Re: [CAD_CAM_EDM_DRO] Finding servo motor voltage without specs.

tompiatt wrote:

>How does one go about determining the voltage and amperage of a servo
>motor with no specs available? The motors are unused but ten years
>old nippon pulse told me they made these for a customer and could not
>release any specs even if they had them. the motor part has 4 brushes
>and I assume the middle part is an electric brake with two brushes
>

No, most likely it is a DC tachometer. Turn the motor and see if you
get a voltage
out of this section. If so, it most likely is a tach. There would be
no need for brushes
on a brake.

>
>and finally an encoder. Any instructions would be appreciated. I
>have 3 of them. I need to find out how to control these things. I'm
>attempting to create a cnc router.
>
>

It sounds like they are DC brush motors, and 4-pole due to the 4
brushes. You apply power
to the brushes in pairs, the brushes directly across from each other are
connected together.

If you need to trace the wires, you can pull the brushes out and poke an
ohmmeter
lead into the brush holder, and see which wire is connected to it outside.

The encoder may be more difficult to dope out.

As for electrical specs, you can attach a low voltage battery or DC
supply, say 12 V,
to the motor and read the speed through the encoder or even with a piece
of tape on
the shaft, and count how many times it slaps your finger. This
establishes the
Kv, or Volts/1000 RPM. You can make extremely rough guesses at the current
rating of the motor by measuring the armature resistance. These
measurements
are very hard to do with a DVM, due to the low resistance, and the flaky
connections of the brushes. You need to take many readings and keep the
lowest one, usually. (A better measurement is made under load.) But, once
you have this, you can make a guess on the current rating using the
resistance
and the size of the motor. Assume it reads one Ohm, and the motor is the
size of a soda can. A motor like that would be likely to have a current
rating
around 10 A continuous, but that is a very rough rating. Some motors have
peak ratings MANY times that. The problem is the peak rating is usually
limited by demagnetization of the permanent magnets, and it is very hard
to figure out what current will demagnetize them without actually testing
a motor to destruction. If you can find a stock motor from the same company
that is close in size and other characteristics to your motors, that can
be a good
guide.

The Kt, or torque constant, is pretty easy to determine. You attach
an arm to the shaft, and place weights on the arm. You apply current until
it lifts the weight. You can then calculate the torque units/amp. A fish
scale can also be used.

Jon