Re: [CAD_CAM_EDM_DRO] Question for Jon was Re: Servo size for Bridgeport?

Walter Luikey wrote:

>Hi Jon,
>
>
>The Servo's I have are AMETEK. There is no HP values given on the
>name plate.
>There is just the following:
>38 VDC nominal
>Peak Current Rating 12 Amps Max.
> They were sold to me as having 600 oz- in. capacity.
>
>Are these big enough?
>
>

Ah, yes, the hideous Ametek motors. If these are the ones with the ~ 4 Ohm
armature resistance, I have some doubts. On the other hand, some people
are using them and think they are fine. I'm running 75 V motors with
a DC supply of about 69 V, and run up to 6 or so amps peak. 38 V at
12 A is about the same power, but there's a problem. If these are the 4
Ohm motors, then 12 A * 4 Ohms = 48 V. So, with a 38 V power supply,
you can only reach 9.5 Amps. If you turn up the power supply voltage
quite a bit, you can get up to full current, but you need a lot of voltage
to do it. For instance, to get the 900 RPM speed of the motor AT peak
torque, you need 38 V for the back EMF, plus 48 V for the IR drop =
86 V. That is way above the rating of a Gecko drive, for instance.
Also, with such high resistance, 12 A squared * 4 = 576 Watts. Obviously,
576 W can only be dissipated in this motor for a fraction of a second
every minute or it will go up in smoke.

If these are different motors with a lower resistance, then they may be a
better choice for this use.

Jon