K-Factor Rated Transformers

A K-Factor rated transformer is specifically designed to handle the additional heating effects caused by harmonic currents generated by non-linear loads, such as computers, variable frequency drives (VFDs), and UPS systems. Harmonic currents distort the current waveform and increase losses in transformer windings and core steel. A K-Factor rating indicates the transformer’s ability to withstand these harmonic stresses without exceeding its temperature rise limits. Rex Power Magnetics designs its K-Factor transformers for dependable performance and long life in environments where harmonic distortion is prevalent.

The K-Factor rating quantifies a transformer’s capability to carry harmonic currents safely. It is determined according to IEEE C57.110, which defines how much harmonic content the transformer can handle without excessive heating. Common ratings include K-4, K-9, K-13, and K-20, with higher numbers indicating greater tolerance to harmonic loads. For example, a K-13 transformer is suitable for most office and light industrial loads, while K-20 units are used in heavy drive or data center applications. Selecting the proper K-Factor ensures safe operation and minimizes the risk of insulation degradation or premature failure.

Non-linear loads draw current in pulses rather than smooth sine waves, creating harmonic currents that increase heating in transformers. Standard transformers are not designed for these conditions and can overheat, leading to reduced lifespan or failure. K-Factor transformers are engineered with larger conductors, reduced flux density, and specialized winding configurations to accommodate the additional thermal and electrical stress. They maintain efficiency and voltage regulation under harmonic load conditions, protecting both the transformer and downstream equipment.

Each transformer features high-grade electrical steel cores and precision-wound copper or aluminum conductors. The core and coils are dipped in polyester resin, which provides mechanical stability, superior insulation integrity, and moisture protection. Windings are arranged to minimize stray flux and reduce eddy-current losses caused by harmonics. The design operates at a lower flux density to manage thermal stress while maintaining high efficiency. These construction details ensure dependable service life even under severe harmonic loading and continuous-duty operation.

K-Factor transformers are designed with increased conductor cross-section, reduced core flux density, and enhanced cooling to offset additional harmonic heating. The polyester resin coating improves thermal conduction between windings and the core, stabilizing temperature rise. Core and winding losses are carefully balanced to maintain rated output even under distorted current waveforms. This design approach prevents localized hot spots, ensuring consistent temperature distribution and extending insulation life under non-linear loading conditions.

Selecting the right K-Factor depends on the harmonic content of the connected load. For typical office or light commercial systems, a K-4 or K-9 transformer is sufficient. For installations with a high concentration of drives, rectifiers, or electronic power supplies, K-13 or higher may be required. Rex Power Magnetics provides engineering assistance to analyze load profiles and recommend the proper rating. Choosing the correct K-Factor optimizes transformer size, efficiency, and cost while ensuring long-term reliability.

Yes. Rex Power Magnetics designs custom K-Factor transformers for specific harmonic profiles and installation requirements. Options include special voltage ratios, impedance levels, temperature-rise ratings, electrostatic shields, and enclosure types. Engineering teams can also incorporate multiple secondary windings or harmonic-filtering features to meet unique system conditions. Every custom transformer is built and tested to the same standards as catalog models, ensuring compliance, reliability, and efficient operation under complex non-linear loads.

Dry-type transformers can generally be connected in reverse (back-fed), but there are some precautions to consider:

• Compensated Windings: Control transformers and distribution transformers below 3kVA are typically designed with an overvoltage on the secondary to compensate for voltage regulation at full load. Reverse feeding these transformers may lead to a lower than expected output voltage.
• Inrush current: The inrush current when energizing a transformer from the intended secondary terminals will be significantly higher than on the primary side as a multiple of the rated current. This high inrush current can cause nuisance tripping of the protective breaker and special considerations may need to be made.
• Voltage Taps: Given that there are typically no voltage adjustment taps on the secondary side, the transformer cannot be adjusted to account for higher or lower than nominal incoming voltages. In order to not damage the insulation or overexcite the core, the input voltage should not exceed the nominal rated secondary voltage. In the case that the incoming voltage is lower than the nominal rated secondary voltage, the taps on the primary winding can be used to boost the output voltage.
• Grounding: When the secondary (wye) of a delta-wye transformer is energized instead of the primary (delta), then the wye side of the transformer is not a separately derived service. As such, the neutral should not be connected to building ground nor should it be bonded to the transformer enclosure.

Always review applicable codes and standards and consult with the local authority having jurisdiction before reverse feeding transformers.

Unless designed for special service conditions / environments, below are the standard service conditions for dry type distribution transformers:

• Ambient Temperature: -40°C to + 30°C (max peak +40°C)
• Relative Humidity: less than 70%
• Altitude: up to 1000m (3300 ft.) above seal level

To ensure proper operation, avoid installing transformers in environments with excessive moisture, extreme temperatures, or direct sunlight. Maintain recommended clearances and keep all ventilation panels unobstructed.

Any transformer which is not installed and energized immediately should be stored in a dry, clean space having a uniform temperature to prevent condensation on the windings. Dry type transformers with resin dipped or epoxy vacuum impregnated coils can be stored at ambient temperatures as low as -50C. Transformers with encapsulated or epoxy cast coils should not be stored at ambient temperatures below -20C to prevent cracking of the epoxy. Preferably, transformers should be stored in a heated building having adequate air circulation and protected from cement, plaster, paint, dirt, and water or other gases, powders, and dust. The floor on which the transformer is being stored should be resistant to the upward migration of water vapor. Precautions should be taken to prevent storage in an area that water could be present, such as roof leaks, windows, etc. Condensation or absorption of moisture can be greatly reduced by keeping the transformer enclosure 5⁰C-10⁰C above ambient temperature. This can be easily achieved by the installation and energization of space heaters (optional). If the transformer is not furnished with internal space heaters, then external, portable heaters can be used. Note: Lamps or heaters should never come in direct contact with the transformer coil insulation.

