Transformateurs de puissance à bobine moulée

The five tap positions (+5%, +2.5%, nominal, -2.5%, and -5%) use cast-in metal inserts and bolted links on the high-voltage coil. Their arrangement is shown on the transformer nameplate and in the Installation and Maintenance Manual .

Tap changes must be performed with the transformer fully de-energized, isolated, locked out, tested for absence of voltage, and grounded. A qualified technician moves all three phases to the same tap position, torques the connections to specification, and verifies the turns ratio before re-energizing.

Rex cast coil transformers use epoxy-encapsulated high-voltage (HV) and low-voltage (LV) windings designed for different electrical and thermal requirements. HV windings use multiple turns of smaller conductors with graded insulation to withstand high voltage, while LV windings use wider conductors to carry higher currents and manage heat and short-circuit forces.

Both copper and aluminum conductors are available. Aluminum offers lower weight, cost advantages, and thermal expansion characteristics closer to epoxy. Copper provides higher conductivity and can support more compact winding designs. Both use the same general casting and insulation process, but coil geometry and reinforcement are adjusted for each material.

For a comparison of losses, weight, and dimensions, use the Rex Product Configurator .

All three, but in different proportions depending on how the unit is used. The standard cast coil design has a 115°C average rise and a 145°C hot-spot rise on a 180°C system, which consumes the full thermal budget at rated load in a 40°C ambient. Ordering 80°C rise on the same 180°C system leaves 35°C of average rise and roughly the same in hot-spot rise unused at nameplate load, and that margin can be spent in one of three ways.

As ambient headroom, it allows the transformer to carry rated load in a room or outdoor enclosure that runs well above 40°C, such as a rooftop or a process area, without derating. As overload margin, it allows a sustained load above nameplate for as long as the hot spot stays under 180°C, which is why an 80°C-rise cast coil is often paired with the AFN fan provision so that the fans are an emergency reserve rather than a continuous rating. As service life, running the standard load on an 80°C-rise unit keeps the insulation far below its limit and provides a design life exceeding 50 years. The cost is a larger, heavier unit with lower load losses and slightly higher no-load loss. The Knowledge Hub article on design, environment and load in transformer lifespan quantifies the life effect.

Each transformer is built with high-quality electrical steel cores and precision-wound copper or aluminum conductors. The coils are placed in a mold and encapsulated under vacuum using epoxy resin to create a dense, homogeneous insulation system free of voids or moisture. The cast coils are mechanically braced and mounted on vibration-resistant supports within a ventilated or sealed NEMA-rated enclosure. The result is a robust transformer that maintains electrical and mechanical stability under severe loading and environmental stress.

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.

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

• Ambient Temperature: -30°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.

Rex pad mounted cast coil transformers require a level concrete pad sized and reinforced according to the unit’s outline drawing and weight. Separate conduit openings allow primary and secondary cables to enter their respective terminal chambers.

The enclosure, core, and frame must be properly grounded, with neutral bonding where required by applicable codes. Tamper-resistant, pad-lockable compartments and code-compliant clearances ensure safe outdoor installation. Refer to the Rex Enclosure Drawings for specific dimensions and installation requirements.

The general purpose configuration of a Cast Coil Power Transformer has primary and secondary bus stubs inside the main enclosure, oriented « stubs up » or « stubs down », for side or bottom cable entry. For cables that arrive from above, from an overhead tray or a bus riser, the terminals may be routed to an air terminal chamber (ATC). The ATC is a separate compartment on the enclosure with its own access panel, in which the bus stubs are extended so that cables are terminated outside the coil compartment.

Specify an ATC when the cable route is from above, when the medium-voltage terminations need the working space that stress cones and lugs require, when the primary and secondary must be in separate lockable compartments for operator safety, or when the installation must allow cable work with the coil compartment closed. The chamber adds width or height to the outline, and the standard Enclosure Drawings table covers stubs-up units only, so an ATC unit is a custom drawing. A related option is routing the stubs to either end of the enclosure to reduce internal cable routing on site. State the cable entry direction and the number and size of conductors per phase on the Technical Data Sheet so that the chamber suits the cable bending radius. The Knowledge Hub’s 3-phase transformer wiring guide covers the termination conventions.

