Transformateurs de commande industrielle

A Rex industrial control transformer with a 120 × 240 V dual secondary has two 120 V coils on terminals X1, X2, X3 and X4. For a 120 V control circuit the coils are paralleled (X1 to X3, X2 to X4) so that the full VA is available at 120 V. For a 240 V output the coils are placed in series (X2 to X3), with the load across X1 and X4. Connecting them in series and grounding the X2-X3 junction gives a 120/240 V three-wire output, which lets one transformer feed a 240 V heater and a 120 V control bus simultaneously, with the constraint that each 120 V half can supply only half the nameplate VA and the two halves should be balanced.

Rex recommends the three-wire arrangement only for larger units where the split does not starve the control circuit, for example a 3,000 VA transformer feeding a 1,000 VA heater on 240 V and a 400 VA control bus on one 120 V leg. For most panels, a single 120 V secondary with the full VA behind it gives better inrush regulation. Where a 24 V circuit is also needed, order a second, smaller transformer rather than a three-voltage secondary. The 3-Phase Transformer Wiring article, although written for three-phase units, uses the same terminal conventions.

Distribution transformers carry 2.5% taps because they are set once and left alone. A Rex industrial control transformer has no taps as standard (tapped windings are available as a custom requirement) because at control-circuit sizes a tap board adds cost and a source of error while providing little benefit: the compensated secondary and low regulation already absorb normal supply variation, and the control devices on the secondary are themselves tolerant of roughly plus or minus 10%.

Instead, Rex offers dual-voltage primaries. Canadian nominal 600 V systems commonly measure 575 V under load and up to 630 V at light load, and a 600 V-rated primary handles the whole band. If a plant runs consistently low, for example a remote mining feeder at 550 V, order the primary rated for the actual voltage; a non-standard primary such as 550 V or 575 V is a normal special requirement (X in the Part Number Builder). Never connect a 600 V system to the 480 V winding to boost the output: the core will be overexcited by 25%, magnetizing current and heating rise sharply and the unit will fail.

Rex Power Magnetics industrial control transformers are typically supplied with dual primary and dual secondary windings—for example, 240×480 V primary and 120×240 V secondary—allowing flexibility in connection for various voltage requirements. These windings can be connected in series or parallel to accommodate single- or three-phase systems. Custom voltage ratings, frequencies (50/60 Hz), and tap configurations are available upon request to meet global application standards. The design versatility makes it easy for OEMs and system integrators to use the same model in different control panels or machinery configurations, simplifying inventory and reducing installation time.

The open core-and-coil Rex industrial control transformer is designed for back-plate mounting inside an enclosure that provides the environmental protection. Position it so the coil is vertical and there is free air above and below for the natural convection that ANN cooling depends on. Keep it low in the panel and away from the components most sensitive to heat, such as PLC racks, power supplies and VFDs, and avoid mounting it directly beneath a wire duct that would trap rising air. Mount 5,000 to 7,500 VA units on the structural back-plate, not a sub-panel.

Place the transformer close to the incoming disconnect so that the primary fused leads are short, and orient it so the primary terminals face away from the secondary control wiring; this keeps 480 V or 600 V conductors segregated from the 120 V control field. Where the panel also contains a VFD, an electrostatic shield on the control transformer and physical separation from the drive’s motor leads reduce noise coupling into the control circuit.

In a Canadian plant the control transformer primary is normally connected phase-to-phase across two lines of the 600 V system, with the primary rated 600 V. Because neither primary conductor is a grounded conductor, both primary leads must be fused or protected; a single fuse in one leg leaves the transformer and the other conductor exposed to a fault through ground. The Rex Primary Fusing Kit mounts a two-pole fuse holder on the transformer frame for exactly this arrangement.

The primary conductors are tapped from the panel’s main disconnect or a feeder in accordance with the Canadian Electrical Code’s control-circuit tap rules, and the primary overcurrent device is sized to protect the transformer rather than the feeder. On the secondary, the X2 terminal is bonded to the panel ground bus and the fuse is placed in the X1 conductor. If the system is a 600 V ungrounded or high-resistance-grounded delta, common in mining and process plants, tell Rex at ordering, since the line-to-ground voltage during a ground fault approaches the 750 V CSA class and the winding insulation should be confirmed for that duty.

Touch-safe terminal covers are strongly recommended on the 600 V primary. The Transformer Grounding and Bonding Explained article covers the system-grounding considerations behind these rules.

No. The Type 1 enclosure offered on the enclosed winding style of the Rex industrial control transformer is an indoor, general-purpose housing that protects personnel from contact with live parts and keeps out falling dirt. It is not rated for rain, hose-directed water, condensation or corrosive atmospheres. Installing a Type 1 unit outdoors, on a rooftop HVAC unit, in a car wash or in a food-plant washdown zone will lead to corrosion of the terminals and eventual insulation failure, and it will not pass inspection.

For those environments there are two approaches. The first, and most common, is to install an Open or Encapsulated industrial control transformer inside the equipment’s own Type 3R, 4, 4X or 12 enclosure, which then provides the rating for the whole assembly. The second is to ask Rex for an engineered-to-order enclosed unit with the required NEMA type; that is handled as a special requirement and typical outlines are shown on the Enclosure Drawings page. In salt or chemical atmospheres, such as marine duty installations, the encapsulated coil inside a stainless enclosure is the durable combination.

Commissioning a control transformer takes minutes but catches the errors that account for most early failures. Working from the installation manual, verify the following before closing the primary disconnect:

• Nameplate versus drawing: confirm VA, primary and secondary voltages, frequency and option codes match the panel schematic.
• Primary jumpers: check that the H-terminal links correspond to the actual supply voltage (series for the higher voltage, parallel for the lower) and that no coil is paralleled with reversed polarity.
• Secondary connections: confirm X2 is bonded to the panel ground at one point only and that the fuse is in the X1 conductor.
• Insulation resistance: a 500 V or 1,000 V megger test primary-to-secondary, primary-to-ground and secondary-to-ground should read in the hundreds of megohms on a new unit; a low reading points to moisture or a pinched lead.
• Electrostatic shield: if fitted, confirm the shield lead is landed on ground.
• Mechanical: mounting bolts tight, terminal screws torqued, touch-safe covers fitted, fuse kit populated with the correct class and rating.

Energize with the secondary load disconnected and measure the no-load secondary voltage; expect it to read a few percent above nominal because of the compensated winding. Then connect the load and confirm the voltage under sealed conditions. The Transformer Commissioning article expands each step, and a certified factory test report is available for projects that require documented evidence.

Industrial control transformers are generally low-maintenance devices, but periodic inspection is recommended for optimal performance. Maintenance involves verifying that terminals are tight, checking insulation for discoloration or cracks, and ensuring adequate ventilation around the unit. Any accumulated dust or debris should be removed to prevent overheating. If the transformer operates in high-humidity or corrosive environments, visual inspections should occur more frequently. Under normal operating conditions, Rex Power Magnetics industrial control transformers provide years of reliable service with minimal attention, thanks to robust construction and high-quality insulation materials.

Yes. When properly installed in rated enclosures or used in conjunction with Rex Power Magnetics’ hazardous-location designs, industrial control transformers can safely operate in demanding environments. These include areas exposed to moisture, dust, corrosive agents, or explosive vapors. Suitable NEMA-rated or explosion-proof housings ensure compliance with environmental protection standards. Additionally, transformers can be designed with epoxy or varnish encapsulation to improve moisture resistance. Proper grounding, ventilation, and insulation practices are essential for maintaining performance and safety in such locations, and Rex provides guidance to ensure compliance with applicable codes and regulations.

Rex Power Magnetics offers industrial control transformers in several configurations to suit diverse installation requirements. Open core-and-coil units are ideal for integration within OEM control panels, while enclosed models are available in NEMA 1, 3R, 4, and 12 rated housings for additional protection. Enclosures can be wall-mounted, floor-mounted, or panel-mounted, providing flexibility for indoor or outdoor installations. Each design supports easy access for wiring and inspection while maintaining safety and ventilation standards. The wide range of mounting and enclosure options ensures compatibility with both standard and custom control system layouts.

The selection table in the Control Transformer Catalogue lists, for each VA rating, the maximum inrush VA the transformer can deliver while keeping the secondary voltage at or above a stated percentage of nominal, at full and reduced primary voltage. The percentage columns correspond to the pick-up voltage the connected coils need. Most NEMA contactors and relays are rated to pull in at 85% of coil voltage, so the 85% column is the usual reference; devices with tighter tolerances, or circuits that must pick up while the primary is already depressed by a motor start, justify using the 90% or 95% column, which returns a larger transformer for the same inrush.

The method is: compute the total inrush VA (all coils that can energize together plus everything already sealed), pick the column matching the lowest acceptable secondary voltage, then read down to the first rating whose inrush capacity equals or exceeds the demand. Check that the sealed VA is also within nameplate. If the supply routinely sags, for example on a long feeder or a generator, use the reduced-primary-voltage columns, since a primary dip appears directly on the secondary before inrush regulation is considered.

Use the 24 V table for 24 V secondaries rather than scaling the 120 V figures. The Transformers Design article gives the general selection logic, and Rex engineering will review a load list on request.

The answer depends on what the panel itself is rated for. A Rex industrial control transformer is a general-purpose component; on its own it carries no hazardous-location marking. In a Class I, Division 2 area, general-purpose equipment that has no arcing or sparking parts and no surfaces above the ignition temperature of the gas is commonly accepted when installed in a suitable enclosure, and a control transformer meets the first condition since it has no contacts. The second condition is the one to check: a Class 180°C unit at full load can have surface temperatures that approach the T-code limits for some gases, so the ratings and the temperature classification should be reviewed with the panel’s certification body.

For Division 1 areas, for panels that must carry their own hazardous-location certification, or where the temperature question cannot be resolved, specify the Rex hazardous location transformer from the specialty products range, which is designed and certified for the classified environment. Many panel builders serving mining and petrochemical customers keep both options in their standards.

Where a control transformer is used, the encapsulated style with touch-safe covers is preferred because it is sealed against dust and eliminates exposed terminals. Rex can advise on the temperature data required for the hazardous-area assessment; use the Contact page with the area classification and gas group.

A properly sized Rex industrial control transformer runs warm by design, but a unit that discolours or smells of overheated varnish is telling you something. Work through the likely causes in order:

• Continuous load above sealed rating: measure the secondary current with everything energized and compare with nameplate VA divided by voltage. Loads added after the panel was designed are the usual culprits.
• Panel ambient: a transformer rated for 40°C ambient in a panel that reaches 55°C has lost a large share of its thermal margin.
• Wrong primary connection: a 480 V winding connected to 600 V overexcites the core; it hums loudly and heats at no load.
• 50 Hz supply on a 60 Hz unit: the same overexcitation effect for export equipment; order the 50 option.
• Blocked convection: wire duct, cables or a second component mounted directly above or below the coil.
• Non-linear load: a large bank of switch-mode power supplies adds harmonic heating out of proportion to the VA.
• Reversed parallel coil: a mis-jumpered dual winding creates a shorted turn; the unit heats rapidly with no load and must be de-energized at once.

Insulation resistance and winding resistance measurements, compared with the factory test report, confirm whether damage has occurred. The Transformer Failures article catalogues these mechanisms, and the Power Quality page addresses the harmonic case.

Contactor chatter on energization is the classic symptom of an under-sized or badly regulated control transformer. When two or more large coils pull in together, their combined inrush drags the secondary below the coils’ pick-up voltage; the armatures start to close, the voltage recovers, they try again, and the result is a buzz or repeated drop-out that can weld contacts. Dropout during operation, rather than on energization, usually means the primary supply sagged, typically when a large motor started elsewhere in the plant.

Diagnose by putting a meter with min/max capture, or better an oscilloscope, on the secondary while the sequence runs, and compare the minimum with 85% of nominal. Then consider the remedies from least to most invasive. Stagger the energization with a short time delay so that the inrush terms no longer add. Replace incandescent pilot lights with LED devices to free up VA. Finally, replace the Rex industrial control transformer with the next rating up, which on an open unit sharing the same mounting pattern is often a direct swap; the Open Dims Table confirms whether the footprint changes.

Where the sag originates on the primary side, an AC line reactor does not help the control circuit; a larger transformer or a motor starting autotransformer on the offending motor addresses the root cause. The Power Quality Issues in Industrial Facilities article discusses supply-side voltage dips.

A primary fuse that clears the instant the disconnect closes, with the secondary unloaded, is almost always reacting to magnetizing inrush, not to a fault. The first half-cycle of flux at energization depends on where on the voltage wave the switch closes and on residual flux in the core. In the worst case the core saturates and the primary draws a pulse that can reach 20 to 30 times rated current for a fraction of a cycle on a small control transformer, decaying over several cycles. A fast-acting fuse sized close to rated current opens on that pulse apparently at random.

The fix is not to keep upsizing the fuse. Use a time-delay fuse of the class specified in the Rex Primary Fusing Kit document, sized within the code allowance for small control transformer primaries, which is deliberately generous for small primaries with separately protected secondaries precisely because of inrush. Confirm the fuse’s time-current curve rides above the inrush point, typically taken as 25 times rated current for 0.01 s.

A fuse that blows after the load is connected, or one that blows in only one leg, points to a secondary fault, a shorted coil or a reversed parallel connection instead. Megger and resistance-test the transformer before re-energizing. The Transformer Fuse Sizing article covers curve coordination.

Rex industrial control transformers are specified below 40 dBA, so a unit audible across a room has a cause worth finding. The sound is magnetostriction of the core laminations at twice the supply frequency, 120 Hz on a 60 Hz system. The most common reasons for a loud control transformer are:

• Overexcitation: a 480 V primary connection on a 600 V supply, a 60 Hz unit on 50 Hz, or a sustained overvoltage on the feeder. The hum is loud even at no load and the unit runs hot. Check the jumpers and the supply voltage first.
• DC offset or waveform distortion on the supply from nearby rectifier loads, which pushes the core asymmetrically into saturation on one half-cycle. This is a power quality issue that an electrostatic shield does not cure.
• Loose mounting: a transformer bolted to a thin back-plate or a resonant enclosure panel acts as a sounding board. Tighten the bolts and add a stiffener.
• Loose laminations or hardware after years of thermal cycling, which rattles rather than hums.

A momentary buzz when a large contactor energizes is normal inrush, not a transformer problem. An encapsulated unit is inherently quieter than an open unit because the epoxy damps lamination vibration, useful in occupied spaces such as data centre electrical rooms. The Transformer Humming Noise article covers each mechanism in more detail.

Most modern control panels feed their electronics from a switch-mode DC power supply, and that supply already provides some isolation from the AC control bus. The question is whether the Rex industrial control transformer ahead of it should also carry the electrostatic shield (option S). The shield is a grounded foil between primary and secondary that intercepts capacitive coupling, and its value depends on what is on the primary side. In a panel where the control transformer shares its 480 V or 600 V feed with variable-frequency drives, welders or SCR heaters, the primary carries fast-edged common-mode noise that a switch-mode supply’s input filter is not designed to absorb, and a shielded control transformer reduces PLC communication errors, analogue noise and spurious safety-relay resets.

In a panel with only motor starters and resistive loads on the supply, the shield adds little. As a rule, specify the shield when the panel contains a VFD or a servo drive, when the control circuit includes networked or safety-rated electronics, when the facility has known power quality complaints, and for EV charging or energy storage cabinets where the supply is shared with power electronics..

Industrial control transformers experience short, high inrush current when loads such as contactors or solenoids are energized. Rex Power Magnetics designs its transformers with optimized magnetic core materials, low-impedance windings, and high thermal capacity to handle these transient demands. This ensures the secondary voltage remains stable, even during peak current events. The transformer’s design includes superior voltage regulation characteristics and minimal magnetic losses, allowing it to quickly recover to steady-state voltage after an inrush event. This performance minimizes voltage dips that could otherwise cause control circuit malfunctions, ensuring smooth and reliable equipment operation.

To size an industrial control transformer, calculate the total steady-state VA of all connected control devices, such as coils, timers, and indicator lights, that will operate simultaneously. Next, multiply that value by an inrush factor (typically between 3× and 10×, depending on coil type) to account for momentary demand. Select a transformer whose continuous VA rating meets or exceeds this calculated requirement. Rex Power Magnetics provides comprehensive sizing charts and engineering support to help customers select the appropriate model for their system voltage, frequency, and environmental conditions. Proper sizing ensures voltage stability, prevents overheating, and extends equipment life.

The nameplate on a Rex industrial control transformer carries the data needed to reorder or replace the unit without guesswork: the Rex part number and serial number, the VA rating, primary and secondary voltages with the terminal and jumper diagram for dual-voltage windings, frequency (60 Hz or 50/60 Hz), the insulation class and temperature rise, the Type (Isolation or Auto), option indicators such as the electrostatic shield, and the CSA and UL marks with the applicable file references. The connection diagram shows exactly how the H and X terminals must be linked for each voltage.

To specify a replacement, record the part number and every nameplate field, note the winding style (open, Type 1 enclosed or encapsulated), and check whether a fuse kit, touch-safe covers or DIN rail assembly are fitted, since these are separate options. If the nameplate is illegible, measure the primary and secondary voltages at the terminals with the unit de-energized from the panel schematic, count the primary and secondary terminals to identify dual windings, and take the overall dimensions against the Open or Enclosed Dims Table to confirm the VA.

Rex can also identify a unit from its serial number. Send the details through the Get a Quote form or your local representative from the Agent Locator, and refer to the Understanding Transformer Nameplates article for a field-by-field explanation.

Four checks, made with the unit de-energized and disconnected, separate a failed Rex industrial control transformer from a panel problem:

• Insulation resistance with a 500 V or 1,000 V megger: primary-to-secondary, primary-to-frame and secondary-to-frame. A healthy unit reads in the hundreds of megohms; below about one megohm indicates moisture or carbonized insulation, and near zero a breakdown.
• Winding resistance with a low-ohms meter: compare primary and secondary values with the factory test report if available, or with an identical spare. An open reading means a broken lead or burned-out winding; a resistance well below expected suggests shorted turns.
• Turns ratio: apply a low AC voltage to the primary from a variac or a small isolated source and measure the secondary; the ratio should match the nameplate within a few percent, remembering the compensated secondary reads slightly high at no load.
• Visual and smell: discoloured varnish, bubbled epoxy, a sharp odour or heat-marked terminals confirm thermal damage.

If all readings are normal, the fault is elsewhere: a blown fuse, a shorted coil in the control circuit, a mis-jumpered primary or a supply-side voltage problem. If it has failed, identify the cause before fitting the replacement; a control transformer rarely fails without an external reason. The Transformer Maintenance article gives the inspection routine, and the Power Transformer Testing article describes the factory tests these field checks approximate.

Neither style of Rex industrial control transformer has moving parts, and a correctly applied unit typically outlasts the panel it is installed in. The difference between styles is in what can go wrong. An open core-and-coil unit is exposed to the panel’s internal atmosphere: dust settles on the coil and reduces convection, conductive dust bridges terminals, and humidity is absorbed into the varnish over years. An encapsulated unit is sealed against all three, which leaves only the terminals and mounting to inspect.

A reasonable routine, aligned with the panel’s own maintenance cycle, is annual for clean indoor panels and semi-annual for panels in dusty, humid or vibrating environments such as mining and marine duty installations. At each inspection, with the panel de-energized: check terminal torque and look for heat discolouration at the lugs; vacuum, rather than blow, dust from an open coil; inspect the fuse holder contacts and touch-safe covers; confirm the shield and X2 bonding leads are intact; and on open units in humid locations take an insulation resistance reading and trend it. A thermal-imaging scan of the energized panel shows a transformer running hotter than its neighbours long before it fails.

The Maintenance, Operating and Installation Manual lists the manufacturer’s requirements, and the Transformer Maintenance article provides a checklist format that panel maintenance teams can adopt.

Rex offers factory-fitted primary and secondary fuse kits for its industrial control transformers, documented in the Primary Fusing Kits and Secondary Fusing Kits documents. Each kit is a fuse holder mounted directly on the transformer frame and pre-wired to the winding terminals, so the transformer arrives as a single tested assembly with its protection in place. The primary kit is two-pole, because both primary conductors on a phase-to-phase connection must be protected; the secondary kit is single-pole, protecting the ungrounded X1 conductor. The documents list the fuse class and ampere rating for each VA and voltage combination, removing the most common cause of nuisance blowing.

A miniature circuit breaker can be used on the secondary in place of a fuse where operators need to reset without opening a fuse holder. Select a breaker with an instantaneous trip characteristic that tolerates the combined contactor inrush and a rating within the code allowance for the transformer and conductors. On the primary, a fuse remains the usual choice because of the higher available fault current and the need for a time-delay characteristic that clears magnetizing inrush.

Whichever device is used, its rating must not exceed the values permitted by the Canadian Electrical Code or NEC for control transformer protection.

The CE compliance design option (code CE in the Part Number Builder) adapts the Rex industrial control transformer for machinery destined for the European Economic Area or other regions using IEC-based rules. Compared with the North American CSA and UL build, the CE design addresses the creepage and clearance distances, insulation coordination, terminal marking and protection-against-contact requirements of the applicable IEC transformer standards, so the machine builder can include it in the machinery’s technical file.

Because most CE-market machinery also runs on 50 Hz supplies at 230 V or 400 V, the CE option is usually ordered together with the 50 Hz nominal frequency option (code 50) and with a primary voltage matched to the destination, such as 400 V or 415 V, and a 230 V or 24 V secondary. The 50 Hz design uses a larger core so that flux density at 50 Hz is the same as a standard unit at 60 Hz; without it, a 60 Hz transformer on a 50 Hz supply overheats and hums. A 50 Hz unit operates at 60 Hz without difficulty, which suits machines tested in Canada before export.

Export panels destined for mining or energy storage projects abroad are the typical users. State the destination country and supply voltage on the order; the Custom vs. Standard Transformers article describes how such variants are engineered.

Yes. Rex Power Magnetics specializes in custom industrial control transformer solutions tailored to unique applications. Options include non-standard voltage ratios, frequency adaptations, fusing, terminal configurations, special nameplates, and mechanical mounting variations. Engineering support is available to assist OEMs and system designers with technical requirements such as space constraints, harmonic considerations, or environmental protection. Each custom unit undergoes the same rigorous testing and quality assurance as standard models, ensuring performance consistency and compliance with applicable standards. This flexibility allows Rex to meet specialized industrial and international market needs effectively.

All Rex Power Magnetics industrial control transformers are CSA certified and UL recognized. They comply with CSA C22.2 No. 66 and UL 5085-2 standards, ensuring adherence to industry requirements for safety, insulation, and performance. Each unit is manufactured under an ISO 9001-certified quality management system that guarantees traceability and process control. The designs meet or exceed temperature rise specifications, while dielectric testing ensures long-term reliability in demanding environments. Rex’s adherence to international standards demonstrates its commitment to consistent quality, durability, and dependable performance across all control transformer applications.