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Guest
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 Posted: Tue Apr 01, 2008 9:05 am Post subject: Re: 3phase induction motor and transformer are working on th |
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On Mon, 31 Mar 2008 08:17:47 -0700 (PDT) VIDHYA <vidhya36@gmail.com> wrote:
| On Mar 27, 6:38?pm, phil-news-nos...@ipal.net wrote:
|> On Thu, 27 Mar 2008 03:10:23 GMT Don Kelly <d...@shaw.ca> wrote:
|> | ----------------------------| "VIDHYA" <vidhy...@gmail.com> wrote in message
|>
|> |news:91ad3917-1444-4d71-bb6e-27bbeb298206@s13g2000prd.googlegroups.com...
|> |> both 3phase induction motor and transformer are working on the same
|> |> principle of em induction only , in motor alone why rmf is produced
|> |> why not in transformer?
|> |
|> | First of all "rmf" is ???. ?I am not familiar with the term but one
|>
|> It's a term often spelled out by people that need to hunt and peck on the
|> keyboard, but glance up while trying to type the "e" and hit the key to
|> its right when they do.
|>
|> --
|> |---------------------------------------/----------------------------------?|
|> | Phil Howard KA9WGN (ka9wgn.ham.org) ?/ ?Do not send to the address below |
|> | first name lower case at ipal.net ? / ?spamtrap-2008-03-27-0...@ipal.net |
|> |------------------------------------/-------------------------------------?|
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| its rotating magnetic field only...
Oh, you mean "rotating EMF" ?
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-04-01-0004@ipal.net |
|------------------------------------/-------------------------------------| |
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bud--
Guest
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| Posted: Tue Apr 01, 2008 6:10 pm Post subject: Re: What happens when you blow a fuse really hard? |
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Paul Hovnanian P.E. wrote:
| Quote: | bud-- wrote:
Ben Miller wrote:
bud-- wrote:
A replacement service downtown [US]was a mere 800A. Right the other
side of the basement wall was a utility transformer vault for part of
a downtown block. The utility said the available fault current was
200,000A. All the service cable connections in the vault were "cable
limiters" - a fuse crimped to the wire on one end and a lug bolted to
the vault bussbars on the other end. I have never seen information on
the cable limiters - probably "current limiting".
Just think of them as big fuses designed for short-circuit protection only.
They are generally used on parallel sets of lines, so a fault on one line
will open its limiter, removing that line from the system. This prevents
more catastrophic damage. The service remains on, but that phase is then
running with one less conductor.
But for a parallel run the wire is still backfed from the load end
(unless there are cable limiters on both ends). The utility is protected
but I would think the customer would still have a failure.
And opening one wire puts the load on the other parallel runs (but what
better option).
Not obvious to me why there is more advantage on parallel runs than
single unless because the wires are usually larger.
Larger? The individual conductors of a parallel run would be smaller
than a single conductor.
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I did not clearly state what I mean. The wire for parallel runs is
likely to be larger than a different run that was just single. The idea
was parallel runs are near the max size wire that is economical, but a
single run is not because the loads vary from small to large.
| Quote: |
That is one reason to use cable limiters. For a single conductor (per
phase), one can protect them with a transformer primary side fuse. On
the other hand, each conductor of a paralleled group is significantly
smaller than the total service rating.
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If a transformer feeds only one set of wires, the transformer primary
fuse may provide short circuit protection. Utility transformers often
feed multiple services.
The one place I saw cable limiters was in a downtown utility transformer
vault. The 800A service I was working on was almost trivial compared to
the transformer rating. But if there was a fault on my wires why
couldn't it propagate into the vault and cause some real excitement? Why
wouldn't cable limiters (utility end only) be useful on my single set of
conductors?
| Quote: |
In the event than one of the group opens, the remaining conductors will
carry more load current, even though the upstream protection will 'see'
nothing unusual. Once the load current exceeds the cable limiters
rating, it will open. As the load current shifts to the remaining
cables, their limiters will open as well.
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Ben, and the Buss data sheets he provided [thanks], say cable limiters
are for short circuit protection. The curves on the data sheets do not
go out to a long enough time to determine what kind of overload
protection the cable limiters provide.
(The data sheets show those limiters are current limiting.)
--
bud-- |
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