A cast coil transformer is moved as a complete assembly by the lifting provisions on the core-and-coil frame or the enclosure base, as identified in the Maintenance, Operating & Installation Manual. The cast coils themselves must never be used as lifting, pushing or rigging points, and no chain, sling or bar should bear on a coil surface; a chipped or cracked casting is a dielectric defect, not a cosmetic one. For crane lifts, all lifting eyes are used together with spreader bars so that the slings are near vertical, and the load is kept level. For floor moves, the unit is rolled on its base channels or skidded on rollers, never dragged by the enclosure.

Before setting, remove any shipping braces or blocking as instructed in the manual, and inspect the coils for transport damage and the clamping hardware for looseness. Set the unit on a level surface; on the anti-vibration or seismic mounting options the isolators or snubbers are installed between the base and the floor at this stage, so their location must be established before the anchors are drilled. Cold weather adds a caution: an unenergized unit that has been below -20°C in transit should be brought to room temperature before it is handled or energized, following the Transformer Cold Start Procedure. The Knowledge Hub’s transformer commissioning article lists the receiving checks.

A bus or fleet charging depot is often supplied at 13.8 kV or 27.6 kV and stepped down to 600 V or 480 V for a bank of DC fast chargers, and it is a natural application for the pad mounted Cast Coil Power Transformer: the transformer sits outdoors on a pad, in a yard where vehicles and the public move, and it runs unattended through every season. The sealed casting handles the condensation of a cold night and the dust of a bus yard, the tamperproof enclosure handles the public exposure, and the 115°C rise and long thermal time constant handle the load profile.

That profile is the key specification point. Charger load is highly cyclic, with full demand when a fleet returns and almost none between shifts, and the chargers are rectifier loads that draw harmonic current. State the number and rating of chargers, the expected coincident demand, the duty cycle and the harmonic spectrum or a K-factor on the data sheet so that Rex can choose the conductor subdivision and rise. A delta primary blocks triplen harmonics from the utility, a lower impedance limits the sag when a group of chargers ramps together, and the future-fan provision lets the depot grow without a second unit. The Knowledge Hub’s transformer considerations for EV charging infrastructure article and the EV charging industry page go further into the sizing.

Conductive dust is the single most damaging contaminant for a ventilated dry-type transformer, because a layer of coal, coke, iron oxide or carbon dust on a coil surface provides a tracking path across the insulation. A Cast Coil Power Transformer is the Rex design for this environment because the coil surface is a smooth, sealed epoxy with no exposed layer edges, so dust cannot penetrate the winding and a surface deposit can be removed without disturbing the insulation. The product page describes the surfaces as « smooth, sealed » for this reason.

The enclosure should be chosen for the dust load. A Type 1 or 3R ventilated enclosure will not provide any degree of protection from dust. A Type 12 enclosure with filtered louvers reduces the ingress at the cost of a larger enclosure or a derated unit, and a sealed Type 4 enclosure requires the transformer to be designed for the reduced cooling. Rex will advise which suits the stated dust concentration. Whatever the enclosure, the cleaning interval is set by inspection: at the first shutdown after commissioning, check the coil ducts and surfaces, and set the interval from what is found. Cleaning is by vacuum and low-pressure dry air with the unit de-energized; never use a water jet or solvent on a cast coil. The mining industry page and the transformer maintenance checklist article cover the routine.

Yes. Cast coil transformers are designed to operate in the most demanding environments, including outdoor, marine, and corrosive locations. The solid epoxy insulation prevents moisture absorption, while enclosures rated NEMA 3R, 4, or 4X provide additional protection from dust, rain, and chemicals. Stainless steel or epoxy-coated housings are available for coastal or industrial applications. These transformers operate reliably in environments that would challenge conventional dry-type designs.

