Re: [CAD_CAM_EDM_DRO] Servo Motor Identification

Dr Brian H Le Page wrote:

> I have just gone from having no servo motors to having 8! My problem is
> that I cannot find out the electrical characteristics of any of them.
>
> The first two came from a Calcomp plotter, the type with a 3 foot or so pen
> movement, possibly a 4XXX. The motors are labelled:
>
> controller. They are about 2 inches in diameter and about 4 inches long,
> excluding the encoder.

These sound a little small for a machine tool. Might work for a Z axis,
however.

> There are no labels on the motors. Hand written on the flange mounting
> plate is:
>
> ST = 14.0 (possibly lb ins)
> SC = 7.1 AMP
> BT = 1.8

>
> If the SC = 7.1 Amps written on the NEMA 34 motors means stall current,
> could I lock the armature and measure the voltage to get to 7.1 Amps, or
> would this destroy the motor?

No, it should not harm it. But, you should be aware the voltage might be
very low. You may need extra series resistors to control the current
with enough precision to get it to 7.1 A so you could make the measurement.
All this does, however, is measure armature resistance, which you can do
carefully with a plain DVM. (You have to wiffle the shaft to get the best
brush contact.)

To get the important motor constants, you can run one amp through the
motor and measure torque with an arm and weight, or a drum, string and
weight. You turn the motor at a known speed, and measure generated
voltage. The first measure is Kt - torque in your favorite unit per amp.
The second is Kv, voltage per 1000 RPM. Combined with the resistance,
you can pretty well characterize a servo motor on paper with these
measurements.

With a permanent magnet motor, the final measure you want to have is the
peak current before the magnets are damaged, and I don't know a way
to get this without harming a motor.

Jon