How Are Custom Transformers Designed?

A custom transformer is designed backwards from the duty it has to survive. Rating and voltage ratio set the starting point, but the things that actually shape the design are the load profile, the ambient it sits in, the fault level it has to ride through, and the space it has to fit into.

Requirements Gathering and Analysis

This stage decides the quality of everything that follows. The inputs fall into four groups.

Electrical. Rated power, primary and secondary voltage with tolerances, frequency, phase, vector group, impedance, and the loss figures the project is working to.
Duty. Continuous, intermittent, or cyclic; the overload magnitude and duration; the daily operating hours; and whether the load is linear or produces harmonics.
Mechanical. Overall dimensions and the maximum weight the floor or structure can take, the mounting arrangement, the enclosure or IP rating, and the terminal and cable entry arrangement.
Environment. Ambient temperature range, altitude, humidity, salt or corrosive exposure, dust, vibration, and any noise limit.

Most of the corrections that appear late in a project trace back to one of these being assumed. Two questions are worth asking early: what happens on the worst day, and what has to be true for this unit to be installed at all.

Electromagnetic Design

The electromagnetic design fixes the core and the windings. Core dimensions follow from the voltage per turn and the flux density the steel can be run at, and the core grade chosen trades no-load loss against cost and size.

Winding design sets conductor cross-section against current density, then divides the winding into parallel paths where the current is high enough that skin and proximity effect would otherwise push the current to the outside of the conductor. Get the transposition wrong and one path carries more than its share, which shows up as a hot winding rather than as a wrong calculation.

Impedance is set at this stage too. It is not a free parameter: it controls fault current, voltage regulation, and the harmonic current the unit passes back to the supply, and changing it moves all three at once.

Thermal Design

Thermal design asks a simpler question than it looks: at the worst duty in the worst ambient, what is the hot spot temperature, and how long will the insulation survive it.

The calculation has to include every source of loss, which means the I²R loss at fundamental frequency plus the eddy losses produced by any harmonic content, plus the stray losses in structural steel around the leads. Where the load is cyclic, the thermal time constant of the winding can absorb a short overload, but only if the overload’s magnitude and duration are known.

Cooling method follows from the heat load and from what the site can support: natural air where the rating allows it, forced air where a fan-cooled rating is acceptable, forced oil with air coolers for large oil-immersed units, and water cooling where air cannot remove the heat.

Mechanical Design

The mechanical design is driven by short circuit forces above everything else. Force between conductors scales with the square of the current, so the fault case, not the running case, sets how heavily the windings are braced and how the leads are supported.

Windings braced and clamped against the calculated fault forces.
Low voltage leads kept short and arranged symmetrically to limit the forces between them.
Structural steel near the leads assessed for stray-loss heating, because a part not designed to carry current can still get hot.
Transport and lifting loads checked separately, since a rigid cast unit and an open winding react differently to being moved.

Insulation Design and Test Levels

The insulation system is chosen against the system voltage and the impulse levels the supply can deliver. Solid insulation in dry type units is assigned a temperature class, and that class sets the limit the thermal design works against rather than a value the unit will actually run at.

Clearances and creepage distances are dimensioned for the impulse level, and on cast resin units the surface profile is part of the design because it determines how the field distributes across the casting and therefore how low partial discharge stays.

Prototyping and Validation

For a design that departs from anything built before, a prototype is often the cheaper route. It surfaces the problems that calculation does not, particularly thermal behaviour under real loading and mechanical response during a fault test.

Testing divides into routine tests performed on every unit, type tests performed on a design to demonstrate it meets the standard, and special tests agreed for a particular project. Which of these are required is normally settled at the quotation stage, because the test programme affects both price and schedule.

Where the Design Decisions Interact

Impedance affects harmonic current, harmonic current affects loss, loss affects cooling, cooling affects physical size, and size affects the fault forces the structure has to resist. Changing one moves the others, which is why a custom transformer is normally evaluated as a whole rather than item by item, and why a specification that fixes too many parameters at once can make a design impossible rather than demanding.

What to Send Us

Electrical ratings and the vector group, or the constraint that sets them.
Duty profile, including peaks and their duration.
Ambient, altitude, and any corrosive or dusty atmosphere.
Space, weight, and access constraints at the installation point.
Applicable standard and the test programme required.
Required delivery date and destination.

See the custom transformer range, or contact us with the requirement.

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