Re: quicky fusible link for fan motor protection

[DeWitt Harrison <de@aztek-eng.com>]

At 08:06 PM 9/16/99 -0400, you wrote:
>Thanks for the pointer, DeWitt.........Along that line, I was glancing
>through a Crutchfield catalog at a friend's house, and saw 70A fuses and
>fuse holders listed in the amplifier section. I think they use them on huge
>trunk-mounted amps that power mega speakers that can be heard blocks away.
>They were called Hi Power fuses. Maybe custom audio stores carry such
>products. Maybe they show up on internet audio shop websites.

Sounds like a possibility if the fuse holder / terminal design is suitably
temperature rated and designed for under-the-hood service. If not,
reliability will be an issue. False tripping is a nuisance at best and maybe
dangerous.

>Another source might be an industrial construction materials outlet that
>sells components for electrical systems in factories.  The voltages are
>different, but essentially what we want is a link that blows when subjected
>to a thousand watt power load.  (80A x 13v = 1,040W). I don't see why a
>fuse for a 120v circuit that would blow under a 8 or 9 amp load wouldn't do
>the same thing (8.67A x 120v = 1,040W).

Fuses and circuit breakers are the subject of an entire engineering specialty
but maybe I can provide a little basic insight into their application.
Fuses open because they melt from heating and they are heated by
current flowing through their small but finite resistance. It's the current
squared times resistance rule. Prior to their melting and opening, fuses
have no knowledge of the voltages available in the protected circuit.

The voltage rating on a fuse reflects the fact that interrupting a high
power circuit, say 80A x 220 V = 17.6 KW, is a hell of a lot more
exciting fire-and-brimstone-wise than interrupting a 12 V circuit.
The stuff needs to be big and designed to contain and suppress the arc
across the terminals as the circuit opens. It is not correct to infer that
the power required for the fuse to operate is the product of it's current
and voltage ratings. High voltage stuff is bulkier and more costly than
low voltage stuff in order to handle the energy during current interruption
when the voltage builds across the melting fuse element. _Then_ you
are dealing with big current times big voltage. In the world of heavy
industry, workers have been killed by the explosive force of an
unsuccessful electrical switch opening operation.

>I visited such a store last week and saw counters full of fuses and fuse
>holders and circuit breakers of all sorts, mainly for 120 and 240 volt
>systems. The place was A-1 Rental, which turned out to be a contractor
>supply house that also rented equipment to contractors. Phone is
>1-800-648-0168.
>
>Another question is "why wouldn't a 1000W circuit breaker do the same job?" 

Again, the fuse begins to open on the basis of its current rating. Whether it
opens successfully depends on its voltage rating which must be equal to or
greater than the circuit voltage. If you use a fuse with a voltage rating much
higher than the circuit voltage, you are, for one thing, spending more money
than necessary. In the present case, you may also have an item which doesn't
like to live next to a motor and behind a heat exchanger. For example, a circuit
breaker may open prematurely if it is pre-heated above it's normal thermal
environment.

This leads into how fuse operation is characterized. Engineering data sheets
for fuses generally provide a chart which shows operating time in seconds
vs. current in amperes. It is a highly nonlinear curve because the fuse element
is thermally connected to the outside world through its housing, terminals
and the wire connections to the terminals, etc. The current rating on the
fuse indicates that at a certain ambient temperature, in a standard lab
fixture, it will open within X seconds or minutes at the rated current. Its
a pretty sloppy number. Ideally, there is a very large difference between
the normal operating current and the fault current in a circuit so that
the fuse rating will fall comfortably above the first yet well below the
latter. I'm real sure the fault current in our fan circuit could be way
more than 80 A, perhaps limited by the battery. (How many "cold
cranking amps" does your battery deliver?) I guess there would be a
fraction of an ohm in the wiring harness too.

>background give it some thought and maybe do some testing. 

You asked some interesting questions. Did I just hear you volunteer?  :^)

>Doyt    87 5kcstq, without fusible link yet.

For 3 bucks, you can buy one of those Buss fusible links from your
car parts store and slap it onto the back of your fan motor while
you decide what you would like to do long term.

>>>>>>>>>>>>>>>>>>>>..
>At 01:52 PM 9/16/1999 -0600, you wrote:
>>Regarding the well known seized fan motor and attendant electrical
>>meltdown problem on the 5000/200 cars: the standard solution to
>>the fusible link retrofit on the type 44 cars has been to install the
>>Audi bracket and terminal box for the 80A link a la the '91 200.
>>
>>That's still the best way to go as far as I can tell. However, I have
>>recently noticed that there are some credible fusible link products
>>displayed on the shelves of my local generic car parts stores these
>>days. Buss makes a couple of such products. [ ... ]