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My A/C unit is fairly old but has worked fine up until now. A few days ago it was getting hot enough to start running it more and I noticed that the fan on the condenser was taking a while (30ish seconds) to spin up. I figured the capacitor needed to be replaced so I stopped running it and ordered a new one. New capacitor on yesterday and it worked fine, fan started as it should (but didn't sound happy and was vibrating a bit) and it cooled the house from 79 to 75, next time it kicked on it the fan ran for a couple minutes then just stopped and hummed. I figured the motor was fried from running with a bad capacitor for an unknown period of time. 

I ordered a new motor (closest I could find on amazon). The part number off the old one was listed in the question section as compatible with this new version. The only difference was 1075rpm for the new one instead of 850 on the old one. Both same 1/3hp rating. 

Today after work I put the new fan motor on with another new capacitor (called for 5uf instead of 7.5) and started it up. All sounded good, house was cooling down, fan motor was running quietly, then after 10-15 minutes it stopped again and was just humming, not spinning. I let it cool down and started it back up, ran fine again for 15 minutes then the fan slowed to a stop and the motor started heating up (it was around 109°F while running, started climbing from there once it stopped before I killed the power). 

Any ideas on what to check next or am I over my head? The contactor looked a bit pitted on the points but pulled down fine when the power kicked on, I didn't check voltage on it yet. 

Are you sure you didn't get a defective fan motor?  You should be able to check the resistance on the windings with a multimeter.  I would also check to see what the amperage draw on the motor is.  Could be bad bearings too.  

In reply to EvanB :

The problem is the 1075 RPM vs. the 850 RPM of the old fan also assuming that you reinstalled the old fan blade.  The pitch of the blade is probably pretty steep to move the most air with the 850 RPM motor.  They used the lower RPM motors because that helps keep the condenser fan noise down.  By using that blade with a higher speed motor you're overloading the new motor as it will require quite a bit more than 1/3 HP to spin that same pitch blade at 1075 RPM.  The motor is trying to do the work (motors are dumb that way) but the higher amperage is causing the motor to overheat and activate the internal thermal overloads.  The solution is to source a comparable 850 RPM motor.  If you have a meter with an amp probe feature you'll be able to see the higher amperage draw vs. the nameplate rating.

The "river" site is great until it isn't.  Poke around on more HVAC / trade related sites and see if you can match something up, SupplyHouse is a good resource for a lot of common HVAC, plumbing and heating parts for example.  If you want to share the make, model number and serial number of the condensing unit I may be able to assist.

Edit: Send a picture of the old motor nameplate if you still have it.  Those can often be sourced through sites like Grainger as well.

In reply to 11GTCS :

Yea, I didn't do enough research to find that out about the RPM ratings until after I had it installed. I'll be out of town for the next few days and the temps are supposed to be cooler so I'll see if I can find an option with the original RPM rating to order so it will be here when I get back. I didn't snap a pic of the old plate but this is the info I wrote down from it:
Emerson Model K55HXPYH-5356
HP 1/3
v 208-230
RPM 850
Part # 38878B001
Frame 48y

Edit: Most of what I am finding is 825RPM instead of 850. Am I correct in assuming a slightly lower rating wouldn't be an issue?

rustyvw said:

Are you sure you didn't get a defective fan motor?  You should be able to check the resistance on the windings with a multimeter.  I would also check to see what the amperage draw on the motor is.  Could be bad bearings too.  

Definitely an option sourcing from the next day delivery low cost source. I'll check that out tomorrow. From doing more research though it seems like the RPM bump with the same HP was not a good decision. 

What about changing the pitch of the fan's blades?

An amp clamp might be able to tell you when you get it good enough, if the motor is overloading.

Pete. (l33t FS) said:

What about changing the pitch of the fan's blades?

 

An amp clamp might be able to tell you when you get it good enough, if the motor is overloading.

Possible, or ordering a new fan. I'd rather return this motor and get one that works if that is the issue though. I've run out of time for fiddling with it until I'm back from SOFR anyway. 

In reply to Pete. (l33t FS) :

Noooooooo!  LOL.  Easier to source the lower RPM motor.  The older aluminum blade would likely crack if you tried to bend it plus balance issues if each blade isn't exactly the same pitch angle, etc.

EvanB said:

In reply to 11GTCS :

Yea, I didn't do enough research to find that out about the RPM ratings until after I had it installed. I'll be out of town for the next few days and the temps are supposed to be cooler so I'll see if I can find an option with the original RPM rating to order so it will be here when I get back. I didn't snap a pic of the old plate but this is the info I wrote down from it:
Emerson Model K55HXPYH-5356
HP 1/3
v 208-230
RPM 850
Part # 38878B001
Frame 48y

Edit: Most of what I am finding is 825RPM instead of 850. Am I correct in assuming a slightly lower rating wouldn't be an issue?

All of the problem is the higher RPM.  (I was a commercial field tech before I started the sales and project management gig).   825 RPM will work great and is likely closer to what the actual speed of the old motor was in the real world application.  (Synchronus speed vs. actual speed with some slip factored in.)  Match the frame (48Y), the voltage, HP and RPM and you'll be fine.   I'll see if I can find any links for you.

Here are a few that I found that were in stock. Not sure about quality of any of the brands. 

https://www.supplyhouse.com/DiversiTech-WG840469-1-3-HP-208-230-VAC-Variable-4-Speed-Condensor-Fan-Motor

https://www.supplyhouse.com/Fasco-D798-D798-1-Speed-825-RPM-Permanent-Split-Capacitor-208-230V

https://www.supplyhouse.com/Century-ORM5488BF-5-5-8-HeatMaster-Condenser-Fan-Motor-Deluxe-Model-208-230V-825-RPM-1-8-1-3-HP

This one is probably the best choice of the ones you have listed:

https://www.supplyhouse.com/Century-ORM5488BF-5-5-8-HeatMaster-Condenser-Fan-Motor-Deluxe-Model-208-230V-825-RPM-1-8-1-3-HP

This can be mounted with the fan shaft facing up or down, is reversible (swap a couple of plugged wires on the motor) and has mounting studs on both ends.  If the studs or fan shaft itself are too long they can be cut to the length you need, just protect the threads and keep any shavings out of the motor windings  Pull the drain plug on whichever end is facing down when installed.

As far as quality both Fasco and Century make OEM motors, this particular one is marketed more to the "van stock" or universal replacement market but quality should be fine.  

They don't list the capacitor value so that may be a question that needs to be asked when ordering  

In reply to 11GTCS :

Appreciate the help. I ordered the Century, googling the part number got me a better picture of the nameplate that listed the capacitor at 7.5 so that matches the original replacement I got a couple days ago. I'll update after the weekend when I get it in. 

You guys are awesome.  It's funny how many general DIY tasks I think about asking for advice on here.

HVAC is one of the few things that seems too daunting to try and fix myself...especially because it's usually the kind of thing that usually needs to get fixed ASAP when it breaks. 

I was going to suggest Grainger for parts.

I spent a good 3 weeks researching our condenser fan before buying one on Grainger. Ours needed a push to get going even after a new run cap.  This is what I used:

https://www.grainger.com/product/5DVX4

Apparently is like multi-fit type motor made to keep stocked on a service truck to get customers going again fast. So far, so good after 2 years.

New motor installed today, everything is working well. Thanks to 11GTCS for the education and I now know more about a/c repair than I did previously.

In reply to EvanB :

Excellent!  Glad it worked out well for you.