Condenser fan motor sizing


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Old 08-20-08, 10:40 AM
K
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Condenser fan motor sizing

I have a 35 year old York 4 ton split ac that came with the house when I bought it. About a year into owning the house, the condenser fan motor died from shot bearings. I put a new motor in with a new fan because I had records showing the system had chewed up a few motors in the past few years. I exactly matched the specs of the dead motor. I had the HVAC supplier match the fan. It's a 24", 4 blade, 27 deg, CCW fan. The motor was a 1/2 hp, 1075 rpm, 230V, Single Phase, 60deg ambient. After i got it all installed and running, for kicks I looked up the specs on this fan and the fan calls for a 3/4 hp motor at 1075 rpm! OK, so I think I am overloading the motor and that the fan motors in my condenser must have been overloaded for years (maybe this is why they are only lasting a year on average). I break out the ammeter and measure the motor input current. 3.2 Amps. The motor is rated at 3.5 amps. If the motor really was overloaded, wouldn't it be pulling a higher than rated current? Do I have a problem here or should I just leave it with the 1/2 hp motor and not worry about it? It seems to be running fine.
 
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Old 08-20-08, 04:34 PM
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My books show that the blade you have requires a 3/4 horse motor. This explains the rather short life of previous motors.

Sometimes a lazy a/c guy will use what they have on their truck rather than make a run to the supply house.

In my experience, when a motor is pulling close to its maximum amp rating, you will have problems. I wouldn't be surprised if it's shutting down on thermal overload when you are not looking.

Best bet, use the correct motor.
 
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Old 08-21-08, 10:12 AM
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Thanks Jim,

The 1/2 hp is in there now and working so I guess I will just keep a 3/4 hp on hand and wait for the 1/2 to die, fully expecting it to have a short life. Thanks for checking. Now I know what motor to buy for next time.
 
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Old 08-21-08, 01:03 PM
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Air Conditioning Repair

You are running at peak amp load so as you can see through your little field test this will shorten the life of your motor many motors get replaced undersized in the real world that was probably the heaviest motor the last service guy had on his truck because you have already hooked up this motor they won't take it back so you might as well let it ride and it will die in a couple of years and you can put on a 3/4 hp (if you remember)
 
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Old 08-22-08, 03:48 PM
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No need to try to take it back. I got it (and three more like it) new in the box on ebay for $65. I'm sure I'll get my $16 worth out of it. It's been running for about 12 hours a day for 3 day now (outside temp is 108 deg now). Jim-Connor brought up an interesting point. The motor does have auto thermal trip (60 deg C) in it but it is not tripping. Even at 110 deg outside, running all day, it's not tripping. Between that and the motor rated at 3.5 amps but only pulling 3.2 amps), leads me to think the motor is running right up at full load but not too much if any over it.
 
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Old 08-22-08, 03:52 PM
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Oh and another reason the unit has been going through motors might be, the Jamoke service company the previous homeowners kept hiring to do the replacement, has been installing open frame motors!
 
 

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