A rectifier transformer is the AC side of a DC installation. It sits between the supply and the rectifier, and it is built for two things a distribution transformer never sees: high secondary current and a load current that is not sinusoidal.
Why the rectifier changes the transformer design
The transformer itself does not convert anything. It steps the supply voltage down to a level the rectifier can work with, and the rectifier turns that into DC. The complication is the current waveform.
A rectifier draws current in short pulses rather than smoothly, so the transformer carries harmonic currents in addition to the fundamental. Those harmonics cause extra eddy current losses in the windings and in structural steel, and a transformer sized as though the load were sinusoidal will run hotter than the calculation suggests. Conductor sizing and cooling both have to be set against the actual harmonic spectrum.
Where the secondary current lands
Rectifier applications include aluminium smelting, chlor-alkali production and electrolysis, where the DC is used directly in a chemical or metallurgical process. These processes run at low voltage and very high current, which is why the secondary of a rectifier transformer looks more like a set of heavy busbars than a conventional winding.
On the largest installations, secondary currents reach into the tens of thousands of amperes. At those levels the conductor and its leads, not the core, dominate the design.
Pulse configurations
Multi-pulse arrangements are how harmonic distortion is reduced, and the transformer is where they are implemented.
- Six-pulse. One secondary winding on a three-phase transformer, feeding a six-pulse rectifier bridge. Simplest arrangement, highest harmonic content.
- Twelve-pulse. Two secondary windings, one connected in star and one in delta, feeding two six-pulse bridges. The fifth and seventh harmonics cancel on the supply side, which is a substantial reduction.
- Twenty-four pulse and beyond. Four or more secondary windings with phase displacement between them. Used where the supply has to meet tight harmonic limits.
The choice depends on the DC output required, the harmonic limits the supply connection imposes, and how much the phase-shifting windings cost against the alternative of adding filters or compensation equipment.
Impedance and short circuit
Rectifier transformers usually run at higher impedance than distribution units, partly to limit the fault current and partly to smooth commutation between the rectifier’s switching devices. Higher impedance also reduces the harmonic currents the supply sees, at the cost of poorer voltage regulation and a lower power factor.
Short circuit duty matters as well. A commutation fault on the DC side or a failure in a rectifier leg reflects into the transformer, so the winding has to be braced against forces it would not normally meet.
Cooling
Losses are high and the duty is usually continuous at or near full load, so cooling is a defining design decision rather than an accessory. Forced oil circulation with forced air over the coolers covers most medium and large units. Forced oil with water cooling is used where the heat load exceeds what air can remove, and direct water cooling of busbars is applied where the current is high enough that air cooling of the leads is impractical.
Where rectifier transformers are used
- Aluminium smelting potlines.
- Chlor-alkali and other electrochemical production.
- Electrolysis and electroplating.
- DC motor drives and large variable speed drive systems.
- Battery charging and welding power supplies at industrial scale.
Send the DC output required, the rectifier topology and the supply characteristics, and we can come back on the pulse configuration and the transformer behind it. See the rectifier transformer range, or contact us.
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Related Applications
- Transformers for Electrochemical Industry
- Transformers for Aluminum Industry
- Transformers for Industrial Equipment
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