RE: [CAD_CAM_EDM_DRO] Re: Batteries in Power Supply

Just some thoughts ...

It was suggested that the capacitors could be eliminated if a battery was
floated across the power supply output. This started me thinking about the
battery problem.

I would be concerned about the high ripple current through the battery, and
constant switching between charge and discharge cycles (120 times per second).

I would not build my own system to work this way, and would put significant
filter capacitance on the output.

I would also used lead-acid batteries, because they are rated for
"floating" on the mains.
I would use a regulated output supply, set to the proper float voltage for
the batteries I was using.
I would make the power supply be able to gracefully and safely handle the
over current condition, and allow power to come from the batteries (current
fold back regulation).

I would oversize the batteries, making the batteries peak discharge current
be five times the expected peak servo load. [This may be too conservative,
and I could change my mind after seeing the battery prices. :) ]

My father would have built this by taking a 12.6 V filament transformer,
running it through a selenium bridge rectifier, across some caps, through a
light bulb, across the battery, and off to the load.

Since we like to use higher voltages as CNC supplies, I'd be concerned
about cell-to-cell variance when placing a large number of cells (lead-acid
or otherwise) in series. Each lead-acid cell has a voltage of about 2
volts, so you must stack 30 cells to reach 60 volts. That allows a lot of
cells for temperature variances, chemistry changes, and other nasty things
to build up and cause problems.

Some one of you who is a battery engineer will probably point out the
fallacies in what I've said. Even if some of my concerns are valid, there
may be rational ways to engineer around them.

It is an area where a little engineering up-front (temp sensors, current
monitoring, maybe a little closed-loop charging control) could save a lot
of grief later.

I'm sure it is possible to make it work. The phone company had 48V
lead-acid batteries floating across the line supply for decades, and maybe
still does. It's been quite a few years since I've been inside a phone
exchange building.

-- Carl


At 03:27 PM 1/21/2003 -0500, you wrote:

>So would this mean that you could eliminate the filter capacitor from the
>power supply leaving only the transformer and rectifier?
>
>John Guenther
>'Ye Olde Pen Maker'
>Sterling, Virginia
>
> > -----Original Message-----
> > From: Mariss Freimanis <mariss92705@...>
> > [mailto:mariss92705@...]
> > Sent: Tuesday, January 21, 2003 14:14 PM
> > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > Subject: [CAD_CAM_EDM_DRO] Re: Batteries in Power Supply
> >
> >
> > Bernard,
> >
> > I posted on this topic about a year or so ago. The main advantage
> > mentioned was servomotors usually have a large ratio between their
> > stall current and continuous rated current (5:1 or more).
> >
> > This means the power supply has to be 5 or more times bigger than
> > needed just to supply very short duty cycle accel/decel current
> > pulses to the motor.
> >
> > The idea was to use low AH batteries to supply this current; they
> > would recharge between accel/decel cycles. The battery would connect
> > to the power supply "+" terminal via a rectifier and it's voltage
> > would be a little less than the power supply's voltage. Under load,
> > the supply voltage would sag, forward biasing the rctifier from the
> > battery, which would now supply the pulse current.
> >
> > A motor stall current 5 times the max rated current implies a 4% duty
> > cycle. Theoretically a 1AH battery could supply 20A for 3 minutes,
> > way longer than you would need for accel/decel (1 or 2 seconds).
> >
> > Now the supply could be sized for the continous current only plus
> > what would be needed to restore the battery charge (5 * 4% = 20% or
> > 1.2 times the max rated motor current).
> >
> > Mariss
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bernard R
> > <bwjarandall@c...>" <bwjarandall@c...> wrote:
> > > In view of the recent interest in power supplies has anyone thought
> > > about using batteries in place of capacitors in some of the larger
> > > power supplies?
> > >
> > > If you used 5 12Volt batteries and 1 6Volt that would give a
> > nominal
> > > 66 volt, with a fully charged cell rising to 2.4 volt that would
> > give
> > > an additional 13.2Volt (33 cells @ .2V) for a total of 79.2 volt.
> > >
> > > In a recent post Les Watts reports routinely seeing 60 amp peaks on
> > > rapid starts, stops and sharp curves, I would think batteries could
> > > cope with these conditions better than conventional caps.
> > >
> > > I'm not sure if there would be any significant price difference
> > > between caps and batteries, a 100Volt 15,000uF cap is almost $30
> > and
> > > for a reasonably stiff supply you probably need at least 3 in
> > > parallel. Without doing the math, I suspect you need more than 3
> > caps
> > > to cope with 60 Amp peaks. 12V 4.5 A/Hour batteries are about $17
> > and
> > > 6V 4.2 A/H $9 for a total of $94. (12V 12 AH cost $$35)
> > >
> > > I'm not sure how effectively batteries kill spikes, I would have
> > > thought much better than caps, they should absorb overvoltage from
> > > motors quite effectively.
> > >
> > > I can see difficulties in disconnecting the batteries from the
> > drives
> > > at power off, but this should be offset by being able to use
> > smaller
> > > transformers.
> > >
> > > Just a thought.
> > >
> > > Bernard
> >
> >
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Carl Mikkelsen
Oasis Semiconductor, Inc.
201 Jones Road
Waltham, MA 02451
(781) 647 8775