Re: Field wound DC Servo

Hi Jon,
Thanks very much for getting back to me, and for your advice. I
will do the tests you suggested this morning.
I don't have the original setup, there was only one drive for the
three motors and the PLC would switch between the motor to be used,
obviously it could only move from point to point.
The original drawings show that the motor was run in a series wound
type of arrangment - for forward operation positive was applied
through one of the field windings and then through one side of the
armature and negative went to the other side of the armature. For
reverse rotation, positive went through the armature (where negative
was for forward operation)and then out the other side of the
armature and through the other field winding to negative.
I hope this makes some kind of sense to you.

Thanks again for your help.
Regards
Andrew

,

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> babinda01 wrote:
>
> >Hi All,
> >
> >I am working on a large milling machine that has field wound DC
servos
> >fitted. I am wondering if there is some way to figure out what
> >voltage to run through the fields.
> >
> Do you still have the original power supply?
>
> >I want to be able to run a constant voltage through the fields
and run
> >the armature through the servo amp, but there is no info on the
field
> >voltage or current draw. If I run these servos this way will
there be
> >trouble with the servos getting excessivly hot
> >
> No, because they were designed for that. Without any info, there
are ways
> to determine the proper field excitation. It is pretty common for
75
> and 150
> V to be used for shunt fields. What you do is apply 60 V DC to
the
> field and
> some known voltage to the armature and read RPM. Then apply 75 V
to
> the field, and read RPM with the same armature voltage as before.
Then
> try quickly with 90 V on the field. If the RPM doesn't decrease
much from
> 75 to 90 V, but you saw a substantial change from 60 to 75, then
it is
> almost certain that the motor was designed for a 75 V field.
Normally, the
> field flux should be fairly close to magnetic saturation at the
rating
> point.
>
> If the RPM changed by the same amount from 60 to 75 as from 75 to
90,
> then it is likely the motor was designed for 150 V field. Try he
> three-point
> test again around 150 V to see if there is a "knee" in the curve
right above
> 150 V. If so, that probably confirms a 150 V field.
>
> Finally, apply the voltage you think you should be using to the
motor for
> three hours. If you can still comfortably hold your hand
indefinitely
> on the motor
> frame after three hours, that is not likely to be overheating the
field
> winding.
>
> > - this machine will be
> >running pretty well continually for 6 days a week.
> >Any help will be greatly appreciated.
> >
> >
> You may want a small computer fan to cool the motor. It depends on
> how hot it gets in operation.
>
> Jon