Re: Transformer and voltage drop

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Anders Wallin
<anders.wallin@...> wrote:

>
>
> Hi Jon, everyone,
>
> > But, for a 1.8 KVA transformer, the 0.7 V drop per amp seems way
> > too much. I'm assuming you must be running the 30 V AC
> > secondaries in series. 1.8 KVA at 60 V AC should be 30 A!
> > So, at no load, you read 60 V AC, and at 30 A you read 39 V?
>
> Yes, the secondaries are in series. I can't load it up to full power
> (30A) with the resistors, lamps, and ovens I'm using for load testing,
> but the trend is there...
>
> > Since you
> > mention the inrush limiter, you don't need (or want) this
> > feature.
>
> Good advice, thanks. I removed the current limiter, and the AC load

test

> now gives a much more reasonable voltage drop of about 0.3 Volts per
> amp. That matches the ca 0.3 Ohm or so resistance that can be measured
> over the secondaries with a multimeter.
>
> But I still see a big voltage drop when connecting the bridge rectifier
> and caps and loading up the psu:
> http://www.anderswallin.net/2007/07/transformer-test/
>
> I tried this with two different bridge rectifiers just to be sure...
>
> Please tell me I don't have to invest in some expensive PFC circuit

like

> suggested in some comments...
>
> thanks,
>
> Anders
>

It would appear from you curve that the DC numbers start to approach
the AC curve at higher loads. What you are seeing is a typical curve
for an unregulated supply. At light loads it sits at peak AC values.
As the load increases the DC will slump but not in a linear fashion.
You provide your own PFC as you load the supply and current is drawn
from the primary across the full waveform. It would be more
meaningful to measure the change from 20% load to 100% load.

I don't know what motors you are going to run with this beast but you
will find that under normal operation and cutting the RMS current will
be pretty low.

The inrush limiter is a form of NTC thermistor. It's resistance drops
with current (and heat). It never goes to zero thus the added loss.
A better arrangement is to have a power relay with a delay timer in
parallel with the inrush limiter and short it out after a few seconds.
No loss limiting.