Tuesday, July 30, 2013

Replacing Condenser Fan Motors

Hot weather always brings on a lot of condenser fan motor replacements. Although it is not too difficult to change a condenser fan motor, it is still possible to do it incorrectly. Of course you need a motor that operates at the same voltage, phase, and frequency of the original motor and it should also be the same horsepower. However, I would like to mention a few items to check that I often find techs have overlooked. First, make sure the motor you plan to use is actually designed for a condensing fan motor. Specifically, is it sealed and is it designed for vertical shaft installation (assuming a typical vertical mount, induced draft condenser fan). Motors with air vents on the sides or top should not be used outside. Also, all motors are not rated for vertical shaft installation. Some motors are designed for horizontal shaft mount only because they do not have the necessary thrust bearing to take the downward force against the bearing when the fan pushes air up. Next, is the motor the correct RPM? Some condenser fan motors turn 1075 RPM while others turn around 850 RPM. They are not interchangeable. If you can’t read the label on the old motor, take it apart and look at the number of poles. A 6 pole motor will be a 1075 RPM motor and an 8 pole motor will be an 850 RPM motor. Does the new motor turn in the correct direction? If you cannot see the markings on the old motor, you can tell by looking at the fan blade. First, the cups of the blades should be facing up on an upflow blower. Next, the blade should turn in a direction that allows the lower part of the blade to hit the air first. If the blade turns clockwise looking down on it, this is called clockwise shaft end, or counter clockwise lead end because the lead end rotation is always opposite the shaft end. Some motors specify the rotation looking at the shaft while others specify looking at the opposite end where the leads come out. If you are using a general purpose replacement motor, such as a Rescue Motor, you can change the motor rotation if you need to. Some of these motors use different size capacitors for different horsepower ranges. Make sure the capacitor you are using fits the application. Most likely, the capacitor for the old motor will not be the correct size. Most sealed condenser fan motors have weep holes on the bottom that come plugged with plastic caps. You should remove these when installing the motor. The idea is to let condensation out. The motor will last longer if you open the bottom weep holes.   After installing the motor and fan, turn the unit on and then off again to see which way the blade is turning. If it is turning in the wrong direction, change the rotation leads on the motor. Changing the power leads will have no effect. Once you establish that the fan is turning in the correct direction, operate the unit and check the motor amp draw. If it operates close to  the manufacturer’s FLA specification for your particular application, you should be good to go.

9 comments:

  1. I have 2 questions for u
    Why do some condenser fan motors have the one side on the bottom of the contactor
    I get that’s it’s the common side (single pole) but wouldn’t that be always energized ??
    I’ve looked at the diagram on those style motors and doesn’t look like there’s a reason for it
    My other question does replacing a 825 rpm with a 1075 have a negative impact
    (Same hp uni motor )
    Only thing I can think of is subcooling faster
    (I would never step down in rpm ofcourse just curious if stepping up has any negative impact
    Thanks

    ReplyDelete
  2. I will answer your second question first. Replacing an 825 RPM fan motor with a 1075 RPM fan motor will almost certainly result in high amp draw of the fan motor and rapid failure. The amount of horsepower required of a fan motor increases by the cube of the RPM increase. In this case, that would be 1.3 cubed, or just over twice the original horsepower. The other problem would be the fan blade itself. I have seen blades ripped by the force created by turning too fast.
    As to your first question, when you say the bottom of the contactor I am assuming you mean the line side where power comes in. Many systems today do use single pole contactors, which does mean that the leg of power connected to the solid brass side (the unbroken side) is energized all the time. For a 240 volt single phase load this is not a problem, breaking just one leg breaks the circuit. Typically these systems also use dual capacitors that are controlled by the single pole of the single pole contactor. So the contactor is controlling both the compressor and the condenser fan motor through that single pole that supplies power to the dual capacitor.

    ReplyDelete
    Replies
    1. Can you explain the math again? You said the hp required is the cubed of rpm increase? So it increases 250rpm so you cube that or divide by 3? Or you are saying to divide 1075/825 which equals 1.3 so 1.3^3 = 2.2 or would it be .3^3 as its rpm increase is .3?

      Delete
  3. is it ok to replace condenser fan motor with low wattage/amps??does it affect the cooling and compressor?

    ReplyDelete
  4. "I love how the colors in this picture blend so beautifully. Mother Nature is an artist!"

    ReplyDelete
  5. "I can't get over how clear the water is in this picture. It's like paradise

    ReplyDelete
  6. "Whoa, talk about #travelgoals! Adding this to my bucket list for sure."

    ReplyDelete
  7. "This photo reminds me why I love exploring and discovering new places. Thanks for the inspiration

    ReplyDelete
  8. Can you replace a 1075 rpm motor with an 825 rpm motor as long as you change the capacitor as well?

    ReplyDelete