On 20/04/17 10:51, The Natural Philosopher wrote:
On 20/04/17 10:17, Tim Watts wrote:
On 20/04/17 08:13, The Natural Philosopher wrote:
On 20/04/17 08:10, Tim Watts wrote:

More poles = slower and higher torque, less poles = higher rpm.


What utter ********.

Perhaps you should check your facts before you go slagging people off:

https://en.wikipedia.org/wiki/Dahlan...changing_motor

I'll accept an apology when you've read that.


I have no idea if this method is used by any car motors but I think
some
trains do - you can hear the "whirrr... drop pitch whirrr" which I
can
only explain as pole changing as the motor is otherwise directly
connected to the axel.

However, I'm pretty sure most traction has at least one set of fixed
gears as motor rpms tend to be higher than desired wheel rpms.

Well even there you would be wrong. Its depends on how many poles the
motor is built with (as opposed to driven).

I could explain the theory, but you wouldn't understand it would you?


Perhaps you could try - or is it beneath you?


People would simply say 'too long, didn't bother to read it' and go back
to believing simple fairy tales as per usual.

Lets just deal with one aspect. Number of poles. There is a given amount
of iron you can cram into a motor, that, plus the magnets you use
(unless they are electromagnets) determines the amount of energy *per
magnetic reversal* you can get in and out of the system

This leads to a simple rule: the more revs (reversals) the more power.
Until other factors limit you, as high frequencies start to impose their
own extra losses, as does friction.

Adding more poles allow for more reversals per shaft rotation. It is
equivalent to a gearbox. However only using *some* of those poles nets
you nothing, because you are simply using less ironwork. Its pointless.
No one does it. It is simply like having a smaller motor that weighs more.

Electric motors are UTTERLY different from other motors.They dont really
have a 'power rating' re se. Only when fed from a given voltage.

I've run motors designed for 3V, generally used at 6-7v, at 11-12 v and
got more than three times the power out of them. The limitation was
brush bounce and wear in that case, and the availability of gearboxes to
get the insane RPM down to something usable..


In practical terms a multipole motor is bigger and more expensive than a
two or three pole motor* BUT if the gearbox you need to use is more
expensive and heavier...then the multi-pole wins slightly.

Some of the model motors used to come with kevlar wound rotors, to stop
them exploding. More revs, more power.


That's part of the simple story. The more complex story involves
understanding copper losses (resistance) iron losses (hysteresis and
frictional motor losses) plus issues with the controlling electronics
(peak currents, inductance, switching losses and so on). And it it uses
permanent magnets, the field strength, weight, and performance versus
temperature of those as well.

Each one of which is a book chapters worth.

*two magnetic, three wound poles is the most basic uni-directional motor.



OK - fine. But you haven't explained why it's "********". I've found
references to train traction using, at some points in history, pole
changing configurations.

Also the type of motor described in the Wiki was an induction motor so
no permanent magnets.

Most EMUs these days use induction motors, even on DC lines so the power
electronics is there either way.

I'm not sure how having a larger number of lighter stator windings is
worse than 2-3 large windings either. Would any large traction motor
really use a 3 pole configuration - it seems unlikely - the starting
torque would suffer.