On 15/02/2018 12:46, DerbyBorn wrote:
Most motors seem to have the same proportions of length to diameter so
there must be some theoretical principal there.
However, I once operated a milling machine that had what was called a
Pancake motor that drove the feeds. It was large diameter and only about 2
inches deep. Then there is the direct drive washing machine.

Any website I should read to clear this in my mind.

I don't know for sure, but it is easy to see how pancake motors might be
a favourable geometry in some applications where space is tight for
other reasons.

But in a pancake motor, I suspect that both the armature and stator
windings might be more difficult to make (and perhaps need more copper)
than in a conventional geometry. There is also a bit more scope for
"flux leakage". I suspect the conventional geometry will have arisen
from manufacturing considerations. It is also now to some extent "locked
in" because there are standard frame sizes.

If you make a "long, thin" motor then the larger separation of bearings
and thinner shaft give you more issues with vibration, especially at
higher speeds.