Control & Automation
Rex Power Magnetics Medium Voltage Control Power Transformers are engineered to provide reliable low-voltage power from medium-voltage sources for switchgear, substation, and protection control applications. These transformers are essential for delivering safe, isolated, and stable control power where direct low-voltage supply is unavailable, ensuring dependable operation of protective relays, breaker controls, and auxiliary systems.
Each transformer is custom-designed to meet demanding electrical and environmental requirements. Utilizing high-quality core materials, copper or aluminum windings, and advanced insulation systems, Rex Medium Voltage Control Power Transformers achieve excellent dielectric strength, thermal performance, and mechanical durability. Resin-encapsulated and dry-type construction options provide superior protection against moisture, dust, and contaminants, making them ideal for indoor or outdoor installations.
All units are designed and tested in accordance with CSA, ANSI, and IEEE standards, ensuring compliance, safety, and long-term reliability. Typical primary voltages range up to 7.2 kV, with a variety of secondary voltages and configurations available to meet customer specifications.
With decades of design and manufacturing expertise, Rex Power Magnetics provides engineered-to-order control power transformers that combine technical precision, robust construction, and field-proven performance — delivering confidence in every critical control application.
| Capacity | 500VA to 3,000VA Single Phase |
| Voltage Class | up to 7.2kV (60kV BIL) |
| Cooling | Air Cooled (ANN / AN) |
| Frequency | 60Hz (50/60Hz Optional) |
| Conductors | Copper (Cu) Windings |
| Insulation System | 180°C (115°C rise) |
| Enclosure Type | Open Construction With Encapsulated Coil |
| Warranty | 12 Months (See Warranty and Limitations) |
| Quality System | ISO 9001:2015 Quality Management System |
| Certifications |
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| Reference Standards |
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| Optional Features & Accessories |
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Configure a control power transformer (CPT) to build its Rex catalog part number. Codes follow the Rex catalog numbering system.
This tool assembles a catalog part number from the options you select, following the Rex Power Magnetics catalog numbering system. Not every combination of options is necessarily a standard or available product, and some configurations may require review or a custom quote. For a voltage not listed, the code ‘X’ is used and the value specified separately. Rex Power Magnetics makes no representations as to the accuracy of the information generated by this tool; part numbers should be confirmed with Rex before ordering. Contact Rex Power Magnetics to verify availability, pricing, and specifications.
A medium voltage control power transformer is designed to step down medium voltage (typically 2.4 kV to 15 kV) to a safe, stable control voltage for auxiliary loads such as switchgear, motor starters, or relays. It provides galvanic isolation between the medium voltage distribution system and low-voltage control circuits, ensuring safety and reliability. These transformers are built for use in environments where dependable control power must be available directly from a medium voltage source, eliminating the need for a separate low-voltage supply.
A Rex MV CPT is single-phase, so the primary is connected across two phases of the bus, and the ordered primary voltage must equal the line-to-line system voltage: 2,400 V, 4,160 V, 4,800 V or 7,200 V, not the line-to-neutral value. Phase-to-phase connection is preferred on both grounded-wye and delta systems because it keeps the transformer voltage independent of neutral shift during a ground fault; on an ungrounded or high-resistance-grounded 4.16 kV delta system a phase-to-ground connection would be exposed to full line-to-line voltage during a fault and to ferroresonance risk.
Both primary terminals are therefore live at up to 7.2 kV and both need fusing, which is why the optional primary fuse clips (code F) are supplied as a pair.
The Rex MV CPT is built as a laminated silicon-steel core with a single coil assembly in which the medium-voltage primary winding and low-voltage secondary winding are wound concentrically and then cast in resin. The encapsulant fills the spaces between turns and layers, replacing air, which is the weakest dielectric in the structure, with a solid insulation that resists partial discharge. Copper windings are standard.
“Open construction” means the core and coil are not housed in their own enclosure. The unit mounts on a base bracket inside the switchgear cell, MCC or control cabinet that provides the environmental protection and the barrier to live parts. This distinguishes it from an encapsulated transformer in the low-voltage range, which is a fully sealed enclosed product, and from a VPI-impregnated coil, which is dipped and baked rather than solid cast.
A voltage transformer and a MV CPT occupy similar positions in switchgear but they are designed for opposite priorities. A VT is built to an accuracy class with a small rated burden, typically tens to a few hundred VA, so that relays and meters see an accurate replica of the bus voltage. Its thermal rating is modest and its impedance is chosen for accuracy, not regulation under load. Connecting a spring-charging motor or heaters to a VT would introduce measurement error, overheat the winding and risk the protection it serves.
The MV CPT is the inverse. It is rated for continuous power delivery at 500 to 3,000 VA and is designed for voltage regulation under intermittent inrush. Its ratio is set for a usable secondary voltage, not for accuracy-class metering.
The secondary voltage should follow the control devices, not habit. 120 V is the common choice for North American switchgear because relay power supplies, spring-charging motors, indication and small heaters are catalogued at 120 V, and a single grounded conductor keeps the wiring simple. 240 V halves the current for the same VA, which is useful where the CPT feeds long runs to outdoor lighting or a bank of compartment heaters; a 3,000 VA unit delivers 12.5 A at 240 V rather than 25 A at 120 V, reducing voltage drop on the control wiring.
A 120/240 V three-wire secondary with a centre tap gives both: heaters and larger loads on 240 V, relays on 120 V, with the neutral grounded. The centre-tapped arrangement requires the two halves of the load to be reasonably balanced, and the secondary protection has to be two-pole. Rex supplies any of these as a standard selection in the Product Configurator; non-standard values such as 110 V, 208 V or export voltages are handled as an engineered variant under the X special-requirement code. The Transformer Turns Ratio Formula article explains how the ratio is fixed at manufacture and why it should be confirmed before the wiring diagram is issued.
Rex MV CPTs appear wherever a medium-voltage assembly needs its own control source. In mining, portable and skid-mounted substations at 4.16 kV and 7.2 kV use a CPT in each switchgear section so protection and breaker charging survive a loss of the low-voltage plant supply; the encapsulated coil tolerates the dust and vibration of a haul-truck-accessible skid. In energy storage and renewable collector systems, the CPT powers the collector switchgear controls and heaters from the MV bus so the site can be re-energized without an external supply.
Data centre medium-voltage lineups and generator paralleling gear use CPTs, often two per lineup with an automatic transfer scheme, so that control power redundancy matches the redundancy of the distribution. MV-fed EV charging hubs and temporary power distribution use them in the MV service section. Medium-voltage motor control centres at 2.4 kV and 4.16 kV use CPTs to supply vacuum contactor coils and starter logic. For shipboard and offshore 6.6 kV switchboards under the marine duty range, the CPT is normally ordered with the 50/60 Hz option and electrostatic shield.
These transformers are used wherever auxiliary low-voltage power is required from a medium voltage source. Common applications include industrial substations, motor control centers, switchgear panels, power generation plants, and remote installations. They supply control power for relays, instrumentation, lighting, or communication systems when a secondary service voltage is not available. The encapsulated coil design ensures reliable operation in demanding environments, providing electrical isolation and protection for sensitive control components even in the presence of high transient voltages.
Rex MV CPTs follow the North American convention: the medium-voltage primary terminals are marked H1 and H2, and the low-voltage secondary terminals X1 and X2, with X3 appearing on a 120/240 V unit for the centre tap. A grounding terminal on the core clamp or base is provided for bonding the core and frame, and the electrostatic shield lead, on units ordered with option S1, is brought out separately for connection to the same ground.
Polarity is subtractive on standard units, meaning that when H1 and X1 are the adjacent terminals and the transformer is viewed from the terminal side, the instantaneous voltage at X1 has the same polarity as H1. Polarity matters more than it might seem on a control transformer: where two CPTs feed an automatic transfer scheme, where a CPT secondary is synchronism-checked against another source, or where the secondary neutral must be common to a station battery charger, the X1/X2 designation must be respected on the drawings. Polarity is verified on every unit during routine testing and shown on the outline and connection drawing issued with the order and posted on the Product Documentation page. The Transformer Polarity Explained and Understanding Transformer Nameplates articles cover the marking conventions.
Ordering a Rex MV CPT with the 50/60 Hz option (code 50 in the Part Number Builder) changes the core design, not just the nameplate. Flux density in a transformer core is inversely proportional to frequency for a given voltage and turns, so a core designed for 60 Hz and energized at 50 Hz runs at about 20% higher flux density. On a compact encapsulated MV coil that pushes the core toward saturation, raising the magnetizing current, no-load losses, temperature and audible noise, and it can increase the magnetizing inrush that the primary fuses must ride through. A 50 Hz-rated core has more iron or more turns, and operates without penalty at 60 Hz.
