[Milsurplus] Fuses to protect transformers

[Richard Brunner]

All true, but, put the fuse in the primary of the transformer. The current 
goes up just as fast as in the secondary and the fuse is in it's natural 
environment, low voltage and ac, and is reliable. There was a report of a 
guy using minifuses to fuse high voltage, and it worked until it didn't.

Richard, AA1P

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
>