fan motor/board shorted

Del

Hey just was on an emergency heat call.  Fan motor on a 90% gas furnace was toasty hot and starting/stopping. Guy had been running it like that for a while. Replaced with a 3 speed 1/2 hp 1075RPM universal motor and 10MFD capacitor.  Got everything wired up, fired up the power.  Inducer turned on, and then BAM! the board shorted out and tripped the breaker.  I looked at the back of the board, and I could see the terminals where the fan connects were burned up.  I checked all my connections, and everything was done according to the wire diagram.  The only difference with the universal fan motor was that it had 2 wires(brown/brown with white stripe) to the capacitor and also a neutral, while the old motor just had one wire plus the neutral hooked to the cap, so I just brought the neutral directly to the neutral terminal on the board instead of to the capacitor like the old one was.  Don't think that would cause a short, as neutral is neutral. Also, I parked the unused motor leads on the "park" terminals. Anyway, I'm wondering if running a burnt out fan motor for so long could have damaged the board so that when I tried to fire the new on up, it shorted out.  I went and turned on the breaker, and of course the fan motor did not start, but the inducer and HSI all started so it only burned up the fan motor section. Thanks.   

Tim McElwain

That is a possibility

what is the Make and Model of the furnace and what make and number is the fan control board?



Try hooking the motor up independent of the board with 120 volts and see if it works okay, that way you know the motor is okay and then replace the board.

Del

Make is....

An older rheem weatherking. I gave the model/serial to our service manager so she could order the part. It is a 90%, and the HSI is mounted on the end of an 8" long ceramic stem that slides into the burner box. This is the first one I've seen if our area, and we're having a heck of a time finding the board. I will definetely test the motor on a separate circuit before I install the new board. Don't newer boards have fuses for protection from this sort of thing? I've had units with shorts in the 24v side, and the fuse keeps blowing till I find the short/shorts with my meter!! Just to clarify, on the back of the board, I could see the short between what looked like the fan relay switch and the fan "heat" terminal. Nothin on the connections on the front of the board looked black or shorted.

Also, would it have made a difference if I had connected the fan motor neutral to the neutral side of the capacitor even though the new fan motor had 2 separate leads for the cap?

Tim McElwain

It may have

made a difference if you had connected all the neutral wires together. Check the recommended wiring on the replacement motor or get in touch with the factory for some assistance.



Not all boards have fuses but you certainly cut one in just to be safe so you do not fry another board.

Del

Good idea

Just to be clear, I'll cut a fuse in before L1 going into the board. What would be the highest amp draw(size fuse) I should use? I know the low voltage side uses 3-5 amp fuse. Thanks for your help.

Tim McElwain

I would put in a temporary

maybe 10 amp fuse just to be safe.

Empire_2

Mistake as I once made......

Dont feel bad.  your original motor  with the com and Brown/White strip is just DANDY, but when you get the new motor what the do not tell you is that your original set up at the cap is either to be wired identically (brown and wht) or with the new motor that usually does not give you this info and is wired in as CAP=Bro/Bro/wht.  But since you wired White or common to the other side of the cap, the result was a DIRECT short.



     Next time Cut the brown wire on the motor, and only use the whit/brown and then....Com on your ciucuit

Del

Hmmmm

Thanks for the replies. I ended up pulling the neutral from the cap off the board, and just wiring the new motor up brown, brown/white to the cap, and leaving the neutral from the fan motor on the neutral side of the board. This all after rewiring the harness for the pressure switch ignition delay. I can see that I created a short circuit somewhere in the cap wiring, but if someone could explain how the new fan motor with the 2 cap leads is different from the old motor with the brown and white(neutral) leads on the cap, and cap tied into the neutral that would be great. I would guess that the leads on the new motor are in series with the starting winding. I'm not sure why the old motor had it's neutral tied into the cap, and why the cap was connected to the neutral on the PCB . I guess I've changed more condenser fan motors than furnace fan motors!!!