Re: [CAD_CAM_EDM_DRO] Re: Power Supply Caps and Xformer

kmslinda wrote:

>
>
>I am by no means an expert on transformers, but the instructions
>I received for a 1400 VA transformer kit from Toroid Corp of
>Maryland, states the the DC seconday Max Load is 880 Watts. In
>all the posts I have read concerning power supply size, no one
>has ever stated that power is lowered when rectified to DC. Is
>this correct?
>
>Below is a chart from their instruction sheet:
>
> Max Load VA DC Max Load Watts
> 80 60
> 200 140
> 400 260
> 700 460
> 1400 880
>
>

Yeah, this is another of those "fine points"! It depends on the way the
transformer was rated. Some are rated for capacitor-input rectifier
service, and no derating is required. Others are rated at pure RMS,
and the rating assumes a perfectly resistive load. The current pulses
that are drawn to refill the charge in a capacitor-input rectifier are of
short duration and high peak current. This current shape distortion
causes excess heating in the transformer windings, as power is I SQUARED
x R.
The transformer you mention is clearly an RMS-rated transformer,
and a correction factor is provided. But, it depends on the nature
of the capacitor filtering. If a very large (value) capacitor is used, then
the current pulses will be huge. This will be the case in big audio amps,
where the DC ripple from insufficient filtering will turn into hum from the
speakers. If the very minimum capacitor is used, the peak currents will
be much lower. Most servo systems tolerate a few volts of ripple quite
well. The only downside to using the minimum capacitance is that if there
is a small power glitch, your servos may stall and ruin the part. I have
rather massive capacitors in the servo system of my main mill, and I have
had the room go totally dark to the point that I reached for a
flashlight before
they came back on. The computer, servo system and the spindle motor's VFD
were all continuing right through the interruption, much to my amazement
and RELIEF! (I have no UPS as our power is very reliable here in MO.)

There are some tricks to using large capacitor banks without the damaging
current pulses in the transformer and rectifier. Mostly, they rely on
putting
just a little resistance in series with the output of the bridge
rectifier. A few
tenths of an Ohm can do wonders to reduce the peak current from hundreds
of amps to tens.

Jon