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Technology / Technical guide
Generator Magnetic Saturation & Flux Density
A magnetic core does not respond with one constant permeability at every operating point. As its response approaches saturation, additional magnetising effort produces a different incremental flux change. This affects how a machine can be represented by a simple linear model.
Read the slope, not a single imagined saturation cutoff.
Saturation describes a nonlinear magnetic response; it is different from a permanent magnet losing its retained magnetic state.
01 / Field quantities
Separate magnetising field from flux density.
Magnetic field strength H and magnetic flux density B describe different quantities.
Magnetic field strength H and magnetic flux density B describe different quantities. In a simple linear material model, B = μH. A ferromagnetic core’s effective permeability depends on its operating state, so that proportional relationship is not universal.
Total flux also depends on the area and distribution of B. The same flux passing through a smaller section can imply a higher local flux density. A machine-wide average can therefore conceal a more strongly loaded local magnetic region.
02 / Nonlinear response
Interpret saturation as a changing relationship.
At lower magnetising levels a core can produce a comparatively large flux change.
At lower magnetising levels a core can produce a comparatively large flux change. As the response approaches saturation, more of the available domain alignment is already expressed, and the incremental B change per added H becomes smaller.
Saturation does not mean B becomes mathematically unable to increase. It means the ferromagnetic contribution no longer increases as it did in the earlier region. A sharp universal maximum drawn across all materials would misrepresent this behaviour.
03 / Curve slope
Distinguish incremental from overall permeability.
The local slope dB/dH describes incremental permeability around an operating point.
The local slope dB/dH describes incremental permeability around an operating point. The ratio B/H is a different, overall quantity. They are equal in an ideal linear relationship through the origin, but need not be equal on a nonlinear curve.
This distinction matters when a model describes a small variation around an already magnetised condition. Using an earlier slope for a much larger excursion can overstate the additional flux that the same magnetising change produces.
04 / Inductance effect
Connect magnetic response with electrical linkage.
Inductance relates current and flux linkage in an applicable model.
Inductance relates current and flux linkage in an applicable model. If added current produces less incremental linkage as the core saturates, incremental inductance can decrease. Coilcraft’s core discussions illustrate that response in magnetic components.
The example is general magnetic behaviour, not a generator parameter. Machine geometry, magnetic bias and coupled windings complicate the relationship. The inductance guide distinguishes a fixed linear L from operating-dependent behaviour.
05 / Two phenomena
Keep core saturation apart from magnet demagnetisation.
A soft magnetic core approaching saturation and a permanent magnet suffering lasting demagnetisation are different changes.
A soft magnetic core approaching saturation and a permanent magnet suffering lasting demagnetisation are different changes. One concerns the response to magnetising effort; the other concerns the retained state and applicable reverse-field or thermal conditions.
The same machine can contain both soft magnetic paths and permanent magnets, but their curves serve different purposes. Use the magnet-temperature guide for that separate material behaviour.
06 / Model boundary
Keep a local approximation local.
A constant-inductance machine model can be useful within conditions where its approximation is justified.
A constant-inductance machine model can be useful within conditions where its approximation is justified. Outside that region, nonlinear magnetic data may be needed. A numerical solver does not make an assumed linear material curve physically exact.
Coilcraft’s ferrite and powder-core examples demonstrate that saturation shape depends on material and construction. No MARSTON core material, magnetic curve, saturated flux density or associated control threshold is specified in this explanation.
Continue the review with Generator Inductance, Reactance & Impedance.
Continue the review with Permanent-Magnet Temperature & Demagnetisation.
Continue the review with PMSG d–q Axes & the Rotating Reference Frame.
General background: Coilcraft: material-dependent saturation and core response · Coilcraft: inductance change with magnetic bias. 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 |
|---|---|---|
| Field strength | H in A/m | Represents magnetising field. |
| Flux density | B in T | Can vary within the magnetic path. |
| Linear permeability | μ in B = μH | Only a stated linear approximation. |
| Incremental slope | dB/dH | Depends on the operating point. |
| Incremental inductance | Change in linkage per change in current | Can vary with magnetic saturation. |
Engineering questions.
Does saturation mean flux can never increase further?
No. The incremental magnetic response changes rather than becoming one universal hard cutoff.
Is B/H always the same as dB/dH?
Only in the applicable linear relationship; nonlinear curves distinguish them.
Can total flux hide a high local density?
Yes. The local area and field distribution matter.
Is core saturation the same as magnet demagnetisation?
No. They describe different magnetic phenomena.
Does a fixed L model capture all saturated conditions?
Not necessarily. Its applicable operating range must be respected.
Is a product saturation limit provided?
No actual MARSTON magnetic curve or cutoff is established here.
Technical enquiry
Discuss the technical question.
A magnetic core does not respond with one constant permeability at every operating point. Share the model and applicable operating or design information for an engineering discussion.