Re: need a transformer

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Jessie" <rebel307@...>
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

Jessie: What analytical tools have you used to reach your
conclusion that your transformer is the root of your motor "acting
crazy"? Have you actually measured the primary or secondary
current? A low cost clamp-on meter will give you average AC current
in the primary leads. You might be surprised by the actual RMS
current your 6A motors actually draw. A DVM on the DC side would
let you know about voltage anomolies. To get the ripple voltages a
scope is needed.

Let's assume your current transformer is too small. A 75V
transformer would result in about 106VDC so what you really want is
a 48V transformer to give you about 72 VDC. A 25A 48 volt
transformer is 1.2KW of power. With big power comes big size.
About 20 watts per pound or 60 lbs for your tranformer.

I know it sounds logical that if a little power is good then more
power is better but having more available power than you need is
like putting a 200 hp motor on a skateboard....fun to look at and
talk about but worthless as transportation.

Actual field measurements indictate that even the 2/3's factor
quoted in a lot of posts for sizing supplies using the nameplate
current is very conservative. Toss in the fact that it's really
watts that you have to supply which is voltage times current times
duty cycle. The latter is important because the duty cycle keeps
the motor within its rated wattage range. Lets say you motors are
6A and 4.5V rated (forget how much voltage you have available) .
Math tells us that that is only 27 watts. Even at 50% efficiency
the net wattage is still less than 60W per motor. Assuming all
three motors are at 100% load its still less than 200W.

I would advise you to get real numbers before you plunk down $$$ for
a mega transformer only to find that you have invested in a nice
boat anchor.

Side note: I toured a CNC machine mfg plant recently and it so
happens they use 6A steppers on their larger machines with heavy
gantries driven by Gecko's I can tell you that they don't use a 25A
transformer or 75V of DC.

Tell us more about your current power supply and motors and how you
are interfaced to the Gecko's from the PC. There is enough
experience and talent on this list to help you get your table
running properly.

>