Technology / Technical guide
Generator Core Loss: Magnetic Hysteresis & Eddy Currents
A changing magnetic field can transfer energy into heat within a magnetic core. Hysteresis and eddy currents arise through different mechanisms, even though both can contribute to a generator’s electromagnetic loss. Understanding the distinction avoids treating magnetic saturation or copper heating as a complete explanation of core behaviour.
Magnetic cycling and conductive loops create different losses.
A field amplitude, cycle frequency and material description need their own boundaries before they become a core-loss model.
01 / Loss locations
Core loss and winding loss are different boundaries.
Winding current dissipates energy in conductive wire.
Winding current dissipates energy in conductive wire. A magnetic core can additionally dissipate energy through changing magnetisation and currents induced inside conductive material. Calling both effects electrical losses does not make their spatial distribution or causal mechanism identical.
COMSOL’s electromagnetic-loss documentation separates resistive heating from magnetic hysteresis and eddy-current contributions. A whole-generator efficiency value combines several paths; it cannot independently reveal how much heat belongs to the core, winding, bearings or other parts.
02 / Magnetic history
The field response follows a path rather than one line.
For a hysteretic material, the relationship between magnetic flux density B and field strength H depends on the preceding magnetisation history.
For a hysteretic material, the relationship between magnetic flux density B and field strength H depends on the preceding magnetisation history. Increasing and decreasing excitation can follow different paths. This history dependence is distinct from a nonlinear curve that is assumed reversible.
The B–H curve explanation illustrates why one straight permeability value cannot represent every magnetic response. A hysteresis loop describes a cycle of that response; its shape belongs to a stated material and excitation, rather than every generator at the same rated power.
03 / Cycle energy
Repeating the same loss per cycle increases average power.
Under a specified repeatable magnetic cycle, let the dissipated energy per unit volume per cycle be wcyc.
Under a specified repeatable magnetic cycle, let the dissipated energy per unit volume per cycle be wcyc. If that same cycle repeats at frequency f, its mean loss density is f wcyc. Joules per cubic metre per cycle become watts per cubic metre through the repetition rate.
As a hypothetical scaling illustration, twice as many unchanged cycles per second gives twice that particular mean loss contribution. A real increase in frequency can also change the cycle and other loss mechanisms, so this restricted comparison is not a universal total-core-loss law.
04 / Eddy currents
Changing flux can drive currents within a conductive body.
A time-varying linked field can induce currents around conductive paths within the material.
A time-varying linked field can induce currents around conductive paths within the material. Their resistance dissipates energy as heat. These are not necessarily the intended winding current, and the paths can exist within the core or surrounding conductive structural parts.
Splitting a conductive region into electrically separated paths changes the possible current loops. Lamination is one general construction concept associated with that mechanism, but its effect depends on geometry, orientation, insulation and material properties. No actual MARSTON core construction is established here.
05 / Waveform effects
Equal peak flux does not imply equal core loss.
Two magnetic waveforms can reach the same peak B yet traverse different paths and change at different rates.
Two magnetic waveforms can reach the same peak B yet traverse different paths and change at different rates. Harmonic content and operating history can therefore matter alongside peak magnitude and nominal frequency. A sinusoidal fit has its own applicable range.
The saturation guide concerns nonlinear field response, while the magnet-temperature guide concerns permanent-magnet behaviour. Neither replaces a time-dependent core-loss explanation or supplies an actual loss coefficient.
06 / Model scope
Keep a causal explanation separate from a material rating.
The generator loss guide addresses the combined operating-point boundary.
The generator loss guide addresses the combined operating-point boundary. Here, separating mechanisms explains why copper resistance, a B–H curve and an efficiency map answer different questions. One item does not uniquely determine the others.
No MARSTON electrical-steel grade, lamination thickness, empirical core-loss coefficients or measured iron loss is assigned. The explanation supplies no excitation setting, magnetic test, material substitution or thermal limit; it concerns the physical meaning of separate loss terms.
Related technical guides: Winding Skin Effect & Proximity Effect: Current Distribution · Loaded Generator Torque Ripple & Cogging Contributions · Thermal Resistance, Heat Capacity & Transient RC Models.
General background: COMSOL: distinct electromagnetic loss mechanisms · COMSOL: nonlinear magnetic response and B–H representation. 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 |
|---|---|---|
| Winding loss | Intended conductor heating | Current distribution and wire conditions matter. |
| Hysteresis | History-dependent magnetic cycle | A reversible nonlinear curve is a different model. |
| Cycle loss density | f wcyc in W/m³ | Same repeatable dissipative cycle is assumed. |
| Eddy-current loss | Heat from induced conductive loops | Paths, geometry and changing fields matter. |
| Total generator loss | Multiple contributions | Core loss cannot be isolated from efficiency alone. |
Engineering questions.
Is core loss just copper I²R loss?
No. Magnetic cycling and induced currents occupy additional physical boundaries.
Does nonlinearity always mean hysteresis?
No. A reversible nonlinear response can be modelled separately from history dependence.
Does twice the frequency always double total loss?
Only the stated unchanged per-cycle contribution follows that scaling; other mechanisms and the cycle may change.
Are eddy currents always the intended winding current?
No. They can circulate within conductive core or structural paths.
Does the same peak B guarantee equal loss?
No. Waveform shape, rate and history also matter.
Are actual MARSTON core materials or coefficients identified?
No. The page explains mechanisms without assigning construction or measured loss.
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