Re: [CAD_CAM_EDM_DRO] Re: Transformer and voltage drop

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