Re: [CAD_CAM_EDM_DRO] Re: P.S question

Your analysis is correct for the time period when the xformer is suppling
current, but to get true RMS current for 1/120 of a second,(full cycle of
the current waveform) you need to include the "dead time" when the xformer
is not conducting to.This reduces the RMS current considerably. Also the
heating that is the basis of xformer rating also includes Core losses in
addition to I^2R losses.


John H. Berg
Berg Engineering, LLC
1133 Kensington
Grosse Pointe Park, Michigan 48230

Phone: (313) 999-1227
Fax: (313) 881-6421
E-Mail: berg1133@...

----- Original Message -----
From: "curious925072002" <ferguson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, June 19, 2002 3:04 AM
Subject: [CAD_CAM_EDM_DRO] Re: P.S question


> Dear Mariss,
>
> First let me say that I have enjoyed reading your posts
> very much. I am starting at groud zero with CNC and am
> very interested in it.
>
> Sizing transformers for electronic power supplies is indeed
> a tricky question. It was answered very well a long time ago
> by a Mr. Schade (O. H. Schade,"Analysis of Rectfier Operation",
> Proceedings of the I.R.E., July 1943, pp. 341-361.) This
> paper may be hard to find now but I have seen his graphs
> reprinted several times--one being in one of Motorola's books
> on rectifiers, I believe.
>
> Your volt-amp analysis does not apply here because the current
> in the transformer is a train of rather sharp pulses--the
> diodes only conduct during the brief time that the secondary
> voltage exceeds that in the capacitor and that voltage does
> not drop very far. The heating effect on the transformer is
> proportional to the square of the current so we need to first
> square and then average and then take the square root. (I
> like to show this to my calculus students as a place where
> they really need to know how to integrate :-) ).
>
> There is a simple rule of thumb that comes out of all this:
> for a full wave bridge rectifier looking into a capacitor
> for the filter multiply the DC output current by 1.6-1.8 to
> get the rms current rating for the transformer. Of course
> the transformer people know this very well. You can see the
> last page of Signal Transformer's catalog for this and other
> rules of thumb. In pdf form it is at:
> http://www.signaltransformer.com/
>
> The upshot then is that I would not draw more than 6 amps out
> of the circuit in question. It will put out 10 amps for a while
> but you are running the transformer way out of specs. This will
> show up in excessive temperature rise and early outright failure.
>
> Sincerely,
> Barry
>
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> > Hi,
> >
> > That's kind of a tricky question. Your transformer is rated at 24VAC
> > and 10A. That is 240VA (volt-amps). Your DC voltage is 1.41 times 24
> > or about 34VDC. That would be 340 watts at 10A. There is usually
> > enough reserve in the transformer to come near the 10A rating. What
> > matters is how warm the xformer gets. If it gets hot, you are asking
> > too much of it. The voltage will stay near 34VDC though.
> >
> > Mariss
> >
> > --- In CAD_CAM_EDM_DRO@y..., "afogassa" <afogassa@y...> wrote:
> > > Hi, All
> > >
> > > If I have a 24v trans. and a bridge ret. and a 10000uf elet.
> > > it gives me about 36v.
> > > now if the transformer is rated at 10amps. should I get 36v. up
> to
> > > 6amps. or only 24v no matter how much courrent is being pulled out
> > of
> > > it?
> > > In other words, will the Voltage be steady at 36v. till I get to
> > > the maximum courrent of 6 amps? then it starts to go down to 24V?
> > >
> > > Thank's
> > > Fogassa.
>
>
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