Typical application in power conditioner
(A) Isolation transformer(Delta/Wye) connected at input terminal
It is a value used to determine how much harmonic current a transformer can handle without
exceeding it’s maximum temperature rise level. K-factor values range from 1 to 50. K-factor of 1 is used for linear loads only, and a K-factor of 50 is used for the harshest harmonic environment possible. A K-factor of 13 is typical. When transformers use a K-factor value, they are said to be Krated.
A K-rated transformer is one which is used to deal with harmonic generating loads. Harmonics
generate additional heat in the transformer and cause non-K-rated transformers to overheat
possibly causing a fire, also reducing the life of the transformer. K-rated transformers are sized
appropriately to handle this additional heat and tested to UL 1561 rigid standards for K-factor rated
transformers.
K-rated transformers use a double sized neutral conductor and either change the geometry of their
conductors or use multiple conductors for the coils. Quality transformers are manufactured with a
high grade silicon steel, copper windings, and more air ducts.
Anywhere non-linear loads are present. New construction, renovations, factory automation,
computer rooms, and office buildings are prime locations for K-rated transformers because of the
high harmonic content in these areas.
Typically a K-13 rated transformer is sufficient for most applications. Loads approaching 100%
non-linear or more than 75% THD should incorporate a K-20 rated transformer.
Aluminum has a tendency to creep due to change in temperature. Creeping is the contraction and
expansion of the aluminum conductors. When they creep: the terminal connections loosen,
creating a hazard; they eventually burn clear; and shorten the life of the transformer. Copper is
more efficient than aluminum. Copper wound transformers general save money; in many cases they pay for themselves over time.