Re: [CAD_CAM_EDM_DRO] Field wound DC Servo

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