What is a three-phase semiconverter (half-controlled bridge)?
A three-phase semiconverter is a half-controlled bridge rectifier: the positive (top) group uses three thyristors (SCRs) — T1, T3, T5 while the negative (bottom) group uses three diodes — D4, D6, D2. Only the top group is triggered a controllable firing angle α after its natural turn-on instant; the diodes commutate on their own. As α increases the average output falls, but the moment the fired line-to-line voltage would go negative the load current freewheels through one thyristor and one diode of the bridge, clamping the output to zero. The semiconverter is therefore a single-quadrant converter — cheaper and simpler than the fully-controlled bridge, with a better displacement factor at high α, and it is widely used for battery charging, electroplating and single-direction DC-motor drives.
Output voltage, PIV & ripple
Here Vm is the peak phase (line-to-neutral) voltage. At α = 0 the semiconverter behaves like the uncontrolled 6-pulse rectifier, Vdc = 3√3·Vm/π ≈ 1.654·Vm. It falls smoothly to zero at α = 180° and, thanks to freewheeling, never goes negative. This simulator triggers each SCR α after its natural commutation instant, lets the diodes commutate naturally, integrates the real load and compares the measured average to (3√3·Vm/2π)(1 + cos α) in the accuracy panel.
Freewheeling & one-quadrant operation
For firing angles up to 60° the output is a continuous 6-pulse envelope, just phase-shifted. Beyond 60° the fired line-to-line voltage tries to reverse; instead of pushing the output negative, the current freewheels inside the bridge (through one conducting SCR and one diode) and the output is held at zero for part of each pulse. During freewheeling the AC line current is zero — the load is disconnected from the supply — which is why the semiconverter has a better input power factor than the fully-controlled bridge at large α. Because the output can never go negative, the semiconverter is one-quadrant and cannot return power to the line (no inversion). The simulator reports the freewheeling percentage live and shades the freewheeling intervals as “FW”.
Advanced options in this simulator
- Device model: add a forward drop
V_fand on-resistanceR_on; current flows through one SCR and one diode in series, so the total drop is2·V_f + 2·R_on·i. The accuracy check stays locked to the ideal envelope. - Transformer: a turns ratio scales the phase voltage that reaches the bridge.
- Load configuration: resistive, inductive (R-L) or with a back-EMF (R-L-E) for battery-charging / motor loads.
- Filter & protection: a series-L, shunt-C or LC output filter, an optional RC snubber, and a live protection-margin check of the device PIV and average current against the ratings you enter.
- Phase sequence & imbalance: swap the sequence or unbalance the phases and watch the ripple and harmonics change.
- Harmonic spectrum analysis: a real FFT of the output voltage or the AC line current with the ripple / THD figure — triplen components appear once freewheeling starts.
- Export & capture: download the full waveform data as CSV, a text report, or a PNG screenshot of the scope.
Semiconverter vs fully-controlled bridge
| Feature | 3φ semiconverter (half-controlled) | 3φ fully-controlled bridge |
|---|---|---|
| Devices | 3 SCRs + 3 diodes | 6 SCRs |
| Average voltage | (3√3·Vm/2π)(1 + cos α) | (3√3·Vm/π)cos α |
| Quadrants | 1 (rectify only) | 2 (rectify + invert) |
| Output polarity | always ≥ 0 (freewheels) | ± (can go negative) |
| Ripple frequency | 6f up to 60°, then 3f | 6f |
| PIV | √3·Vm | √3·Vm |
| Input power factor at high α | Better | Poorer |
See the three-phase fully-controlled 6-pulse bridge for the six-SCR version, the uncontrolled 6-pulse bridge for the diode version, or the single-phase semiconverter for the 1φ half-controlled bridge.
Applications
Battery charging, electrochemical / electroplating plant, single-direction DC-motor and traction drives, heaters and any adjustable DC supply where reverse power flow is not required. The lower device cost (three SCRs instead of six) and the improved power factor at high firing angles make the semiconverter a popular medium-power choice.
Frequently asked questions
What is the average output voltage of a 3-phase semiconverter?
Vdc = (3√3·Vm/2π)(1 + cos α) ≈ 0.827·Vm·(1 + cos α) in continuous conduction, where Vm is the phase peak. It is maximum (3√3·Vm/π) at α = 0 and falls to zero at α = 180°.
Why can't a semiconverter invert?
The three diodes plus the freewheeling action clamp the output at zero whenever the fired line voltage would go negative, so the average output never becomes negative. It is a one-quadrant converter — use a fully-controlled bridge or a dual converter for inversion / regenerative braking.
When does freewheeling start?
For firing angles above 60°. Below 60° the output is a continuous 6-pulse (6f) envelope; above 60° a freewheeling interval appears each pulse, the output is held at zero, the line current is zero during that interval, and the ripple becomes 3f. The simulator shows the freewheeling percentage live.
What is the PIV of each device?
PIV = √3·Vm — the peak line-to-line voltage — for both the thyristors and the diodes.
What causes discontinuous conduction?
A resistive or lightly-inductive load lets the current fall to zero between firings, especially at large α. The simulator flags continuous (CCM) or discontinuous (DCM) conduction and only compares against the (3√3·Vm/2π)(1 + cos α) formula in CCM.