EE eng. advice pls on furnace blower OEM replacement

Problems related to residential installations.

EE eng. advice pls on furnace blower OEM replacement

Postby StephenS » Sun Jul 17, 2016 6:33 pm

I tried to have an elec. motor shop provide a replacement for the original furnace fan blower motor after I removed it. A new aftermarket motor worked fine on bench tests (3 speeds) but when installed would not draw any contact voltage from the control panel. Several HVAC shops (similar practice as auto dealer shops) would only discuss a manufacturer's (Carrier) replacement to the tune of $500 - $700 ($150 motor rest is R&R sheet metal labor).

I gave up and reinstalled a Carrier motor and it works fine of course. So, while I am happily and comfortably back in operation, the technical side of me seeks an explanation of what kind of circuitry in the motor signals the furnace logic board to not provide any voltage. Manufacturer''s Technical Support is buried beneath numerous layers of non-technical "Customer support" and unavailable for discussion.

Manufacturer's certified HVAC people I have spoken with are similar to dealer auto support - e.g. replace OEM parts from a flat rate manual only. None have an electrical engineering background. The electric motor shop is similarly at a loss as to why the replacement won't work and I look forward to sharing any answer with them for our mutual enlightenment.

TIA
StephenS
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- EE eng. advice pls on furnace blower OEM replacement

Postby Freon » Mon Jul 18, 2016 7:12 am

How old is this Carrier furnace? What prompted you to seek a new motor when, apparently, the old motor works perfectly?

If you look at the schematic diagram of the furnace control board, does it show any other wires from the motor other than the 3 speed taps and the common? I have never heard of a control board having the capability to "sense" what motor was connected but then if you have a newer furnace, maybe this is how manufacturers get you to buy their parts. However I doubt this. But I have seen it in laptop computer batteries.

The schematic should show simple relays, one for heat, one for cool, that powers the motor when there's a call for the fan. Without any motor connected to the control board, I'd set the thermostat to FAN-ON and then check the voltages at the 3 speed tap locations where the motor would connect. I would also check the control voltage (24 volts AC) between the G terminal and common where the thermostat wires connect to the control board. You should see 24 volts.

Lastly I would double check the connection between the new motor and control board. Is the plug identical to the old motor plug? Did you have to adapt the new motor to the control board plug?
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- EE eng. advice pls on furnace blower OEM replacement

Postby StephenS » Mon Jul 18, 2016 10:38 pm

ow old is this Carrier furnace? What prompted you to seek a new motor when, apparently, the old motor works perfectly?
IT IS 19 YEARS OLD. THE FRONT BEARING APPEARED TO BE WORN AND PRODUCE A 10 SECOND CHATTER AT STARTUP, WHICH SEEMED LIKE 10 MINUTES PARTICULARLY DURING THE NIGHT. THIS WENT ONE FOR A YEAR OR MORE DURING WHICH I ASKED A COUPLE OF HVAC COMPANIES IF IT WAS FEASIBLE TO DO SOMETHING ABOUT THE BEARING. AS STATED THEY WOULDN'T TOUCH IT PARTICULARY DUE TO THE AGE AND WERE ONLY INTERESTED IN SELLING A NEW SYSTEM WHICH I RECONIZE WILL BE COMING UP BEFORE TOO LONG. SO AFTER REMOVING IT MYSELF I TOOK IT IN TO ELEC MOTOR SHOP. AFTER GOING TO ALL THAT WORK AND TRIPS BACK AND FORTH ACROSS TOWN I WAS PREPARED TO SIMPLY INSTALL A NEW ONE AFTER ALL THE SHEET METAL WORK. TO MAKE A LONG STORY SHORT, AFTER BEING UNABLE TO GET NEW ONE TO WORK AS DESCRIBED, LEARNED THAT MOTOR WAS SLEEVE BERAING AND LUBRICATION GOT IT WORKING SMOOTHLY.

If you look at the schematic diagram of the furnace control board, does it show any other wires from the motor other than the 3 speed taps and the common? I have never heard of a control board having the capability to "sense" what motor was connected but then if you have a newer furnace, maybe this is how manufacturers get you to buy their parts. However I doubt this. But I have seen it in laptop computer batteries.
THE "SENSING" WAS MY PRESUMPTIVE CONCLUSION AFTER ALL THE STEPS WERE EXHAUSTED, LARGELY BECAUSE ON BENCH ALL SPEEDS WORKED WHEN INDIVIDUALLY CONNECTED WITH TEST LEADS. NO, THERE WERE NO OTHER LEADS. BUT YOUR COMMENT ABOUT BATTERIES AND CAPTIVE AUDIENCE IS WHAT I THOUGHT ALSO. SCHEMATIC OR NOT, I WAS THE ONE CONNECTING & DISCONNECTING R&R SEVERAL TIMES SO I PERSONALLY MARKED EVERY LEAD.

The schematic should show simple relays, one for heat, one for cool, that powers the motor when there's a call for the fan. Without any motor connected to the control board, I'd set the thermostat to FAN-ON and then check the voltages at the 3 speed tap locations where the motor would connect. I would also check the control voltage (24 volts AC) between the G terminal and common where the thermostat wires connect to the control board. You should see 24 volts.
THOSE WERE ALL AS YOU DESCRIBE.

Lastly I would double check the connection between the new motor and control board. Is the plug identical to the old motor plug? Did you have to adapt the new motor to the control board plug?
TO BE SURE OF THAT WE CUT OFF THE LEADS AND CONNECTED THEM TO THE NEW MOTOR AND RE-USED THEM AS IS; WHEN WE GAVE UP, WE CUT THEM OFF THE NEW MOTOR AND RECONNECTED THEM TO THE OLD, OILED THE BEARING AND REINSTALLED, RETURNING THE NEW MOTOR TO AN UNDERSTANDABLY UNHAPPY MOTOR SHOP..

THANK YOU FOR YOUR TIME. WE ARE IN OPERATION BUT I WANTED TO GET TO THE BOTTOM OF THIS MORE FOR PROFESSIONAL SATISFACTION AS STATED.
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