Machine Blowing a Fuse

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Little Bill

Little Bill

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Location
Tennessee NEC:2017
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Semi-Retired Electrician
I was called to troubleshoot a machine that was not working. I discovered a blown fuse. It was a "midget fuse" 250V 7A dual element, FNM-7. The only fuses available locally was a 6.25A so I replaced it with one of those. It was late and I told them there was probably something wrong to cause the fuse to blow and I didn't feel secure that I fixed anything.

Well, I was right. They called back and said it blew again. I went back when I had time and dug into it. The machine has several functions/operations. So I had them turn them on one at a time until the fuse blew. It was a circuit that operates a gas blower motor and igniter. After taking it apart I found a cut wire that was shorting to the motor housing. I repaired the wire and had them run the machine. Everything worked as it should and no blown fuse.

Now two days later they call and said the fuse blew again. They changed it and it ran for several hours and blew again. I know that I fixed that one wire and it doesn't appear that the fuse is blowing from a short this time. No, I didn't check the amp draw when I was there as it was late when I finished and everything was working. My question is, do you think it is blowing from overload since the original fuse was 7A and the replacements are 6.25A?
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
190506-2114 EDT

Little Bill:

You said the original fuse was a dual element. This I believe means it is a slow blow and it was 7 A. This should allow motor starting current without blowing.

Apparently you used a standard blow fuse and at just slightly lower current rating. A standard blow at 10 to 15 A could probably handle the starting current.

Replace with the correct fuse and see what happens. You could also measure and record vs time the motor starting currernt. This would help you see what is happening.

A blower is a high inertia load, and this extends the starting time, but it still should be quite short. If there is high bearing friction then starting time will be longer. Ideally you would use a scope to measure starting current.

.
 
M

MTW

Senior Member
Location
SE Michigan
Bill, the combustion blower on my oil fired steam cleaner, includes, attached to the blower shaft, a tiny oil pump, calls for 12A @120V.
Ignition transformer is 240VA @120V.

Likely you need a bigger fuse. I doubt that a 15A would hurt anything. Nameplate should tell.

gar said:
190506-2114 EDT
Little Bill:
A blower is a high inertia load, and this extends the starting time, but it still should be quite short. If there is high bearing friction then starting time will be longer. Ideally you would use a scope to measure starting current.
Click to expand...

Gar, you haven't been away from the bench in a long time. A clamp on could easily get the job done.

MTW Ω
 
Little Bill

Little Bill

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Staff member
Location
Tennessee NEC:2017
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Semi-Retired Electrician
MTW said:
Bill, the combustion blower on my oil fired steam cleaner, includes, attached to the blower shaft, a tiny oil pump, calls for 12A @120V.
Ignition transformer is 240VA @120V.

Likely you need a bigger fuse. I doubt that a 15A would hurt anything. Nameplate should tell.



Gar, you haven't been away from the bench in a long time. A clamp on could easily get the job done.

MTW Ω
Click to expand...

After I discovered that the fuse blew immediately when turning on the blower I didn't check any current draw as I was sure it was a dead short, which I found.
I did see some schematics (not full set) that showed the fuse @ 7A and that's what was in it. I had to order the 7A so when they come in I will see if that holds. If not, I will tell them to let me check the current on startup and running and not keep "throwing" fuses at it.

Just a little more info...........
The circuit is 120V with the 120V coming from a 240V to 120V transformer
There is a small relay on/around the motor, the coil voltage is 24V from a 120V to 24V transformer
When the switch is turned on the relay pulls in and powers the blower motor and (I think) the igniter.
The 7A fuse is in series between the 120V side of the 240/120 transformer and the small 120/24V transformer. relay, and motor.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
190506-2438 EDT

MTW:

A clamp-on may or may not tell you what you need to know. Its peak hold may be too slow to get a good indication.

I have searched for a comparison curve set for slo blo vs fast acting and could not find what I was looking for. I did not find a Littlefuse document with the curves overlaid.

Page 6 of http://www.cooperindustries.com/con...03.1766744011.1557208156-662246525.1557208156
gives some comparison.

.
 
J

jim dungar

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Staff member
Location
Wisconsin
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PE (Retired) - Power Systems
gar said:
... Littlefuse document with the curves overlaid.
Click to expand...


Just to be picky.:D The fuse company name is spelled Littel even though it is pronounced little.

The documents you shared shows 'midget' fuses.
 
FionaZuppa

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
Almost guaranteed something is tired on the load side, and it's drawing more amps, or some amps for too long, probably at startup.

It sounds like one of those "been fine for years; now all of a sudden; we didn't change or touch anything", scenarios. This typically indicates aging load gear.

Diagnostics has to be amp clamp on inrush mode all of the parts involved. Are any of the motors cap start?


As far as Littel fuse, I always write "little" because I have always just dealt with little/small fuses, none of the monster HV 200A+ ones.
 
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