Carrier 5 amp fuse blowing

[Bill26]

I have a Carrier 5 amp fuse that blows on an indoor air handler heat pump. Any subjestions? Thanks Bill26

[HayZee518]

well........ the system was designed with a 5 amp fuse for protection. find out what's blowing the fuse. could be something in the control circuit. try a six amp fuse but go no higher.

[jnaas2]

If the 24 volt transformer is going bad it can blow the fuse, or a shorted wire or lose connection on the control board

[LazyPup]

I doubt very seriously if a 120/24v or 240/24v stepdown transformer could produce enough load to blow a 5a fuse. In almost all cases when a stepdown transformer fails the failure is in the secondary side.

I would be more inclined to believe that the motor bearings are dry and causing the motor load to increase as it runs or you have a run capacitor that is breaking down under load.

On both end bells of a motor there is a small diameter tube from the outer circumference of the motor to a reservoir surrounding the motor shaft. On the ouer end of the tube there is a tiny plug in the tube. In most instances the plugs are red plastic but on some motors they may be a small metal insert that looks like a rivet and may be painted the same color as the motor body. Inside that reservoir there is a felt wiper surrounding the sleeve bearing. The flt retains the oil and the bearings are porous bronze. The oil from the felt wiper passes through microscopic pores in the porous bronze sleeve bearing to lubricate the motor shaft. The oil ports should be lubricated with fine machine oil at least once a year, although in the real world they generally go unattended for years unless the unit is annually serviced by a trained technician, and even then there is no guarantee they are oiling the motor bearings.

In some instances if you catch a motor when it first starts to reduce RPM and oil the bearings you can save the motor, but generally if the problem has gone long enough that it is causing the bearings to partially seize under load and trip the fuse or breaker, it is highly unlikely that oiling the bearings will save the motor.

You can check the motor bearings by first checking the motor data plate to see what is the correct amperage for your motor; then attach a snap on amp meter to the common wire from the motor and measure the actual amps as the motor starts and runs. There should be a momentary spike in amperage as the motor first starts. That spike will normally be about 3 times the rated amperage for the motor, but the start amperage spike should only last about 1 to 2 seconds, then the amperage should drop down to the rated amperage and remain steady at the rated amperage. Continue monitoring the amperage, if it begins to climb significantly after the motor has run 10 or 15 minutes either the bearings are dry or you have a run capacitor that is breaking down under load.

NOTE: The amperage rating on the motor data plate is computed to a theoretical line voltage, however the line voltage at your location may vary significantly from the rating on the motor. Example, the motor may be rated at 125v 2a but when you measure the actual voltage at your location it may only be 110v. (line voltage may vary from 90 to 125v).

To get the most accurate measurement of the motor amps you must compensate for your local line voltage. Begin by multiplying the voltage x amperage rating on the motor data plate to determine the watt rating then divide the watt rating by the actual measured line voltage and the result will be the correct operating amperage for your motor at your location at the time the test is being made.

From our example;
the data plate is 125v x 2a= 250w.
The actual line voltage at your location at the time of the test is 110v so the correct run amps for the motor at this location should be 250w / 110v = 2.27a.

If you oil the motor and allow about 1 hour for the oil to saturate the porous bronze sleeve bearing before testing and it is still showing high amperage either the pores of the bearing are by now obstructed with dirt and debris or you have a bad capacitor.

The make capacitor testers that can test a capacitor under load, but they are rather expensive and generally not a tool that a homeowner would have access to. Capacitors are relatively inexpensive so when in doubt, change the capacitor. If that does not resolve the problem you will have to change the motor. On the upside, when changing a motor it is recommended that you always change the capacitor with it, so even if changing the capacitor did not resolve the problem you are not out any money because you should get a new capacitor for a new motor anyway.

[HayZee518]

this all above may be well worth the reading but I have never seen a load [i.e. a motor] fed off the control side of a control transformer. the motor comes off the line side of the circuit. the OP says a 5 amp fuse is blowing. A quarter hp motor draws 1.5 amp when its running. I would look into the control circuit for a burnt up control relay coil or a reversing valve IF it's 24 volt!

[LazyPup]

Quote from the original post:

"I have a Carrier 5 amp fuse that blows"

Nowhere does it mention anything about a control transformer or the control side of the transformer.

