Re: Toroidal transformer ratings?? What after being rectified?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

> R Rogers wrote:
>
> >Thanks to all who responded. Now, what happens to this amperage

rating after it goes through a rectifier? I know the DC voltage will
increase by a factor of 1.416 due to the RMS. Does the amperage stay
the same or decrease due to the increased voltage?

> >
> >
> This gets REALLY messy, fast. The problem is the rectifier is a
> non-linear component,
> and in this application the transformer is required to deliver

short

> pulses of high
> current as it recharges the capacitor bank only at the voltage peak

of

> the sine wave.
> There are some rough rules of thumb, but they depend strongly on

transformer

> leakage inductance and the ratio of capacitor size to current draw

(or

> the DC ripple
> is another way of expressing it). Power supplies with very low

ripple

> are the worst,
> because the cap is charged for the shortest duration. I've seen

numbers

> varying
> from roughly 50% to 75% of the pure resistive rating for capacitor

input

> filters.
> Sometimes adding a small series resistance (in this case a fraction

of

> an ohm)
> somewhere between the transformer and cap can reduce the current

peaks

> without
> seriously affecting regulation.
>
> Jon

When I first started looking for components, I was thinking 21 amps.
(3 motors, 7 amps each) but after a little contemplation, I realized
how the power moves.

My motors are 1.7 volts, 7 amps. or roughly 12 watts. ( volts times
amps = watts)

at 25x for the power supply, I was hoping for a 42 volts power supply.

working backwards, that is 42 divided by 1.414 or a 29 volt AC
transformer. I had a 24 volt transformer so used that.

I used a pair of the MPJA 10 amp transformers to get 20 amps although
I supposedly will never need more than about 13 amps (66% of full
load for the Geckos)

so... 24 volts times 20 amps = 480 watts after the transformer.
then the loss of about 1.2 volts thru the bridge (23.8 volts) times
1.414 (peak voltage) = 33.6 volts.

480 watts at 24 VAC = 20 amps
480 watts at 33.6 volts = 14.2 amps, Quite a difference.
and, 480 watts at 110VAC = 4.36 amps

So, I needed to size wires according to the amp load at the voltage.

As Jon mentioned, there are peak demands for power at the top of the
AC wave, but I did not considder that as part of my sizing.

In reality, my machine runs one axis at a time so I could actually
size for one motor and scale things back quite a bit. I didn't as
the possibility I will run all three axis at some future time does
exist.

What I am unclear about is the 66% of rating for the power supplies.
using Gecko 201's, and a 7 amp motor, with a nameplate of 1.7 votls,
I theoretically need 12 watts, 24 watts if both phases need to be
included. But, at the higher voltage, the supplied wattage is HUGE
compared to the raw motor calculations.

So the selection of correct amperage for a power supply still is
confusing. Since it works, I can't conmplian too much, but still
wonder if I am vastly over sized.

Hope that helps.

Dave