Rex Power Magnetics cast coil transformers are used in mission-critical and heavy-duty environments such as marine systems, mining operations, offshore platforms, industrial plants, hospitals, data centers, and renewable energy installations. They are ideal for medium-voltage distribution, motor control, and step-up or step-down applications requiring enhanced protection and reliability. Their solid insulation and rugged mechanical design ensure stable operation in locations subject to vibration, dust, or corrosive atmospheres.

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 -30°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.

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.

Both designs are used in hospitals, and the choice turns on the room and the operating regime rather than on electrical performance. A hospital’s medium-voltage transformers usually sit in a dedicated electrical room, run continuously at moderate load, and are transferred to and from generator supply under test each month. In a dry, climate-controlled room, a VPI/VPE Power Transformer at 115°C or 80°C rise meets the reliability requirement at lower cost, and its lower mass makes it easier to move into an existing building.

A Cast Coil Power Transformer is the better choice where the room is below grade and subject to humidity or flooding risk, where the unit sits next to a chiller plant or in an unconditioned penthouse, where the transfer test involves the transformer being energized cold after a long de-energized period, or where the essential-power unit has to ride through a generator-fed block load at the moment of transfer, which the long thermal time constant and partial-discharge-free insulation tolerate well. Its flame-retardant, self-extinguishing casting is also easier to justify to a hospital’s fire authority. Either way, specify winding thermometers with alarm contacts to the building automation system, an NGR where the secondary is resistance grounded, and the partial discharge test report as a submittal. The VPI vs. cast coil Knowledge Hub article works through the same trade-off.

A hazardous location, in Canadian Electrical Code Section 18 or NEC Article 500 terms, is an area where flammable gas, vapour or combustible dust may be present in sufficient quantity to ignite. A cast coil transformer is not certified for such an area by virtue of its construction. Its ventilated enclosure admits the atmosphere, its terminals and tap links are potential arcing points, and its coil surfaces run hot. The Rex Hazardous Location Transformer is the product certified for a Class I, Division 2 or similar area, and it is a low-voltage unit; medium-voltage transformers are, as a rule, kept out of classified areas entirely.

The correct approach is to locate the Cast Coil Power Transformer in an unclassified electrical room or outdoor pad outside the area boundary, and to run the low-voltage distribution into the classified area through certified equipment. A pressurized or purged enclosure near the boundary is a specialised engineering solution requiring a certification review, not a standard option. What the cast coil design does offer to petrochemical, grain and mining sites is resistance to the corrosive and dusty atmospheres that surround, but do not enter, the classified areas, and freedom from oil, which reduces the fire load adjacent to them. Confirm any installation near a classified boundary with the authority having jurisdiction. The Specialty Products category page lists the hazardous-location unit alongside the other special-environment designs.

A Rex cast coil transformer is generally heavier than a comparable VPI/VPE unit due to its epoxy-encapsulated windings, although their footprints may be similar.

For existing electrical rooms, verify floor loading, door and corridor widths, rigging access, and installation clearances. Refer to the Rex Enclosure Drawings for exact dimensions and weight before planning a retrofit.

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.

Several clauses inherited from VPI or liquid-filled specifications produce a query or a non-standard quotation when applied to a Cast Coil Power Transformer. The most frequent:

• Temperature rise stated as 150°C. The cast coil standard is 115°C on a 180°C system, with 80°C optional; 150°C rise is a VPI/VPE value.
• Insulation system stated as 220°C. The cast coil uses a 180°C (Class F) system; a 220°C requirement points to VPI/VPE.
• Partial discharge listed as a type test to be priced separately. On cast coil it is a standard production test on every unit.
• Reference to an IEC test standard alone. Rex cast coils are CSA certified and UL listed to CSA C22.2 No 47, CSA C9, IEEE C57.12.01 and IEEE C57.12.91; state these as the basis and add IEC clauses only where the site requires them.
• A Type 4 sealed enclosure specified without accepting the cooling implications,
• A 10 kV low-voltage BIL applied to a medium-voltage secondary.
• Sound level, losses or impedance copied from a liquid-filled unit’s data.

