What is Power Factor? Causes, Effects, and Methods of Improvement

 

Introduction

Welcome, future engineers! If you have ever looked at an electricity bill from an industrial unit or read the nameplate of a large motor, you have probably come across the term "Power Factor." It’s a crucial concept in electrical engineering that bridges your theory exams and real-world industrial practice. Understanding it not only helps you score marks but also saves lakhs of rupees in electricity costs. Let’s demystify this important topic together.

What is Power Factor?

Definition

In the simplest terms, Power Factor (PF) is a measure of how effectively electrical power is being used. Think of it like this: When you buy a glass of cold drink, you pay for the full glass (what you get). But what if it’s half-filled with foam? You paid for the full volume, but you only get half the actual drink. Similarly, in an AC circuit, not all the power supplied by the electricity board (called Apparent Power) is converted into useful work (called Real Power). Power Factor tells us the fraction of the supplied power that is doing real, useful work.

Formula

The standard formula is:
Power Factor (PF) = Cos φ
Here, φ (phi) is the phase angle difference between the voltage waveform and the current waveform. When current lags or leads voltage, they are "out of phase," leading to a low power factor. When they are perfectly in sync (in phase), the power factor is 1, or "unity."

Relationship between kW, kVAR, and kVA

This is best explained with a famous triangle—the Power Triangle.

• kW (Kilowatt): This is the Real Power or True Power. It is the power that actually powers your motors, heats your heaters, and lights your bulbs. It performs useful work.
• kVAR (Kilovolt-Ampere Reactive): This is the Reactive Power. It is the power required to create the magnetic fields in inductive devices like motors, transformers, and chokes. It doesn’t do any useful work but is necessary for the equipment to function. It constantly shuttles back and forth between the source and the load.
• kVA (Kilovolt-Ampere): This is the Apparent Power. It is the vector sum of kW and kVAR, i.e., the total power supplied by the utility.

The relationship is: (kVA)² = (kW)² + (kVAR)²
Power Factor (Cos φ) = kW / kVA

Types of Power Factor

• Lagging Power Factor: This is the most common type in industries. It is caused by inductive loads (like motors, transformers, fluorescent lights) where the current lags behind the voltage. The power factor value is less than 1.
• Leading Power Factor: This is caused by capacitive loads (like capacitor banks, long underground cables) where the current leads the voltage. This is also undesirable for a utility system.
• Unity Power Factor: This is the ideal scenario (PF = 1). Here, the current and voltage are in perfect phase (φ = 0°). All the supplied power is used for useful work. Purely resistive loads like incandescent bulbs or electric heaters have unity power factor.

Causes of Low Power Factor

A low power factor (typically lagging) is almost always an outcome of industrial and commercial operations. The primary causes are:

1. Inductive Loads: These are the biggest culprits. Most industrial machinery runs on induction motors, which have winding coils that create strong magnetic fields.

2.    Variable Loads: Equipment that operates at less than its full rated load (like a motor running idle or a machine on standby) tends to have a poorer power factor.

3.    Industrial Heating Systems: Arc furnaces and induction furnaces have a very low power factor.

4.    Transformers: They themselves are highly inductive, especially when operating below their full capacity.

5.    Discharge Lamps: Traditional magnetic ballast-based fluorescent and HID lamps contribute to a low power factor.

Effects of Low Power Factor

A low power factor is costly and inefficient for both the power supplier and the consumer.

• Increased kVA Demand: For the same amount of real power (kW), a lower PF means a higher kVA demand from the utility. This forces the electricity board to install larger generators, transformers, and cables.
• Higher Electricity Bills: Many electricity boards in India impose a power factor penalty in their industrial and commercial tariffs. If your PF falls below a specified limit (often 0.90 or 0.95), you pay a heavy surcharge on your bill. On the other hand, maintaining a high power factor can get you a discount on your bill. On the other hand, maintaining a high power factor can get you a discount on your bill.
• Large Conductor Size: To carry the higher current resulting from low PF, thicker and more expensive cables are required.
• Poor Voltage Regulation: A low PF can lead to a greater voltage drop in cables, resulting in lower voltage at the equipment terminals, which affects performance.
• Reduced System Capacity: A transformer or cable already supplying a load with low PF cannot supply additional useful load until the PF is improved. It wastes the available infrastructure.

