I have a cast iron wheel with magnets around the inner radius as a braking mechanism. If I were to add additional magnets around the outer radius, would the amount of Eddy currents increase or crease if the polarity of the outer magnets was opposite?


Here's a diagram

enter image description here

where black circle, red and green rectangles depict the wheel, existing magnets and new magnets, respectively. The existing magnets have the S side facing the wheel. So my question is, if the green magnets have the N side facing the wheel, will Eddy currents increase or decrease? What if the S sides face the wheel? In fact, do the polarities of any of the magnet matter?

  • $\begingroup$ Can you include a diagram? There are a few different ways to interpret this. $\endgroup$
    – rob
    Sep 23, 2018 at 6:32
  • $\begingroup$ @rob Sure. I've drawn something up which I hope is understandable. $\endgroup$
    – John M.
    Sep 23, 2018 at 6:43
  • $\begingroup$ If you increase the magnetic field in which the conducting wheel moves then this will increase the induced emf, the eddy currents and hence the braking effect. $\endgroup$
    – Farcher
    Sep 23, 2018 at 8:05
  • $\begingroup$ Related question by same user : Increasing Eddy current induced within surface by attaching a magnet on surface $\endgroup$ Sep 23, 2018 at 9:41

1 Answer 1


If the magnets in the two sets were facing each other (i.e., if green outer magnets were shifted to the $6$ o'clock position), it would be pretty obvious that, in order to increase eddy currents, the outer magnets would have to be installed with their north poles facing the wheel, so that the magnetic fields of the two sets boosted each other rather than canceled each other.

When the outer magnets are located as shown on the diagram (at about $3$ o'clock), the interaction of the magnetic fields of the two sets is not as significant.

If the wheel was made out of aluminum or other non-ferromagnetic material, we could say that the polarity of the outer magnets would not matter, i.e., in either case, they would generate roughly the same additional eddy currents and similarly increase braking action.

In your example, through, the wheel is made out of cast iron, a ferromagnetic material, and, as such, it would bend magnetic lines and increase the interaction between the two sets. It is hard to predict the degree of this interaction, but, to be sure (and since it costs nothing), it makes sense to install the outer magnets with their north poles facing the wheel.

  • $\begingroup$ Thanks. A more specific question: Should each outer magnet have the same or different orientation? $\endgroup$
    – John M.
    Sep 23, 2018 at 15:04
  • $\begingroup$ @JohnM. That is a trickier question. Eddy currents are generated when the magnetic field changes, i.e., if you had the same magnets everywhere (around the clock, so to speak), there would not be as much braking. So, if each individual magnet is big enough relative to its distance to the wheel, I would expect that alternating magnet polarity would increase the total effect. Smaller magnets located far from the wheel would likely weaken each other's fields, if they had different polarities. But, to be sure, I would try it out:) $\endgroup$
    – V.F.
    Sep 23, 2018 at 15:26
  • $\begingroup$ @JohnM. Alternatively, if this is an option, you could place individual magnets far enough from each other, in which case, they won't interfere much and their polarity would not matter much. $\endgroup$
    – V.F.
    Sep 23, 2018 at 15:29

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