Re: best TORQUE-WEIGHT RATIO

On Fri, 19 Oct 2007 13:01:43 +0000, colin wrote:

"Sven Wilhelmsson" <sven.wilhelmsson@xxxxxxxxx> wrote in message
news:OlZRi.11690$ZA.7697@xxxxxxxxxxxxxxxxxx
On Thu, 18 Oct 2007 15:49:06 +0000, colin wrote:

Hi,
Im considering a direct drive for an actuator.
mainly to do away with backlash and anti backlash gears etc,
presumably there will be less noise too.


---


if the force on a wire is proportional to current and field strength,
how does the number of poles affect this presuming the total wire length
remains about the same ?
yet stepper motors seem to have more torque the more steps it has.



The torque/copper ratio is not so much affected by number of poles for
a normal motor design. With fewer poles you will need more iron, however,
since the field lines become longer. On the other hand, at a fixed and
high speed, there is more iron loss for multipole motors, since there are
more field reversals per turn. So efficiency normally drops with nr of
poles for high speed motors at fixed weight.

But for maximum torque/weight at low speed, use as many poles as possible.

Stepper motors are made differently, but they are still extreme cases of
low speed and high torque.

thanks, I'm realy thinking of a stepper pancake type motor,
hopefully to get the benefit of both, the point about iron path length is a
good one,
and the speed is extremly low as there is no gear reduction.

I cant quite get my head around what makes a stepper motor more
weight/torque efficient
I know the construction of them is quite different as I have taken many
apart.


The key point is the 'air' between the cogs. It does about the same job as
copper & current, however much better. To have one tesla over 1 mm airgap
would require about one kA, and it is hard to get such a high current
through copper in this very small volume. So a 'normal' motor with 100
poles is not practical, unless the diameter is very big.

reluctance type motor is easier to see why increasing poles increases
torque.

maybe a similar reasoning can apply to the types of configuration of PM
steppers ?

Colin =^.^=



--
Sven Wilhelmsson http://home.swipnet.se/swi
.

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