Re: [CAD_CAM_EDM_DRO] 90v DC motor as servo?

Chris Stratton wrote:

> I'm curious if someone could comment on how well one of the 90v DC
> brush motors (designed presumably for SCR speed control) would operate
> as a servo motor?
>
> Alternatively, what are the basic properties a motor needs for
> positioning servo use? Is 'cogging' bad?

Yes, cogging is bad, as it introduces a discontiuity (or at least an additional
external fluctuation) into the control system. There are really two kinds of
'cogging'. Reluctance cogging can be felt when the motor is turned without
power (at least in a PM motor). That is a variation in the magnetic circuit
caused by the armature poles cutting in and out of the field at the field
magnet poles. The fix for this is to 'twist' the laminations of the armature
when it is built, so the magnetic circuit is constant at all angles of rotation.
The second kind is velocity ripple, which can be seen by connecting the
motor to a very low voltage source (around 1-3 volts) and watching the
shaft to see if the speed varies. The twisted laminations help greatly with this
problem, as well.

Since many cutting operations require the machine to feed at very slow speeds, a
motor that runs smoothly down to very low speeds is a plus for servo work, as
the control loop doesn't have to compensate for these variations.

The other thing is that many motors have the brushes optimized for rotation
in one direction, and may not perform well if the motor is reversed. So, a motor
designed for bi-directional operation is just about a must. Also, series-wound
motors are not reversible (without opening them up) and so are generally
unsuitable.

Jon