It is not advisable to store a dry type transformer outdoors, but in the case that it is unavoidable, protective measures should be taken to prevent moisture and foreign debris from entering the transformer enclosure. The plastic wrapping supplied during shipment should be left in place, and a suitable drying agent such as silica gel packs should be used. The unit should also be checked periodically for indications of condensation on the windings, coil support blocks, core, core clamping system and bus/cables.

In dry-type transformers, the surrounding air plays a critical role in their operation. Generally, low ambient temperatures do not negatively impact an energized transformer, as the no-load losses typically generate enough heat to maintain proper conditions, even in environments as cold as -40°C. However, transformers stored at low temperatures present two primary concerns:

• Insulation Brittleness: At low temperatures, the insulation in the coils may become brittle. Expanding conductors when a cold transformer is loaded, or contracting conductors during cold storage, may cause cracks in the insulation, leading to internal faults.
• Condensation Risk: Low temperatures can cause condensation inside the transformer enclosure, as well as on the transformer coils. Energizing a transformer with condensation present on the coils can lead to internal faults and insulation damage.

Rex Power Magnetics recommends testing transformers (megger testing), warming them to above 0°C, or following a drying-out procedure if moisture is suspected. Refer to Rex Power Magnetics’ cold start procedures to ensure safe energization in cold conditions. Energizing a transformer with compromised insulation due to moisture can cause damage and potential safety hazards.

Cold Start Procedure

The minimum required clearances of a dry type transformer to walls, floors or other equipment must adhere to the local electrical code requirements.

In the absence of such requirements, Rex Power Magnetics recommends that dry type transformers be mounted so that there is an air space of no less than 150mm (6”) between the enclosures, and between the enclosure and any adjacent surface except floors. When the adjacent surface is a combustible material, the minimum permissible separation between the transformer enclosure and the adjacent surface should be 300mm (12”). Where the adjacent surface is the wall on which the transformer is mounted, the minimum permissible separation between the enclosure and the mounting wall should be 6mm (0.25”) so long as the surface is of a non-combustible material.

Temperature rise refers to average increase of temperature of the transformer windings at full load above the ambient temperature. when operating at full load. In addition to the average temperature rise of the windings, transformers also experience a “hot spot” temperature, which refers to the highest temperature point in the windings.

For example, a transformer with a 220°C insulation system may be designed with a 150°C average temperature rise and a 30°C hot spot allowance. This means that Above a 40C ambient, the total absolute temperature will not exceed 220°C. Transformers with lower temperature insulation systems (180°C or 200°C) will be designed with lower temperature rises (115° or 130°C) and hot spots so they can be installed in the same ambient temperature and still not exceed the temperature rating of the insulation system.

The table below shows the maximum average winding temperature rise, maximum hot spot temperature rise and maximum winding temperature for the most common insulation classes. Note that these are based on a max average ambient of 30°C during any 24-hour period and a maximum ambient of 40°C at any time.

Customers occasionally specify a transformer of a particular insulation class to be designed with an average temperature rise below the average temperature rise values shown in the table above. The benefits of doing so include:

• Longer Transformer Life: Lower temperature rise means the transformer can operate at a lower overall temperature, extending its life expectancy.
• Handling Higher Ambient Temperatures: Transformers with lower temperature rise ratings can operate safely in higher ambient temperatures without exceeding their insulation limits.
• Increased Overload Capacity: These transformers can handle continuous or short-term overloads without overheating, making them ideal for environments where transformers may be subject to occasional overloading.

The life expectancy of a dry-type transformer is primarily determined by the insulation system and the operating temperature. According to IEEE Std. C57.96, the deterioration of insulation is directly related to the time and temperature the transformer experiences during operation. Insulation materials degrade faster at higher temperatures, so the transformer’s life expectancy is closely tied to how well it is kept within its design temperature limits.

In most transformers, the highest temperature occurs at a specific point in the windings, known as the hot spot. This area undergoes the most significant wear over time, making it the primary factor in determining the transformer’s ageing process.

All of Rex Power Magnetics’ dry-type transformers are designed using UL-listed insulation systems with a maximum hot spot temperature that ensures a design life of at least 30 years under standard operating conditions (continuous rated load, typical ambient temperatures, and no sustained overloads). Transformers designed with reduced temperature rise can extend this design life expectancy to over 50 years, as operating at lower temperatures slows the insulation’s ageing process.

Factors That Affect Life Expectancy:

• Temperature: As discussed, the most significant factor is the operating temperature of the transformer. Operating continuously at higher temperatures reduces the expected lifespan.
• Humidity and Condensation: Humid environments or condensation can affect the insulation material and lead to premature failure. Proper storage and maintenance help mitigate this risk.
• Short Circuit Events: Sudden surges or short circuits can damage internal components and shorten the transformer’s life.
• Overloading: Continuous overloading beyond the transformer’s rated capacity generates excess heat, which accelerates insulation degradation.
• Environmental Conditions: Extreme environments, such as exposure to dust, moisture, or chemicals, can also lead to earlier-than-expected failure.

By following proper installation and maintenance practices, such as avoiding overloading and ensuring the transformer operates within its designed ambient temperature, you can significantly extend its lifespan. Rex Power Magnetics’ high-quality transformers are built for durability, ensuring reliable performance for decades under standard conditions.

It`s normal for new transformers to release some harmless odors from the varnish impregnation used in the coils for a week or two after energization. Older Transformers can also release some odor if loaded to a higher level than they have experienced previously in their history.

Rex Power Magnetics’ ventilated distribution transformer terminals are rated 90°C. Conductors with at least a 90°C insulation rating at or below their 90°C ampacity rating should be utilized.