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Technology / Technical guide
Rectifier Output Ripple, Pulse Number & Frequency
Rectification changes the direction of an electrical waveform, but it does not automatically produce perfectly constant voltage. The output can contain an average DC component and repeating variation. Pulse number describes that repeating pattern under a defined source and rectifier model.
Positive voltage can still be strongly time-varying.
Pulse repetition, generator electrical frequency and switching frequency are different quantities.
01 / Average and ripple
Decompose a periodic output conceptually.
A periodic rectified voltage can be written as its average plus a time-varying component with zero average over the chosen cycle.
A periodic rectified voltage can be written as its average plus a time-varying component with zero average over the chosen cycle. The average describes the DC level; the remaining component describes ripple.
RMS ripple, peak-to-peak ripple and ripple as a percentage of a reference are different measures. One cannot be substituted for another without the waveform and definition. A positive average says nothing by itself about how flat the output is.
02 / Single-phase pulses
Compare half-wave and full-wave idealisations.
With an ideal sinusoidal source, a half-wave rectifier retains one polarity of each cycle, producing one output pulse per source period.
With an ideal sinusoidal source, a half-wave rectifier retains one polarity of each cycle, producing one output pulse per source period. A full-wave bridge uses both half-cycles, producing two similar pulses per period.
MathWorks’ rectifier introduction distinguishes these pulse patterns. This comparison assumes the stated ideal source and rectifying action; smoothing and a particular load change the voltage or current actually observed.
03 / Six-pulse case
Keep the three-phase conditions explicit.
For a balanced sinusoidal three-phase source and an ideal diode bridge without smoothing or commutation overlap, the output follows the highest phase potential minus the lowest.
For a balanced sinusoidal three-phase source and an ideal diode bridge without smoothing or commutation overlap, the output follows the highest phase potential minus the lowest. The selected pair changes as the phase waveforms pass one another.
The ideal output pattern repeats six times per electrical cycle, so its basic ripple repetition is 6fe. In a hypothetical case with fe = 40 Hz, that pattern repeats at 240 Hz. Neither value is a MARSTON generator or rectifier specification.
04 / Voltage and current
Avoid assigning one waveform to every quantity.
A pulse count for ideal output voltage does not define the current drawn by every connected load.
A pulse count for ideal output voltage does not define the current drawn by every connected load. A resistive load follows the voltage differently from a path with substantial inductive or capacitive behaviour.
Source impedance and commutation overlap can also reshape the ideal waveform. Keep source voltage, rectifier-output voltage, load current and any smoothing-element current as separate quantities; their ripple components need not have identical magnitudes.
05 / Smoothing physics
Relate variation to stored energy and load demand.
An ideal capacitor follows i = C dv/dt.
An ideal capacitor follows i = C dv/dt. It can exchange charge during a repeating waveform and supply part of a load between charging episodes. That changes the output variation without making the rectifier a regulated supply by definition.
Actual resistance, inductance, load demand and additional conversion stages influence the resulting spectrum. Machine speed variation changes electrical cycle timing, while a downstream switched converter can add another frequency family. The capacitor guide covers nonideal losses.
06 / Scope boundary
Do not infer hardware from a pulse formula.
The rectifier-interface guide distinguishes stages in an actual package.
The rectifier-interface guide distinguishes stages in an actual package. Here the waveforms are ideal explanatory cases, not a reconstruction of a supplied circuit.
No MARSTON rectifier topology, phase arrangement, ripple percentage or smoothing component is confirmed by this explanation. The relation 6fe applies to its defined balanced six-pulse model, not every AC-to-DC converter or every operating record.
Continue the review with Diode & Active Rectification: Devices and Control Roles.
Continue the review with DC Capacitor Ripple Current, ESR & Frequency Dependence.
Continue the review with PWM Switching Frequency, Carrier & Fundamental.
General background: MathWorks: half-wave, full-wave and six-pulse rectifying patterns · MathWorks: ideal and dynamic three-phase diode-bridge modelling. 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 |
|---|---|---|
| Average DC | Cycle-average voltage | Does not imply zero variation. |
| Ripple measure | RMS, peak-to-peak or defined ratio | Requires waveform and reference. |
| Pulse number | Repeats per source electrical cycle | Depends on the rectifying model. |
| Six-pulse repetition | 6fe in Hz | Balanced ideal three-phase bridge conditions. |
| Load waveform | Voltage and current separately | Changes with impedance and smoothing. |
Engineering questions.
Does rectification make voltage constant?
No. Rectified output can retain repeating variation.
Is peak-to-peak ripple the same as RMS ripple?
No. Their relationship depends on waveform shape.
Does every rectifier have six pulses?
No. Pulse number depends on the source and topology.
Is 6fe always the generator-side output ripple?
Only within the stated ideal balanced six-pulse model and its defined quantity.
Does a capacitor make the output regulated?
Not by itself. Charge buffering and regulation are different functions.
Are 40 and 240 Hz product values?
No. They are a hypothetical illustration of pulse repetition.
Technical enquiry
Discuss the technical question.
Rectification changes the direction of an electrical waveform, but it does not automatically produce perfectly constant voltage. Share the model and applicable operating or design information for an engineering discussion.