Understanding Power Transformer "K
A K-Factor rating is an index of the
transformers ability to supply harmonic content in its load current
while operating within it temperature limits. For Dry Type Transformers
a K-Factor calculation is made to determine the amount of the Harmonic
Content present in a power system.
|To Order Power Transformers Call
K-Rated transformers are sized to
handle 100% of the fundamental 60 Hz load, plus the non-linear load
specified. The neutral of the K-Rated transformer is sized at 300% of
the current rating of the phase connections. Industry
literature and commentary refers to a limited number of K-factor
ratings: K-1, K-4,K9, K-13, K-20, K-30, K-40. Aa transformer
could be designed for other K-factor rating in between those values, as
well as for higher values. The commonly referenced ratings calculated
according to ANSI/IEEE C57.11-1986 are as following
K-Factor 1: A transformer with this rating has been designed to handle only the heating effects of eddy
currents and other losses resulting from 60 Hz, sine-wave current
loading on the transformer. Such a transformer may or may not be designed to
handle the increased heating of harmonics in its load current. Applications are motors, incandescent lighting, resistance heating,
motor generators (without solid state drives).
K-Factor 4: A transformer with this rating has been designed to
supply rated KVA, without overheating, to a load made-up of 100% of the
normal 60 Hz, sine-wave, fundamental current plus: 16% of the
fundamental as 3rd harmonic current; 10% of the fundamental as 5th; 7%
of the fundamental as 7th; 5.5% of the fundamental as 9th; and smaller
percentages trough the 25th harmonic. The "4" indicates its ability to
accommodate four times the eddy current losses of a K-1 transformer.
Uses are HID lighting, induction heaters, Welders, UPS with optional
input filtering, PLC and solid state controls.
K-Factor 9: A K-9 transformer can accommodate 163% of the
harmonic loading of a K-4 transformer.
K-Factor 13: A K-13 transformer can accommodate 200% of the
harmonic loading of a K-4 rated transformer. These transformers
are used for multiple receptacle circuits in health care facilities, UPS
without optional input filtering, Production or assemble line equipment,
Schools and classroom facilities.
K-Factor 20, K-30, K-40: The higher number of each of these
K-factor ratings indicates ability to handle successively larger amounts
of harmonic load content without overheating. Some of these transformers
are used in SCR variable speed drives, Circuits with exclusive data
processing equipment, Critical care facilities and Hospital operating
Today's modern electronic, electric components and circuitry such as
computers, copiers, printers, fax machines and display terminals utilize
switching mode power supplies for their operation. These switching mode
power supplies are non-linear in nature and can cause significant power
- Circuit breakers and fuses blowing far below current rating.
- Neutrals in transformers and panel boards are much hotter than
- Distributions Transformers are overheating even when operating
well within their specified nameplate rating.
Solid state switching
elements such as SCR, Transistors and capacitors which are found
in computers, fax machines, solid state drives, energy efficient drive,
etc. The Continuously switching on/off producing non-linear or
non-sinusoidal wave shapes in the current supplied from the power
source. These problems are the results of Harmonics.
A linear load uses current from the power source continuously over the
sinusoidal cycle. A non-linear load uses current in large pulses from
the power source thereby creating harmonic distortion.
Harmonic components are represented by periodic waves that have a
frequency that is a multiple of the fundamental frequency.
- Fundamental Frequency=60Hz
- 3rd Harmonic Frequency =3 x 60Hz=180Hz
- 5th Harmonic Frequency=5 x 60Hz=300 Hz
Harmonic Frequencies superimpose themselves upon the fundamental
waveform distorting and changing its magnitude.
Effects of Harmonic Components on the Sinusoidal Cycle:
The major components in the harmonic currents of switching mode power
supplies are the 3rd and the 5th Harmonics. Harmonics which are even
multiples of 3, such as 3rd, 6th, 9th, etc. are called Triplen
Harmonics. When Triplen Harmonics are present in a 3-Phase system, they
add together in the neutral conductor. 3rd Harmonics result in high 180
Hz current flowing through the transformer's neutral terminal. The core
steel of the transformer has stray losses that will increase
dramatically with Harmonic Distortion. These effects cause the
transformers to operate at significantly higher temperatures.
Great Power Transformer Articles
1. Top 20 Things to Know before Selecting a Power
Transformer - This is a great article to help educate and
inform you on how to select the right transformer before you buy. It
will help you make an informed decision and covers questions including,
what voltage power output do you need?, will it be single phase or three
phase?, etc. We answer the 20 most asked questions so you can be well
informed and choose the right transformer for the job. Choosing the
right transformer can be a daunting task for the inexperienced. This
section takes the first step toward becoming a confident, knowledgeable
consumer. This article addresses the process of choosing these
transformers at its most fundamental level.
How Does Transformer Manufacturer Company Brand
Affect Power Transformer Quality? - For manufacturers of large power
transformers, product design and features seem fairly standard. But
different manufacturers offer unique features. there are several
standards such as ASTM D 3487 and IEEE Standard C57.12.90. Quality
transformers can have a significant impact on cost. Did you know that
some transformers brands improved materials, design and quality can save
you 30%, or more, in energy cost? Understanding the differences can play
a key role in making an informed selection.
How Does Winding Metal Type Change a Transformers
Properties? - gain knowledge on how the winding and types
of metal used can change transformer properties. The conducting material
used for the winding depends upon the application. Small power and
signal transformers are often wound with solid copper wire. Larger power
transformers may be wound with copper wire, or aluminum and may include
rectangular conducts. When copper wiring is used it will increase the
efficiency of the transformer and will generally generate a lot less
heat. Read more to learn about the many other efficiencies gained by
choosing a transformer with a metal composition that is fit for your
Understanding Power Transformer "K-Factor Rating" - A great
information article on what "K-Factor Rating" is and the effects they
have on transformer choice. The K-Factor rating assigned to a
transformer and marked on the transformer case in accordance with the
listing of Underwriter Laboratories. It is an index of the
transformer's ability to supply harmonic content in its load current
while remaining within its operating temperature limits.
Read Addition Power Transformer Articles -
Additional informal helpful articles about power transformers.
Power Transformer Information: