FAQs

Dependent on the transformer model, but all flow requirements are listed on the drawings.

We reference RWMA Bulletin 14 in our Universal Manual.

We currently have products in operation for over 20 years in the field. 

Yes, we offer an ERT, which consists of AC water-cooled transformers in combination with SCR Power Controllers, Breaker and CTs inside a cabinet. It allows you to remove the old VRT and place the smaller ERT cabinet in its place.

MFDC stands for Mid Frequency Direct Current, a type of inverter heating technology that uses a mid-frequency AC input (1000 Hz) and converts it to a direct current (DC) output for heating.  The MFDC power supply includes a water-cooled transformer and rectifier potted in a case.

An IGBT (Insulated Gate Bipolar Transistor) functions as a high-speed electronic switch, converting DC power into AC power for controlling power flow in a system.  It is the major component of an inverter which also includes an SCR bridge that converts 3-phase AC to power a DC bus. This technology has been used for automotive resistance welding since the early 1990’s. RoMan Manufacturing currently produces 3000-5000 MFDC power supplies a year to function with IGBT controls.

Frequency and flux density are inversely related in applications like transformers, where increasing the operating frequency allows less core to be used while maintaining the required flux density.  Additionally, MFDC systems are single-phase requiring one core where 3-phase transformers have 3 cores. 

Direct Current provides more stable and steady power flow, reducing thermal cycling, stress, and oxidation on the heating element.  This results in more efficient heat transfer and a longer operating life for the element.

Given the significant reduction in weight of the MFDC power supply, less structural steel is required to support the equipment.  This also allows the MFDC power supplies to be located much closer to the terminal box, reducing the length of the secondary conductors.  Additionally, since the MFDC system is single-phase, one bus from the 3-phase system is no longer required.

Electrical energy is reduced in several ways:

1. The line reactor filters harmonic content from the 3-phase AC line supplying the inverter.  This reduces the gauge of the cable feeding the system.
2. The SCR bridge in the inverter converts the incoming AC power to DC, increasing the power factor.
3. The water-cooled MFDC power supply has less inductive losses than the AC counterpart.
4. The close-coupled MFDC power supply reduces losses in the secondary conductors.
5. The IGBT Control Converts 3 phase AC to 1 phase AC and in the process increases primary voltage to the MFDC power supply. By increasing primary voltage, the transformer can utilize more primary turns. The more primary turns, the less primary current it utilized to reach a given secondary current.   Additionally, the 3 phase input is balanced benefitting the plant’s loading.

MFDC technology has been used in Resistance Welding for nearly 50 years.  It is also used in commercial heat treating, sintering, melting, and electrogalvanizing. 

The highest current at rated voltage that a device is identified to interrupt under standard test conditions. [NFPA 70]

Interrupt rating applies to devices that are intended to interrupt currents, i.e. fuses, circuit breakers.

The prospective symmetrical fault current at a nominal voltage to which equipment is able to be connected without sustaining damage exceeding defined acceptance criteria. [NFPA 70]

SCCR is applied to equipment specific equipment, e.g. industrial control panels, industrial machinery, surge protective devices (SPDs), etc., where the equipment is intended to withstand the applied fault current without causing damage that could create a safety hazard

The National Electric Code (NFPA 70) requires that all electrical equipment that has a SCCR be installed in a location where the SCCR of the equipment is equal to or greater than the calculated fault or short circuit current of the point of application.

An assembly of two or more components consisting of one of the following:

1. Power circuit components only, such as motor controllers, overload relays, fused disconnect switches, and circuit breakers;
2. Control circuit components only, such as push buttons, pilot lights, selector switches, timers, switches, and control relays.
3. A combination of power and control circuit components. These components, with associated wiring and terminals, are mounted on, or contained within, an enclosure or mounted on a subpanel. [NFPA 70]

Industrial control panels are typically used to provide power and controls to industrial machinery and processes. For glass furnaces, the industrial control panel provides overcurrent protection, power control, monitoring, and in some cases, the step-down or step-up transformers used to melt or refine the glass composition at various sections of the process.

The electrical power system has two main configurations: Wye (Y) and Delta (Δ). Within the Wye system, there is a Solidly Grounded and a High-Resistance Grounded (HRG) connection to earth. In a Solidly Grounded Wye configuration in the US, the Neutral and Ground conductors are tied together and tied to the common point of the three-phase conductors.

The advantage of a Solidly Grounded Wye system is that there is less voltage variation between the phases and ground as they are “anchored” together and to the earth during normal and abnormal operation. The disadvantage of the Solidly Grounded Wye system is that in a fault to ground – approximately 90% of all equipment failures are a fault to ground – The increase in current flow to ground will cause the overcurrent protection device to activate or trip

The electrical power system has two main configurations: Wye (Y) and Delta (Δ). Within the Wye system, there is a Solidly Grounded and a High-Resistance Grounded (HRG) connection to earth. In a HRG configuration a resistive element is placed between the Ground and the Neutral or Common point of the three-phase windings.

The advantage of a HRG system is that there is less voltage variation between the phases and ground as they are “anchored” together and to the earth during normal operation. During abnormal operation, the maximum voltage deviation any phase can see is the Square-root of 3 times the Line-to-Neutral voltage. The HRG system is ideal for continuous operating processes where a ground-fault could cause a disruption in the process. Whenever HRG systems are used, ground-fault monitoring is required to inform the user when a ground-fault occurs.

An extended delta transformer is a special transformer that has modified secondary windings intended to create a phase shift between the secondaries for the mitigation of harmonic currents developed by semiconductor devices. Common phase shifts are ± 7.5 degrees.

To create a 24-pulse rectifier with an extended delta transformer will require 4 secondary windings.

Extended delta transformers are one method of mitigating harmonic currents – as seen by the source – from the power system. Passive harmonic filters that are tuned to absorb specific harmonic frequencies can also be installed upstream of the industrial control panel where the SCRs are installed.

An active harmonic filter that senses the harmonic current on the power system and injects and opposing current back into the power system is a new technology that has been deployed.

Each type of harmonic filtering has advantages and disadvantages. It is important for the user to understand each before choosing a solution to mitigate harmonic currents.

Harmonic currents can be generated when the SCR is operated in phase-angle control mode and is phased back. The more the SCR is phased back, the more harmonic currents are generated. At full-load the SCR is not phased back, so no harmonic currents are generated.

When the SCR is operating in zero-cross mode, there are no harmonic currents generated by the SCR. Some SCRs operating in zero-cross start-up using phase angel mode. During start-up, the SCR will generate harmonic currents until the SCR reaches full zero-cross mode.

SCRs can be controlled using Power, Voltage, or Current. When the SCR is in Power Control, the set-point is identified by the control system. The SCR will multiply the voltage and current to determine the power. As voltage fluctuates up or down, the SCR will allow less or more current respectively to ensure that the specific amount of current is provided to the load.

When the SCR is in Voltage Control, the voltage set point is defined by the control system. As either the source or load changes, the SCR will limit the voltage applied to the load based on the set-point.

When the SCR is in Current Control, the current set-point is defined by the control system. As the temperature of the process changes, the SCR will provide more or less current based on the demand. If temperature decreases, this typically results in more current being applied. Conversely, if temperature increases, less current will be applied.

In addition to the control methods of SCRs, modern digital SCRs have additional capabilities that can be used to augment the control of the process. These features include power, voltage, and current limits. Regardless of the control methods, maximum power, voltage, and current limits can be added to the programming of the SCR to provide added user flexibility.

All electrical devices and equipment require overcurrent protection. The overcurrent protection is determined by the manufacturer, but is generally defined by either the National Electric Code (NFPA 70), or the applicable safety standard, in the US typically standards from Underwriters Laboratories (UL) are the guide. 

For electrical devices, e.g. SCRs, overcurrent protection is required. As part of the Listing process, SCR manufacturers are required to undertake short circuit current testing with the applicable overcurrent protection, i.e. fuse or circuit breaker, installed upstream of the SCR. 

The Listing organization will then require that the installation instructions and the “Conditions of Acceptability” define the specific overcurrent protection that is required. Where the overcurrent protection is installed is dependent on the topology of the SCR. Some SCR manufacturers have built the overcurrent protection inside of their device, whereas others require users to install it as part of the overall industrial control panel.

See FAQ “Do Silicon Controlled Rectifiers (SCRs) require a fuse and/or circuit breaker for overcurrent protection?” If the SCR manufacturer has conducted the short circuit current testing with the circuit breaker that has been chosen to be installed upstream of the SCR, NO, an additional fuse IS NOT REQUIRED. However, if the SCR manufacturer has conducted the testing with a different overcurrent protective device than what has been chosen, YES, and additional fuse IS REQUIRED.

There are compliance answers and practical answers to this question. From a compliance standpoint, SCRs that contain the CE Mark are required to be tested to various standards associated with EMI and transients. If testing shows that harmful (as defined by the applicable IEC standard) EMI can be conducted back onto the powerline or can negatively impact the operation of the SCR, then EMI filters are required. Similar to EMI testing, if the SCR showed an impact of transient testing, surge protective devices (SPDs) are required to be installed. The requirements to add EMI filters and/or SPDs are required to be documented in the installation instructions.

It is best practice to ensure that the SCR does not impact other equipment and that the environment does not impact the SCR. EMI filters are not required directly upstream of the SCR if other filtering devices are deployed. SPDs are also not directly required to be installed upstream of the SCR if protection from transient conditions are effectively installed in the power system feeding the SCRs. If the installation of EMI filters or SPDs are unknown, or the protection that is to be provided is questionable, it is best practice to install EMI filters and SPDs in the industrial control panel where the SCRs are located.

Every electrical device rated 480 vac is required by the Listing agency to undergo a dielectric withstand test of 1,960 vac for 1 minute. Every RoMan transformer is subjected to a dielectric withstand voltage test of 4,000 vac as part of the routine test procedure.

Impedances between the source and the load can create standing waves at the primary of the transformer terminals exceeding the dielectric strength of the transformers insulation system.

When SCRs are closely coupled (less than 50 cabling feet) with the primary windings of a transformer, no transient or electromagnetic interference (EMI) filters are required. In this configuration, there is insufficient inductance to set-up a standing voltage wave that can impact the dielectric insulation of the transformer.

If the SCR is not closely coupled (greater than 100 cabling feet) from the primary winding of the transformer, it is recommended that a special EMI filter be applied to the primary windings of all transformers, regardless of manufacturer. An impedance difference between the SCR and the transformer winding can create a standing voltage wave that can exceed the dielectric capabilities of transformer insulation systems.

When the cabling distance between the SCR and the primary of the transformer is greater than 50 feet and less than 100 feet, the requirement to use a specialized EMI filter is determined by the engineer and is based on the capabilities of the transformer’s insulation system. If the dielectric testing of the transformer meets the minimum requirements of the Listing Agency, e.g. 1,960 vac for 480 vac system, then it is recommended to install a specialized EMI filter to the primary terminals of the transformer. If the dielectric testing of the transformer is 4,000 vac or greater, a specialized EMI filter is generally not required.

There are compliance and practical answers to this question. From a compliance standpoint, unless specifically defined in the SCR manufacturer’s Conditions of Acceptability, which is issued by the Listing Agency, a contactor located upstream of the SCR is not required.

When an SCR is in the OFF-State, with the control system providing OFF-State control signals to the SCR, no voltage is present at the output terminals. Voltage will be present at the input terminals of the SCR and at all points upstream of the SCR. When the control system provides any control signal that turns the SCR ON, voltage will be present at the output terminals of the SCR. The amount of voltage present will be based on the control signal provided.

It is best practice to provide a contactor upstream of a SCR, especially when there are circuits that need to be de-energized because of operator interaction. Additionally, some SCR manufacturers recommend that a contactor be installed upstream of the SCR to ensure that the SCR does not provide power to downstream circuits. In these cases, the necessity of a contactor installed upstream of the SCR is solely dependent on the user and their associated design standards.

The National Electric Code (NFPA 70), Article 110.2 states “The conductors and equipment required or permitted by this code shall be acceptable only if approved.” Enhanced content denotes that the Authority Having Jurisdiction (AHJ) has the ultimate authority to determine this requirement. In residential, commercial, medical, and other similar facilities, the AHJ is a government inspector, e.g. electrical inspector. In manufacturing facilities or areas without inspection services, the facility manager or other qualified person is responsible for carrying out the function of the AHJ.

Regardless of the Listing requirement, Industrial Control Panels are required to meet the requirements described in the National Electric Code (NFPA 70), Chapter 409. It is recommended that all Industrial Control Panels be evaluated to UL508A or NFPA 79 either by the equipment manufacturer or a third party resource. Documentation should be provided by the equipment manufacturer stating compliance with UL508A or NFPA 79.

Both the manufacturer and the end-user are responsible for the safe operation of the equipment. The manufacturer is required to ensure that the equipment is designed, manufactured, and tested to meet the applicable regulatory requirements. The manufacturer is required to ensure that the equipment does not create a safety hazard during normal operations. The manufacturer is also required to ensure that the equipment will not create a safety hazard when operating at any foreseeable abnormal operating conditions. The equipment manufacturer is required to provide sufficient operation instructions to the end-user on the intended use of the equipment.

The end-user is responsible for ensuring that the equipment is installed in accordance with the National Electric Code (NFPA 70) and any specific requirements defined by the equipment manufacturer. The end-user is required to conduct a hazard identification risk assessment (HIRA) to ensure that all hazards with using the equipment in the process have been identified and mitigated in accordance with regulatory requirements.

A power distribution unit (PDU) is an electrical device that takes one or more input sources, conditions the power, and distributes to one or more outputs. PDUs are low-voltage equipment (rated 1,000 vac or less). PDUs are highly configurable and are application specific. PDUs typically transform the voltage and currents from the input and to different voltage and current amplitudes at the output. PDUs are used extensively in data center applications, but can also be used in industrial applications (semiconductor manufacturing, industrial furnaces, glass manufacturing, steel and aluminum manufacturing, etc.). PDUs can only be used in facility distribution locations.

Components used in PDUs various widely, but common components include overcurrent protective devices (fused disconnects or circuit breakers), transformers, bus bars and cabling, power monitoring, and surge protective devices.

The maximum size or rating of the Power Distribution Unit (PDU) is based on the overcurrent protective devices available. The largest commonly available overcurrent protective device is rated at 5,000 A. Using Method 2 for overcurrent protection in accordance with the National Electric Code (NFPA 70), Article 450.5(B), a 5,000 A circuit breaker can be installed upstream of a 3.5 MVA, 480V 3-Phase transformer.

Heating from non-linear loads can significantly impact the thermal capabilities of a transformer’s windings. K-rated transformers identify the type of linear or non-linear load that can be applied. Transformers with a K-1 rating are intended for linear loads only. Transformers with a K-4 rating are intended for applications where no more than 50% of the load is non-linear. Transformers with a K-9 rating are intended for applications where 100% of the load is non-linear, but the application is considered non-critical. Transformers with a K-13/14 rating are intended for applications where 100% of the load is non-linear and the application is considered to be critical.

K-rated transformers are typically associated with air-cooled, cast-coil, and oil-cooled transformers as their ability to dissipate heat associated with harmonic currents is limited. RoMan water-cooled transformers do not have standard K-ratings as they are connected to an external water source that provides continuous cooling. RoMan recommends that transformer temperatures, and inlet and outlet water temperatures be monitored regardless of the applied load.

Power distribution units (PDUs) that use air-cooled or cast-coil type transformers should have K-rated transformers associated with the intended load and criticality. RoMan water-cooled transformers do not have standard K-ratings are they are connected to an external water source that provides continuous cooling. RoMan recommends that transformer temperatures, and inlet and outlet water temperatures be monitored regardless of the applied load.

US Department of Energy (DOE) implemented transformer efficiency requirements in 1977. DOE requirements looked at reducing transformer losses at 35% of the transformer’s power rating. K-rated transformers included in older DOE standards (TP-1) exempted K-rated transformers. The DOE 2016 requirement is applicable to all low-voltage dry transformers, including K-rated transformers. DOE 2029 requirements passed in 2024, which will require identified transformer to meet the new requirements by 2029.

Transformers that are not required to comply with DOE requirements include: Autotransformers, Drive (Isolation) Transformers, Grounding Transformers, Non-ventilated Transformers, Rectifier Transformers, Sealed Transformers, UPS Transformers, Welding Transformers, and more.

Transformers in PDU or industrial power control applications generally supply power to rectifier based power supplies and are exempt from DOE requirements.

A switchboard is an electrical device that takes one or more input sources and distributes to multiple outputs. Switchboards are used in low-voltage applications (1,000 vac or less). Switchboards are highly configurable and are application specific. Switchboards do not condition or transform the electrical voltage or current, they only distribute and provide overcurrent protection to the distributed branches. Switchboards are used in data centers, industrial, medical, commercial applications. Switchboards can be used as service entrance or facility distribution locations.

Common components included in a switchboard are overcurrent protective devices (fused disconnects or circuit breakers), bus bars and cabling, power monitoring, and surge protective devices.

PDUs intended to have secondary voltages of either 480Y/277 vac or 208Y/120 vac and intended to be connected to information technology equipment (ITE), it is recommended that the Neutral conductor be sized for 200% of the rated current.

In 3-phase non-linear loads, such as those associated with 6-pulse rectifiers, harmonic currents are out of phase and non-additive. In 1-phase non-linear loads, such as those associated with common ITE equipment distributed across the electrical system, triplen or zero-sequence harmonics (3rd, 9th, 15th, etc.) are additive. Using a 200% Neutral conductor ensure that heating from the triplen or zero-sequence harmonics does not overheat the Neutral conductor.

Water-cooled transformers are assembled with the insulated primary and secondary coils physically adjacent around the core.  This close-couples construction provides the least inductive loss of any transformer design.  The internal cooling water allows the transformer to be installed in locations with elevated temperatures and does not require any cooling fans.  Locating the transformer close to the load also reduces losses in the secondary conductors.  Additionally, the transformer is assembled in potting material eliminating the potential for contamination of debris.

RoMan Manufacturing recommends the following water quality:

• pH between 7.0 and 9.0
• Maximum Chloride content of 20 PPM
• Maximum Nitrate content of 10 PPM
• Maximum Sulphate content of 100 PPM
• Maximum Solids content of 250 PPM
• Maximum Calcium Carbonate content of 250 PPM
• Resistivity greater than 2,000 ohm-cm at 77 “F (25°C)
• Deionized water should not be used in a closed water system unless the water is constantly recirculated through the deionizer. Deionized water is ion-hungry and still contains many water soluble gases which will quickly reduce the resistivity of the water if not continually run through the deionizer.

A Scott-T transformer (or Scott connection) is a specialized transformer configuration used to convert a three-phase primary connection to two single-phase secondary connections. It uses two single-phase transformers—the main and the teaser—to balance loads across the three-phase supply.  The primary benefit is the ability to supply two single-phase loads, such as electrodes, with a balanced three-phase input.  It can be used to control two pairs of electrodes together with a three-phase SCR on the transformer primary.  Individual control of electrode pairs can be achieved using a single-phase SCR control on each of the secondary outputs.  Consideration should be given to the balance of individual secondary currents in maintaining a balanced three-phase primary current.