Capacitor matching for fan motor

gtojohn · 08-14-15, 10:05 AM

Yesterday I had a problem. I replaced an outdoor fan motor on an a/c. The box wasn't marked for which capacitor. This happens sometimes, but this time the motor wasn't labeled either. I new they are usually 5mf or 7.5mf. This was an 825rpm motor which is about 10% of the ones I see so I wasn't completely sure. I tried the internet with my phone without quick success. I ended up trying both and using my amp meter to decide. The 5mf drew 1.9 amps per leg. And the 7.5mf drew 1.5 amps. Deciding less amps is better I went with the 7.5mf.
Later at home I looked up the us motors 1784 and found it listed with a 5mf capacitor...So now should I drive back and swap it for the right one, or rest on my laurels telling myself I put in a better, more correct for the application capacitor? My main concern is to not burn up or shorten the lifespan of the new motor, however saving a little power, 96 watts is also a good thing.

jeff5may · 08-14-15, 05:47 PM

If the motor starts up ok with the larger value, then it's not larger enough to not start the motor. If it is permanently connected (no centrifugal disconnect), it may lower the power factor of the motor enough to heat it up. You may never know until that hottest day of the year comes around. When you need the a/c the most, poof! Either the motor cuts out on overtemp or it just gives up after being slow cooked. Then again, it might work good enough for something else to fail first (bearings or bushings) many hours down the road.

It's pretty hot this time of year. If heat is an issue, you'll find out shortly. If nobody calls to complain soon, it'll probably be ok for the long haul. Not perfect, but good enough.

Just remember the rule of thumb with electronic devices of most every type: increasing the operating temperature by 10 degC cuts its life in half.

MN Renovator · 08-14-15, 07:45 PM

Less amperage draw but it might be running slower too and hurting COP by not rejecting as much heat. Hard to tell without using a tach on it. I doubt it will hurt an air-over motor.

gtojohn · 08-15-15, 08:03 AM

This one had a small fan blade, my thinking was the load might have been small enough vs the motors rated load could have been the difference. The diff between a 5mf and 7.5mf is marginal. However as is turns out it ran about 20hours straight and shut down yesterday afternoon in the 102f heat. So, new 5mf capacitor it is!

08-14-15, 10:05 AM	#1
gtojohn Journeyman EcoRenovator Join Date: Jul 2014 Location: Austin, TX Posts: 314 Thanks: 39 Thanked 50 Times in 45 Posts	Capacitor matching for fan motor Yesterday I had a problem. I replaced an outdoor fan motor on an a/c. The box wasn't marked for which capacitor. This happens sometimes, but this time the motor wasn't labeled either. I new they are usually 5mf or 7.5mf. This was an 825rpm motor which is about 10% of the ones I see so I wasn't completely sure. I tried the internet with my phone without quick success. I ended up trying both and using my amp meter to decide. The 5mf drew 1.9 amps per leg. And the 7.5mf drew 1.5 amps. Deciding less amps is better I went with the 7.5mf. Later at home I looked up the us motors 1784 and found it listed with a 5mf capacitor...So now should I drive back and swap it for the right one, or rest on my laurels telling myself I put in a better, more correct for the application capacitor? My main concern is to not burn up or shorten the lifespan of the new motor, however saving a little power, 96 watts is also a good thing.

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08-14-15, 05:47 PM	#2
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	If the motor starts up ok with the larger value, then it's not larger enough to not start the motor. If it is permanently connected (no centrifugal disconnect), it may lower the power factor of the motor enough to heat it up. You may never know until that hottest day of the year comes around. When you need the a/c the most, poof! Either the motor cuts out on overtemp or it just gives up after being slow cooked. Then again, it might work good enough for something else to fail first (bearings or bushings) many hours down the road. It's pretty hot this time of year. If heat is an issue, you'll find out shortly. If nobody calls to complain soon, it'll probably be ok for the long haul. Not perfect, but good enough. Just remember the rule of thumb with electronic devices of most every type: increasing the operating temperature by 10 degC cuts its life in half.

08-14-15, 07:45 PM	#3
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	Less amperage draw but it might be running slower too and hurting COP by not rejecting as much heat. Hard to tell without using a tach on it. I doubt it will hurt an air-over motor.

08-15-15, 08:03 AM	#4
gtojohn Journeyman EcoRenovator Join Date: Jul 2014 Location: Austin, TX Posts: 314 Thanks: 39 Thanked 50 Times in 45 Posts	This one had a small fan blade, my thinking was the load might have been small enough vs the motors rated load could have been the difference. The diff between a 5mf and 7.5mf is marginal. However as is turns out it ran about 20hours straight and shut down yesterday afternoon in the 102f heat. So, new 5mf capacitor it is!