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INDOOR BLOWER PSC MOTOR
PURPOSE;
The indoor blower fan assembly (fig.1) is responsible for moving air through the heat exchanger, through the duct work and into the living space. Typically, the assembly consists of a direct drive motor, a centrifugal blower wheel with forward curved blades and a double inlet blower wheel housing. The assembly must overcome the resistance of the furnace air passageways, filter, and ductwork to provide adequate supply air to the living space. Most furnaces use a simple PSC motor with a capacitor to drive the indoor blower assembly.
THE INDOOR BLOWER MOTOR CIRCUIT
When a call for heat is received by the IFC, the pre-purge (induced draft furnace) and the ignition sequence begin. Once the main burners are firing and sufficient flame current is being detected, the furnace begins a timed on indoor blower delay warm up period. (Usually around 45 seconds) Once the delay period has elapsed, the IFC energizes the indoor blower motor assembly through the heat terminal. (fig.2) If the indoor motor fails to start or quits running during the heat cycle, the high limit control will trip and shut down the burners.
When the thermostat is satisfied, the gas valve is de-energized, the induced draft motor begins a 5 second post purge cycle, and the IFC begins a timed off indoor blower delay period. At the completion of the blower delay off period, the indoor blower motor is de-energized and the cycle is finished.
INDOOR BLOWER FAN PSC MOTOR CHECK
The blower motor features an internal thermal overload to protect the motor winding from overheating. If the motor is hot, allow it to cool down before checking the motor.
TOOLS NEEDED;
MULTIMETER
PROCEDURE; SINGLE SPEED PSC MOTOR WITH INTERNAL OVERLOAD
1. Disconnect power to the furnace.
2. Using an ohmmeter, check the indoor blower motor capacitor. (Remember, always discharge the capacitor with an insulated screwdriver BEFORE attempting to test it.) This is to prevent shock and the possible destruction of your meter. Place one ohmmeter lead on each of the capacitor terminals. If the capacitor is working properly, the ohmmeter should ramp up and then ramp down. If the capacitor fails to respond, it is bad and must be replaced. If it is good, go to the next step. (If the capacitor is swollen or damaged, the above step is unnecessary)
3. Using an ohmmeter, check for resistance between the common and run windings, and between common and start windings of the indoor blower motor. You should measure resistance between each winding (fig.3) If you measure infinite resistance from both common to run and common to start windings, the internal overload is open and the motor must be replaced. (The motor should be cool to the touch when testing.) If you measure infinite resistance from common to only one winding, the winding is open and the motor must be replaced.
4. Check the resistance from the motor winding leads to the motor case (fig. 4) You should measure infinite resistance. If you measure any amount of resistance, the motor is shorted to ground. Replace the motor. If all of the motor windings check out OK, proceed to the next step.
5. Restore power to the furnace and initiate a call for heat. When burner flame is established, use a voltmeter to measure the voltage at terminals HEAT and CIR N on the IFC board, (fig. 5) The IFC should send out 120 volts through these terminals to the indoor blower motor after the blower delay ON period has expired. If no voltage is present after the blower delay ON is completed, replace the IFC board.