An eddy current brake resists the motion of a conducting part through a magnetic field. This page shows a rotating disc passing behind a stationary permanent-magnet array without rubbing against it. Its calculator evaluates an explicitly assumed torque–speed curve from supplied peak data; it does not infer brake capability from air gap alone.
Eddy Current Brake Interactive Calculator
Explore an illustrative torque–speed curve using measured or fitted peak data for your actual gap. Compare clearances separately; gap alone cannot establish torque.
Equation Used
- Supplied peak data correspond to the actual magnetic arrangement and clearance.
- The selected fitting curve describes the speed range being explored.
- Speed is prescribed; no stopping-time or thermal calculation.
- Dimensions other than labelled clearance are schematic.
The curve is an explicit fitting assumption, not a brake rating or electromagnetic field solution. Supply new curve data for a changed gap.
The disc moves past stationary magnets
In the front view, the conducting disc and shaft markings rotate while the magnet carrier stays fixed to its support. The separate edge view shows clear space between the magnet faces and the disc. It is enlarged to make the entered clearance visible.
Relative motion through a magnetic field induces currents in the conductor. Their electromagnetic interaction resists the motion, and energy is dissipated as heat. In this illustrative permanent-magnet disc arrangement, the changing field is encountered as disc material travels past the fixed magnets.
The animation represents that component arrangement, not a particular product. Magnet size, carrier shape, disc thickness and bearings are schematic. It does not plot solved current paths or magnetic field lines, and it does not animate the magnets rotating with the disc.
Rotor speed is prescribed. The opposing-torque arrow shows direction only; the page does not solve rotor deceleration, inertia or stopping time.
Explore a measured torque curve
Enter a measured or fitted peak torque and the speed at that peak, both for the actual brake configuration and actual clearance. The calculator uses these values to scale the stated illustrative curve. Check that the curve fits measured data over the speed range of interest before using its outputs quantitatively.
Reference and actual clearance remain adjustable. They control the magnified edge view and the signed clearance difference. Clearance changes update the diagram and clearance comparison; the torque curve still uses the peak data you entered. Obtain new curve data when changing the actual gap.
The sketch illustrates one axial disc arrangement. Other eddy-current machines have different magnetic circuits and heat paths. A small demonstrator’s geometry cannot establish the capacity of a vehicle retarder or dynamometer.
Fitted curve and mechanical power
Define x=n/np, where n is entered rotor speed and np is the entered fitted peak speed. The illustrative normalized curve is f(x)=2x/(1+x²). Calculated torque magnitude is T=Tp f(x), where Tp is the supplied peak torque for the actual setup.
This chosen curve is zero at rest, rises to 1 at x=1, and decreases beyond that point. It is a fitting shape, not a universal electromagnetic brake law. If measured torque does not follow this shape, these two parameters are insufficient to represent that brake.
Mechanical power dissipated is P=T×2πn/60000 in kW for torque in N·m and speed in rpm. It is energy per unit time at the stated point, not a continuous cooling rating or a predicted temperature.
Clearance difference is Δg=g−gref. Positive means the actual face clearance is larger than the reference. The relation between clearance and torque depends on the magnet geometry and magnetic circuit. Use measured torque data for each actual clearance.
Compare three points on the same fitted curve
For an entered peak torque of 3000 N·m at 1000 rpm, the assumed curve returns 3000 N·m at 1000 rpm and mechanical dissipation of about 314.159 kW.
At 500 rpm, x=0.5 and the curve factor is 0.8. Torque is 2400 N·m and power is about 125.664 kW. At 2000 rpm, x=2 and the factor is also 0.8, but power is about 502.655 kW because speed is four times the 500 rpm case.
These values demonstrate the entered curve, not a product rating. Set n to zero and both eddy-current torque and mechanical power become zero. The static magnet field can still exist; there is no motion-induced braking torque in this model.
If reference clearance is 3 mm and actual clearance is 8 mm, the comparison reads +5 mm. The page still needs measured or fitted torque data for that 8 mm configuration. Changing the clearance comparison does not change the entered peak torque or peak speed.
Limits of the calculation
Brake behavior depends on conductor material and thickness, magnet geometry, field strength, magnetic circuit, temperature and speed. A single clearance is not enough to calculate these effects.
The two-parameter curve omits thermal change, excitation control, bearing drag, transient electromagnetic response and speed-dependent changes not captured by its shape. Power is not checked against a cooling limit.
An eddy-current-only brake of the type modeled here does not supply a holding torque at zero relative speed. Use the actual brake manufacturer’s data for permissible clearances, temperature limits and operating ratings.
Eddy current brake questions
Why do the magnets stay still?
The drawing places them on a fixed carrier. The conducting disc rotates past them.
Is there contact between magnet and disc?
No. The edge view shows the entered face clearance. It is not a friction-pad brake.
Why does the gap slider not automatically change torque?
Gap alone does not determine the field or the torque curve. The slider changes the physical clearance comparison. Enter measured or fitted peak data for the new gap to update the curve.
Does the peak torque input mean a guaranteed rating?
No. It is supplied curve data. The calculator does not verify the measurement, cooling capability or suitability of that operating point.
Is the curve correct for every brake?
No. It is an explicit illustrative fitting shape. Use a measured torque map or a validated device-specific model when this shape does not describe the machine.
Why does the disc not slow down during playback?
Speed is an input held fixed for comparison. The animation is not a stopping-time solver.
References and calculation basis
Almeida and Nicoletti, Mathematical modeling and experimental validation of an eddy current brake with permanent magnets, figure 1, shows an axial magnet array facing a conducting disc. The illustration uses a similar axial arrangement; its fitted-curve calculator does not implement the paper’s electromagnetic model.
PHYWE: Eddy current brake with Cobra DigiCart explains motion-induced eddy-current braking in a conductor.
The calculator uses the stated illustrative curve. Check it against measured data for the actual brake before using the results to assess an operating point.
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