Squirrel Cage Motor In Air Handler...no Start - HVAC - DIY Chatroom Home Improvement Forum

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03-15-2009, 10:12 AM   #1
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## Squirrel cage motor in air handler...no start

Recently my squirrel cage motor will not engage or start. It is getting signal and current. Today I opened the front of the unit to see if i could trouble shoot the problem. When the signal comes to the fan motor I hear a humming sound (which i assume is current) and clicking (which i assume are the controls), but the motor won't engage. I gave the squirrel cage a spin with my hand and the motor engaged and ran fine. It seems the motor isn't getting enough initial power to start. I have no idea what the problem could be. The motor and capacitor are less than a year old.

03-15-2009, 10:55 AM   #2
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First guess is the capacitor.

You can check it by putting a 1 Megohm resistor across it until the voltage drops below a volt or so. If the voltage is already less than 120v you can discharge it faster by putting an incandescent lamp across the cap.

Then put a 9v battery across the resistor/capacitor combo and measure the voltage with your DMM. If the voltage doesn't immediately go up to about 9v, the capacitor is shorted.

Then remove the battery, and time how long it takes to drop to 37% of initial voltage. So if the initial voltage is 9.1v then you're looking for a voltage of 3.4v.
Time in seconds = capacity in uF, so taking 30 seconds to reach 3.4v would mean 30 uF.
Compare this value with the capacitor's marked value, taking into account the marked tolerance on the capacitor value.

A part failing this soon falls into the category of "infant mortality" [as opposed to normal wearout after years of use] and usually means a factory defect. I don't think there was an external root cause making this part fail.

Last edited by Yoyizit; 03-15-2009 at 11:04 AM.

03-15-2009, 11:09 AM   #3
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## thanks

Thanks, Yoy...that makes perfect sense. Unfortunately i don't have access to the resistor you mentioned and i'm not sure lowes or home depot would carry it or even a capacitor for that matter. It looks like i'll be running the space heater until monday or tuesday when i can get to a supply house. thanks again for your help.

03-15-2009, 11:17 AM   #4
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Quote:
 Originally Posted by andyinrichmond i don't have access to the resistor you mentioned
Radio Shack has them, though it's probably a good gamble to just replace the cap, but don't throw out the old cap until you're sure the problem is fixed.

Troubleshooting by parts replacement is generally a bad idea.

Last edited by Yoyizit; 03-15-2009 at 11:51 AM.

03-15-2009, 12:43 PM   #5
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Quote:
 Originally Posted by Yoyizit Radio Shack has them, though it's probably a good gamble to just replace the cap, but don't throw out the old cap until you're sure the problem is fixed. Troubleshooting by parts replacement is generally a bad idea.

True, but if you got no meter a 20 buck cap ain't much of an investment to change.

 03-15-2009, 12:47 PM #6 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,561 Rewards Points: 6,086 Maybe someone left the lead squirrel out of the squirrel cage motor. That can happen if you leave the cage door open. ROFL
03-15-2009, 01:00 PM   #7
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Quote:
 Originally Posted by beenthere Maybe someone left the lead squirrel out of the squirrel cage motor. That can happen if you leave the cage door open. ROFL
UUhhhhh.....Been... stick to the serious stuff..You're better at it than i am ..I'l do the jokes

 03-15-2009, 01:15 PM #8 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,561 Rewards Points: 6,086 Couldn't resist the squirrel cage motor. LOL
03-15-2009, 01:27 PM   #9
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Quote:
 Originally Posted by beenthere Couldn't resist the squirrel cage motor. LOL

it's the vernacular that's used here to describe the furnace blower (I am sure you already know that though) around here. Do they call it something else in your part of the country?

 03-15-2009, 01:43 PM #10 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,561 Rewards Points: 6,086 We just call it a blower motor. Or direct drive blower motor. Only a couple 1940s and 50s units had prop/blade type fans/blowers. The old ones that were installed for gravity or forced air application. And they were belt drives. With the blower on top.
03-15-2009, 02:30 PM   #11
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## Update

Alright...point taken. It's just that carpenters such as myself are accustomed (and forced) to use terminology that other trades can grasp. It's actually a relief (all jokes, of course). That being said, I removed the capacitor and reversed the terminals before remounting (as a last ditch effort to get some heat), flipped the breakers, turned on the thermostat and the fan immediately turned on. I'm guessing i'm going to get out of this with a new capacitor, unless you guys see it differently. Thinking back, this has been a slow progression. I have noticed that it has taken more and more time for the fan to cut on over the last two weeks.

Thanks again for all of your responses...and jokes. It's a good feeling to know that i don't know everything.

 03-15-2009, 02:34 PM #12 Member   Join Date: Oct 2008 Location: Suburbs of Detroit Mi Posts: 3,704 Rewards Points: 2,000 Go to an hvac supplier and have them test the capacitor. Bring the motor just in case the cap checks good. If it does your motor should be replaced.
03-15-2009, 04:43 PM   #13
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Quote:
 Originally Posted by andyinrichmond I removed the capacitor and reversed the terminals before remounting (as a last ditch effort to get some heat), flipped the breakers, turned on the thermostat and the fan immediately turned on.
Very strange.
There might be a microgap in the internal connection of the cap. Maybe it opens and closes depending on temp. If they check it with a tester have them put stress on the terminals to check it for an intermittent.

http://www.rclambert.com/supra/blowe...rel%20cage.JPG

03-15-2009, 04:53 PM   #14

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Its a capacitor.
Just replace it, and see if the motor keeps working better.

If not, motor start windings or switch is going out.

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