Parallel rectifier groups are used in high-current DC supplies, electrochemical systems, traction, industrial drives, and multipulse converters. Their instantaneous output voltages are not identical, even when average DC voltage matches. An interphase transformer supports the difference voltage and allows groups to share load rather than circulating uncontrolled current.
What the interphase transformer does
Windings are connected between rectifier outputs so differential or ripple voltage creates opposing magnetic action while DC load current divides between paths. The magnetic component presents impedance to circulating components but passes the intended combined DC output through its winding arrangement.
Its waveform is neither a conventional power transformer sine wave nor a simple DC reactor waveform. The applied voltage depends on phase displacement, rectifier conduction, commutation overlap, source impedance, firing angle or diode behavior, and load.
Start from the time-domain differential voltage
Derive the voltage between rectifier group outputs over the full line cycle and all operating modes. Integrate this voltage to obtain flux swing for the selected turns and core area. Average volt-second balance should be checked; small phase, ratio, firing, or source-impedance mismatch can create DC flux bias.
Multipulse systems may include 12-, 18-, or 24-pulse arrangements. Transformer phase shifts and rectifier sequence determine ripple frequency. The interphase transformer can also influence harmonic cancellation when current sharing is imperfect.
Current sharing and winding resistance
Equal turns do not guarantee equal current. Rectifier voltage drops, source impedance, transformer leakage, bus resistance, connection length, temperature, and winding resistance all affect sharing. Symmetrical buswork and Kelvin sensing where control is involved help reduce imbalance.
Each interphase winding carries substantial DC current plus ripple. DC resistance creates loss and can reinforce current imbalance as temperature rises. Parallel foil or busbar conductors require balanced terminations to share current.
Core selection and gap
The core must support differential flux and tolerate DC bias from mismatch. Laminated steel, cut cores, gapped structures, or other materials may be used depending on line frequency, ripple frequency, power, loss, size, and acoustic requirements. A deliberate gap can stabilize inductance and bias margin but increases magnetizing current and fringing.
Core loss should be calculated from the actual nonsinusoidal flux trajectory. Minor loops and unbalance can add loss beyond an ideal symmetrical waveform.
Fault and abnormal conditions
A failed rectifier device, missing phase, gate misfire, blown fuse, open connection, or bus fault can apply sustained unbalanced voltage or force current through one winding. Define protection clearing time and maximum fault current. Saturation may sharply reduce circulating-current impedance during a fault.
Thermal and mechanical design
High DC current creates copper heat and electromagnetic forces. Terminal joints, busbars, winding supports, core clamps, and cooling must withstand steady and fault duty. Proximity to rectifier heatsinks can raise ambient. Water-cooled conductors or forced air may be needed in high-current systems.
Validation checklist
- Measure group currents, differential voltage, and flux balance across load.
- Check current sharing at line and component tolerance extremes.
- Measure winding and core temperatures after stabilization.
- Verify harmonic spectrum and multipulse cancellation.
- Test startup, current transfer, missing phase, rectifier fault, and protection clearing.
- Inspect bus joints and conductor temperatures with production hardware.
BaoHui Tech needs the rectifier topology, source transformer vector groups, line frequency, differential voltage waveform, DC output range, current-sharing target, harmonic requirements, fault cases, cooling, insulation, terminal geometry, and size constraints.
Frequently asked questions
Is an interphase transformer the same as a DC choke?
No. A DC choke smooths current in one path; an interphase transformer magnetically coordinates voltage and current between parallel rectifier groups.
What causes DC flux bias?
Unequal phase shifts, turns ratios, firing, voltage drops, source impedances, bus resistance, or faults can produce nonzero average differential voltage.