Re: [CAD_CAM_EDM_DRO] Servo Brushes

Ozzie@... wrote:

> Why do some servo motors have two brushes and others four, and some
> none?????

Motors with no brushes (if indeed they are servo, not just AC motors or
steppers)
would be either DC brushless (with a permanent magnet in the rotor) or
AC
flux-vector (which are really AC induction motors designed to operate in
a
servo system).

The difference between two brush and four brush motors is the field
arrangement.
with 2 brushes, you have two field poles, with 4 brushes, you have 4
field
poles. On the 4 pole motors, brushes opposite each other are connected
together, either at the motor or at the amplifier.

Now, there is also the DC motor/DC tach arrangement, that has what looks
like
a big motor with a smaller motor piggybacked. This may have 4 brushes,
but
they won't form one circle around one commutator. Some of them are
built using
the same permanent magnet field poles and armature poles, but with a
long
winding section for the motor, and a short winding section for the
tach. These
will generally have 2 brushes on each end of the motor. If wires are
still
attached, the motor wires would be heavier gauge than the tach wires.
If no wires are left, the armature resistance of the tach would be much
higher
than the motor.

I think I covered most major types of servo motors here.

Jon