Kevin wrote:
The Natural Philosopher wrote:
Kevin wrote:
The Natural Philosopher wrote:
Fred wrote:
"The Natural Philosopher" wrote in message
...
robgraham wrote:
Why is that when you are at one's intellectual worst that the more
complex of discussions occur - solution of the world's problems,
etc !!!

This particular one revolved round ac and dc motors - covering
single
phase v. 3 ph induction motors and their relative power /torque
capabilities and how they work, which was hard work 45 years after
university, but what triggered the whole discussion was how Bosch
can
claim that a lawnmower with a 36v Li-polymer powered dc motor can
have
the same capability as a 1700w ac motor. OK 1700w is a comfortable
2hp but single phase induction motors are not too good on torque,
and
if I remember that is where dc motors score but are modern ones
going
to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

certainly 36v and about 50A is not unusual in the largest 'DC'
model aircraft motors. Mind you there is no such thing as a DC motor.

They are all 3 phase. - a DC brushed motor simply uses a
commutator to generate the AC..

??? In a DC motor the field is DC.

So what?

Its still a three phase motor with the poles switched on one (or
two) at a time via a 2 brush system on a Nx3 commutator element.



The commutator is to ensure the field
in the rotor is aligned wrt the stator to provide continuous
torque. Not sure where you get 3 phase from.

Each winding gets an AC signal. Each winding is switched at 120
degrees phase to the next one: If I told you that and told you
nothing else, you would say 'ah, a 3 phase AC motor!!'


I think you are wrong a dc motor is not a 3 phase or ac , some motors
might have three poles but that only 1 configuration , is a five pole
motor 5 phase? and a 7 pole 7 phase?

I cannot think of any motor that does not have a multiple of 3 actual
windings. But I suppose its possible. So OK, they are 5, 7 whatever
phse motors!

you have not done much research, most model boat motors are 5 pole along
with trains
http://www.zscale.org/articles/fivepole.html

but they still are not 3 phase and no dc motor is, as there are not 3
rotating magnetic/electrical feilds there is only 1 field that is
switched in sequence by the commutator


and you cant feed a permanent
magnet motor AC current

Yes you can.

lets see you feed ac into any normal aero style model motor

Well that's what the electronic controllers do.

Three wires coming out. Hard to connect DC to that and get it to work.
Put a scope on any two pairs and you will see AC. Put a scope on all of
them and you will see three phase AC, albeit with a lot of trash due to
throttling and commutation, but motor inductance smooths that out somewhat..





I have about 9 of them myself.

so to call a dc motor ac is simplifiing things to far



I think not. All electric motors have alternating current in the
windings.

are you sure its alternating and not just feeding one set of windings
then the next etc

Yes, in the sense that later on the current gets fed backwards to the
winding that was forwards last time.