Re: electrical advice needed

> My question is, will that be 67VDC @
> 4A? Or would that turn into 67VDC @ 2.8A, so as to conserve the 192 Watt
> rating ?

Well, in the absence of any of the really smart guys answering, I'll take
a shot at this.


The full answer is WAY more complicated than first appears. I'll try to
explain a bit of it. ( There's good news at the end, though )

In actual fact, at 4 amps, it will be 48 volts !

In a capacitor input filter ( the only kind we see these days ), the
unloaded voltage of a supply rises to the peak value of the AC waveform,
which is roughly 1.4 * the RMS value.

At very low voltages, the forward drop of the diodes needs to be
taken into account, but at these sorts of voltages, we can more or less
ignore this error.

Remember, this is for an unloaded supply. Things change a whole lot for a
supply that's actually doing genuine work. Remember too, that a variable
voltage is being used to charge that capacitor. At it's greatest, that
voltage is (in your case ) about 67 volts. At it's lowest, in everybody's
case, it's 0 volts.

Current, as it is drawn out of the supply, is first supplied by the capacitor.
The voltage on the capacitor begins to fall, as a function of several things.
The two most important considerations are

More current drawn, lower voltage out.
Larger capacitor, higher voltage out.

There are a number of other factors, but those are enough for now.

So long as there is sufficient charge in the capacitor, the capacitor will
be supplying most of the current. Only during those times in the AC cycle
when the voltage out of the transformer is equal to, or greater than, the
voltage across the capacitor will the transformer actually contribute any
current to the circuit. At such times as the voltage is greater, it will
both supply current to the circuit as well as charge the capacitor.

It does these TWO things just to screw us up, and make the calculations
more difficult.

This all means that as the current demands increase, the voltage decreases.
At roughly the rated current of the transformer, the DC voltage out will
equal the rated AC voltage, and it does so because it was designed that way.

In a theoretical world, the DC output voltage would always be 1.4 * the
AC voltage. Unfortunately, this would require 100% efficient transformers
wound with superconducting wire, connected via superconductors to a nearby
fusion generator. And oh yeah, capacitors the size of Jupitor, made
from thin sheets of pure gold suspended in a vacuum harder than
intergalactic space.

So, to keep it simple and "real world",

0 amps = 67 volts
4 amps = 48 volts

and in the middle is in the middle.

Assuming you are driving a simple resistive load,
like a heating element !

Now comes the weird part. But it's also the good news part. When used
with a modern "switching" controller, such as Geckos or Camtronics, the
stepper motor itself acts as both another storage device ( a wee bit
capacitor-like ) and also a transformer. These weird traits are what allow
us to use a 67 volt supply with steppers rated for 2 or 3 volts.

Steppers don't really care ( within limits ) about the voltage. They
care about the current.

It is the function of the stepper controller to "fill em up" with current, as
fast as possible, without over filling them. The higher the voltage, the
faster they "fill" and the faster they turn.

There are a number of other limitations to stepper speed, but voltage is
the biggie.

The magic is in preventing the overfilling, and that's why we spend the money
on fancy controllers.

Now, how much power does the stepper require out of the power supply ? Well,
a pretty good approximation can be had by multiplying the stepper's rated
current by its rated voltage. My pretty good sized NEMA34s are rated at 5 amps
at 2 volts = 10 watts. Let's be pessimistic engineers and double that
figure .. 20 watts. Now let's double that again, as we remember that there are
two windings and so we're up to 40 watts. Your transformer can easily power 5
of my motors.

And that's the good news. Steppers today, using modern electronics, need less
power than we would intuitively believe.

Now, how much power from the AC line ? Figure a crappy transformer to be
75% efficient. Take the whole 192 watts, add 50% because you're paranoid
and you're still well under three amps total. More likely, you're well
under two amps.

Hope this helps.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta