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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).

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    Can you turn the fan itself freely? This could be a case of the fan being bound up mechanically... – ThreePhaseEel Aug 5 '17 at 3:09
  • Yep! Fan turns freely and smoothly. – Erik Reynolds Aug 5 '17 at 5:26
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    Motors can be bad right out of the box but that is rare. If more than 1 speed wire is hooked up try removing all but 1 of the speed wires to see if the motor starts. Depending on the motor wiring you could be energizing more than 1 speed. – d.george Aug 5 '17 at 9:11
  • Could you provide the motor type information. Some motors have 3 phase electronics for use on single phase, other than the cap and contactor this is the only thing I know of that can cause this. – Ed Beal Aug 5 '17 at 16:45
  • Motor is a FASCO D724 - 4 speed 3/4HP 115V motor. Matches to a 15mfd 370v capacitor. – Erik Reynolds Aug 6 '17 at 2:36
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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...

  • P.S. I assume that the shaft turns freely... and don't let it buzz that long. – Steve Aug 5 '17 at 1:19
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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.

  • I appreciate the detailed reply! We've actually replaced both the capacitor, motor, and contactor relay as part of this. We get the same result before and after each of these items. So strange. – Erik Reynolds Aug 5 '17 at 15:25
  • Did you ever get this resolved? If so, what was it in the end? I have the same problem -- same buzz, motor won't turn. But I haven't replaced anything yet. Totally different brands. – Kris Morness Jan 27 at 15:19
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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!

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