Carrier Blower Motor not turning on, i think low voltage

[bradlawson]

I am testing the motherboard to see why the blower motor is not turning on. I get 120 volt on the main power source and when it cycles to turn on the motor during a test it only puts on 82 volts to the power wire, so it never turns on. I removed the power wire for the motor and attached it to 120 volt line and it worked fine. So why would the motherboard be only putting on 82 volts?

THanks

Brad

[HayZee518]

first things first - can you supply me with a schematic of the unit? the blower probably uses a triac in place of a mechanical contactor. a computer typically uses voltages of +5 and -5, +18 to ground for control, a gating signal is necessary to the triac to turn on. its voltage is lower than the circuit it controls. to keep from burning up the triac uses a resistance to limit current to the device - this is probably what you are seeing. as you stated the motor runs normal under a full 120 volt supply. look for a burnt 2 or 3 watt resistor near the triac.

[bradlawson]

I can get you the schematics but quick question

First of all it is a carrier single stage heating and A/C. The reason why i think the line should be getting 120 is because when i give it 120 it turns on the motor but when i measure what the board is giving it, it is only 82 which doesn't not turn on the board. So if the triac was bad then why would the motor turn on with 120 and not 82. Again, i dont' know any thing about A/C just trying to figure it out so i am sorry in advance for my ignorance

Thanks,

Brad

[HayZee518]

the motor has a threshold voltage that it will turn on with. I have a variac connected to a drill motor that won't even budge at 80 volts but at 90 it will turn labored.

[hvacwiz]

Whats the model# of the furnace?