A rectifier transformer differs from a conventional power transformer in one fundamental way: the load current is not sinusoidal. Several design decisions follow from that single fact, and skipping any of them shows up as overheating in service rather than in the calculation.
Electrical design
The electrical side starts with the same items as any transformer and then adds the rectifier’s particular demands.
- Voltage ratio, set by the DC output required and the rectifier configuration.
- Impedance, chosen to limit fault current and smooth commutation. Higher impedance reduces harmonic current on the supply side but worsens regulation and power factor, so the value is a compromise rather than a maximum.
- Harmonic loading, calculated against the actual current spectrum rather than assumed. Harmonic components cause extra eddy losses in windings and structural steel.
- Insulation co-ordination between the transformer and the rectifier, so that the two see the same impulse levels consistently.
- Voltage regulation across the load range, which matters most where the process is sensitive to DC voltage.
Thermal design
Thermal design carries more weight than on a conventional unit because the harmonic losses are real but easy to underestimate. The calculation has to include the eddy losses the harmonics produce, not just the IĀ²R loss at fundamental frequency.
Cooling method follows from the heat load and the site: forced air for smaller dry type units, forced oil with forced air for most medium and large oil-immersed units, and forced oil with water cooling where the heat load exceeds what air can remove. Where the secondary current is very large, the busbars are water-cooled directly.
Temperature rise limits are often specified below general-purpose values in rectifier service, because the duty is continuous and insulation ageing is the mechanism that ends the unit’s life.
Mechanical design
The mechanical design is driven by short circuit forces rather than by transport or vibration, although all three have to be covered.
- Windings braced and clamped against the forces produced at short circuit.
- Low voltage leads kept short and arranged symmetrically to limit electromagnetic forces between them.
- Structural steel around the secondary leads assessed for stray loss heating, so that hot spots do not develop in parts not designed to carry current.
- Support structures verified against the calculated forces, which is where finite element analysis is normally used.
Where the design decisions interact
Impedance affects harmonic current, which affects loss, which affects cooling, which affects physical size, which affects the short circuit forces the structure has to resist. Changing one moves the others, which is why rectifier transformer design is normally evaluated as a whole rather than item by item.
The practical consequence for a specification is that the harmonic spectrum and the rectifier topology belong in the enquiry. Given those, the rest of the design follows; without them, the thermal calculation is a guess.
What to send us
- DC output required and the rectifier topology.
- Expected harmonic spectrum and any supply-side harmonic limits.
- Primary voltage and system short circuit level.
- Cooling medium available and ambient conditions.
- Duty profile and the temperature rise limits applicable.
See the rectifier transformer range, or contact us with the design data.
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