Furnace blower motor makes buzzing noise and won't start

Question

I'm having a bizarre issue with my HVAC forced air blower motor. Specifically, it will not start and just buzzes while showing a great deal of startup current (20-22 amps).

Background: This is a TEMPSTAR Model# NUG5150BKA1 furnace that is ~8 years old. I purchased the home last December, and I noticed back then when the blower fan would turn on, I would hear a loud buzzing sound for 1-2 seconds and then the fan would kick on. When the old motor failed a few weeks ago, it was just buzzing continuously for a minute and by the time I ran downstairs the motor was smoking.

A friend and I just replaced the motor with an equivalent new model (same brand - FASCO). We went through the entire wiring and verified to the best of our ability that it matched up with the wiring diagram on the blower door. When we power on the system and have the thermostat call for cool / cycle on the fan, the motor BUZZes loudly and will not turn over. A clamp ammeter on the high speed motor line shows 20-22amps (running full load is rated at 9.5 amps). When the motor terminals are disconnected from the board, the terminals for the motor show 117volts as they should when the system thinks the motor is on.

Link to video detailing noise.

We have replaced the blower motor with a brand new unit as well as a brand new matching capacitor. The capacitor test setting of the clamp meter indicates the capacitor is good (reading 15.2mfd on a 15mfd rated cap). We have also tried replacing the contactor relay in case that was the source of the noise. No difference.

I'm absolutely stumped. Is it possible the brand new motor is also bad or am I missing something obvious (or perhaps less obvious).

Answer 1

There are 2 main or common things that can cause buzzing and not starting. The first is a starting capacitor that has gone bad. Starting caps provide a phase shift to help the motor start. The capacitor(s) are usually close to the motor in a electrical box on the unit (some motors have the caps mounted on the motor but more often in a HVAC system they are behind a panel). The second is the contactor (motor relay) that connects the high amperage to the motor using low amperage / voltage to energise the motor.

To check a cap. It needs to be removed from the circuit power off for a few minutes before removal and if it has a bleed resistor this disconnected after removal the easiest way is to use a meter with a capacitor function and see if the value in uf (microfarids ) is within 10% of the value stamped on the cap. A more complex way and less accurate is to use a ohm meter put the leads on the cap and watch for the value to start increasing, switch leads and the value should start high and drop ( the meter is charging the cap in 1 direction and discharging in the other) if there is no change or the cap dosent charge and discharge it is bad.

If the cap appears bulged or leaking it should be replaced.

With the contactor sometimes they just get weak and can not pull in from rust or the coil is swolen because of overheating. The electrical contacts can be pitted and not making a good contact it will need to be replaced. For any of these issues.

I find the starting cap to be the problem more often than the contactor but have had both fail.

1 additional note there may be a second capacitor or 2 capacitors in 1 can the second if it has it is usually the run cap. Make sure to label the wires when you take them off I even do this especially with dual caps.

Sorry this got long but was trying to give enough info for you to troubleshoot and repair without waiting for multiple steps as you check things out.

Answer 2

I can only add that the high current is normal for a motor that is starting. It's called locked rotor current. But if left in that condition the motor will indeed overheat. I would have suspected the cap but you eliminated that...

Answer 3

For my unit (similar issue) I think the issue is the blower relay, which should be an easy swap when the part comes in tomorrow. I'm just a novice that is very safe, does a lot of research, and follows directions well. I have read on other forums that a failing relay can cause the buzzing sound - lets hope that's the case because the next step is blower motor for me!

Answer 4

A blower motor is typically a permanent split (phase) capacitor motor. A blower has a load line that leaves the motor lightly loaded until it’s close to full speed so it does not need additional starting torque and the motor does not have a starting capacitor to increase starting torque.

It is possible that the motor that is buzzing has a bad capacitor but it’s far more likely that it has a bad bearing at the pulley end.
The typical repair choice is to replace the blower motor, the 2023 cost of a 1/2 hp blower motor is around $150.

If the motor has sleeve bearings replacement is the only choice, the shaft will be worn where the bearing failed.

A skilled individual can remove the motor, pull the pulley from the shaft, remove the end plate, sand the shaft with 600 grit paper to remove surface corrosion to allow the ball bearing to be pulled, press a new bearing into place and reassemble the machine in 20 minutes or less. Total cash outlay to replace the ball bearing roughly $5, puller tools, sandpaper and mechanical expertise not included.
Talented DIY enthusiasts should budget $50 for pulley puller, bearing puller and a tool to press the bearing onto the shaft pressing only on the inner race. The replacement ball bearing should be, or have been, greased with a polyurea electric motor grease like Mobil Polyrex EM. A white lithium or molybdenum grease, even one using a synthetic oil, has a short service life in an electric motor.