🚀 go-pugleaf

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
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 Physical:  Shailendra Save,      Logical:  ssave@caen.engin.umich.edu  
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