On Sun, 14 Aug 2016 16:54:37 +0100, Fredxxx wrote:

On 14/08/2016 13:54, The Natural Philosopher wrote:
On 14/08/16 13:32, Tim Streater wrote:
In article , Tim Lamb
wrote:

In message ,
harry writes
On Saturday, 13 August 2016 20:22:17 UTC+1, Tim Lamb wrote:
These are expensive and wear out so these days it's replaced with
electronics to turn DC to AC where ever possible.

Hmm.. suppose the commutator is simply switching the current to a
different winding on the armature? Direction of flow may not change
which appears to me to be DC.

The current direction reverses as the commutator segnebt passes
under the brush.

You mean the armature is wound such that the magnetic flux generated
by the winding is reversed?

What's all this about brushes that harry is rabbitting on about.
Didn't think motors had those these days (unlike the cheapies that ran
toy electric trains when I was a nipper).

Some do, some don't.

If they are DC motors and they dont have brushes, they have electronics
doing the same job.

I agree with you, and there is a consensus here that they are still
called DC motors.

Actually, the phrase used is "DC Brushless Motor". In this case, the
motor itself is actually an AC motor but the incorporation of a DC to AC
converter which allows them to be powered from (and *only* from) a DC
voltage supply allows them to be used in place of the classic
mechanically synchronized commutated motor using brush gear (but without
the problems of commutator and brush wear and friction) hence their name,
"Brushless DC Motor".

The homopolar motor is a unique form of true DC motor where only slip
rings are required to energise the rotor without any form of commutation
being required. The major problem is their limitation to very low voltage
high current applications unless you're prepared to design them for
extremely high speeds of rotation on even very low voltage high current
supplies (say a large 2v lead acid accumulator cell or a very large NiCd
or KniFe cell).

--
Johnny B Good