Furnace failure to run

I took apart a window motor that failed to run and drew too much current. I happened to notice the gaps between commutator segments were black. On closer inspection, the black was carbon from the brushes - a conductor shorting out the motor! Easy, no cost fix, very satisfying.

One thing that you could relate to an air flow issue is the fan blade spinning on the motor shaft.
The fan could have been prevented from turning by some type of debris inside the fan housing.

BJean wrote:
I have read the replies and the motor does not have a flat spot but I have encountered this before on other items. It appears that I have a air flow switch problem and will address is Monday.

That is a standard 12 volt Circuit breaker. Either your motor was pulling too many amps or you had a direct intermittent short. A BAD motor can run and still intermittently pull too many amps causing the CB to trip. Air flow has nothing to do with it. Your furnace is 6 years old, the motor may be bad. On your model, the 12 volt power goes to the CB from the furnace On/Off switch. The output side goes direct to the Ignition module and a tstat wire to the wall tstat. IF the wall tstat wire was a short, that will trip the CB but will usually burn out the wall tstat as the CB is a higher amp than the connections on the wall tstat. The odds are you Motor is intermittently pulling too high amps causing the CB to trip. Doug

Air flow should not have anything to do with the motor circuit breaker tripping.
A flat spot can be hard to find unless you tear the motor apart. and look at the commutator closely and test the armature fields.

I have read the replies and the motor does not have a flat spot but I have encountered this before on other items. It appears that I have a air flow switch problem and will address is Monday.

The 12V DC power from your dist. panel goes to that furnace CB/on-off switch then to motor.

They are a heat sensitive device to protect motor....circuit.
They are of a slow blow style.
Heat from furnace or short amperage spikes will not cause them to trip.
The motor will have to produce an excessive and prolonged amperage draw to trip it.
When it trips, a problem exists.
Voltage and amp readings should be taken after resetting.
You could have a power source, breaker or motor problem.
(Low voltage/higher amp draw------weak breaker/on-off switch or failing motor)

Take voltage/amp readings with blower running on both sides of switch.

Could be flat spot on the motor.
Circuit breaker could be failing
Circuit breaker could have got drop of water inside it.