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