Re: [CAD_CAM_EDM_DRO] Re: mill servo conversion ?'s

Roy J. Tellason wrote:

>On Thursday 05 August 2004 11:22 pm, Stephen Wille Padnos wrote:
>
>
>>Roy J. Tellason wrote:
>>
>>
>>>On Thursday 05 August 2004 11:57 am, Jon Elson wrote:
>>>
>>>
>>>>For performance. To get full peak torque, you need to deliver 12 A
>>>>to the motor. With a 36 V power supply, you can't deliver 12 A to a 4
>>>>Ohm motor, even standing still. At 900 RPM, the back EMF of the motor
>>>>is approximately 36 V, so you can produce zero torque! This severely
>>>>limits performance. I have a client in town who has an 800 Lb Grizzly
>>>>style machine with these motors and a 36 V supply, and the performance
>>>>is pretty weak. To get full peak torque at rated speed, you need 36 +
>>>>12 * 4= 84 V minimum, appled to the MOTOR itself. You need some margin
>>>>for the modulation scheme of the servo amp. too. So, a 120 V supply
>>>>sounds good, but if using Gecko 320 drives, 75 V or so is the limit.
>>>>With a 5 TPI screw driven directly, you get 180 IPM, which should be
>>>>plenty for most users. But, if belt reduction is used, or if the screw
>>>>is finer than 5 TPI, then the max rapid rate goes down, even with the
>>>>higher voltage.
>>>>
>>>>
>>>This part of your post I don't quite follow. How are you figuring out
>>>what the back EMF of a motor will be? I feel like I'm missing something
>>>here...
>>>
>>>
>>That's a rating that usually appears on the label of the motor as Ke.
>>It will be in Volts per Radian/Sec or Volts/kRPM.
>>
>>
>I guess all the (small) motors I have on hand here must be missing that info
>or something, as I've never seen that anywhere.
>

Yeah - I think motors on the surplus market and possibly the "small"
motor market don't have all the info. Of course, different motors have
different subsets of "all" the information. I have an Ametek from
Surplus Center that has only the part number (found on the web only at
the Surplus Center) and the nominal voltage on it.

It can be measured though. If you have an encoder or a tachometer on a
motor, you can drive it with a variable bench power supply. Make sure
there's no load, and turn up the voltage to some level. Note the RPM
from the tach/encoder. Do this at another point or two, and then fit a
line through the points. The line chould go through (0,0), and the
slope is Ke. (of course, it's easiest if you record points like 1000
RPM, 2000 RPM, etc :)

You can try taking the max terminal voltage and dividing by the max
speed, but this isn't foolproof. I have Baldor motors that list 150V as
the max voltage and 2500 RPM as the max speed, which would give
60V/kRPM. The Ke listed on the spec plate is 40V/kRPM. Go figure.

- Steve