Re: [CAD_CAM_EDM_DRO] need a transformer

Jessie wrote:

>I am having all kinda of trouble with my setup. My steppers are acting
>crazy but I am sure I have rounded my trouble down to the transformer.
>I dont think its putting out nameplate amps. I am running three 6amp
>steppers. I wouldnt mind having about a 25amp transformer that will
>output about 75V. I am running my steppers through geckos, have
>noticed (still a novice with stepper controls) that voltege makes a
>slight difference but putting resistors on the geckos and limiting the
>amps makes a major difference. Does anyone have an old transformer
>that will do what I need? 25amp or so around 75V? send me a private
>email if anyone can help me with one. I dont get a chance to look
>through every post due to my work hours. rebel307@... is my
>email. Shoot me some lines with some prices.
>
>

Why do you feel the transformer is the problem? What is the DC voltage at
the capacitor when the drives are enabled, but not moving? What is the
DC voltage when the motors are moving? If the DC voltage doesn't drop
much, the transformer is not likely to be the problem. The current setting
resistors MUST be matched to the motors for them to work properly at all.
Leaving the resistors out, except with the largest motors, will damage them,
as the drive will pump out 7 A peak, no matter what the motor's actual
rating is. Running 7 A through steppers rated at 6 A doesn't sound too
bad, but is it possible you are using the full winding, and the motor's
6 A rating assumes only half the winding is powered at a time? This is
a very typical way of rating them, and if you run 6 or 7 A through the
full winding, it will saturate, and maybe even damage the magnets.

25 A at 75 V is 1875 W, WAY excessive for the typical stepper system.
The greatest current I can imagine a Gecko stepper drive drawing at the
6 A setting is about 8.5 A (6 A per winding * 1.414 for the combined power
to the two windings as 90 degree sine waves). But, that would be delivering
full torque at a speed where the voltage applied to the stepper windings was
75 V. This is not a reasonable scenario. The true current draw of the
drives is FAR less, and will be under one amp when moving at slow speed.
I think the highest current draw will be no more than 3 or 4 amps under the
worst case acceleration and mechanical load, and that will be a transient
condition, in most cases. A transformer capable of delivering an average
current of 10 A should be more than adequate for any milling application
using steppers that a Gecko drive can handle, and maybe a 15 A transformer
for a high speed router that has a well matched drive, motor, and linear
motion system.

What value capacitor have you used for the DC supply? Have you read the
instructions for the drive (you don't mention model) as to external
capacitors
and wiring? What software are you using for the step pulses? How carefully
have you tuned the settings of the software?

Jon