Re: [CAD_CAM_EDM_DRO] Constant voltage transformer

dave_ace_me wrote:

> Hi all,
>
> Thanks to you who advised me that what I have is a constant voltage
> transformer or as some called it, ferro-resonant (sp) transformer.
>
> In doing some further checking, and going by the size of the frame
> and using the sq cm as the basis, it is of the same capacity as
> the 'standard' transformer that was in the same unit.
>
> My question is whether there is any plus (or minus) of using this
> constant voltage transformer over the standard transformer.

They are heavier, dissipate a lot of heat, and sometimes buzz annoyingly.
If it produces the correct voltage output, though, it should be fine.
One thing to note, ordinary transformers have a large current surge
capability. By that I mean that they can provide 2 - 5 times rated output
for a short burst, which can allow you to use a much smaller transformer
for servo and stepper systems. Ferroresonant transformers have a
LOT of leakage inductance, that is how the regulation is accomplished,
and so they have very little surge capacity. If you try to draw more current
than the rating, the output voltage drops. This can actually be an advantage
in some cases.

> The second question is to the reason why there is a 100 ohm
> (measured) resistor across the taps of the cap. Does the constant
> voltage transformer require a minum load or something ? I would have
> expected a much smaller bleed resistor over such a large cap.
>
> The resistor is a 100 ohm, 55 watt connected directly across the
> 68,000 / 75 volt capacitor.

No, generally, the transformer doesn't need such a load, but they may have
had some reason for this, possibly safety. I suspect you can replace
it with a higher value, smaller wattage resistor without much trouble.

Jon