Re: Torroid Question

Hi Chris

I would aim at 80 degrees or less.
3 identical transformers all wired in parallel would give you 27 amps at 44 volts ac which would theoretically rectify to 62 volts DC at full load. If the load current is somewhat less than the rated load then you will likely get a higher output voltage as the capacitors will lift the mean voltage higher in the rectified sine curve.

Regards
Bob Thomas

--- In CAD_CAM_EDM_DRO@y..., Chris L <datac@l...> wrote:
>
> Thanks Bob,
> You make sense here.... Sure couldn't determine much without a long term
> test. Obviously, If I ran it for a full day at moderate temps, that
> would be pretty good.
> I assume that we don't want to rely on "Fan Cooling" for a little more
> ooomph.......
> Do you have an opinion on what is a good target temp ?? Caudlet
> mentioned 80C I believe....
>
> Thanks,
> Chris L
>
> echnidna wrote:
>
> >Chris,
> >If you use Caudlets method to establish a safe working current you need to allow about an hour from memory so it fully heats up. but you must monitor the temp all the time and stop as soon as it gets too hot, let it cool , then restart at a lower temp. Its generally better to start too low and increase the load (current) gradually. You could tape an old car temp guage to the coil for a better temp indication.
> >
> >Bob Thomas
> >
> >--- In CAD_CAM_EDM_DRO@y..., "caudlet" <tom@t...> wrote:
> >
> >
> >>>If I can get some bigger numbers, maybe I can use these, otherwise
> >>>
> >>>
> >>they are starting to look like landfill material. (Ok, recycleable
> >>material.....) Maybe they are 2000VA !??
> >>
> >>
> >>>Thanks,
> >>>
> >>>Chris L
> >>>
> >>>
> >>If you have some time Chris, you can do what we used to do with
> >>unmarked and military transformers.
> >>
> >> Measure the wire diameter of each winding. You will need to find a
> >>wire chart (type in "wire chart" in Google; use a copper wire chart)
> >>that will give you circular mils and cross that over to a standard
> >>wire gage. Using 750 circular mills per amp, calculate the suggested
> >>amperage. That gives you some real rough ball park numbers to start
> >>with. For example you can run .5 amps through a #24 wire (pretty
> >>small). Now hook up the transformer to full AC (use insulated
> >>connections). Using the secondary voltages, calculate a load
> >>resistance to give you the amps you want to test (R = E/I) You will
> >>have to have some pretty good sized load resistors to give you the
> >>load. You may have to get creative and use Auto headlamps in series
> >>or a similar type load. I actually used to use heating elements. An
> >>AC ammeter on either side of the transformer can be a big help, but
> >>you can do it all with a multimeter and ohm's law (E=I * R). Watts =
> >>I*E =VA. If you design up a 3A load at 44VAC the dissipated power is
> >>132VA(watts). At five lbs each and the size you listed I would
> >>expect them to be at least 100 to 150 VA units....just a guess.
> >>
> >>The objective is to run the unit long enough under load to determine
> >>the temperature rise of the core. Once they get too hot to hold your
> >>hand on for more than a few seconds, (80 deg C) it's time to crank
> >>the amps back. The temp will rise slowly because of the transformer
> >>mass and cool slowly as well. If it rises to a reasonable temp and
> >>stays there you are probably operating within the range of the unit.
> >>Your load WILL get hot and may have to be fan cooled. You may have
> >>to make several runs at the test using different loads to get a
> >>profile.
> >>
> >>The wire diameter is usually a pretty good indicator. Look at this
> >>way. A transformer designer wants to build the smallest most cost
> >>efficient transformer he can. He starts out with the required input
> >>voltage, input frequency (60HZ) and output voltages and the worst
> >>case current draw. The secondary current dictates the primary
> >>current which in turn dictates the size wire you have to use on
> >>each. Then using another formula you calculate number of turns you
> >>need to keep a given core area of a given core type for saturating.
> >>Now you calculate the "fill" of the window area for the selected
> >>core. Can you get all the wire on the core you selected? If not you
> >>have to adjust the core and recalulate the primary and secondary
> >>turns.
> >>What all of this points to is that wire diameter predicts winding
> >>losses and the size core you need to fit everything together. It is
> >>wasteful to have too large a core by making the wire size larger than
> >>it has to be and small wires will not carry enough current to dictate
> >>using a larger core to stay out of saturation.
> >>
> >>You can tell a lot about a transformer from the physical size and the
> >>wire diameters.
> >>
> >>Bottom line: If you burn up one of your surplus transformers below
> >>the current you need then its no big loss.
> >>
> >>One last note of caution. Make sure you have a piece of metal under
> >>the transformer to protect your bench top and fuse the primary at 2X
> >>the worst case predicted current draw. Make sure no voltages can
> >>short to the metal.
> >>
> >>
> >
> >
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