2-Speed AC motor humming loudly only at 1 speed

I just finished installing a whole house fan (Air Vent 54301), which utilizes a Genteq AC motor (5KCP39EG R540 P). The capacitor is Wilspec CA370-07V. It is a 2-speed motor.

After initially wiring it up, I tested it out without the fan blade installed (i.e. zero load). On LOW, the motor spun quietly (almost unnoticeable) just like I expected. On HIGH, however, the motor hummed very loudly, vibrating throughout the house. Not something I expected. The sounds reminds me of when you put a big load on a motor and it hums to overcome that load...but in my case there is zero load.

The weirdest thing was that there was not a noticeable change in RPM. I held the shaft lightly between my fingers as someone switched between HIGH and LOW and I couldn't tell any change.

I am getting in touch with the fan manufacturer to see what they say, but I do not think this is normal. The fan is wired up through a HI/LO/OFF switch and digital countdown timer, both of which are rated to 15A. The fan is on its own dedicated circuit (14 gauge, less than 40 feet to the electrical panel). I found one source online that says the motor pulls around 3 amps (on HIGH I am assuming). My next step after I recheck my wiring is to pull out the countdown timer to make sure that isn't causing problems.

Has anyone experienced this before with an AC motor?

 

So I did a little more troubleshooting this evening. I double checked the wiring throughout and everything looked correct. I rewired it w/o the countdown timer as well as trying out every combination of wiring (e.g. switching HIGH and LOW wires) to see if I got inconsistent results. Results were the same: normal sounding on LOW, loud humming on HIGH.

I borrowed an optical tachometer from work to check the RPM of the motor. On LOW I got 893 RPM. On HIGH I got...guess what...893 RPM. So obviously something is not correct if I am getting the same revs of HIGH and LOW.

Made some resistance measurements, though I don't know how to interpret them:

Black/White (HIGH winding): 185 ohms
Red/White (LOW winding): 277 ohms
Red/Black: 165 ohms
Ground/Case: 55 ohms

I'm not sure what else to do besides request a replacement motor...

 

Is the wiring diagram, including the capacitor, posted on the motor ?

Ground/case 55 ohms. Not quite sure what measurement this is ??
Usually ground to case is 0 ohms.

 

I believe I'd talk to the dealer where you bought the fan.

 

Yes, it is. In my first post I posted a picture of the motor tag. Click on it to zoom in.

The ground measurement I did isn't useful. Thought I read something about checking that, but was mistaken. it was the winding wire/ground that is more useful, both of which showed open circuit which is good.

I got a response from the manufacturer. They weren't very helpful, but they did mention the RPMs should be 600/800.

 

Notice on the motor data sticker the A 2.00? The motor draws 2 amps on high speed. The RPM is 800. That's also high speed; low speed will be slower. You've said you're testing with the fan blades off. With no load on the motor, the speed difference will be imperceptable which is exactly what you saw. You also saw the RPM to be a little higher than nameplate. With no load, I would expect that. Your resistance measurements sound right. Low speed will have more resistance than high speed. Red to black is an intermediate reading- not that important at this juncture. I'm not sure what's up with your ground to case reading, but it's low so not important now. When switching from high to low, additional resistance is inserted into motor circuit [internally] which slows the motor down [under a load that is]. There is a run capacitor wired to the 2 brown wires, correct? Notice on the sticker it says CAP 7.5/370? That means the capacitor is 7.5 mfd and 370 vac. What happens if you install the blades? Same results? If yes, return it.

 

Thanks, daddyjohn.

I installed the fan blade and you were right, the problem was fixed. The high and low worked fine and I measured ~625 RPM and ~800 RPM, respectively. With the fan on HIGH, it was hard to tell if the humming was still present just over the sound of the fan itself, but it seems like it is gone. So it looks like I'm all good, now.

Now that my problem is solved, the rest is just to learn a bit more about AC motors (I'm more familiar with DC motors).

After doing a little research I now understand that the RPM on the motor nameplate is the full load speed, which is the RPM at the rated voltage/current.

I'm still puzzled about the no-load (synchronous speed), which is determined by the frequency (60 Hz) and the number of poles. The HIGH speed windings have less poles than the LOW speed windings (which agrees with my resistance measurements). From what I read online, the motor should run very close to its theoretical synchronous speed if there is not a load on the motor.

Just knowing that the HIGH and LOW have different number of windings, their synchronous speeds will be different. However, was something limiting the motor from getting close to this speed? Especially on HIGH? Could the run capacitor be limiting the speed?

 

The motor is what's known as a PSC motor [Permanent Split Capacitor] The no load speed is determined by the number of poles the motor windings have and the frequency. The synchronous speeds are:

2 poles- 3600 rpm
4 poles- 1800 rpm
6 poles- 1200 rpm
8 poles- 900 rpm
12 poles- 600 rpm
16 poles- 450 rpm

These are all unloaded synchronous speeds on the drawing board. In the real world, motors have slippage which is the difference between the theoretical speed and what the motor actually operates at under load. So, for example, an 1800 rpm, 4 pole motor will be tagged 1725 to 1750 rpm. Your fan motor is an 8 pole motor tagged at 800 rpm. It's a 2 speed, PSC motor. When low speed is chosen, the resistance of both the high speed windings and the low speed windings are placed across the power source. On high speed, only the resistance of the high speed winding is in play. In actuality, shifting to low speed and adding resistance changes the horsepower of the motor which causes it to slow down. Why? Because the load doesn't change. The fan blades are still the same size and pitch. The capacitor's function is to shift the start and run windings out of phase with each other which changes the lead/lag of the current flow in the windings. This allows the motor to start. The capacitor is a run capcitor, which is intended to be in the circuit all the time. Notice the 2 brown wires? That's the start winding. If the motor windings all saw the same phasing, the motor would not start; it would just sit there and hum. Can you see that an open capacitor would cause a PSC motor to not start? The motor would sit here and hum, draw too much current , overheat, and shutoff on its' thermal protector because the start winding would not be in play. Notice where the sticker indicates thermally protected? PSC motors are low starting torque motors but are fine for fan duty. So, it's not a matter of high and low having different poles, just different resistances. Your fridge compressor motor will have more starting torque because it has to start against a higher load.

Do you have central a/c in your house? Take a look at the indoor and outdoor fan motors. You'll find that they are both PSC motors if they are both direct drive. Translation- direct drive= the fans are connected directly to the motor shaft.

 

Got it! Thanks for taking the time to explain. Makes total sense now...I was thinking there were different poles for HIGH and LOW. Adding resistance makes more sense.

So when I was running it with no load, the LOW speed horsepower was enough to spin up the rotor to near synchronous speed, and adding additional HP (HIGH speed) would have no effect on the speed...correct? Any idea what the humming would be that I heard on no-load HIGH? Just curious...

 

Welcome

With no load, the motor will just ramp up on either speed. Don't know why it was so noisy, but it certainly needed some load on it. With this information and some Internet research, you could repair a malfuctioning fan motor on your a/c sytem and save $$$. Often the problem is no more than a failed run capacitor. $$$ if you call a service company, but a cap might cost you $10 to $15 and some sweat equity.