JLLN200 fuse and DC motor

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
About once per year we replace a 200 amp fuse that is associated with the DC power wiring to a Civil Defense Siren. Have since a year or two after installation.
Normal run time for these are three minutes or less on Alerts, fire or unit calls. It does pull the full 200 amps. 400 amps would take the fuse at about 100 seconds.
About a $1200 service call if we have to get a lift up there to inspect the motor itself. We will meg it from the controller up today or tomorrow, but I would hazard a guess we've done it before.

Any thoughts?
 
Does it blow when first energized or after running for some time?
What's the local weather when it blows?
Does it sound the way it should before blowing?

I'm wondering if there's some condition that binds the motor and drives up the current into the trip zone.
 
zbang said:
Does it blow when first energized or after running for some time?
What's the local weather when it blows?
Does it sound the way it should before blowing?

I'm wondering if there's some condition that binds the motor and drives up the current into the trip zone.
Click to expand...
All good questions.
We give it a short run test at fuse replacement. Maybe we will let it run the whole three minutes this time.

We aren't called until the neighbors surrounding the siren, mention to the Fire Chief that they haven't heard it for some time. The County tests them once a month but obviously no one sits there waiting at 5:30 PM on the last Thursday of the month.
 
How is the DC derived? Plain-ol' rectifier, across the line? SCRs, soft-start? M-G set?
Have you looked at the dynamic current draw? Perhaps the current rises after it's been playing for a minute or two, or surges each time it's facing a particular direction?
(I'm guessing it would be a trifle challenging to listen closely for mechanical parts binding)
Is there another identical unit "nearby" that you could inspect and measure its current draw?
 
I don't know much about the field. It is my understanding that there are some "moving parts" involved. Is the system under a service contract? I'll assume it is a mechanical siren. A lot more info is needed to venture a guess.

I would start by determining if it is a wiring/insulation issue or if the equipment is drawing more current than it should due to a mechanical problem. I'm inclined to think mechanical. Now if it is an electronic siren that changes things. If you have batteries (and capacitors) associated with chargers or AC to DC conversion all of those have a life expectancy and can cause high currents.
 
drcampbell said:
How is the DC derived? Plain-ol' rectifier, across the line? SCRs, soft-start? M-G set?
Have you looked at the dynamic current draw? Perhaps the current rises after it's been playing for a minute or two, or surges each time it's facing a particular direction?
(I'm guessing it would be a trifle challenging to listen closely for mechanical parts binding)
Is there another identical unit "nearby" that you could inspect and measure its current draw?
Click to expand...
We service most of them in the County, or at least the ones we installed.

Transformer to 48 then rectifier across the line.

Well Dang and double dang. Nominal load is 100 amp. RTFM. !@$#%!

eta: I thought this had posted, but I guess not.

Nothing worked but the rotation When we got there. Siren control and power fuse where both blown. The contacts of a relay base on a printed circuit board had finally quit their intermittent contact over the years. That interposing relay controlled the contactor for the siren. It always functioned while we’re there, of course.

Inrush is the fastest setting we have on our meters.
Did some field engineering to make it run. 600+ at start, dropping eventually to 100 while in that mode.
Simple record shows run at about 50.

Intermittent starting at 600 amp would not take long too blow the 200.
 
It might be useful if you could remotely monitor whether a fuse has blown but without activating the siren. This might be done by turning on the DC power supply and measuring the output voltage from the fuse. Or it could be done with a small DC supply and a series resistor to apply a current through the fuse, rectifiers, etc. to check whether the fuse is intact (in which case the voltage from fuse would be small but non zero due to diode drops).
By such monitoring you might tell if the fuse blew during the latter part of an extended siren activation but was not noticed until the next the time it was needed. Also checking could be done on a more regular basis to verify that the system is ready to respond when necessary instead of finding out after the fact. Whether doing this would be practical in your situation I don't know.
 
Yep, when I was a kid, the local VFD (volunteer fire...) used one about a block away to call in the crew. I think they stopped using it in the late 1970s.

Just like living next to RR tracks, most people filter it out within a week or so.
 
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