roy@alanine.phri.nyu.edu (Roy Smith) (12/15/90)
We had an air conditioning failure in our machine room a couple of
weeks ago. Turned out to be the 1/3 HP induction motor in the blower had
died. Seemed like some sort of internal short, since it just sort of hummed
but didn't turn, even with no load on it, and got *real* hot. Anyway, they
replaced the motor, and yesterday, one of the 15A cartridge fuses protecting
it blew. We replaced the fuse and the A/C came back on, but we're worried
that there is more to it that just a chance fuse failure.
The motor only draws about 6A (yes, I know about startup surges), so
I think the fuses are the right size. My current theory is that when the
previous motor died, it wasn't quite drawing enough current to blow the
fuse, but "damaged" it enough that two weeks later a normal startup surge
was enough to blow it completely. Is this possible? Do fuses act that way?
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy
"Arcane? Did you say arcane? It wouldn't be Unix if it wasn't arcane!"ssave@caen.engin.umich.edu (Shailendra Anant Save) (12/15/90)
In article <1990Dec14.171020.3615@phri.nyu.edu>, roy@alanine.phri.nyu.edu (Roy Smith) writes: > replaced the motor, and yesterday, one of the 15A cartridge fuses protecting > it blew. We replaced the fuse and the A/C came back on, but we're worried > that there is more to it that just a chance fuse failure. > > The motor only draws about 6A (yes, I know about startup surges), so > I think the fuses are the right size. My current theory is that when the > previous motor died, it wasn't quite drawing enough current to blow the > fuse, but "damaged" it enough that two weeks later a normal startup surge > was enough to blow it completely. Is this possible? Do fuses act that way? Yes, fuses act this way. A very untechinical explanation would be that the repeated cooling and heating of the fuse would put enough stress on it that it would just plain break. I don't know what would happen if you left the motor running on continuously after it has been turned on, since then there would be no temp. cycling. I do know that my car fuses blow quite often, usually when I am in real bad weather. I haven't quite figured that out yet. Some related thoughts. Many applicatons nowadays no longer use fuses for engineered weak-points in the circuit. Most are now converting to circuit-breakers, which are solid-state devices, reasonably cheap when doing a lot of experimental work, where you are liable to blow a lot of fuses. These breakers do have reset switches which can do the equivalent of replacing a blown fuse. The other thing is that you said that you used a 15A fuse for taking startup surges into account on a normal 6A load. A better alternative is to use a 6A 'slow-blow' fuse. Meaning that it takes a longer time to melt and hence not blow during start-ups. I have found that to be safer. --Shailendra -- Physical: Shailendra Save, Logical: ssave@caen.engin.umich.edu 2303 Conger Baits II, UUCP: ...!umix!caen.engin.umich.edu!ssave Ann Arbor. MI 48109. Audible: 313-763-1627(H) 313-764-8033(O) ICBM: 42 33'W 83 71'N Fax: 313-747-1781 Eagles may soar, but weasels don't get sucked into jet engines. (For those who don't know, a weasel is a wolverine)
depolo@eniac.seas.upenn.edu (Jeff DePolo) (12/15/90)
Fuses can "wear out" for a couple of reasons. If a fuse starts to blow, but never blows completely, it usually ends up having a thin center portion of the fuse as compared to the ends. This means that the temperature will tend to be higher in the center of the fuse, so it will take a lower current than what it's rated for to blow the next time full current is pulled through the fuse. Also, as fuses get hotter and then cool, they suffer from thermal expansion and contraction. This can cause fractures and weakens the fuse. Also, if the cartridge isn't air-tight, the glowing phenomenon that occurs when the current is high enough to cause the fuse to glow bright red but not enough to make it melt will tend to cause the fuse material to oxidize faster, making it's resistance increase, which in turn makes it hotter for any given current flow so that eventually it will blow, even when drawing under its rating. There are probably other reasons that I didn't think of. --- Jeff -- -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Jeff DePolo N3HBZ Twisted Pair: (215) 386-7199 depolo@eniac.seas.upenn.edu RF: 146.685- 442.70+ 144.455s (Philadelphia) University of Pennsylvania Carrier Pigeon: 420 S. 42nd St. Phila PA 19104
mcovingt@athena.cs.uga.edu ( Michael A. Covington) (12/15/90)
Yes. Fuses die of old age. They heat up somewhat (but not enough to blow) under normal loads. This puts stress on them.
harding%b56vxg.dnet@kodak.com (JON HARDING) (12/17/90)
In article <1990Dec14.171020.3615@phri.nyu.edu>, roy@alanine.phri.nyu.edu (Roy Smith) writes... > > We had an air conditioning failure in our machine room a couple of >weeks ago. Turned out to be the 1/3 HP induction motor in the blower had >was enough to blow it completely. Is this possible? Do fuses act that way? >-- >Roy Smith, Public Health Research Institute >455 First Avenue, New York, NY 10016 >roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy >"Arcane? Did you say arcane? It wouldn't be Unix if it wasn't arcane!" Over thew years I have replaced many (20-30) fuses which blew for no reason: subsequent tests of quiescent and surge current were negative. It is, however, a known phenomenon that when one part of a system is replaced, other parts may subsequently go west ... hence the preventive maintenance practice of replacing related parts of systems when they fail. (Within reason, of course.) N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J | Jon Harding, N2KZJ email: harding%b56vxg.dnet@Kodak.COM | | * I don't represent KODAK by word or deed. * | N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J N 2 K Z J