• • • COVER STORY • • •
Use of chokes and resistors in power factor (PFC) equipment
nductive loads, such as motors, welding machines and transformers, cause a lagging power factor. This means more current is required to deliver the same amount of useful (real) power, which increases the burden on cables, transformers, switchgear and other infrastructure. In practical terms, a low power factor leads to:
I
• Higher energy consumption and operational costs
• More significant current flow through conductors, increasing I²R losses
• Greater demand on electrical infrastructure and reduced system capacity
• Voltage drops and instability • Penalty charges or reduced tariffs from utility providers
To quantify an installation’s power factor (PF), the ratio of real power (kW) to apparent power (kVA)is taken. A system with a power factor of 1.0,
or 100 per cent, uses all its electrical power for productive work, while a system with a power factor of 0.7 wastes 30 per cent of its apparent power handling non-productive reactive energy. It may be surprising to know that since 2010 the UK’s National Grid has been including an excessive reactive power charge for customers with a power factor of >0.95. See
https://www.nationalgrid.co.uk/smarter- networks/losses/what-causes-losses for more information. Power Factor Correction (PFC) reduces or eliminates reactive power in a system, bringing the power factor closer to unity (1.0). This is typically achieved by installing capacitor banks, which supply leading reactive power to offset the lagging reactive power drawn by inductive loads. However, many loads in modern industrial environments are non-linear, such as variable speed drives (VSDs), UPS systems, or LED lighting and generate harmonic currents. These harmonics
can interact with capacitor banks and cause resonance conditions, amplifying current distortion and stressing components, leading to premature failure.
That’s why the use of inductors in PFC equipment is often standard practice in quality- focused installations to detune the system to avoid the resonances at the triple frequencies, odd multiples of the 3rd harmonic, which usually appear as a result of non-linear loads, see Figure 1. Detuned systems incorporate series inductors (chokes) and other passive components, like resistors, to shift the resonant frequency away from potentially harmful harmonic frequencies, allowing capacitors to perform power factor correction safely and effectively. The choke, or series reactor, in a detuned PFC system is more than just a simple coil, it plays a critical role in protecting capacitors, improving power quality and ensuring the system’s long-term reliability.
8 ELECTRICAL ENGINEERING • APRIL 2025
electricalengineeringmagazine.co.uk
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