Methods of Power Factor Improvement

The goal of power factor improvement is to reduce the phase difference between voltage and current. Since industries have a lagging PF (inductive), we need to add leading PF devices (capacitive) to cancel out the lag. This is called Power Factor Correction.

1.    Static Capacitors (Most Common Method):

o   These are capacitor banks installed across the supply terminals of inductive loads.

o   The capacitor draws a leading current, which neutralizes the lagging current drawn by the inductive load.

o   They are inexpensive, require little maintenance, and can be easily installed at individual motors or at the main panel.

2.    Synchronous Condenser:

o   This is a synchronous motor running without a mechanical load (over-excited).

o   When over-excited, it acts like a capacitor and supplies reactive power (kVAR) to the system.

o   It is used for very large-scale correction in substations and is more expensive but offers smooth control.

3.    Phase Advancer:

o   This is a special device used to improve the PF of induction motors specifically.

o   It is mounted on the motor shaft and supplies exciting current to the rotor circuit, reducing the lagging current drawn from the supply.

4.    APFC Panel (Automatic Power Factor Correction Panel - Industry Standard):

o   This is the intelligent, automated solution used in modern plants.

o It consists of multiple capacitor steps controlled by a microprocessor-based controller.

o   The controller continuously monitors the system's PF. When it detects a drop, it automatically switches ON the required number of capacitor steps to bring the PF back to the desired set value (e.g., 0.99). When the load decreases, it switches OFF the extra steps.

o   An APFC panel ensures optimal correction at all times without manual intervention.

Advantages of Power Factor Improvement

• Elimination of Utility Penalty Charges: Direct and significant reduction in electricity bills.
• Increased System Capacity: Frees up capacity in existing transformers and cables to add more productive loads.
• Improved Voltage Profile: Reduces voltage drop, leading to better equipment performance and longer life.
• Reduced Energy Losses (I²R Losses): Lower current means less heat loss in cables and switchgear, improving efficiency.

Practical Industrial Example

Imagine a small textile factory with 50 sewing machines (induction motors) and lighting. Without any power factor improvement, the plant's overall PF might be 0.75 lagging. The electricity board charges a penalty for PF below 0.90.

The factory owner installs a properly sized APFC panel at the main distribution board. This panel automatically injects the right amount of capacitive kVAR into the system. The plant's PF is now maintained at 0.99.

Result: The kVA demand from the utility drops significantly for the same amount of sewing work (kW). The monthly electricity bill is reduced by 15-20% due to the removal of the PF penalty. The transformer runs cooler, and there is spare capacity to add a few more machines if needed.

Importance of Power Factor in Exams and Industry

• For Exams: Power Factor is a fundamental concept in AC circuits, electrical machines, and power systems. Questions on its definition, causes, effects, and correction methods are very common in university and competitive exams. Drawing the power triangle and doing simple calculations is a guaranteed marks-scoring area.
• For Industry: As a junior engineer, you will see APFC panels in almost every plant. Understanding PF helps you troubleshoot low voltage issues, reduce operational costs, and contribute directly to the company's bottom line. It’s a practical skill highly valued by employers.

Conclusion

Power Factor is not just a theoretical concept with a formula (Cos φ); it is a vital measure of electrical efficiency with serious financial implications. As future electrical engineers, your ability to understand the causes of a low power factor and implement the right power factor improvement techniques will make you an asset in any industry. Remember the cold drink analogy, master the power triangle, and you'll see this concept everywhere—from your exam paper to the factory floor. Keep learning, and keep improving efficiency