Technology / Technical guide
Rectifier Commutation Overlap: Finite Current Transfer
An ideal rectifier diagram can show current moving instantly from one conducting path to the next. A model that includes source inductance instead permits a finite transfer interval. Commutation overlap describes that interval within a specified converter model, rather than a universal delay for all power electronics.
Current-path transfer and switch-command timing are different concepts.
Understand the source-inductance constraint before interpreting overlap angle or an average-voltage expression.
01 / Illustrative model
State the conditions behind the current-transfer picture.
Use a hypothetical line-commutated three-phase bridge with ideal valves, finite source inductance and approximately constant continuous DC current.
Use a hypothetical line-commutated three-phase bridge with ideal valves, finite source inductance and approximately constant continuous DC current. This is a generic teaching model. Its topology and load-current condition are not descriptions of an actual MARSTON rectifier, converter or electrical package.
In that model a new source phase takes over current from a previously conducting phase. Ignoring source inductance permits an instantaneous exchange. Including it changes the transfer behaviour, even before resistance and semiconductor nonidealities are added. A finite transfer is therefore not solely a gate-command delay.
02 / Inductive constraint
A finite driving voltage cannot instantaneously change current.
For constant linear inductance, an oriented voltage contribution follows vL = L di/dt.
For constant linear inductance, an oriented voltage contribution follows vL = L di/dt. The voltage difference available around the transfer loop determines how rapidly its currents can redistribute. With finite voltage and nonzero inductance, an instantaneous finite current change is outside that ideal differential model.
The ETH converter thesis describes this mechanism in a line-commutated setting. During the illustrated transfer, the outgoing and incoming phase currents coexist. Their redistribution differs from commanding complementary devices in one switching leg to conduct together; overlap terminology must retain its circuit context.
03 / Current sharing
The outgoing and incoming paths carry portions of one total.
Under the assumed constant DC current Id and the chosen path references, ia + ib = Id during one transfer.
Under the assumed constant DC current Id and the chosen path references, ia + ib = Id during one transfer. Thus dia/dt + dib/dt = 0 in that interval. As the incoming contribution rises, the outgoing contribution falls; the sum remains constant within this simplified model.
If DC current changes appreciably during the interval, the derivative of that sum is no longer zero. Different load behaviour, source conditions or topology changes the equations. The constant-current identity must not be imposed simply because a converter has a DC output label.
04 / Angle definition
An overlap angle represents an interval at a stated frequency.
With constant electrical angular frequency ω in rad/s, a transfer duration Δt corresponds to overlap angle µ = ωΔt in radians.
With constant electrical angular frequency ω in rad/s, a transfer duration Δt corresponds to overlap angle µ = ωΔt in radians. If frequency varies during the interval, the angle is the integral of instantaneous ω over that duration. A duration and an angle are not interchangeable without this frequency reference.
Hypothetically, a 1 ms interval at 10 Hz corresponds to 0.0628 rad, or 3.6°. Those assumed numbers illustrate units only. They define no MARSTON transfer duration, generator frequency, acceptable overlap angle or valve-control setting.
05 / Waveform effects
A changed average voltage is not automatically resistor heating.
Sharing between source paths modifies the voltage and current waveforms relative to the no-overlap idealisation.
Sharing between source paths modifies the voltage and current waveforms relative to the no-overlap idealisation. In the stated line-commutated model this can reduce the average rectified voltage. A fitted average-voltage term proportional to current may resemble a resistance algebraically without representing an actual ohmic heat source.
The source thesis distinguishes an equivalent commutation resistance from a dissipative resistance. Real winding and device losses need their own terms. The energy-accounting guide explains why a port-voltage difference alone does not identify the physical destination of energy.
06 / Separate phenomena
Recovery, dead time and source overlap need distinct models.
Semiconductor reverse recovery concerns charge and transition behaviour in applicable devices.
Semiconductor reverse recovery concerns charge and transition behaviour in applicable devices. Switching dead time concerns intentional command separation. Source-inductance overlap concerns finite redistribution between conducting paths. They can interact in a detailed model, but their durations and causes are not automatically identical.
The reverse-recovery guide addresses the device phenomenon. This guide supplies no actual rectifier topology, timing adjustment or commutation test. PMSG confirmation for FD14-50KW also establishes no particular bridge or source-inductance value.
Related technical guides: Generator Flux Linkage and Leakage: Define the Reference · Snubber Concepts: Transient Energy, Peak Voltage and Damping · Filter-Capacitor Reactive Power and Cyclic Stored Energy.
General background: ETH Zurich: Line-Commutated Converter Overlap Models. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Model class | Line-commutated three-phase bridge | Illustrative ideal valves and continuous DC current. |
| Source inductance | Finite L and transfer-loop voltage | Constrains current redistribution. |
| Current balance | ia + ib = Id | Constant Id and the declared path references. |
| Overlap angle | µ = ωΔt | Constant electrical angular frequency, radians. |
| Equivalent drop | Average-voltage representation | Not automatically an ohmic loss mechanism. |
Engineering questions.
Does every rectifier use the same overlap model?
No. Topology, source and load assumptions determine the transfer equations.
Is overlap just a delay in the gate command?
No. Source inductance constrains the redistribution after a new path becomes available.
Can overlap angle be calculated from duration alone?
No. An electrical-frequency reference is also required.
Are the two phase currents always constant in sum?
That identity belongs to the stated constant-DC-current transfer model.
Does an equivalent commutation resistance necessarily dissipate heat?
No. An algebraic average-voltage term can describe reactive commutation behaviour.
Are the 1 ms and 10 Hz numbers product settings?
No. They are hypothetical unit-conversion values.
Technical enquiry
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