. Due to low p.f current increases which impact the cost of Generation and Transmission lines as it requires thick wires and bigger switches etc.
It can be explained as:-
We know that R=ρ l/A Where R, ρ, and A are resistance, resistivity, and cross-section area of the conductor.
Again, R=ρ l/A =ρ l A/A 2 =ρ v/A, where v is the volume of the conductor, if we fix the volume constant then R∝ 1/ A i.e. resistance is inversely proportional to the cross-section area of the conductor. Hence if p.f is low, current increases and further resistance decreased as R=I/V (Ohm’s law) where I and V are current and voltage.
Due to decreased resistance cross-section area of the conductor is increased from the above relationship so the thickness of the wires increases.
4. Voltage regulation generators, transformers, and transmission lines are high.
5. In generating plant, an inductive load must be running with a high power factor(pf). If it does not maintain good pf then inductive load draws more reactive from Generator. Generators connected with the Grid draw reactive from Grid, thus apparent power (KVA) increases rather than useful power (KW). So you cannot import much useful power. Also, it impacts on Generator as the PF of the Generator decreases and the current of the Generator is high.
6. Winding temperatures of the Generator increase.
How to improve pf:
- In generating plant you could increase pf by decreasing the excitation voltage of the Generator or increasing the Tap position of the Power Transformer.
- For maintaining good pf you must install a capacitors bank to the inductive load side in parallel with the load.
- Inductive load should run with a full load so that efficiency is good and pf high.