What is a three-phase to three-phase full-wave cycloconverter?
It is the highest-quality direct AC-to-AC frequency changer: a three-phase supply is turned into a balanced three-phase output at a lower frequency with no DC link, using three single-phase-output cycloconverters — one per phase — each built from a positive and a negative three-phase full-bridge (6-pulse) group. That is 36 thyristors in total (3 phases × 2 groups × 6 devices).
Balanced three-phase output by cosine modulation
Each output phase is fired with cos α(t) = r·sin(ωot + φ), with φ = 0°, −120°, −240°. Because each 6-pulse bridge has mean output Vd0·cos α, the three local averages form a balanced low-frequency three-phase set built from six input segments per input cycle — far smoother than the 3-pulse version.
Fundamental output voltage (per phase)
where Vm is the peak phase voltage. The three fundamentals are equal and 120° apart. This simulator synthesizes all three phases with the actual cosine-crossing firing and validates the phase-A fundamental against r·Vd0.
Output frequency limit & waveform quality — topic-specific
With p = 6, each output phase is assembled from 6·fi/fo segments per cycle, so a full-wave three-phase cycloconverter keeps low distortion up to about fo ≤ fi/2 — the widest usable range of the family.
Applications
This is the topology used for the very largest low-speed synchronous-motor drives (10s of MW) — ore mills, gearless mine hoists and ship propulsion. Compare with the three-phase to three-phase half-wave cycloconverter (3-pulse), the three-phase to single-phase half-wave cycloconverter, the three-phase to single-phase full-bridge cycloconverter and the single-phase to single-phase cycloconverter.
Frequently asked questions
What is a three-phase to three-phase full-wave cycloconverter?
A direct AC-to-AC frequency changer that produces a balanced three-phase output at a lower frequency using three single-phase cycloconverters (one per phase), each with positive and negative 6-pulse bridge groups — 36 thyristors, no DC link.
How many thyristors does it use?
Thirty-six: three output phases, each with a positive and a negative three-phase full-bridge group of six thyristors (3 × 2 × 6 = 36).
What is the per-phase fundamental output voltage?
Vo1(peak) = r·Vd0 with Vd0 = (3√3/π)·Vm ≈ 1.654·Vm; the three phases are equal and 120° apart.
Why is the 6-pulse three-phase cycloconverter preferred for big drives?
Six segments per input cycle give the lowest distortion of the family and allow output frequency up to about fi/2, which suits very large low-speed synchronous-motor drives.