RE: [CAD_CAM_EDM_DRO] Re: Power Supply for 4-axis CNC stepper driver

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.

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

What's n? That's the time it takes for the inductor voltage to reach 2.4V
and is based on the inductance, the resistance, the back emf and the
position of the armature.

Remember, at the time that you are reaching 6A another motor may be at time
0 and wants to see 60VDC (+/-5% ripple).

Once the chopper part turns off the driver transistor, (another step
function) current coming out of the supply stops or reverses as the motor
field decays. When the current decays enough, the chopper turns it on again
and current reverses once more and _continues_ at 6A.

And through all of this, the scope is your friend and your DVM lies to you.

As for throwing the percentages around they are usually just for rule of
thumb back of the envelope calculations.

The transformer supplies an RMS value of say 42V. My understanding is that
the manufacturer provides the rated amperage at that voltage. Put a DC load
on the transformer with the appropriate size filter and you'll bet pretty
well 0V ripple at 42V at X amps. A bigger cap won't make any difference, a
smaller cap will show ripple on the DC rail.

So instead of specifying a 60VAC transformer, filters and regulating it to
exactly 60VDC we use the fact that the steppers in fact don't draw full
current at 60V. In fact they draw at most (full step mode) 12A at 2.4V
which is well below the 42VRMS rating of the transformer. But they do need
12A.

Hope that makes more sense.

John Dammeyer

>
> > I'll stop here.
>
>
>
> no, wait....
>
> The premise is that the transformer is designed for a resistive load
> and should be sized 1.8 times for the inductive load.
>
> then times 66%
>
> but, is that for the AC or for the DC ?
>
> four, 6 amp motors times 43 volts AC = 1,032
> four 6 amp motors times 60 volts DC = 1,440
>
> Wouldn't the DC filter caps decouple the load to some degree ? or is
> that what the 66% is for ?
>
> Dave
>
>
>
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