Iron loss is the key performance index of electrical steel. It decides how much of the energy fed to a motor, generator or transformer is turned into heat instead of useful work, and it is the number on the mill certificate that buyers compare first. The subject also attracts confusion, because iron loss and zinc loss belong to two completely different processes.
What Iron Loss Means in Electrical Steel
Iron loss, also called core loss, is the ineffective electrical energy consumed when the core is magnetised under an alternating field of not less than 50 Hz, expressed in watts per kilogram. The energy is dissipated as heat inside the core, which raises the temperature of the machine and reduces its efficiency.
Electrical steel is normally sold against two guaranteed magnetic properties: core loss and magnetic induction intensity. Because the loss index is what separates one grade from another, producers classify and name their products by loss value, and every effort in alloying and processing is directed at reducing it.
The Three Components of Total Loss
Total loss Pt has three parts: hysteresis loss Ph, eddy-current loss Pe and anomalous loss Pa.
Hysteresis loss (Ph) - caused by the irreversible movement of domain walls. Inclusions, crystal defects, internal stress and unfavourable crystal orientation all hinder domain wall movement and make the magnetic induction lag behind the applied field. Ph is proportional to the area of the direct-current hysteresis loop.
Eddy-current loss (Pe) - caused by circulating currents induced around the magnetic flux as it changes direction, following Faraday's law. It is calculated with the classical eddy-current formula and is strongly dependent on sheet thickness and resistivity.
Anomalous loss (Pa) - the difference between the measured total loss and the calculated sum of Ph and Pe, caused by the behaviour of magnetic domains in a real microstructure.
| Material group | Hysteresis share of Pt | Eddy current plus anomalous share |
|---|---|---|
| Non-oriented low-carbon and mid-grade silicon steel | About 75-80% | About 20-25% |
| High-grade grain-oriented silicon steel | About 30-60% | About 40-70% |
The split is not fixed. It moves with silicon content, grain size and texture: in oriented high-grade steel the grains are much larger, the hysteresis share falls, and the anomalous component can become one to two times larger than the classical eddy-current term. Published shares therefore vary between grades and thicknesses, and the reliable figure for a specific order is the guaranteed loss index on the mill certificate.
How Loss Is Measured and Specified
Specific total loss is measured on the Epstein frame in accordance with IEC 60404-2, with corresponding procedures in ASTM A343 for alternating-current magnetic properties. The test uses standard strip specimens at a defined frequency and a defined polarisation, and results are quoted as P15/50 (1.5 T at 50 Hz) or P17/50 (1.7 T at 50 Hz).
Grade names encode the same information. Under GB/T 2521, the first digits give the nominal thickness in hundredths of a millimetre, the letter shows the type (W for non-oriented, Q for grain-oriented), and the closing number is the loss value multiplied by one hundred. Two commonly quoted examples make the logic clear:
| Grade | Type | Thickness | Guaranteed loss |
|---|---|---|---|
| 50W470 | Non-oriented | 0.50 mm | P15/50 not more than 4.70 W/kg |
| 30Q120 | Grain-oriented | 0.30 mm | P17/50 not more than 1.20 W/kg |
Loss-Reduction Measures in Practice
Higher silicon content raises electrical resistivity and cuts eddy-current loss, although it also raises hardness and reduces saturation induction, so the addition is balanced against the duty of the machine.
Thinner laminations, typically 0.23-0.65 mm, interrupt the eddy-current paths and are the main lever in high-frequency applications.
Grain orientation - the Goss texture of grain-oriented steel aligns easy magnetisation directions with the rolling direction, narrowing the hysteresis loop in that direction.
Laser scribing and domain refinement on grain-oriented grades subdivide magnetic domains and lower loss further.
Insulating coatings on the strip surface limit inter-laminar currents when the core is stacked.
Process control on inclusions, surface quality and residual stress keeps hysteresis loss at the level the grade is capable of.
Why Zinc Loss Is a Different Subject
Zinc loss belongs to hot-dip galvanizing, where it describes zinc consumed from the bath through dross formation, oxidation and coating take-up on the strip. Electrical steel does not carry a zinc layer: it carries a thin inorganic insulating coating, usually phosphate based or chromate free, whose job is inter-laminar insulation and punchability rather than corrosion protection.
Mixing the two terms leads directly to specification errors. A certificate for electrical steel should be checked for the P15/50 or P17/50 loss index, magnetic induction and coating type; coating mass designations such as Z275 or Z180 belong to hot-dip zinc-coated sheet under EN 10346 and have no place on an electrical steel certificate.
Practical Impact for Motor and Transformer Design
Low iron loss saves electrical energy directly, extends the operating time of motors and transformers and simplifies or shrinks cooling equipment. The scale of the effect is large: reported studies put the electricity lost through core loss at roughly 2.5-4.5% of annual generation in major economies.
Because core loss runs continuously whether or not the machine is loaded, and because core loss in transformers and large machines is much greater than copper loss, the loss index drives lifetime running cost far more than the purchase price of the steel does. That is why buyers specify a maximum P15/50 or P17/50 value on the mill certificate rather than accepting a generic grade name.
Frequently Asked Questions
Q: Is iron loss the same as core loss?
Yes. The two terms describe the same quantity: the energy dissipated in the magnetic core under alternating magnetisation, expressed in watts per kilogram. Iron loss is the term used in the electrical steel industry, core loss is the term used in machine and transformer design.
Q: What do P15/50 and P17/50 mean?
They are specific total loss values measured in watts per kilogram at a defined polarisation and frequency. P15/50 is measured at 1.5 T and 50 Hz, P17/50 at 1.7 T and 50 Hz, generally on the Epstein frame in accordance with IEC 60404-2.
Q: How do I decode a grade name such as 50W470?
The 50 gives the nominal thickness, 0.50 mm; the W shows a non-oriented material, against Q for grain-oriented; and 470 is the guaranteed loss value multiplied by one hundred, so P15/50 is not more than 4.70 W/kg.
Q: Does zinc loss form part of electrical steel losses?
No. Zinc loss is a hot-dip galvanizing term describing zinc consumed in the coating bath. Electrical steel carries an insulating coating rather than a zinc layer, so its loss figures are always iron loss, quoted as a P15/50 or P17/50 index.
Q: Why does higher silicon content reduce iron loss?
Silicon raises the electrical resistivity of the steel, which suppresses the eddy currents induced in the core and therefore lowers eddy-current loss. The penalty is a harder, more brittle material with lower saturation induction, so the addition is optimised for the intended frequency and flux density.
Q: Why are grain-oriented steels specified for transformers?
Transformers operate continuously with flux along one direction, and the Goss texture of grain-oriented steel aligns the easy magnetisation direction with the rolling direction. Hysteresis loss is therefore much lower in that direction, at the cost of poorer performance if the flux path leaves the rolling direction.
