In article 6,
DerbyBorn writes:
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.

Larger diameter motors have lower max speeds above which the
forces on the armature will break it. Larger diameter motors
also allow space for more poles giving lower speed higher
torque characteristics. To increase torque with a given
diameter, the motor body can be made longer.

Obviously, there may be physical constraints on fitting a
motor into a given space too. Some mains tools/appliances
with limited space will use a DC motor with a rectifier,
because it enables replacing the field windings on what
would have been a universal motor with a permanent magnet
which can be made significantly smaller than a set of field
windings, and reduce the outer diameter.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]