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

Roy J. Tellason wrote:

>On Thursday 05 August 2004 11:57 am, Jon Elson wrote:
>
>
>
>>>If they are, where is there a better deal? And why
>>>isn't it more widely publicized? Also, why do you recommend a 80-120V
>>>power supply for the Ameteks when camtronics only sells a 36V for them?
>>>
>>>
>
>
>
>>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...
>
>

This is a VERY important motor parameter. The two main parameters are
Kt and
Kv. Kt is torque/amp. Kv is the voltage at 1000 RPM, and is generally
measured
by spinning the motor at 1000 RPM and measuring the voltage, so the
armature current
is essentially zero. If it is not clearly stated, it is often given in
some less specific
manner, such as 900 RPM at 36 V, no load. That is pretty close to the
Kv value
which looks like about 40 V/KRPM. Even if NOT given, I happen to KNOW these
particular motors, as they are UBIQUITOUS! UPS is receiving 50% of its
income, it seems,
from people trading these same motors back and forth, ad infinitum! (I
am exaggrating
slightly here :)

Jon