Posted: 9/3/2008 2:11:56 PM EDT
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The other day the condenser fan motor on my home A/C unit failed. It's a 1/4 horse 230 volt shaft down motor. It's got 3 wires and is on a double capacitor. I went to the local appliance supply store and they had a 1/3 hp motor they told me would work fine. A local A/C repair guy who was waiting in line confirmed that and gave me a primer on how to wire it since it had 4 wires instead of 3. I walked out with the motor and a 12.5 mf 370 volt run capacitor. Fast forward to having it mounted the two voltage wires in place on either side of the contactor, and the run capacitor connected to the brown and brown/white wires. Turned it on, fired up and worked fine. 2 hours later it shut itself down, the motor was very hot but still turned freely, my infrared thermometer read 228 degrees on the motor housing (it's 105 out today). I checked my wiring and called the A/C guy I talked to at the store. He confirmed my wiring was correct and said I probably got a bad run capacitor, to go buy another one. I did, came home today and hooked it up. It ran for two hours and the fan stopped and again the temp is 228 at the motor housing. The motor is marked 208-230 volts (as is the old one) and I've still got 237 volts at the contactor terminals I'm connecting it to. The fan IS running in the correct direction. The capacitor is the correct one as called for on the motor label. It's a GE motor and says it's a condenser fan motor on the box. If I let it cool down again it will start back up and run till it hits 230 again and shut down. I know these motors are thermally protected so I'm assuming it's hitting it's max temp and cutting itself off. Do I just have a bad motor? The appliance parts store said no returns on motors and I foolishly paid in cash. Any other things it might be? |
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The new motor may have a much lower Celsius rated thermal overload. Most motors operate at 40C-70C on the overload. This means that the overload opens at 40C-70C over ambient temperature. If you put a bigger horsepower fan motor in, I doubt the motor is over-amping. Compare the Celsius rating on the new motor to the old one... AC |
Yes, I'm running one cap, the original one for the compressor, and a new one for the motor. I'm not using a jumper. I'll try this and see if it works. Thanks. |
The old motor was a 1/4 horse 1.5 amp motor. The new one is a 1/3 horse 2.4 amp motor. I checked and it's drawing 2.1 amps. Is there any reason the more powerful motor can't run the same fan the less powerful one did without overheating? |
I can't find any rating on the old motor, it just says "thermally protected". The new motor is marked 60C Ambient. It's a 2.4 max amp motor and it's drawing 2.1. |
Not sure what you mean. Is this the same question JoeyP asked?
If so, I'm about to try that now. |
*buzzz* wrong. a 825rpm motor uses a larger fan blade/more blades, less rpms. the heavier fan blade placed on wrong motor would, indeed, overwork the new higher rpm motor |
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that motor is too big. get another 1/4 horse, firstly. the amperage a fractional horse motor draws is a better indicator of properly matching motors than the actual horsepower. manufacturers judge their horses differently. the 1/4 horse motor will use a 5 or 7.5 uf cap. a motor with insufficient load will do exactly what you've described. think of it this way. that new motor is gonna generate X power. as the static drop thru the condenser and the associated load of the prop are too easy for this one, but its gotta do SOMETHING with the excess power, either kineticly, audibly, or thermally, it's droppin that power via heat. get another motor, one with FLA of around 1.8a, matched capacitor, make sure its wired right, (only the brown and brn/wht going to new capacitor, abandon the head of the dual run one, and the black and the white going to respective sides of contactor), make sure its the right/same RPM, and voltage, make sure the fan sits at approximately the same height, as close as you can, in the condenser, tighten the set screw, and wait for the coolness. |
the brn/wht striped wire is connected internally to white, eliminating any need for a jumper. besides, it wouldnt run 2 minutes if the had it shorted pole to pole across that cap. |
Yes, the rotation is correct and the blades are on the way they came off the old motor. Putting them on upside down is very obviously wrong. The motor/fan is at the top of the vertical unit and air movement through the coils, past the fan and out the top. |
A condenser fan motor that's not over amping ,is the right rpm, has the correct voltage , has the correct uf capacitor, is wired right and is moving the air in the proper direction but trips on klixon ? Check all three legs for amperage ? From memory since I don't work on single phase very often, the Start leg pulls about 1/5th the amps of the common and run. Don't hold me to that, it's been a long time and maybe things have changed in the last, oh 15 yrs or so  but if something is wrong and one of them(start) pulls too much it will overheat. That uf requirement sounds high ! I'd think 5 or 7.5...max. Don't forget to pull that little aluminum plug out of the drain hole. |
I respectfully disagree with this statement. If I had room and mounted a 3/4 hp motor in place of the existing 1/3 hp motor it would work fine. If the motor is running under amperage and kicking off on thermal overload, the motor is not rated at a high enough thermal ambient in Celsius, the motor overload is bad, it is somehow wired wrong or has a bad run capacitor. AC Licensed Master Mechanic and business owner |
Forgive my ignorance on AC motor terminology. I've got 4 wires, black and white which the diagram says goes to power and brown and brown/white which go to the run capacitor. Amperage across black is 2.06. White is 2.06. Brown is 1.7 and Brown/White is 1.7. I'm not sure which of these are start, run and common. I'm very familiar with running amperage draw tests on DC systems, I'm assuming it's done the same on AC systems except with the meter set to AC instead of DC.
