Re: [CAD_CAM_EDM_DRO] Re: What makes a motor a servo? calculating power

Mariss Freimanis wrote:

>I figure on about $35 per lb for motors. Check it out:
>
>1) USDigital MS-23 single-stack 23, 1.6 Lbs, $59
>2) Vexta PK268 double-stack 23, 2.25 Lbs, $80
>3) Camtronics IH34-112 double-stack 34, 5 Lbs, $165
>4) Machine Tool Camp 34-size servo, 10 Lbs, $350
>
>It makes sense. The motor market is competetive, profit margins are
>small. The ingredients for the product are primarily copper, iron and
>aluminum, commodities that are priced by weight.
>
>

If only hobby RC motors for cars and airplanes were similarly priced.
In their case the price depends mostly on how cool the stickers on the
case look. :)

I'm curious about brushless motors. They are built somewhat like
steppers, but operate more like servos. The brushless motors I'm
familiar with from RC cars typically consist of a 4 pole rare earth
magnet mounted on the shaft, with three coils in wye configuration
(evidently delta configuration is preferred for motors that don't
normally run in one direction) and connected to a brushless controller.
The motor may or may not have magnetic or optical feedback sensors.
These motors typically have horrendous cogging as they are intended to
run at high speed.

Steppers seem to be essentially the same kind of beast, magnet rotor
with a smart controller running some coils to make the magnets chase a
rotating field around (or stand still in the same place). And yet while
brushless servos are smaller, lighter, more powerful and more efficent
than brushed servos, steppers are slower, bulkier, weaker and less efficent.

What is it about the design differences that make brushless servos good
for high speed, high power applications (everything from RC toys to
electric cars for example), and take steppers in the opposite direction?

cs