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Old 07-06-2012, 05:17 PM   #1
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Swamp cooler keeps blowing buss fuse on low speed


My swamp cooler keeps blowing the low speed fuse every so many days 15 amp, I have new motor 3/4 hp new belt and cage bearings has been oiled, everything seems to be wired correctly and wires tight. My neighbor HVAC tech amped it out and far below what the motor draws. The disconnect box has 2 15 amp buss fuses. One that controls the pump which I thought the high was connected to it but it's not and the other controls the speeds maybe because each fuse is lined up under ea. Switch. I disconnected the wires to it and used a wire brush and electrical cleaner as far as the connections. Neighbor says i should remove the box and wire it directly without the box. He amp it at the switch and it was only pulling 4.7 steady on the low side, Amped it at the motor side at the cooler side it was pulling about 9, but when I had to replace fuse today the box has 2- on-off switches and the one blown controlled both speeds but at the switch high still runs, I figure the high has to be connected to the high switch sinse its works along with the pump. And the other fuse low side is the one blowing. Could this box be the culprit? Need help

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I amped it out at the switch on the low side and tried turning the dial (rotary dial type)on and off to see and as soon as I turned it on with the amp meter connected it went from 49 to the 4.7 amps within 2-3seconds is this normal?
The other night I walked in and the swamp was off hadn't called for air yet on the dial..so I turned the dial to a lower temp setting and as soon as it kicked on I turned it just a hair more to stay on longer and it blew and motor wasn't hot at that point. That's why I tried amping it and messing with the dial on and off is the switch the bad guy in this?
HELP.... so confused and frustrated...I have gone through about 13 fuses
The cooler is on a 20 amp breaker with a few other circuits but we also when amped it at the switch turned on most of those circuits and still the ampage didnt change. it has been running about 1 1/2 wk up to the time I messed with the dial last night. And I had changed the dial within that week and strange thing it didn't blow. so at this point I put in a new fuse and it's running on low again. I hope this makes sense please ask if not
I would appreciate any help HELP!!!

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Old 07-06-2012, 06:20 PM   #2
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Swamp cooler keeps blowing buss fuse on low speed


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My swamp cooler keeps blowing the low speed fuse every so many days 15 amp, I have new motor 3/4 hp new belt and cage bearings has been oiled, everything seems to be wired correctly and wires tight. My neighbor HVAC tech amped it out and far below what the motor draws. The disconnect box has 2 15 amp buss fuses. One that controls the pump which I thought the high was connected to it but it's not and the other controls the speeds maybe because each fuse is lined up under ea. Switch. I disconnected the wires to it and used a wire brush and electrical cleaner as far as the connections. Neighbor says i should remove the box and wire it directly without the box. He amp it at the switch and it was only pulling 4.7 steady on the low side, Amped it at the motor side at the cooler side it was pulling about 9, but when I had to replace fuse today the box has 2- on-off switches and the one blown controlled both speeds but at the switch high still runs, I figure the high has to be connected to the high switch sinse its works along with the pump. And the other fuse low side is the one blowing. Could this box be the culprit? Need help

Additional details
I amped it out at the switch on the low side and tried turning the dial (rotary dial type)on and off to see and as soon as I turned it on with the amp meter connected it went from 49 to the 4.7 amps within 2-3seconds is this normal?
The other night I walked in and the swamp was off hadn't called for air yet on the dial..so I turned the dial to a lower temp setting and as soon as it kicked on I turned it just a hair more to stay on longer and it blew and motor wasn't hot at that point. That's why I tried amping it and messing with the dial on and off is the switch the bad guy in this?
HELP.... so confused and frustrated...I have gone through about 13 fuses
The cooler is on a 20 amp breaker with a few other circuits but we also when amped it at the switch turned on most of those circuits and still the ampage didnt change. it has been running about 1 1/2 wk up to the time I messed with the dial last night. And I had changed the dial within that week and strange thing it didn't blow. so at this point I put in a new fuse and it's running on low again. I hope this makes sense please ask if not
I would appreciate any help HELP!!!
49/4.7 seems a bit high for the LRA/RLA ratio for a motor but I'd first try getting a slo-blo fuse with a clearing time curve that can handle this startup surge.
Two or three seconds is probably normal for a high inertia load and a low starting-torque motor.

Of course you want the fuse to blow if the motor behaves abnormally so don't go too high with this time delay fuse.

Since the fuse takes a while before it fails the clearing curve of your present fuse choice is probably close to OK.

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Old 07-06-2012, 06:28 PM   #3
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Swamp cooler keeps blowing buss fuse on low speed


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49/4.7 seems a bit high for the LRA/RLA ratio for a motor but I'd first try getting a slo-blo fuse with a clearing time curve that can handle this startup surge.
Two or three seconds is probably normal for a high inertia load and a low starting-torque motor.