The old 1/4 motor required a 5 uf capacitor, with it hooked in the 1/3 hp motor won't even start. The capacitor specs are on the motor housing, so unless it's mismarked 12.5 is correct.
The local A/C guy warned me about that so I did it first thing. This has me whipped. The only way it runs is the way I first wired it, the way the A/C guy told me to, white to one side of the contactor, black to the other and brown and brown/white to opposite sides of the run capacitor. The jumper from C on the old dual capacitor to the new capicitor didn't work, it just popped the circuit breaker on one side and on the other side the fan wouldn't run. The temp goes up a degree every few minutes, but doesn't cool that fast, so after a few run cycles it overheats and shuts down. It will run all night, but as soon as it gets hot out the motor can't cool enough. If I run a box fan at it so it cools between run cycles, then it's fine. Something is wrong. If I can't figure it out tonight or tomorrow morning I'm going to return the motor, ask for my money back and get a 1/4 hp motor with the same amp rating as the original. If they won't give it to me then I'll accept an exchange for another 1/3 hp motor, which is all they've got. I'm having a hard time believing that all the extra heat is caused by going from 1/4 hp to 1/3 hp, but I'm grasping at straws now. |
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The two brown wires took the place of the old 3 wire setup. You checked both sides of the capacitor so that covers it. The amperage on that still seems about double what it should be to me. It sounds to me like you've got a defective motor. I ran a 1/2hp motor in place of a 1/4hp on my house for about 5 years so highly I doubt the "extra" hp adds appreciably to the heat. |
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Trouble shooting without seeing it is kind of hard but it sounds like you connected the 2 brown wires to the start/run capacitor for the compressor. If you have 2 brown wires on the motor, it should have it's own capacitor and not be connected to the compressor cap. It should be around 5mf/370 vac for that size motor. Also check the cap to see if it is swollen and ohm it out too...after disconnecting the power, take the wires off the cap and short it out as they can hold a charge, just put a screw driver across the terminals to discharge it then use your multi meter to ohm it out, touch the leads to each side of the terminals and the reading should go up and then back down, you have to reverse the leads each time you check it or it won't do anything, check it for ground by putting one lead on a terminal and one on the case of the cap, do this for each terminal. you also have an indentefied side for the hot lead, the brown/white lead, it will have a mark on it or a bump on the top of the case, you will be able to tell if it is marked. PM me if it works i would really like to know. good luck. ETA. Page 2 ownage since the above poster didn't claim it. 
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The motor has it's own capacitor, I bought it new with the motor and it's the specs listed on the motor info plate, which is 12.5 uf, 370 volts AC. Brown to one side, brown white to the other. I see no difference between one terminal and the other, no marks, bumps or anything to make me think it makes any difference which terminals the wires connect to. Does it make a difference? I'll try switching them, but by now I've had it disconnected and reconnected so many times I'd think I would have had to have it both ways by now, but I'll switch it to make sure. I even tried a second capacitor thinking that the first one I bought could be bad. It overheats with both. The old dual capacitor is only used for the compressor now. |
Tell me about it. It's even pissing me off more than the fact that the original motor fell off my bench and landed on my big toe and made it hurt like hell and turn strange colors. |
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I would almost guarantee you are doubling the capacitors. The old wiring was going to the split capacitor right? Have you disconnected that wiring to the old cap? The new motor uses a internal ground for the cap, hence the extra wire. Abandon the old wiring, hook you black and white directly to the contactor and the brn, and brn/wht to the new cap and it should work fine. |
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I sell replacement motors almost daily for a major HVAC/R Wholesalere and oversizing a motor will definitely cause it to overheat and trip on internal thermal overload. Download the "FascoFacts" motor manual and it will tell you virtually the same thing. Your motor is oversized for the load, causing it to overheat and trip on internal thermal overload. You need to select a replacement motor based on amp draw of the defective motor. Here is text copied from this manual. The chart is on page 33 www.fasco.com/pdf3/fasfacts.pdf When selecting a replacement motor using current (amp) rating comparisons, it is important to choose a replacement motor that has the correct load carrying capability. An overloaded motor will overheat causing the thermal protector to trip. Selecting too strong a motor will also cause overheating. The table below gives the amp range of an acceptable replacement. For instance, a defective motor with a nameplate rating of 2.35 amps would be closest to 2.4 amps and can be replaced with a motor of the same voltage having a nameplate current rating between 2.4 and 3.0 amps. Basically you oversized your replacement motor. According to the replacement chart you should replace your motor that used to draw 1.5 amps with one that draws between 1.5 and 1.87. The motor you selected at 2.4 rating and only drawing 2.1 is oversized and your problem. Sorry, but I go through this regularly with techs that don't go with Motor Mfg. recommendations. Replace you motor with one in the correct amp draw for the original motor/blade combination and your problems will go away. |
The new motor was never hooked up to the original split cap, it was always hooked up exactly as you describe, to it's own cap, a new one that matches the motor that was sold to me with the motor. |
Apparently this is the correct answer, which Briansmech also mentioned. I replaced the 1/3 hp 2.4 amp motor with a 1/4 hp 2.0 amp motor (and matching run cap) yesterday morning and it's been fine since. Thanks to all that took the time to respond. Edited to add: That pdf is a great resource, thanks. I've saved it to my local drive. |