Re: [CAD_CAM_EDM_DRO] How to select power supply???

Weyland wrote:

> Hey Jon~!
> Thanks for the help -
> Questions interspersed.
>
> From: "Jon Elson" <elson@...>
>
> > > Do you choose the power supply first, or the driver boards?
> >
> > Choose motors first (in general), then choose drivers that can
> > power the motor properly, then choose power supply to suit the
> > needs of the driver.
>
> The motors I got were the specs I posted.
> So I look for a driver now, huh?
> I haven't seen a chopper driver for something with these specs,
> but that's part of the problem. ( I don't know the voltage rating of them)

The voltage rating of the motor is practically irrelevant. If I assume
the resistance is 1.3 Ohms per half winding, then 3.5 A x 1.3 Ohms = 4.55 V
That is the voltage drop across the winding at rated current. You should
be able to run 40 to 80 V on these motors without much trouble. The
driver will handle the voltage to prevent excessive motor current.
You really DON'T want to run an RL driver on these motors, unless you
need a space heater in the shop for the winter!

> > > Do you *add* all the amperages for *each* motor?
> >
> > No. You need to make some sort of computation based on actual
> > power draw of the motors. Since almost all new drivers are swtiching
> type,
> > current into the driver is much less than current in the motor windings.
> > The current draw rises as motor speed increases. But, in general,
> > the drivers will rarely draw current equal to the rating of one winding,
> > assuming a sufficient DC supply voltage is chosen. Even if this
> > current is drawn, it will be for short bursts. Unregulated DC supplies
> > can supply peak power above the rating point for short periods.
> > Also, a slight dip in DC voltage will not affect the drivers.
>
> When you say "switching", are you saying that the driver is turning off and
> on,
> or that it's acting like a relay, and thus not handling the voltage and
> current itself?
> Sorry, trying to learn.

The driver switches on and off based on the measured current in the winding.
When below rated current it is on, when it reaches the rated current, it turns
the voltage off for a while, and lets the current recirculate through the
winding
through some diodes. It does this very fast, thousands or tens of thousands
of times a second.

>
>
> > > (for me adding three motors?)
> > > And how can I tell at what voltage these are safe to run at?
> > > 12V or 48V????? Or somewhere in between?
> >
> > This is mostly determined by the specs of the drivers. Some can
> > handle no more than 35 V, some can go to 80 or so. The driver
> > limits winding current to whatever you set it to, so you don't really
> > need to worry about DC supply voltage versus the motor voltage
> > rating.
>
> Sorry, I'm pretty ignorant in the electronics area.
> (probably other areas as well)
> Are you saying that drivers have an adjustable current setting
> so I can tailor it to the motor specs?

Yes!

> The motors are rated at 3.5A uni, and 5.0A bipolar.
> So I should look for a driver that is 5A capable (or rated) ?

Yes, again.

> So, I should look for a power supply that is capable of supplying the
> amperage, and just go with a "in the ballpark" voltage?
> If the motors are rated at...let's say 5A, and it is possible that all three
> motors will be
> engaged at the same time (three axis move) wouldn't that draw 15A
> theoretically?

Power out must equal power in plus the small losses in the driver.
So, when it is driving your motor windings at, say 5A x 0.65 Ohms (windings
in parallel for bipolar) there would only be a 3.25 V drop across the
winding. Lets assume a 50 V power supply. Power in will need to
equal power out, which is V * I = 3.25 * 5 = 16.25 W. Now, if the
particular drive scheme requires both windings to be energized at
the same time (full step always has both windings on, half step alternates
both, just one, both, just one, etc.) that needs to be multiplied by 2,
so you get 32.5 Watts (neglecting losses in the driver). Current draw
from the power supply will be I = P / V = 32.5 / 50 = 0.65 Amps!
So, you see the current draw from the higher voltage supply is
DRAMATICALLY reduced. When the motor is delivering power
at high speed, the current draw of the driver will go up quite a bit.
How much depends entirely on the motor and driver.

So, yes, it is possible that you could occasionally draw 15 Amps
from the power supply, but it would likely be for very short bursts.
A large capacitor bank could tide a smaller power transformer over
during the surge. 50 V x 15 A = 750 W, roughly a full horsepower.
My guess is that you could get by with a 5-7 Amp supply, as long as
it has some surge capability.

>
> > > I have *some* schematics available to me,
> > > but haven't been able to make heads or tails of them.
> > > Let me know what I should be looking for on them, maybe?
> >
> > Schematics of what, motors, or drivers? Without comments on them,
> > a raw schematic may be of little use. You can't tell what voltage or
> > current it can handle just from the schematic, unless it says so in
> > words, without looking up the component ratings in databooks.
>
> I think both.
> The schematics I got are found here:
> http://sisp.net/~sclark/SIGMA%20STEPPER/
> The guy that sold them to me posted them.
> Being this ignorant, I can figure out *some* of what it's saying,
> but not much, and definitely don't see anything about voltage.

I saw a performance curve that indicated some of the motors were
run with a 40 V DC source. I couldn't wait the 15 minutes for that
chart to download from a very slow server.

Jon