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Magnetic bearings work by levitating a rotating shaft so that it is not in contact with its supports. This greatly reduces the friction of the system.

In all of the literature that I have seen on magnetic bearings, the bearings are described as "low friction" and not "no friction".

Wikipedia

... they do not suffer from wear, have low friction ...

Synchrony

... they do not suffer from wear, they have low friction ...

Steorn

... A low-friction bearing ...

Calnetix

... extremely low friction and wear ...

It would seem that there wouldn't be any friction since the magnets are keeping the rotating shaft from touching anything.

Where does the friction come from in magnet bearings?

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The bearings themselves may have ideally 0 friction, but there will still be some losses of rotational energy in the system as a whole.

Even if the vacuum was perfect, anything that conducts electricity even a little becomes a transformer secondary in the presence of changing magnetic fields. Conducting material in the rotating thing will dissipate power due to eddy currents caused by any external magnetic field, including the earth's. Clearly there are strong magnetic fields in the magnetic bearing. Even a small asymmetry in that field will cause a apparent alternating magnetic field to the spinning part.

This works in reverse too. If the spinning part is magnetized, then anything on the outside that is conductive will dissipate some power.

Small imbalances and asymmetries in the magnetic fields of both the fixes stuff and the rotary stuff are very hard to avoid. Conducting materials are also hard to avoid. In the end, the best engineering tradeoff is usually to accept some small losses.

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    $\begingroup$ So for a bearing in a vacuum, it would seem that using the term "friction" is misleading. It is a "loss", but there isn't friction. It may just be convention then. In other types of bearings, friction is the source of the losses. $\endgroup$
    – hazzey ♦
    Commented Mar 21, 2015 at 18:25
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There are two types of losses in magnetic bearings, windage and electromagnetic losses. Windage or aerodynamic loss is the dissipation of rotational energy due to the viscosity of air or other gases trapped between the rotating and stationary portions of the system. These effects are more significant in high speed applications and clearly non-existent in a vacuum sealed chamber. The electromagnetic losses can come from hysteresis and/or eddy currents in the magnets. These effects have to do with a variation in the magnetic flux due to the rotating ferromagnetic materials. Below are some references I have found useful.

References:

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  • $\begingroup$ the ideal would then be a superconductor shaft in a true vacuum? $\endgroup$
    – ratchet freak
    Commented Mar 22, 2015 at 23:58

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