Any danger to running an AC motor without a run capacitor?

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,800
The exhaust fan in my garage was just humming, not running. If I flip the fan with my hand it will slowly speed up to about 1/2 of normal speed. On further inspection the run capacitor is swollen and deformed, so I think this is the problem. It's a small 150w 110v AC motor, the run cap is 10uF 250vac rated. I've ordered a new one, but is there any risk to the motor if I run it as-is until the new one arrives? It sounds happy, but it is running about 1/2 of normal speed.
 

#12

Joined Nov 30, 2010
18,210
It sounds happy, but it is running about 1/2 of normal speed.
Right there is the crux of the biscuit. An AC synchronous motor that can't synchronize is not developing the back EMF to minimize the stator current and that larger than normal current will be wasted as heat instead of motion.
 

shortbus

Joined Sep 30, 2009
7,746
The exhaust fan in my garage was just humming, not running. If I flip the fan with my hand it will slowly speed up to about 1/2 of normal speed. On further inspection the run capacitor is swollen and deformed, so I think this is the problem. It's a small 150w 110v AC motor, the run cap is 10uF 250vac rated. I've ordered a new one, but is there any risk to the motor if I run it as-is until the new one arrives? It sounds happy, but it is running about 1/2 of normal speed.
May be wrong in this answer, but, are you sure the capacitor is a "run" capacitor? Most small AC motors use a "start" cap not a run cap. And manually turning the shaft to get it to run makes it sound more likely that it is a run cap.
 

MaxHeadRoom

Joined Jul 18, 2013
20,859
Most small AC motors use a "start" cap not a run cap.
My experience is the opposite, generally motors over 1hp use a start capacitor, some together with a run cap.
Under 1hp are typically just a start/run combined (single cap) and remain in circuit.
Induction motors cannot run in synchronism , but must have slip frequency (min ~5 cycles)
Giving a motor with a bad start cap a spin at turn on provides just enough out of phase condition to come up to speed in order to function.
Max.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,800
Right there is the crux of the biscuit. An AC synchronous motor that can't synchronize is not developing the back EMF to minimize the stator current and that larger than normal current will be wasted as heat instead of motion.
I think your explanation is what happened. I noticed the fan was dead silent and not moving, so I power cycled it and that's when it started humming. So it must have over heated and the thermal protection tripped, and was reset when I power cycled it.

It must have been degrading for some time because the time it was taking to go from off to full speed seemed to be getting longer, but it always seemed to get up to full speed so I didn't really pay much attention to it. Until today. ;)

I will say after going through about 1 fan per year (they run almost 24/7) I found this one and it has been solid as a rock for about 8 years, very high build quality and it runs smooth and quiet. Hopefully this run cap will fix it.
 

#12

Joined Nov 30, 2010
18,210
it must have over heated and the thermal protection tripped,
I used to see this several times a year. Arrive at dead air conditioner, the outside fan is not moving and too hot to touch. Poke it with a stick to be sure the bearings are still good, replace the capacitor, and let it cool off. Bingo. That's a $4 capacitor to save a $100 motor.:)
 

MaxHeadRoom

Joined Jul 18, 2013
20,859
It has become common knowledge among the importers of Chinese equipment that they can pretty much expect a start capacitor failure within the first 6 mths to a year on any AC motors.
Replaced with a reliable make.
Max.
 

crutschow

Joined Mar 14, 2008
25,098
...................
I will say after going through about 1 fan per year (they run almost 24/7) I found this one and it has been solid as a rock for about 8 years, very high build quality and it runs smooth and quiet. Hopefully this run cap will fix it.
I can't help but wonder, why a garage exhaust fan runs near 24/7. :confused:

But if you want to use the fan before you've installed a replacement capacitor, you might want to try removing the old capacitor and the see if it comes up to speed after you give it a spin.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,800
I can't help but wonder, why a garage exhaust fan runs near 24/7. :confused:
It minimizes the gasoline fumes in the garage, plus it maintains negative pressure in the garage which keeps any remaining fumes out of the house. With all the lawn equipment Plus the motorcycles plus the boat motor , some kind of ventilation is needed and this actually does a really nice job and almost eliminates the gasoline fumes in entirely. Gas fumes are heavy so the air pick up is down on the floor.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,800
The garage is built for cars, which hold a ton more gas than all those other things combined! Plus here anything left in a shed would be stolen, or rusted to pieces by the next year.
 

Hypatia's Protege

Joined Mar 1, 2015
3,223
The garage is built for cars, which hold a ton more gas than all those other things combined! Plus here anything left in a shed would be stolen, or rusted to pieces by the next year.
I hear ya!:cool: -- It's just that, in my case, my structure underwriters won't let me place vehicles (nor other fuel powered machinery) within 50 yards of my main building! --- And I've never 'had' a fire nor made a claim of any sort!:(:(:(

Best regards
HP:)
 

#12

Joined Nov 30, 2010
18,210
my structure underwriters won't let me place vehicles (nor other fuel powered machinery) within 50 yards of my main building!
Interesting. My shed is 43 feet from the house, and the house originally had a garage where there are now 2 bedrooms. You can't park much closer than inside the house!:D
 

Hypatia's Protege

Joined Mar 1, 2015
3,223
Interesting. My shed is 43 feet from the house, and the house originally had a garage where there are now 2 bedrooms. You can't park much closer than inside the house!:D
:confused::confused::confused: -- Yebut -- my 'main building' is my house (i.e. residence):):):) --- Can it be? A case of 50 states separated by a single language?!:eek::oops:

Best regards
HP:cool:
 
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