Why does laminating a transformer core reduce eddy current losses rather than eliminating them?
ALaminations reduce the core's magnetic permeability, weakening the induced EMF
BLaminations confine eddy currents to smaller loops, reducing the induced EMF and increasing the effective resistance of each loop, dramatically cutting dissipated power
CLaminations cool the core by increasing surface area, reducing resistive heating
DLaminations replace the magnetic material with a non-conducting material between sheets
Laminations work by confining current paths — each insulated layer limits eddy currents to loop within that thin sheet rather than across the full cross-section. Smaller loops mean smaller enclosed flux and thus smaller induced EMF; thinner conduction paths also mean higher resistance. Since P = V²/R, the combination of smaller V (EMF) and larger R reduces power loss dramatically — scaling as the square of the lamination thickness. Laminations do not eliminate eddy currents; residual currents still flow within each lamination sheet.
Question 2 Multiple Choice
A transformer core operates at twice the frequency. By approximately what factor does eddy current power loss change?
ADoubles — power is proportional to frequency
BIncreases by a factor of 4 — power is proportional to frequency squared
CStays the same — eddy current losses depend only on material resistance
DHalves — higher frequency means current has less time per cycle to flow
Eddy current power loss scales with the square of frequency. At twice the frequency, the magnetic flux changes twice as fast, doubling the induced EMF. By P = V²/R, doubling voltage quadruples power. This is why high-frequency power devices (switching supplies, radio-frequency transformers) cannot use laminated steel cores — even thin laminations are insufficient at high frequencies. Ferrite cores with very high resistivity are used instead, nearly eliminating eddy current paths altogether.
Question 3 True / False
Eddy currents in a falling-magnet experiment produce a braking force because the induced currents oppose the change in flux that created them.
TTrue
FFalse
Answer: True
This is Lenz's law applied to eddy currents. A falling magnet increases the downward magnetic flux through the conducting plate below it. By Lenz's law, the induced eddy currents flow in a direction that opposes this increase — they create an upward magnetic field that pushes back on the falling magnet. The result is a braking force: the magnet falls more slowly than it would under gravity alone. This is the operating principle of magnetic brakes in trains, roller coasters, and laboratory damping systems.
Question 4 True / False
Lamination eliminates eddy currents mostly in a transformer core.
TTrue
FFalse
Answer: False
This is explicitly listed as a misconception. Lamination reduces eddy currents by confining them to smaller loops within each insulated sheet, but it does not eliminate them. Eddy currents still flow within each lamination layer — they simply cannot cross the insulating gaps between layers. The power loss is dramatically reduced (scaling as the square of lamination thickness), but only approaches zero in the limit of infinitely thin laminations or infinitely resistive material (like ferrite). In practice, some eddy current loss always remains.
Question 5 Short Answer
Why do eddy current losses scale with the square of frequency, and what practical consequence does this have for high-frequency power devices?
Think about your answer, then reveal below.
Model answer: Eddy current losses scale as frequency squared because EMF is proportional to the rate of flux change (Faraday's law), which is proportional to frequency. Doubling frequency doubles EMF, and since P = V²/R, power quadruples. At high frequencies (switching power supplies, radio-frequency transformers), even thin laminated steel cores produce unacceptable losses. The practical consequence is that high-frequency devices use ferrite cores — ceramic magnetic materials with very high electrical resistivity — which nearly eliminate eddy current paths and thus keep losses manageable at frequencies where steel would turn the core into a resistive heater.
The f² scaling means that eddy current losses become the dominant loss mechanism as frequency increases, outweighing other considerations. This is why AC power grid frequency is low (50–60 Hz) — keeping eddy current losses in transformers manageable. Switching power supplies operate at 50–500 kHz and require ferrite cores for the same reason. Understanding the f² relationship allows engineers to predict how losses scale and choose the right core material for the operating frequency.