[Milsurplus] Fuses to protect transformers

[Bob Camp]

Hi

As Peter points out, distribution transformers have for a long time used inductance to protect themselves. Back at the start of AC distribution this was not the case. Transformer mutual inductance got better and better (80 -> 90 -> 95 -> 99% ..). As it did efficiency went up and up. The accountants were happy people. At some point they found that protecting the transformers got pretty tough. Even with fusing, they had transformers blowing up. They backed off on the mutual inductance and traded efficiency for reliability. The surge current dropped and the transformers stopped blowing up before the fuse did it’s thing. 

The inductance required is fairly easy to work out. If you want a 100 ma 300V secondary to limit at 3A you need a reactance of 100 J ohms. That same reactance will cause your transformer to droop 10V from no load to full load. Just like a resistor, the device needs to handle about 900 KVA in the limit condition.  At 60 Hz, you would need about 260 mHy. The choke would need to hold up it’s inductance (not saturate) at the 3A RMS current. Typically the inductor does saturate a bit, so your regulation is worse than the 10V drop. 

Bob

On Mar 16, 2014, at 6:38 PM, hwhall at compuserve.com wrote:

> >
> You might try placing some amount of inductance (RF choke?) in the
> transformer secondaries.? I can't help you with an inductance value, but
> it's worht considering and researching.
> >
> 
> Thanks for the thought, it's novel & sometimes that leads to breakthrough methods, but I expect that an inductance large enough to save a secondary rated for just a couple hundred milliamps from a short circuit would, for 60 Hz, be larger than there is room in or on the chassis to install. It would be interesting to experiment with, but wouldn't such a choke also impede the normal current flow? Might require careful design & contruction. An RF choke would be next to invisible in its effect, I think. I really do believe that what is needed in the short run is something to disconnect the winding as fast as possible.
> 
> So far my findings seem to be that HV rated fuses exist (450, 500 & 1000 VAC) but often are not available in smaller current sizes, such as less than one or three amps. I'm not done looking, but have had to pause for another project. I am heartened by several folks who indicate the 250VAC fuses have been found to serve adequately. I may run a few through a short test to see if they part clean or flash a lot of vapor to the interior surfaces. Ought to be fun!
> 
> Wayne
> WB4OGM
> 
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