The rectifier transformer does not change what a transformer is. It applies the same electromagnetic principle under conditions that make the surrounding design unusual: a non-sinusoidal load current and a secondary that may carry thousands of amperes.
The basic principle, unchanged
Alternating current in the primary winding produces a changing magnetic field in the core, and that changing field induces a voltage in the secondary. The ratio of primary to secondary turns sets the voltage ratio, and there is no electrical connection between the two circuits.
Everything after this point is rectifier-specific.
Why the current waveform matters
A rectifier does not draw current smoothly. It draws in pulses as the switching devices conduct, so the current reaching the transformer has a ragged waveform containing harmonic components well above the supply frequency.
Two consequences follow. The harmonic currents produce extra eddy current losses in the windings and in nearby steelwork, so the transformer runs hotter than a sinusoidal load of the same RMS current would suggest. And the harmonic currents travel back onto the supply, where they can disturb other equipment or breach connection limits.
How multi-pulse operation reduces harmonics
The trick is to run two or more rectifier bridges from secondary windings that are displaced in phase relative to each other. The harmonics each bridge produces are then displaced as well, and certain orders cancel when they meet on the supply side.
- Six-pulse. A single three-phase secondary feeding one bridge. Harmonics at the fifth, seventh and higher orders appear on the supply side.
- Twelve-pulse. Two secondaries, one star and one delta, feeding two bridges. The fifth and seventh harmonics cancel, which is the most common step up from six-pulse.
- Twenty-four pulse. Four secondaries with 15-degree phase displacement between adjacent groups. Used where harmonic limits are tight.
The phase displacement is built into the transformer by how the secondary windings are connected. This is why the transformer specification and the harmonic mitigation strategy are the same decision rather than two separate ones.
Current distribution in a high current secondary
Where the secondary current is large, the conductor is split into parallel paths rather than made into one heavy bar. Parallel conductors have to be arranged so that each carries its share of the current, which is not automatic: at high frequency the current crowds toward the outside of a conductor, and adjacent conductors influence each other. Getting this wrong produces localised overheating in conductors that are well within their nominal rating.
What the design has to account for
- Harmonic loss in the windings and in structural steel parts.
- Impedance selected for fault current limitation and commutation, not just regulation.
- Mechanical bracing against short circuit forces.
- Cooling sized against a continuous load with harmonic losses included.
- Insulation co-ordination between the transformer and the rectifier.
If you are specifying a rectifier transformer, the rectifier topology and the expected harmonic spectrum are as important as the kVA figure. Send those and we can come back on the configuration. See the rectifier transformer range, or contact us.
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