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Bob Engelhardt

wrote:
I've got a couple DC servo motors ... I would
like to know if there is a way to figure out what max voltage and
current might be. ...

Eric, I'm having the same problem with a motor I'd like to use as a
servo on my lathe feed. I've come to these conclusions:

- the max current is determined by the motor's insulation. It has a
class (A, B, F, or H) which sets the maximum temperature that the
insulation can handle (for a given life). Without knowing the rating,
you (we) would have to assume the lowest (class A), which has a
temperature rating of 105C. The max current, then is that which causes
the motor to heat to 105C (loaded).

The empirical approach might be: apply a voltage that gives a
"reasonable" no-load speed. Apply a load & measure the current. Allow
the temperature to stabilize, monitoring to avoid 105C. Increase the
load & repeat until 105C is reached. This is the maximum (steady) current.

There is also a maximum peak current, which will demagnetize the
magnets. I have no idea how to determine that.

- I'm having a bigger problem with the maximum voltage. It seems that
the voltage is more of a dependent variable. I.e., a function of the
speed desired and load. There is a maximum speed based upon mechanical
limits & the maximum voltage is what produces that speed, with a given
load. So, under the maximum load (i.e., for the maximum current), the
maximum voltage will be greater than the maximum voltage under no load.

There is a voltage limit based upon insulation break down, but I think
that it is far greater the the speed limiting maximum voltage.

I'm not very confident of these conclusions, so I'd be glad to hear some
feedback.

Bob
--
Nota for President