Understanding Power in AC Circuits
In alternating current (AC) circuits, power is more complex than in direct current (DC) circuits. Unlike DC where power is simply the product of voltage and current (P = VI), AC circuits involve three different types of power that describe how electrical energy is utilized:
- This is the actual power consumed by resistive loads to perform useful work
- Examples include heating elements, incandescent lamps, and resistors
- This is the power that gets converted to heat, light, or mechanical work
- This is the power associated with magnetic and electric fields in inductive and capacitive loads
- Examples include transformers, motors, and fluorescent ballasts
- This power oscillates between the source and load but does no useful work
- This is the vector sum of real and reactive power
- It represents the total power that the electrical source must supply
- It’s what you measure with a voltmeter and ammeter (S = VI)
Defining Power Factor (PF)
Power factor is a dimensionless number between 0 and 1 that describes how effectively electrical power is being used. It is defined as the ratio of real power to apparent power:
Power factor can also be expressed as the cosine of the phase angle (φ) between voltage and current:
Power factor indicates the efficiency of power utilization:
- PF = 1: Perfect efficiency (purely resistive load)
- PF = 0: Completely inefficient (purely reactive load)
- 0 < PF < 1: Typical for practical loads with both resistive and reactive components
Why a Low Power Factor is Bad
A low power factor has several negative consequences for both consumers and utility companies:
For a given amount of real power (kW), a lower power factor requires more current:
This means that with a lower power factor, more current is needed to deliver the same amount of useful power.
- Power losses in conductors increase with the square of current (Ploss = I²R)
- Higher current due to low power factor results in significantly higher losses
- This wastes energy and reduces system efficiency
- Transformers, switchgear, and conductors must be sized for higher currents
- This increases capital costs for electrical infrastructure
- Existing equipment may become overloaded
- Many utility companies charge penalties for power factors below 0.95
- Industrial customers often face significant financial penalties
- Utilities may impose demand charges based on kVA rather than kW
Improvement
Power factor correction involves reducing the reactive power component to bring the power factor closer to 1. The most common method is using capacitor banks:
- Capacitors provide leading reactive power that cancels lagging reactive power from inductive loads
- They are connected in parallel with the load
- Automatically switched based on load requirements
- Can improve power factor to 0.95 or higher
- Synchronous Motors: Operated at leading power factor to compensate for lagging loads
- Static VAR Compensators: Electronic devices that provide dynamic reactive power compensation
- Proper Motor Sizing: Avoiding oversized motors that operate at low power factors
- Load Management: Balancing inductive and capacitive loads
- Reduced electricity bills due to lower kVA demand
- Elimination of utility penalty charges
- Improved voltage regulation
- Increased system capacity
- Reduced I²R losses in conductors and transformers
Typical Power Factors
Different types of loads have characteristic power factors:
- Resistive Loads: Heaters, incandescent lamps (PF ≈ 1.0)
- Inductive Loads: Motors, transformers, fluorescent ballasts (PF = 0.7-0.9 lagging)
- Capacitive Loads: Capacitor banks, some electronic equipment (PF = 0.8-1.0 leading)
- Electronic Loads: Computers, LED drivers, variable frequency drives (PF = 0.6-0.95, often corrected)
