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!



and you cant feed a permanent
magnet motor AC current

Yes you can.

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.

Realising that takes a lot of bull**** about the so called difference
between 'AC' and 'DC' motors away.

Really there are only two basic types of motor: those that are driven,
either electronically, or by means of commutators, with a phased AC that
synhronises to the motor speed, and those that are synchronous to a
fixed frequency AC source.

The only other variable is whether permanent magnets or electromagnets
provide the field. And if electrical whether they are series or shunt
connected.

Apart from eddy current motors, which are off the topic here.