Re: Transformer and voltage drop

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@...> wrote:

>
> caudlet wrote:
> > --- 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.
> The only problem with all these schemes (NTC thermistor,
> slow-start relay) is that they don't solve the problem on short
> power interruptions. That can cause a current surge that can
> pop things. So, a little bit of resistance and leakage
> inductance in the transformer is a good thing. There's no way
> you need a 1.8 KVA supply, anyway, unless you are planning on
> running a Kearney & Trecker gantry mill that weighs 10 tons.
>
> Jon

Hi, as an elctronics engineer who designs these things once in a
while I have to say that what you have got there is pretty much how
it should be.
The bit that most people miss with this kind of power supply is that
the transformer only has a very short time during the peak of the
output waveform in which to charge the capacitors. This means that if
the average output current of the load is (say) 10 amps then the
actual charging current will be in excess of 100 amps since the
changing time will be only 10% or less of the cycle. The output
impedance / circuit of the transformer and the wiring connecting the
rectifier to the capacitor is working in 100 amp pulses so if you
want to minimise voltage drop or losses you need a very heavy gauge
of connection wire and short lengths.
To fix this kind of problem there are several solutions.....none of
them will really help you though. One is to use a three phase supply
and rectifier, then there are six charging pulses per cycle instead
of two. Another is to put a DC choke after the rectifier in the lead
to the capacitor. The choke allows the rectifier to conduct for more
of the time giving a better load regulation. The catch is to be any
use the choke has to be similar size to the transformer !!
Not much help I'm afraid but there is probably not much wrong with
your kit although heavier wiring will help.

Regards Bob

>