Explain the K-Factor rating?
K factor is defined as a ratio between the additional losses due to harmonics and the eddy current losses at 60Hz. It is used to specify transformers for non-linear loads. Transformers with a rated K factor of 4, 9, 13, 20 are available. For balanced loading, a transformer with a K factor of 4 should be specified when no more than 50% of the total load is non-linear. A transformer with K factor 9 should be specified when 100% of the load is non-linear. For critical applications, K=13 can be considered.
As the K-factor of the transformer increases, generally the size and cost increase, low load efficiency (<25% load) decreases and impedance decreases. That is the trade-off to be able to handle higher harmonic factors.