Diagnosing Condenser Fan Motor and Blade Failures

cond fanThe system ran trouble free for years (don’t they all?) Then the condenser fan motor failed. The failure may be electrical or mechanical, it doesn’t really matter because the end result is the same; you need to replace the motor.
You install a new motor (OEM or after market; again, it doesn’t really matter). You even took the due diligence of replacing the capacitor like any technician worth their salt would. Two months later the owner calls back with another “no cooling” call. You arrive at the job to find the condenser motor failed again.

What happened? Are the new motors and blades inferior to the old ones? Not likely.

There are three possible causes: The condenser coil, the application, or you.

The first, and most likely, culprit is a dirty condenser coil. Everyone knows the load on a forward curve blower increases when negative pressure is decreased. It’s easy to see, when you remove the blower compartment door, the fan motor amperage increases. But propeller type fans are just the opposite, when you increase the negative static on them, their load increases.
What would increase the negative static on a condenser fan? A plugged coil for starters. A dirty coil will increase the load on the motor, stress the fan blade, and as an added bonus, it will cause the discharge air temperature to increase. So think about this for a moment; the motor and blade are under higher stress due to the plugged coil, the motor is air cooled, and because the coil is plugged, the air flowing over the motor is hot. This all equals a stressed out, overheated motor and blade; can you say “recipient for disaster”?
The second likely cause of a repeat failure is the application. Condenser fan motors come in three flavors: shaft down, shaft up, and horizontal shaft. Some motors are position sensitive due to the location of the thrust bearings. If you install a motor that’s only rated for shaft up operation with its shaft down, the thrust bearing may not be able to take the load. The good news is, most replacement motors are not position sensitive.
The third likely cause is you. I know you measured where the blade was on the motor shaft, but guess what, that doesn’t matter. What does matter is where it is located in the fan deck venturi. The venturi is the flared out portion of the fan deck, it’s where the seal between the fan blades and the deck is formed. If the blade is not positioned correctly the air seal will “slip” causing the blades to flex up and down as the seal makes and breaks. This continuous flexing will eventually fatigue the blades to the point of cracking.
To summarize;
• Dirty coils increase the load on prop fans and motors.
• An incorrectly positioned blade can flex until it cracks.
• Some motors are position sensitive and will fail if misapplied.

Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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2 comments on “Diagnosing Condenser Fan Motor and Blade Failures
  1. Great post Thanks for sharing.

  2. Darrel Naone says:

    Keep working ,terrific job!

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