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!!'




From a standard single pole pair DC motor
I would call it "single phase". 2 pole pairs on a DC motor is common place
for starter motors. I haven't come across with 3 pole pairs with 3 sets of
brushes?

The simples motor is a single phase. But its not self starting.
Therefore the simplest that works is three phase with two magnet
poles.It ameks no dss as to whether the current is originally DC with
permanent magnets, or AC with a field winding. It will work.

Unless you actyally go for synchronuus motors, the actual AC that drives
the MOTOR as opposed to a high frequency ripple on the magnetic fields,
is generated by brushes and commutators or an electronic switch.






If you are sticking 1700W in, its got to come out as heat, noise or as
shaft power.

I remain unconvinced about power in and useable power out for mains motors.
At present I would have hoped green thinking would have had some meaningful
measure of real power out, not just turned into heat.

On a mains motor, the frequency of commutation is nor far off mains
frequency for typical motors speeds - about 3000 RPM say. So there is
not much extra iron loss using a mains motor or a permanent magnet motor
or an induction motor off DC.

All are capable of better than 90&% efficiency if built well, or as low
as 50% if stamped out in thousands in Taiwan, and operated well off
decent efficiency points.


There is still the
concept that the more power in the better the motor! Generally small
induction motors are very inefficient, DC motors are inherently very
efficient. AC (brushed) motors are less efficient than DC and the best
thing that could be done would be to rectify the mains first.


I dont see why they should be really.

I doubt efficiencies are any less on a brushed PM versus induction motor,
..


yup


Rob