Non-linear loads such as variable-frequency drives, UPS systems, and switching power supplies draw current rich in harmonics. Those harmonics cause extra eddy-current heating in a transformer’s windings, and because the loss scales with the square of frequency, high-order harmonics are far more damaging than their amplitude alone suggests. The K-factor quantifies this so you can specify a transformer built to handle it.
How it works
The K-factor weights each harmonic current by the square of its order:
Σ ( I_h^2 × h^2 )
K-factor = -----------------
Σ ( I_h^2 )Here I_h is the RMS current of harmonic order h and the sum runs over every
harmonic present. The denominator is the total RMS-squared current, so the
result is independent of whether you enter amps or percent of fundamental. A
purely linear load (only the fundamental) gives a K-factor of exactly 1.0.
Selecting the rating
Transformers are manufactured in standard K-ratings: K-1, K-4, K-9, K-13, K-20, K-30, K-40, and K-50. The tool rounds the computed K-factor up to the next standard rating. For example, a spectrum that computes to 7.2 calls for a K-9 transformer.
Example and notes
A typical six-pulse VFD load with a 33 percent third harmonic, 20 percent fifth, 14 percent seventh, and smaller higher orders computes to a K-factor near 6 to 8, which means a K-9 transformer. Always measure the spectrum at the actual load rather than estimating — drive front-end design and the presence of line reactors dramatically change the harmonic content and therefore the required K-rating.