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K-Factor Rated Transformers

Overview of K-Factor Rated Transformers

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 system problems:

  • Circuit breakers and fuses blowing far below current ratings
  • Neutrals in transformers and panel boards are much hotter than their ratings
  • Distributions Transformers are overheating even when operating well within their specified nameplate rating

 These problems are the results of Harmonics. Solid state switching elements such as SCR, Transistors and capacitors which are found in computers, fax machines, solid state drives, energy efficient drives, etc. Continuously switching on/off producing non-linear or non-sinusoidal wave shapes in the current supplied from the power source.

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= 3x 60Hz=180Hz
  • 5th Harmonic Frequency= 5x 60Hz=300Hz

Harmonic Frequencies superimpose themselves upon the fundamental waveform distorting and changing its magnitude.

Effects of Harmonic Components on the Sinusoidal Cycle

Effects on Transformer

distortedThe 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 180Hz 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.

K-Factor Ratings

A K-Factor Rating applied to a transformer is an index of the transformers ability to supply harmonic content in its load current while operating within its 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. K-Rated Transformers are sized to handle 100% of the fundamental 60Hz load, plus the non-linear load specified. The neutral of the K-Rated transformer is sized at 200% of the current rating of the phase connections.

K-Factor 1: Motors, Incandescent Lighting, Resistance Heating, Motor Generators (without solid state drives)
K-Factor 4: HID Lighting, Induction Heaters, Welders, UPS with optional input filtering, PLC and solid state controls
K-Factor 13: Multiple receptacle circuits in health care facilities, UPS without optional input filtering, Production or assemble line equipment, Schools and Classroom facilities
K-Factor 20: SCR Variable Speed Drives, Circuits with exclusive data processing equipment, Critical care facilities and Hospital operating rooms

What about other K-Factor Ratings? We can calculate K-Factor Ratings or any magnitude when Harmonic Values are given


Rex K-Factor Rated Isolation Transformers are specifically designed to meet the requirements of AC or DC Variable Speed Drives or Rectifier outputs.


  • Anticipating that harmonics generated by the rectifiers will be present the steel core is increased to operate the transformer at a lower flux density to prevent saturation.
  • Extra bracing to withstand the mechanical stresses of the current reversals and short circuits associated with SCR drives
  • Isolates the power source from low frequency noise generated by SCR voltage spikes and transient feedback
  • Lower than normal losses and temperature rise for greater life expectancy and overload capability
  • Reduces short circuit currents
  • Full current rated neutral
    • Class 220 Insulation
    • 150°C Temperature Rise
    • Primary Taps ±2.5% & ±5% (±5% up to 14kVA)
    • CSA certified, File No. LR34493
    • UL Listed, File No. E108255
    • ISO9001 Quality Certification
    • Available in Aluminum (Al) or Copper (Cu) windings
    • CAN/CSA 802.2-00 Energy Efficient (Part numbers will have /Z suffix)

Available Options Include

  • Temperature Rise (115°C, 80°C)
  • Other Primary or Secondary Voltages
  • Different frequency (other than 60Hz)
  • Different Nema Rated Enclosure (2, 3R, 4, 4X, 12, etc)
  • Electrostatic Shield (single, double, triple or multiple/ultra)
  • Custom sizes (specific kVA ratings)
  • Open type (Core & Coils only)
  • Special Paint/Finish
  • NEMA TP-1 Energy Efficiency (USA); Part numbers will have /Z suffix