Re: [CAD_CAM_EDM_DRO] Transformer and voltage drop

Anders Wallin wrote:

> Hi All,
>
> I'm assembling a PSU for my servo setup.
>
> It's a simple design:
> - AC input, fuse, 2-pole switch, inrush-current limiter
> - 1.8 kVA toroidal transformer w. 2x30 V secondaries
> - rectifier bridge
> - four 10 000uF caps
>
> I'm testing it with various loads, and the DC voltage drop I'm seeing
> looks surprisingly big. This is all explained in my blog at
> http://www.anderswallin.net/2007/06/1800-w-80-v-psu-for-servos/
> (scroll down, there are pics in the comments too!)
>
> Testing only the AC side (no rectifier and no caps) gives me a voltage
> drop of around 0,7 V for every amp of current. But with the rectifier
> and the caps connected I see 1.9 V of drop, which sounds quite big!
>
> any ideas on what is going on here? Should I be worried, or is this
> typical for large toroidal transformers?

Hmm, this seems strange. Now, the DC may droop a bit from no
load to small load, and then level off. Leakage inductance may
cause the no-load voltage to be a bit above the predicted 1.414
X AC voltage, and as soon as you apply a load it drops to the
predicted value. I have seen this.

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?
Yikes! OK, assuming this is all copper loss in the secondary (a
total worst-case, and unlikely) you would be dissipating 21 V at
30 A or 630 W in the transformer. Have you tried this? (Hmm,
you'd need many toasters to build such a load bank.) I would
suspect if this was the case, you'd have the transformer smoking
within a minute. Is this a Plitron transformer? They have a
line of inrush-limiting transformers that have a high leakage
inductance designed into the transformer to eliminate the inrush
surge no matter the line phase when you turn it on. Since you
mention the inrush limiter, you don't need (or want) this
feature. It will act this way, although this seems like a bit
much for such a rating.

Jon