Of course you want the fuse to blow if the motor behaves abnormally so don't go too high with this time delay fuse.

Since the fuse takes a while before it fails the clearing curve of your present fuse choice is probably close to OK.
Or we could just size everything according to the NEC.
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Old 07-06-2012, 07:20 PM   #4
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Swamp cooler keeps blowing buss fuse on low speed


I do have the time delay fuses in there... I had talked with my brother and his son had a similar occurrence and he said its that disconnect box the contacts in the back of that box where that back of the screw of the fuse would contact the back of the box being such an old box is worn so..first thing in the morning that's coming out and direct connecting it. It kinda makes sense at start up I would assume on the vibration it's losing it's contact not even sure why I kept it in cause I still have the plugs for disconnect I also asked my neighbor if that start amp 49/4.7 is normal and he said yes at about that 2-3 seconds I will post here what I found after its been direct connected and tested .. Thanks a lot guys
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Old 07-06-2012, 10:19 PM   #5
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Swamp cooler keeps blowing buss fuse on low speed


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Or we could just size everything according to the NEC.
Pray tell, Sire, what sayeth the NEC?
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Old 07-06-2012, 10:33 PM   #6
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Swamp cooler keeps blowing buss fuse on low speed


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Pray tell, Sire, what sayeth the NEC?


this'll be good............
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Old 07-06-2012, 11:24 PM   #7
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Swamp cooler keeps blowing buss fuse on low speed


Whats the voltage of the 3/4 hp motor?
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Old 07-06-2012, 11:33 PM   #8
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Swamp cooler keeps blowing buss fuse on low speed


Gred
If you could post a model number and a mfg. we could look up some information that could help you.....of course you could do that yourself
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Old 07-06-2012, 11:36 PM   #9
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Swamp cooler keeps blowing buss fuse on low speed


I dont think that loose/dirty connections are causing your problems !
I think you need to use the correct type and size of fuse / breaker.
It is well know in the trade that motors pull a LARGE amount of
power in start up mode,
it can be quite large depending on type of motor
and the load on the motor.
This is why they use slow blow fuses / breakers.
I would try a 20a slow blow.
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Old 07-07-2012, 07:47 AM   #10
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Swamp cooler keeps blowing buss fuse on low speed


the LRA is blowing the fuses not the RLA as suggested go to a 20AMP that 2-3 seconds on start and your seeing what any motor sees 10X RLA on start,and the fuse is taking the heat and draw....are you working a line voltage 115 switch to turn that motor on..thinking the distance from the cooler to the switch isn't hepling the situation.might consider adding a contactor to limit the amperage tripping out to a switch

Last edited by biggles; 07-07-2012 at 11:46 AM.
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Old 07-07-2012, 08:01 PM   #11
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Swamp cooler keeps blowing buss fuse on low speed


I was told that 20 amp buss fuse would be to much for the motor. I thought bout that. Well I took off the old disconnect box this morning and wired it directly and will wait to see if this will pop the 20 amp breaker in the panel and if it does than I really have a problem. ....Missouri! Model num of the motor? If this runs more than wk or doesn't pop when messing with the dial then I think I fixed it but if it pops the panel breaker than Houston we have a problem. ...keeping my eye on it.. And will wait it out and see...thanks again guys
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Old 07-08-2012, 06:24 AM   #12
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Swamp cooler keeps blowing buss fuse on low speed


If its a 20 a quick reacting type
then i think you will still have the odd blow out,
you should try a 20 a slow reacting type !
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Old 07-08-2012, 07:40 AM   #13
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Swamp cooler keeps blowing buss fuse on low speed


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I was told that 20 amp buss fuse would be to much for the motor. I thought bout that.
Whats the voltage of the motor? Most likely it contains internal thermal protection since it's only a 3/4 HP motor, depending on the voltage, 20 amp could still be too low to start the motor. Take a picture of the motor nameplate, or the appliance nameplate on the unit itself.
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Old 07-08-2012, 11:59 AM   #14
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Swamp cooler keeps blowing buss fuse on low speed


You know, if you can get the fuse spec's it shouldn't be too much trouble to figure out if the presently used fuse is supposed to sufficient.
You already know the starting transient duration and magnitude and the steady-state current draw.
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Old 07-08-2012, 07:43 PM   #15
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Swamp cooler keeps blowing buss fuse on low speed


Well I removed that old disconnect box yesterday must of been up there over 33 yrs. now it's wait and see if it pops that 20 amp panel breaker and there was no way to test the old box unless I poped the rivets on it from the backside. I'm on day 2 of the direct wiring of it. So far so good...you know yoyizit I was trying to remember when I first had to replace that fuse and can't and know what size it actually was in there.
Man do I appreciate all the advise thank a lot.

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