The option is required for export switchgear destined for 50 Hz networks, for shipboard and offshore installations in the marine duty range where the vessel’s generation may be 50 Hz, and for temporary power equipment that may be deployed on either system. Where a lineup is built to IEC voltages such as 3.3 kV, 6.6 kV or 11 kV, state that on the order too, since 11 kV is above the standard 7.2 kV class and falls within Rex’s CSA-certified envelope to 15 kV as an engineered design. The Understanding Losses in Transformers article explains the frequency dependence of core loss.
The secondary of a Rex MV CPT is a separately derived system, and the CEC and NEC both require one conductor of a 120 V control circuit to be grounded. Standard practice is to ground X2 at the transformer or at the switchgear ground bus with a bonding jumper, and to run all secondary fuses, breakers, switches and contacts in the ungrounded X1 conductor. On a 120/240 V three-wire secondary the centre tap X3 is grounded and both outer conductors are protected. Grounding one conductor ensures that a control-wiring ground fault is a bolted fault that the secondary device clears immediately, rather than a floating condition that could energize a device unexpectedly.
Three further connections are needed. The core and frame of the CPT are bonded to the switchgear ground bus through the grounding terminal on the base. The electrostatic shield lead, if option S1 was ordered, is bonded to the same point. And the secondary ground must be the switchgear ground bus, not a remote building ground, so that the fault return path is short and impedance is low. The Transformer Grounding and Bonding Explained article sets out the code reasoning, and the Rex installation manual gives the terminal details.
The primary difference lies in construction and insulation. Medium voltage control power transformers are designed to handle much higher primary voltages than standard control transformers. They use encapsulated coil construction, high-dielectric insulation systems, and extended creepage distances to safely manage the electrical stress of medium voltage operation. The encapsulated design also protects windings from dust, moisture, and contamination. This ensures long-term reliability, stable performance, and compliance with the requirements of switchgear, substations, and heavy industrial systems.
Reliability is achieved through careful material selection and encapsulated coil construction. The encapsulant protects windings against moisture, dust, and vibration while enhancing heat dissipation and dielectric strength. The magnetic core is optimized for low losses and excellent voltage regulation. The combination of encapsulated design, precision winding, and rigorous testing ensures consistent operation under high-stress conditions such as switching transients, inrush currents, and voltage fluctuations. These features make Rex Power Magnetics transformers ideal for mission-critical control applications.
Where a substation has a low-voltage station service board, a designer can choose between a Rex MV CPT fed from the switchgear bus and a Rex industrial control transformer fed from the 480 V or 600 V board. The ICT is less expensive, is available from 25 to 7,500 VA and needs only low-voltage fusing, so it is the natural choice where the station service supply is highly reliable and backed by a UPS or generator.
The MV CPT wins on independence. Its supply is the bus it protects, so it does not depend on a low-voltage feeder, a separate transformer, an ATS or a UPS, any of which can be out of service during maintenance or a fault elsewhere. That is why utilities and mining operators commonly specify a CPT in every switchgear lineup even where station service exists, and use the ICT-fed source as the alternate in a transfer scheme. The MV CPT also avoids running 120 V control power across a substation from a remote board, with the voltage drop and exposure that implies.
A combined approach is often best: MV CPT as normal source, station service ICT as alternate, with a transfer switch on the control bus. The What Is an Isolation Transformer? article describes the isolation both products provide, and the Control & Automation category page compares the ranges.
A primary fuse that clears when the bus is energized, with no damage evident, is nearly always an inrush-coordination problem rather than a transformer fault. Work through the following in order on a Rex MV CPT:
• Confirm the fuse’s E-rating and type against Rex’s recommendation for the VA and voltage. A fuse sized purely on rated current, for example 0.5E on a 2,000 VA, 4,160 V unit, will have a minimum-melt curve that inrush crosses.
• Check whether the fuse was replaced with a different class after a previous event; a general-purpose fuse and a current-limiting E-rated fuse of nominally similar ampere rating have very different melt curves.
• Isolate the secondary and re-energize. If the fuse holds, the problem is on the control circuit: a bolted fault, a shorted heater, or a secondary breaker set too high so that the primary fuse operates first.
• If the fuse still blows with the secondary open, test the transformer: insulation resistance primary-to-ground and primary-to-secondary, turns ratio, and winding resistance, compared with the test report. A shorted turn shows as a ratio error and elevated no-load current.
Repeated energization while troubleshooting adds thermal stress to the fuses and should be limited. If tests indicate a winding fault, contact Rex through the Contact page with the serial number and test values. The Transformer Failures article covers the common failure modes.
