Re: Transformer and voltage drop

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Anders Wallin
<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?
>
> thanks,
>
> Anders
>

.7 Volt/amp seems too high. That means at 20A you drop 14V!!!!

There will be some IR losses in the transformer core. Do you have the
inrush current limiter bypassed? With the typical Thermal type in the
circuit it has low resistance at higher loads BUT not zero so it would
contribute to the voltage drop. Being on the primary the percentage
would be higher. Also if you have multiple primary windings the
transformer will be designed to use the primary windings in parallel
at 120 or in series at 220VAC. At 120 you pull twice the primary
current and IR losses will be 4 times as much using only one winding.