Jnass made the first mention of a control transformer:
"If the 24 volt transformer is going bad it can blow the fuse"

Unless there is a direct short in the control transformer I don't think it is capable of creating enough load to blow a 5amp fuse,

On the other hand, as you mention, the motor would normally draw well under 5amps, but it is the only load capable of exceeding 5amps unless there is a direct short in the circuit.

[lespaul32]

You always want perform a process of elimination, so starting at the fuse, tracing the wire back to the transformer checking the connections and check its primary and secondary voltage.. please note that u must Never Use a fuse rated higher than the specifications.. Even though it most likely will not hurt a thing, the engineers have it rated for a purpose.. people are guessing as to what the problem is although they are good guesses, even the best technician cannot guess the problem all the time. tracing the wires and using the manufacturing schematics to determine voltage ratings. fuses blow due to a surge / short wire / bad breakers/ or corrosion on connections.. it is somthing small i am sure and good luck in correcting the problem...

[HandymanTrainer]

As a factory trained Carrier technician, my first step would be to eliminate any shorts in the t-stat wire by simply removing them all from the control board. Then, with the power to the furnace off, I would use a continuity tester to check from each one of the two terminals the fuse plugs into to ground. If either one shows a short to ground, you know you have a control board that needs to be replaced. If there is no short to ground on the board, then process of elimination sends us to check the stat wires and stat for shorts. IN ANY CASE the five amp fuse located on the control board is strictly for protecting the transformer in the event of low voltage shorts. Never replace a board fuse with any other amp rating then that which is factory installed.

HandymanTrainer

[tennesseefix]

Need some Carrier expert assistance please

Hello, I have an old carrier a/c split unit model 40vu004300. The inside air handler appears to have "missing fuse" as I unscrewed the little panel and it appears that either one or two fuses are possibly missing from the panel. I tried to find an owners manual to no avail and tried to find an image so I could see if indeed what I am seeing is missing fuse/fuses. Could someone please verify that it is missing fuse/fuses and if so what size it needs. I'm dying of a heat stroke and have very limited funds so I'd very much appreciate your help!

[HayZee518]

I would install two glass 5 amp fuses to start. seems odd that a motor load would be protected by such a small amperage fuse. motors draw 125% of nameplate amperage when they start, then go down in amps when they are spinning. could say its a direct short when they start, that is why most a/c use a time delay fuse. see what happens. I'll go do some poking around.

[paul52446m]

I would install two glass 5 amp fuses to start. seems odd that a motor load would be protected by such a small amperage fuse. motors draw 125% of nameplate amperage when they start, then go down in amps when they are spinning. could say its a direct short when they start, that is why most a/c use a time delay fuse. see what happens. I'll go do some poking around.

You and lazy pup are way off base. The unit will have a 10 to 15 amp fuse.
The 5 amp fuse that the OP is talking about is on the electronic board and it is used to protect the 24 volt only. In most units they are only a 3 amp. fuse.
You have had a couple of guys saying how to start testing. Like they said inspect and test all low volt wires. Then you have to start checking each 24 volt load, one at a time for pulling too many amps. I would bet he might have to have a tech. find the problem. Paul

[HayZee518]

the OP is talking about a control fuse. I know a window unit will take a 15 or 20 amp breaker or fuse. you said it yourself although you said a 3 amp fuse.

[HandymanTrainer]

I have a Carrier 5 amp fuse that blows on an indoor air handler heat pump. Any subjestions? Thanks Bill26

Disconnect all thermostat wires from the control board, replace the three amp fuse, and power up the unit...if the fuse still blows, the problem is the control board. If not, replace all the wires back on the control board one at a time, cycling the power off in between, Or you can twist them all together at the air handler end and co to the thermostat end, and begin testing for open circuit,, then separate them all and check for short in thermostat wire. Often there is a spare unused wire in the sleeve to replace the shorted wire with.

Noah

[HayZee518]

if you had the control circuit diagram you'd see how the fuse is connected in the circuit. the blower is usually 240 volts, the compressor is 240 volts, the reversing valve is 24 volts. Control relay coils are 24 volts. Thermostat is probably 24 volts.

[ned]

blown 3 amp 24 volt fusenin carrier air handler

I recently got my thermost and system going bybreplacing the 3 am fuse in the air handler....it lasted three days and has blown again...any ideas why the delayed failure? Thanks Ned