A healthy Rex MV CPT produces a low 120 Hz hum from magnetostriction of the core, twice the supply frequency, that is barely audible inside a closed switchgear compartment. The encapsulated coil and clamped core keep lamination vibration low, and units are physically small, so the hum is usually inaudible beside the switchgear itself. A change in hum is worth investigating: a louder hum after energization on a new lineup usually means the bus voltage is above the CPT’s nominal, driving the core toward saturation, and should prompt a check of the ratio ordered against the actual system voltage.
Two other sounds indicate a fault. A sharp crackling or hissing, sometimes with a faint ozone smell, is surface discharge or partial discharge across the primary terminals or coil surface, usually caused by contamination and moisture on the creepage path, and the unit should be de-energized and inspected before the tracking damages the encapsulation. A buzzing or rattling that varies with load is a loose mounting or a loose terminal connection, which will overheat and should be tightened to the torque values in the installation manual.
The Transformer Humming Noise article explains the magnetostriction mechanism, and the Partial Discharge in Dry-Type Transformers article covers the discharge signature.
All medium voltage control power transformers are designed and tested in accordance with CSA C9, CSA C22.2 No. 66, and IEEE C57 standards. Units are CSA certified and manufactured under Rex Power Magnetics’ ISO 9001-certified quality management system. The encapsulated coil construction provides superior environmental and electrical protection while ensuring compliance with applicable North American and international safety requirements.
Encapsulated medium voltage control power transformers are low-maintenance devices. Routine inspection should include verifying terminal connections, cleaning the enclosure exterior, and checking for signs of overheating or corrosion. The encapsulated coil design eliminates the need for varnish re-application or insulation reconditioning. When operated under normal load and ambient conditions, these transformers deliver long service life with minimal upkeep. Periodic visual inspection ensures continued electrical integrity and early detection of mechanical wear in high-duty installations.
Yes. Rex Power Magnetics specializes in custom designs for specific industrial and utility applications. Customization options include primary voltage ratings up to 15 kV, special secondary voltages, insulation levels and integrated protective devices such as fuses. Our engineering team can assist in optimizing form factor, thermal performance, and environmental protection. Each custom-built transformer undergoes the same rigorous testing and quality control as standard models, ensuring long-term reliability and compliance with project specifications.
Commissioning a Rex MV CPT takes less than an hour and prevents most early failures. With the unit isolated and the secondary disconnected from the control circuit:
• Inspect the encapsulated coil surface and primary terminals for shipping damage, cracks or contamination, and clean with a dry cloth; verify the mounting hardware is tight.
• Measure insulation resistance primary-to-ground, primary-to-secondary and secondary-to-ground with a 2.5 kV or 5 kV megohmmeter on the primary and 500 V on the secondary. Values should be in the hundreds of megohms or higher at 20°C; compare with the factory value on the test report rather than a generic threshold, and correct for temperature.
• Confirm turns ratio with a TTR set against the nameplate and test report, remembering the compensated no-load ratio.
• Verify polarity marking and the X2 (or X3) ground connection.
• Check that the primary fuses installed match the specified E-rating and that both clips make firm contact.
• Confirm the secondary breaker rating and that the control circuit is free of ground faults.
A field applied-voltage test is not normally required and, if a specification demands one, should be run at no more than 75% of the factory level. After energization, check the secondary voltage at no load and during the first spring-charging operation. The Transformer Commissioning article and the Rex installation and maintenance manual provide the full sequence.
An encapsulated Rex MV CPT needs no scheduled maintenance beyond inspection, which is best aligned with the switchgear’s own outage schedule, typically every one to three years and more often in dusty or humid locations. With the cell isolated and grounded, look for: dust or salt film on the coil surface and terminals, which shortens the creepage path and should be removed with a dry cloth or vacuum, never solvents that attack resin; discolouration or darkening of the encapsulant near the terminals, which indicates overheating from a loose connection or overload; white or carbonized tracking marks across the surface, which are the residue of surface discharge; and cracks or crazing in the resin.
Record insulation resistance at each inspection and trend it. A steady value confirms the encapsulation is intact; a falling trend, particularly primary-to-secondary, means moisture ingress or tracking. Retighten terminals to the manual’s torque values, since thermal cycling loosens connections on fine primary wire.
Replace rather than clean when tracking has penetrated the resin surface, when insulation resistance does not recover after cleaning and drying, or when a ratio test shows a shorted turn; encapsulated coils cannot be rewound economically at this size. The Transformer Maintenance article gives the checklist, and the Role of Design, Environment and Load in Transformer Lifespan article explains what drives ageing.
Wednesday, 10 September 2025
Transformers are essential components in electrical systems, allowing the efficient transmission and distribution ...
