First External Rotor Electric Motors

Papst had asynchronous motros like this. I think they were used in Revox A77 tape decks.

Sold since 1967. In the bottom two pictures on that page you can see what looks like the capstan motor. Internal coil, external rotor.

Thomas

Also see

Thomas

In article , snipped-for-privacy@aol.com mentioned...

ISTR seeing such motors decades ago. All of the brushless DC fan motors I've experimented with have a similar design. I believe those designs have been around for several decades. But when you think about it, a regular DC motor, with brushes, is no different. The magnets are around the outside, and the armature is ineide the magnets. All you have to do is affix the commutator shaft to the mounting point, and let the outside of the motor revolve. And since the commutator is no longer moving in relation to the mounting point, the brushes can be eliminated. But you still have to keep the magnetic field moving inside, so that's where the electronics comes into action. Seems like eliminating the brushes is a fair tradeoff.

Does one need to point out that gas turbines are not found in consumer equipment?

Most of those ceiling fans use external rotors. Many (most?) of the designs are approaching 100 years old.

Is there a reason why (or why have I never seen) this type of motor used in power situations. The reason for asking is that I hired a big belt sander to sand a maple floor and it was ridiculously convoluted in design because of the need to drive a wide drum from a shaft and it seemed so obvious to make it with the rotating part of the motor on the outside.

Why? It would require a custom motor design that is heavier, and would cost more. By using an off the shelf motor, they save on initial design cost, they can change brands, series, or other specs without a full redesign, and they are easier to repair in the field.

in

sander to

of

make

Normally I would agree but in this case the motor was made by the maker of the sander and was a totally custom thing, I tried talking to the guy that made them and he had never seen or heard of a motor where the outside rotated. I would have thought that for this type of application there are advantages that would out weigh the disadvantages though as the motors do exist that must not be the case unless there is some reason that they can't be made to produce enough power and still be a reasonable size. I don't see why it should be heavier or bigger for a given power, its only a case of which bit you denote as being stationary.

I'm almost certain Tesla did something with the idea but can't find a reference.

I don't think a Tesla turbine could be turned inside-out. ;>)

Mark L. Fergerson