Harmonics
What a drive puts
back on the line.
On its input side, a six-pulse drive draws current that is not a sine wave, and the supply has to carry that distortion.
THE CAUSE
Pulsed input current
The drive's diode rectifier charges its DC-link capacitors only near the peaks of the supply voltage. The current comes in two pulses per half cycle.
THE CONTENT
5th, 7th, 11th, 13th ...
A six-pulse rectifier's current contains harmonics at orders 6k ± 1. THDi is their combined size relative to the fundamental.
THE FIX
Filter at the drive
A passive filter in front of the drive gives the harmonic current a local path through its trap, so the supply current stays close to the fundamental.
Traces are illustrations of waveform shape, not measured data.
Upstream effects
Why it matters.
- Heating
Harmonic current adds loss in transformers, cables and generators upstream of the drive.
- Voltage distortion
Harmonic current through the source impedance distorts the voltage that every other load on the bus sees.
- Specifications
Project specifications and utilities commonly set distortion limits at the point of common coupling, often by reference to IEEE 519. Those limits are total demand distortion at that point; a filter’s THDi is at its own input, and a system study relates the two.
For a selection
What to send.
| Drive | Make, model and rating (kW or hp) |
|---|---|
| Input current | Drive input current at full load, from the drive or equipment submittal |
| Supply | Voltage and frequency |
| Target | THDi 5 % or 10 %, or the specification clause |
| Location | Indoor, outdoor, or mounted on the equipment |
| Quantity | Number of drives and whether any share a filter |
| Drive input | Whether the drive already has a DC choke or line reactor |
| Source | Transformer kVA and impedance, and whether the drive runs on generator backup |
Send us the drive.
Drive rating, input current, voltage and how many. We'll come back with a filter selection.