Re: Power Supply for 4-axis CNC stepper driver

--- In CAD_CAM_EDM_DRO@yahoogroups.com, John Dammeyer <johnd@a...> wrote:

> I'll put in a 'that depends'. If you want to build 10,000 of

something then

> designing to the wire is important for cost considerations.
>
> The motor driver expects a DC supply. You create one by using a
> transformer, rectifier and filter assembly. Power supply design is an
> engineering stream all to itself. For example. One big cap isn't

as good

> as a bunch of smaller ones in parallel. That's because capacitors

get warm

> if they are constantly charged and discharged. The OSMC motor

control group

> has had some interesting posts about that. Since each cap has an

ESR this

> resistance results in heat. Parallel resistors means less heat per

unit.

> But really each cap also just stores and releases less energy too.
>
> If the caps are really large, often it's a good idea to put a

temperature

> dependant resistor in series with the transformer. When cold, it

has high

> resistance and prevents a massive load on the transformer when the

supply is

> first turned on but when hot has almost no resistance and therefore

drops

> minimal voltage. That allows you to fuse the transformer closer to
> operating characteristics rather that power up surge.

I think that most of us are only looking for enough information to
make one machine work well enough. and then use E-bay parts with
questionable histories and mis-matched parts. acetpting that the rule
of thumb is really only to get us in the same parts of the city as the
ball park, if not in the ball park itself. I have seem some that were
in never-never land.....

back to the quesitons at hand....

> There's the 80,0000 formula (can't remember off the top of my head) for
> determining what size cap you need.

Mariss posted it as (80,000 times total motor current) / power supply
DC voltage. that is for 60 cycles, 100,000 for our 50 cycle friends.

>What you're really after is defining
> ripple voltage. Think about how rough the motor would run at high

speeds if

> one step had 60VDC applied to it and then the next step had 30VDC

applied to

> the motor. It would sound and run rough. So 30VDC ripple is not

what you

> want.
>
> So we aim for what? 5% (that's 3VDC) or 10% (that's 6VDC).
>




> What you are aiming for is a power supply
> that can provide 60V into an
> inductor. Using the step function concept,
> at the instant the switch is
> closed, the voltage across the
> inductor is 60V. (Across a capacitor it's
> 0V). After time t, the current through
> the inductor is infinity and the
> voltage is 0. (Across the capacitor
> the current is 0 and voltage is 60V).
> I'm ignoring resistance here.
>
> For the inductor,
> the higher the applied voltage,
> the faster the current
> tends toward infinity or the shorter
> the time t. So if you want to maintain
> that 60V drive (+/-5% ripple),
> then your supply at time n has to be able to
> deliver 6A.

here is a part I may have missed or
gotten wrong.
At time n, the voltage would be about 2.4 volts
and current would be 6 A. ?
I have not allowed for anything
to go from zero to n
If the potential has to counter any stored
energy or any genrated energy in the inductor
then that would have to be accounted for.


I gotta run, kids in the car......

I'll pick this up and read it a few times when I get back.

Dave