Key takeaways
- Grade is chosen by specific loss at a stated flux and frequency, not by thickness alone.
- The insulation coating is part of the product. Damage it and the eddy current loss rises even if the steel is correct.
- Burr is the main in-process indicator. A burr that bridges laminations creates a short circuit in the stack.
- Punching raises loss through stress. Annealing can recover some of it, but only if the coating survives the cycle.
What a lamination has to do
A lamination is one thin sheet of electrical steel in a stack that forms a magnetic core. The stack guides flux, and the thin sheets are insulated from each other so that eddy currents cannot circulate through the whole core. The steel carries the flux with as little loss as possible, and the punch decides the shape of the magnetic path.
Two losses matter. Hysteresis loss depends on the grade and the flux density. Eddy current loss depends on thickness, frequency and, critically, the resistance between laminations. A stack of the best steel in the world will run hot if the coating is damaged or the burrs touch. That is why the press shop controls both the geometry and the edge.
Grades, thickness and the loss number
Non-oriented electrical steel is the usual choice for motors and generators, where the flux runs in several directions. Grain oriented steel is used in transformers, where the flux runs along the rolling direction and the loss can be much lower. The grade name usually carries the thickness and the maximum specific loss.
A designation such as M400-50A means a maximum specific loss of 4.00 W/kg at 1.5 T and 50 Hz, in a strip 0.50 mm thick, in a fully processed condition. A lower loss number means a better grade and usually a higher price. Thinner strip lowers eddy current loss but reduces the stacking factor and raises the number of sheets in a core of a given height.
| Family | Thickness | Loss character | Usual application |
|---|---|---|---|
| Standard non-oriented | 0.50 to 0.65 mm | Higher loss, lower cost | General motors, pumps, fans |
| High efficiency non-oriented | 0.35 to 0.50 mm | Lower loss, tighter process control | EV traction motors, efficient drives |
| Thin non-oriented | 0.20 to 0.35 mm | Low eddy current loss at high frequency | High speed motors, inverters |
| Grain oriented | 0.23 to 0.35 mm | Very low loss along the rolling direction | Transformers and reactors |
| Nickel iron alloys | 0.10 to 0.35 mm | High permeability at low flux | Sensors, small transformers, shielding |
Illustrative families. Loss values depend on the exact grade, the test standard and the thickness. Ask for the mill certificate and the test method.
The coating is not a luxury
Electrical steel is supplied with a thin insulation coating, often a few micrometres thick. The coating provides interlamination resistance, improves punchability by reducing friction and, in some types, survives a stress relief anneal. Coating types are graded by their thermal class and their suitability for welding, annealing or punching.
A coating that is too thick lowers the stacking factor, because more of the stack is insulation and less is steel. A coating that is too thin, or damaged in handling, allows contact between sheets. The result is a local short circuit, a hot spot and a rise in loss that no dimensional inspection will catch.
The press shop should treat the coating as a surface to protect. Sharp die entry, poor strip guidance, excessive stripper pressure and dirty feed rolls can all mark it. If the part is annealed after punching, the coating must be compatible with the annealing temperature and atmosphere, or the anneal will destroy the insulation it was meant to preserve.
Loss is a system property
Steel grade, coating, burr and stack pressure all contribute to the final loss. Testing only the incoming strip misses the damage the press can do.
Burr and the interlaminar short
A burr on a lamination is more than a handling problem. If the burr is tall enough to press through the coating of the next sheet, the two laminations make electrical contact. That creates a path for eddy current, raises loss and can produce a hot spot in service. The effect is worse in a stack that is compressed or welded, because the clamping force pushes the burrs together.
Burr height depends on clearance, punch sharpness, die wear and the coating. Electrical steel is abrasive, so punches and dies wear steadily. A die that starts at a good burr will drift upward, and the drift is easy to miss because the part still measures correctly. Monitoring burr at a fixed interval is the usual control.
