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Power Factor plays a critical role in electrical power transmission and distribution. A low power factor means your system draws more current than necessary to deliver the same useful power — leading to higher losses, voltage drops, and penalty charges on your electricity bill. In this article, we will study the causes of low power factor and the most commonly used methods of power factor improvement.
What is Power Factor?
Power factor is the ratio of real power (kW) to apparent power (kVA) in an AC circuit. It indicates how effectively electrical power is being used.
A power factor of 1 (unity) means all the power drawn is being used productively. In practice, most industrial loads operate between 0.7 and 0.9 due to inductive equipment like motors, transformers, and welding machines.
The reactive power component (kVAR) does no useful work but is necessary to maintain magnetic fields in inductive loads. Power factor improvement aims to reduce this reactive component.
Causes of Low Power Factor
Most industrial and commercial loads are inductive in nature. The main causes of low power factor include:
- Induction motors — especially when running at light load (PF can drop to 0.3–0.4)
- Transformers — magnetising current is highly lagging
- Welding machines and arc furnaces — operate at very low PF (0.3–0.5)
- Fluorescent lighting — with magnetic ballasts
- Lightly loaded systems — equipment running below rated capacity
When the power factor is low, the supply system must carry more current for the same useful power output. This increases I²R losses in cables, reduces voltage regulation, and requires larger conductor sizes.
Methods of Power Factor Improvement
Power factor can be improved by connecting equipment that supplies leading reactive power (capacitive kVAR) to cancel the lagging reactive power drawn by inductive loads. The three main methods are:
1. Static Capacitor (Capacitor Bank)
This is the most widely used method of power factor correction in industries. A capacitor draws leading current — meaning current leads voltage by 90°. When connected in parallel with an inductive load, the capacitor supplies the reactive power locally, reducing the reactive current drawn from the supply.
For three-phase loads, capacitors can be connected in star or delta configuration. The required capacitor bank size can be calculated based on the existing and desired power factor.
Where P = real power (kW), φ₁ = original PF angle, φ₂ = desired PF angle
Advantages of Static Capacitor
- Low maintenance — no rotating parts
- Easy installation, no foundation needed
- Low losses (typically < 0.5W per kVAR)
- Can be installed at individual load points
- Performance unaffected by atmospheric conditions
Disadvantages of Static Capacitor
- Shorter lifespan (8–10 years typical)
- Easily damaged by overvoltage or harmonics
- Once damaged, repair is not economical
- Fixed compensation — cannot adjust smoothly to varying loads
2. Synchronous Condenser
An over-excited synchronous motor running at no load is called a synchronous condenser. When over-excited, a synchronous motor draws leading current and behaves like a capacitor. It is connected in parallel with the inductive load to supply reactive power and improve the power factor.
The key advantage is that by varying the field excitation, the reactive power output can be continuously adjusted — providing smooth, stepless power factor control. This is why synchronous condensers are preferred in large substations and power plants.
The relationship between excitation and power factor is shown in the V-curve of synchronous motor — as excitation increases beyond normal, the motor transitions from lagging to leading power factor.
Advantages of Synchronous Condenser
- Smooth, stepless control of power factor by varying excitation
- Longer lifespan (25+ years)
- Faults can be easily repaired
- High thermal stability against short circuit currents
- Can absorb or generate reactive power (both lagging and leading)
Disadvantages of Synchronous Condenser
- High initial cost and installation complexity
- Significant running losses (motor losses)
- Requires auxiliary starting equipment (not self-starting)
- Produces audible noise
- Higher maintenance cost compared to capacitors
3. Phase Advancer
A phase advancer is a special AC exciter mounted on the shaft of an induction motor. It is used specifically to improve the power factor of induction motors.
Normally, an induction motor draws its magnetising current (which is lagging) from the supply. A phase advancer supplies this excitation current at slip frequency directly to the rotor circuit. This means the motor no longer draws the lagging magnetising current from the supply — effectively improving the power factor.
Key Points about Phase Advancers
- Used exclusively for induction motors
- Can improve PF to unity or even leading
- Economical for motors above 200 HP
- Not suitable for small motors due to cost
Comparison of PF Correction Methods
Advantages of Power Factor Correction
Improving power factor provides significant technical and economic benefits:
- Reduced electricity bills — eliminates kVAR penalty charges and may earn PF incentives
- Lower I²R losses — less current flows through cables for the same real power
- Improved voltage regulation — reduced voltage drop across the system
- Released system capacity — transformers and cables can serve additional load
- Smaller conductor sizes — reduced current allows thinner cables in new installations
- Reduced carbon footprint — lower losses mean less generation required
Penalty Charges for Low Power Factor
In India, electricity boards impose penalty charges when the power factor falls below a threshold (typically 0.9 or 0.95). The penalty appears as kVAR charges on your electricity bill.
Conversely, maintaining a high power factor (above 0.95) can earn incentives — typically a 1–2% rebate on the energy charges for every 0.01 improvement above the threshold.
After correction to PF 0.95, it draws only 526 kVA — saving 188 kVA of system capacity.
Modern Approach: Today, smart solar inverters (IEEE 1547-2018 compliant) can also provide reactive power compensation to the grid, acting as distributed power factor correction devices — especially useful for rooftop solar installations in India.
FAQs
What is the ideal power factor for industries?
The ideal power factor for industries is between 0.95 and unity (1.0). Most electricity boards in India mandate a minimum of 0.9 to avoid penalty charges. Maintaining PF above 0.95 can earn incentive rebates.
Which method of power factor improvement is most commonly used?
Static capacitor banks are the most commonly used method due to their low cost, easy installation, minimal maintenance, and suitability for all types of loads. They are used in over 90% of industrial PF correction installations.
Can power factor be greater than 1?
No, power factor cannot exceed 1. It ranges from 0 to 1. A value of 1 means all power drawn is real (useful) power with zero reactive component. However, over-correction with capacitors can make the PF leading, which is also undesirable.
What happens if power factor is too low?
Low power factor causes increased current flow, higher I²R losses in cables, poor voltage regulation, overloading of transformers and generators, and penalty charges from the electricity board. It also reduces the available capacity of the entire distribution system.
What is the difference between leading and lagging power factor?
Lagging power factor occurs when current lags behind voltage (inductive loads like motors). Leading power factor occurs when current leads voltage (capacitive loads). Most industrial loads have lagging PF, which is why capacitors (leading) are used for correction.