Magnet configuration for spinning permanent magnet induction heating

[positron96]

Watch this video:

Permanent magnets moving relative to a stationary copper tube generate Eddy currents which result in Joule heating of the copper. Simple enough.

Let's change the problem to a rotor with 4 (circumferential) permanent magnets rotating inside a stationary (aluminum or copper) tube at some given RPM.

1. Which would heat the stator faster: magnets placed in alternating polarity (NSNS) or the same polarity (NNNN) around the rotor? I think it would be alternating because an area element on the stator would see more variation in the field per revolution.

2. Which would require more torque to turn the rotor: alternating or constant? I think they would be the same because the field strength and relative velocity are the same as the stator element passes each magnet, so the Lorentz force should be the same for both configurations.

3. Why does the alternating magnet configuration generate more heat but require the same amount of torque? Where did I go wrong?

Any links or references would be much appreciated.

 

[NascentOxygen]

If one configuration achieves greater heating, it follows that its rotation will demand greater torque. There is no such thing as a free lunch!

I think you should build both and experiment.

 

[Gordon]

Any eddy currents produced in the pipe would produce a counter magnetic field apposing the "changing" magnetic field of the rotating magnets. if the magnets have all the same poles, then the change in the magnetic field is mainly a change in field intensity, but if you have alternating field poles you have a complete cycle of the magnetic field from a maximum positive direction, then through a point with no magnetic influence, then a maximum field strength in the opposite polarity, resulting in a much larger change in the magnetic field over the same time period. Only a change in the magnetic field across the pipe creates eddy currents. You get much the same effect from running a single current carrying conductor through an electrical metallic conduit due to the alternating magnetic field created by the alternating current traveling through the wire. that's why you always run the return conductor through the conduit to cancel out the magnetic fields of the feed conductor, or you run multi phase conductors all together in the same conduit for the same reason. The alternating field poles would create more heat at the same angular velocity as an armature with the same poles, and would require more torque to operate.