Help me diagnose this electric motor

I have this motor from my whole-house fan. The fan was new about a year ago, and worked fine until a couple of weeks ago, I went to turn it on, and it did, but wasn't generating enough RPM to even open the louvers. OK, fine, it's less than a year old, so I emailed the company, and they sent out a replacement. Exactly the same thing. After verifying that there's nothing mechanically wrong with the fan, belt, bearings, etc, I decided to call my BIL, who's a former electrician. He put a meter on it and it was only getting about 90V. So we started checking other outlets on the same circuit, and some were getting 120V, some were getting 90. In the process of all this checking, we must have jostled something into place, because now everything is getting 120. BIL think s that maybe I had a loose common (white) wire somewhere that caused the low voltage situation. But even with the correct voltage, the fan still turns slowly.

The fan is controlled by a simple on-off switch. The motor has two speeds, but I only use the higher one. The switch checked out OK.

Here are some pics of the first motor:

In the last pic, you can see that there are both red and black wires. The red is used for the lower speed, so I just left it disconnected. 

I've verified all the connections a bunch of times.

Although it's possible that I had one motor die and they sent me another one with the exact same issue, that doesn't seem too likely. And I doubt they will send me another one. It seems to me that the low-voltage condition must have damaged both motors. 

So, how can I diagnose the issue? Both motors run, but not at full speed. Full speed, according to the label, is 1100 RPM. I, of course, have no way of measuring this, but I think I'll know when it's running "right" again. I do have a meter, and can do basic checks. The capacitor on there is supposed to help with startup, maybe if it has failed, the motor can never get up to full speed?

Trying to figure this out on my own without having to take the motor somewhere or spend $150 on another new one.

It's possible that the low voltage over-amped the capacitor, but that would likely cause it to struggle to get started, not cause a low RPM situation.  I have an old radial arm saw here at the shop with a bad cap and I just give it a little spin to help it start. (totally safe)

Is it a blade fan or a squirrel cage?  If it's blades you can match the pitch of the blades to a piano or tone generator app on your phone to get the frequency.  Divide by the number of blades, and you should be able to get an RPM.

You can also get a cheap sirometer that will tell you the RPMs.  Might have to put a big piece of tape on one fan blade to induce harmonics.

Curtis73 (Forum Supporter) said:

It's possible that the low voltage over-amped the capacitor, but that would likely cause it to struggle to get started, not cause a low RPM situation.  I have an old radial arm saw here at the shop with a bad cap and I just give it a little spin to help it start. (totally safe)

Yeah, that was the failure mode for the one I had before this. If you gave the fan a push, it would get going and be fine. We used it like that for a year or two before finally replacing it.

Curtis73 (Forum Supporter) said:

Is it a blade fan or a squirrel cage?  If it's blades you can match the pitch of the blades to a piano or tone generator app on your phone to get the frequency.  Divide by the number of blades, and you should be able to get an RPM.

Blade fan. It's this model. Here's a pic:

what other outlets are on the same circuit? have you plugged anything new in recently that could be suspect? Maybe you have another high amp appliance on the same line?

Can you reset the themal protector?

Have you tested the motor on the bench?  Find someone with a laser tach and you could test it...

Harbor Freight has one for $40.

Belt driven.  I assume the blade spins freely with the belt off?  Belt is properly tensioned?

Weird that both motors would fail in that way.  I can't help but feel that there is still some issue with your supply voltage.  The only thing done to "fix" it was a theoretical "jostling" of a theoretical loose wire?

1988RedT2 said:

Belt driven.  I assume the blade spins freely with the belt off?  Belt is properly tensioned?

Weird that both motors would fail in that way.  I can't help but feel that there is still some issue with your supply voltage.  The only thing done to "fix" it was a theoretical "jostling" of a theoretical loose wire?

Yep on the fan turning freely and the belt tension. If anything, the tension is kind of loose.

I agree that it's odd for both motors to fail the same way. As for the low voltage, I'm not sure what else to do. All the outlets are now putting out 120V again, and the fan is getting 120V, too. The only thing I can think is that when the voltage was low, it damaged the first motor, and since it was still low when I put the second in, it did the same thing. IF that's the case, a new motor should fix me up now, but I'm trying to avoid that if possible.

I guess what I'm looking for is if there's some way to check the motor or see what if anything needs to be repaired on it. Can I check the capacitor? The windings?

noddaz said:

Have you tested the motor on the bench?  Find someone with a laser tach and you could test it...

Harbor Freight has one for $40.

Didn't know that such a thing existed. And Amazon has one for $19.

If you hook up the red wire instead of the black do you get the 900 RPM?

triumph7 said:

If you hook up the red wire instead of the black do you get the 900 RPM?

Good question. Haven't tried that, but I will.

It sounds like it was a brownout condition - which probably damaged the motor.  Voltage goes down & current goes up and stuff gets hot! (P=VI)

Are you checking the voltage while the fan is running?  My guess is that your getting 120 with no load but still have a bad connection that's preventing the fan from getting full power.

APEowner said:

Are you checking the voltage while the fan is running?  My guess is that your getting 120 with no load but still have a bad connection that's preventing the fan from getting full power.

This is where I would start. If you didn't find and fix the bad connection, you still have one. 

APEowner said:

Are you checking the voltage while the fan is running?  My guess is that your getting 120 with no load but still have a bad connection that's preventing the fan from getting full power.

Yes, it's now 120V with the motor running.

In reply to Tom_Spangler (Forum Supporter) :

Then I would check the amp draw and see if it is in spec. You may have a shorted winding.