Harmonics & non-linear loads
Why electronic loads distort the current waveform — and the overheating and nuisance tripping that follows.
Where they come from
A simple resistive load draws a clean sine-wave current that follows the voltage. Electronic loads — VSD front ends, LED drivers, switch-mode power supplies, UPS — draw current in sharp pulses instead. Those pulses are the same as adding currents at multiples of the supply frequency: the harmonics.
The more non-linear load on a circuit, the more distorted the total current becomes. The voltage waveform itself can start to distort once the harmonic currents are large compared with the supply's strength.
What goes wrong
Harmonics cause heating out of proportion to the apparent load: cables and transformers run hot, and motors can overheat from harmonic torques. The classic surprise is the neutral — certain harmonics (the triplens) don't cancel between phases but add up in the neutral, so a shared neutral can carry more current than any phase conductor.
They also trip protection that wasn't expecting them: electronic RCDs can nuisance-trip, and meters or controls fed from a distorted supply can misbehave. A board running hot with no obvious overload is a prompt to look at the load mix.
Managing them
Options range from sizing the neutral and derating equipment, through to line reactors on drives and dedicated harmonic filters on larger installations. The first step is recognising the load mix — lots of electronic equipment on one board is the tell.
Related faults
Motor noisy or whining when run on a VSD
The motor runs but is noticeably noisy on the drive — a whine, whistle, or growl that isn't there on direct supply, sometimes worse at certain speeds.
Three-phase equipment running hot from supply imbalance
Three-phase equipment (motors, heaters, large gear) runs hotter than expected or nuisance-trips, traced to a voltage/current imbalance between phases rather than the equipment itself.
RCD nuisance tripping with lots of electronics
An RCD trips intermittently with no single faulty appliance — typically where many electronic devices (with filters/SMPS) share one RCD, each adding a little standing leakage.
Related definitions
VSD (Variable Speed Drive)
Controls the speed of an AC motor by converting the supply to a variable frequency and voltage.
Neutral vs earth
The neutral carries return current; earth is for safety. Confusing them causes real, dangerous faults.
Real, apparent & reactive power (kW · kVA · kVAr)
Three different 'powers' on an AC system — what each one is, and why they don't simply add up.