Write the specification around the Rex specification table, complete the Technical Data Sheet on the product page, and list accessories by their Rex option names. The Certifications page confirms the marks, and the power transformer testing article explains the test terminology so that the two lists align.

A cast coil is designed to survive both events, but each is an opportunity to confirm that it did. After a through-fault, de-energize and lock out, then inspect each coil for cracks, crazing or chips in the casting, especially at the coil ends and around the terminal and tap inserts, and check the clamping hardware, the bus bracing and the coil supports for movement or loosened fasteners. Winding resistance and turns ratio tests compared with the factory report will reveal a displaced or shorted turn, and any change is grounds to contact Rex before re-energizing. Insulation resistance should be measured and compared with the commissioning values.

After a lightning strike or a switching surge event, the checks are dielectric rather than mechanical: look for tracking marks or discoloration on the coil surface and at the coil ends, inspect the surge arresters if fitted, and, if the unit is critical, arrange a partial discharge test, which is the sensitive indicator of internal damage to a cast coil. Record what was found in the maintenance log with the fault current and duration from the relay if available, because the through-fault curve counts cumulative events. The Knowledge Hub’s transformer failures and lightning and surge protection articles describe the damage mechanisms, and the Maintenance, Operating & Installation Manual gives the inspection procedure.

Work through the causes in order of likelihood.

1. Load: compare the current on all three phases with the nameplate, including harmonic content, which increases winding loss without increasing the fundamental current; a true-RMS meter or power quality analyser will show it.
2. Ambient and airflow: the 115°C rise is on a 40°C peak ambient, and a poorly ventilated room, a unit closer than 150 mm to a wall, blocked louvres or a dust-loaded filter will raise the intake temperature.
3. Coil ducts: dust in the air duct between the high- and low-voltage coils reduces convection markedly, and this is the most common cause on a unit a few years old in a dusty plant.
4. Cooling Fans (if fitted): confirm that they run when the thermostat calls and rotate in the correct direction.
5. Connections: a loose joint heats locally and can raise a nearby sensor reading; an infrared scan finds it.
6. Tapping: a unit on a low tap carries more current for the same kVA.

Only after these are excluded is the transformer itself suspect, at which point resistance and ratio tests against the factory report and a thermometer calibration check are the next step. The Knowledge Hub’s power quality issues in industrial facilities article covers the harmonic contribution and the power quality industry page lists the Rex products that address it.

A new unit’s sound level is verified against IEEE C57.12.01 and CSA C9, and the hum at that level is the 120 Hz magnetostriction of the core with its harmonics. An increase after years in service points to something that has changed. The most common cause is loosening of the core clamping or coil supports after thousands of thermal cycles and the occasional through-fault; the laminations then vibrate and the sound acquires a rattle. A torque check of the clamping bolts at the next shutdown is the first action. The second cause is the enclosure: a panel or louvre with a loosened fastener resonates with the core, and a fastener or damping strip resolves it.

The third cause is the supply. DC offset or even-harmonic content on the primary, from a nearby rectifier, drives the core into partial saturation and produces a louder, rougher hum with a strong 60 Hz component; harmonic-rich load current has a similar effect. A power quality measurement identifies this. Fourth, a move to a lower tap raises the flux density and the sound with it, and operation above rated voltage does the same. If none of these apply, Rex should be consulted, since a shifted coil is a mechanical concern. The Knowledge Hub’s transformer humming noise article covers the mechanisms, and the anti-vibration mounting option is the remedy for structure-borne noise.

Yes. Rex Power Magnetics designs and builds custom cast coil transformers to meet specific electrical, thermal, and mechanical requirements. Options include special voltage ratios, kVA ratings, impedance levels, enclosure types, cooling enhancements, and seismic or vibration-resistant configurations. Engineering support is available to ensure the optimal design for each project’s application and environmental conditions. Every custom transformer undergoes full electrical and mechanical testing to ensure compliance, safety, and dependable operation.