On Fri, 16 Feb 2018 14:01:39 +0000, Andrew Gabriel wrote:

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.

Not only that but the use of the latest rare earth magnets means less
copper can be used for a more efficient motor or else the space within
the motor taken up by *both* rotor and stator windings can be
concentrated to the stator (DC Brushless motor example) allowing even
heavier gauge wire again than the reduced turns that the much stronger
magnetic field produced by powerful rare earth magnets allows, resulting
in a considerably more powerful motor for a given volume or mass.

It's the use of rare eath magnets in those tiny motors used in drones
that allows them to achieve more useful endurance times out of their LiPo
battery packs (much greater power to weigh ratios out of what generally
makes up most of their mass).

Indeed, I'm surprised that they haven't replaced the complex mechanical
transmissions that seem to still curse most electric cars today by being
incorporated as part of each wheel where the transmission is entirely
heavy duty cable (with a specially flexible section to feed the power
past the suspension to the hub motors), reducing the 'gearbox' to nothing
more than a sophisticated program controlled heavy duty switch mode
converter. This might not suit the handling demands of a more extreme
performance road car but it should do nicely for a typical family saloon
or small runabout.

--
Johnny B Good