A common approach is to set a burr limit from the coating thickness and the stack pressure, then measure at the start and end of each run. Values in the region of 0.02 to 0.05 mm are often used for thin laminations, but the right number depends on the coating and the application. The limit should be agreed with the core designer, not copied from a general table.
Punching stress and annealing
Shearing and bending damage the crystal structure near the cut edge. That damage raises the local loss, and in a small lamination the damaged edge can be a significant share of the magnetic path. The effect is larger in narrow teeth and small slots, where the cut edge is close to the flux path.
A stress relief anneal after punching can recover part of the loss. The cycle is usually in the region of 750 to 800 C in a controlled atmosphere, and it must be matched to the coating. A coating designed for annealing will survive; a standard organic coating will burn off, and the stack will lose its insulation. Annealing also changes dimensions and flatness, so the fixture and the stack sequence matter.
Not every part should be annealed. The cost, the distortion and the handling can outweigh the loss gain, especially in a large motor where the cut edge is a small fraction of the path.
Die design and maintenance for laminations
Lamination dies run at high stroke rates and produce large volumes, so they are usually built with carbide inserts or a premium tool steel in the cutting stations. Clearance is typically around 5 per cent per side, and the die is designed for quick sharpening and insert replacement rather than for a single long life.
The press needs good strip guidance, a flat stripper and reliable slug removal. A slug that stays in the die can mark the strip or break a punch, and a single miss-hit can damage a carbide insert.
Sharpening policy should be based on burr and punch load, not only on a stroke count. The punch edge rounds gradually, the burr rises, and the point at which the burr crosses the limit is the real maintenance trigger. Record the grind amount so the die height and the punch length stay within the design window.
- Carbide or premium tool steel in the cutting stations
- Clearance set for the coating and the grade, not a generic steel value
- Flat, guided strip with controlled stripper pressure
- Reliable slug clearance and die protection sensors
- Burr measured at a fixed interval and recorded by station
- Sharpening triggered by burr and load, with grind depth recorded
What to inspect and what to ask for
A lamination inspection plan covers three things: the geometry, the edge and the magnetic result. Geometry means the slot profile, the tooth width, the bore, the keyway and the stack height. The edge means burr height and coating damage. The magnetic result means specific loss and, for a stack, interlamination resistance.
For a new part, ask for a first article with the dimensions, the material certificate, the coating type and thickness, the burr record and a loss test on a trial stack. If the part is annealed, add the anneal cycle and the flatness after annealing. If the stack is welded, add a weld section and a check for coating damage around the weld.
For production, agree the sampling and the reaction plan. A burr result that is close to the limit should trigger a die inspection, not just a concession.
Frequently asked questions
What does M400-50A mean on electrical steel?
It is a grade designation: a maximum specific loss of 4.00 W/kg at 1.5 T and 50 Hz, in strip 0.50 mm thick, in the fully processed condition. The exact test method and tolerances come from the relevant standard and the mill certificate.
Why is burr a problem on a lamination?
A tall burr can break through the insulation coating on the next sheet and create an electrical path between laminations. That short circuit allows eddy currents to circulate, which raises loss and can produce a hot spot. Burr also changes the stack height.
Should laminations be annealed after punching?
Sometimes. Annealing can recover part of the loss caused by punching stress, but it must suit the coating, and it can distort thin parts. The decision should be based on a loss test on a trial stack and on the cost of the extra process.
Which coating should I specify?
The coating has to provide interlamination resistance and survive the process. If the part is annealed, choose a coating rated for that cycle. If it is welded, check the welding behaviour. If punchability is the priority, choose a coating with good lubricity. Agree the type and thickness with the steel supplier and the die designer.
Sources and further reading
- IEC 60404-2, Methods of measurement of the magnetic properties of electrical steel strip and sheet
- IEC 60404-11, Method of test for the determination of surface insulation resistance of electrical steel strip and sheet
- Qumek lamination die and inspection notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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