Re: [CAD_CAM_EDM_DRO] Wound vs PM DC Motors

jmw@... wrote:

> Upon close examination a couple of my recent DC servo motor
> acquisitions (for X and Y) appear to be wound field motors and not
> permanent magnet as claimed by the seller. Inside they look like the
> generator on my old MG, with field coils wrapped around a shoe. I
> foresee a potential problem because my servo amps specify permanent
> magnet (brush) DC motors.

No problem! A properly designed shunt motor behaves exactly like a
properly designed PM field motor. The field flux can be considered
as totally constant throughout the normal operating range of the motor.
If you exceed some threshold armature current the armature flux will
overpower the field flux, and the generated back-emf will be reduced.
In a PM motor, this is getting dangerously close to the point at which
the magnets are permanently damaged. In a shunt motor, the performance
of the motor declines at this point, but it will not cause damage if it is
a momentary condition.

Now, as long as these motors have totally separate connections for the
field and armature, you should be OK. This will be the case for all true
servo motors. The difference between a shunt and series field motor
is that the shunt winding is designed to drop a pretty large voltage
at low current. The series field winding is designed to drop a low
voltage at high current (equal to the armature current), and would be
harder to excite properly from a DC source.

> My thinking was that the motor can't tell if its stator is a
> permanent or an electro-magnet. But apparently it's not that simple
> when the field coils are not independently energized. The makers
> plate on the motors says "SPLIT SER". My Golden Book of Motors
> discusses the basics of series wiring of field coils, but doesn't
> discuss split series.

Oh OH! This is NOT a servo motor. Generally, split series is a
way to make a series motor run at several different speeds
form a constant voltage supply, without series resistors.
The series field winding has taps at several different points,
and you apply power to one of those field taps. One end of
the field winding connects to one of the armature brushes.

> Apparently wound DC motors can, depending on configuration, require
> additional amplifier functionality to switch the field coil polarity
> for reversing, etc. My amps don't appear to be equipped to do this.

No, you don't do that in a servo system, as it takes too long to
change field flux, due to the inductance. Flipping field polarity
on a traction motor drive may be desirable as less current needs
to be switched. On a servo drive, you need to flip the direction of
armature current quickly to keep the system balanced. So, the
servo amp needs to be able to provide both polarities. A PWM
drive would use a full bridge set of switching transistors to do
this.

> Would it be possible to re-wire the field coils so that they are
> independently engergized at a constant, unswitched level so as to end
> up with a virtual PMDC motor with nameplate torque and speed
> characteristics?

Yes. But, if this is really a series-wound motor, the series field
may want 10 A at 2 V or so for proper excitation. That could be
a mess. You might want to rewind the field coils for something more
like 1 A at 50 V. Get me more info from the nameplate and I'll
try to give more specific advice.

I am using GE shunt-field motors on my CNC Bridgeport conversion,
and they work very well. (They are even nice to lean against on cold
nights in the shop!)

Jon