Blower motor - Control board not telling the capacitor to give an assist at startup?

[oliverc]

On my old Carrier 58GSC furnace, there is a humming sound when the motor is supposed to kick in, and if I give the fan a spin it will go fine. Everybody on the web says it is the run capacitor, but I replaced the run capacitor with another one of the same rating (12.5 uF, 370 VAC) and it still doesn't go.

I got the replacement through ebay for $5 including shipping, which makes me worry that maybe they just sent me a bad part? I don't have a multimeter to check the capacitor.

The other possibility, I figured, is that it's a control board issue. Maybe the control board isn't telling the capacitor to give an assist at startup? Anyone know if this might be it?

Thanks!

[oliverc]

Possible answer

I just read this on an old post in this forum:

"There is a centrifugal switch inside the motor which disconnects the capacitor when the motor is up to speed".

This makes sense. Do all motors work this way?

[Speedbump]

Motors like yours that have only run caps don't have centrifugal switches. The capacitor is in line all the time, so a cut off switch isn't needed. You may have gotten a bad cap on Ebay. If the motor runs fine with a spin of the fan, doesn't sound funny or get overly hot, I don't know what else it could be. I am not an HVAC guy and don't work on these items so I'm not sure what the control board actually does.

[akaothermick]

This might help explain the capacitors purpose and aid in the diagnosis https://en.wikipedia.org/wiki/Motor_capacitor

[Lili88]

The Wiki article is great! That's like a small tutorial.