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outdoor unit fan speed

oldtrail
oldtrail Member Posts: 2
in A-C, Heat Pumps & Refrigeration
Is it proper, safe and efficient for the unit to increase the fan speed from 875 to 1025 rpm. We have 3 identical units and one was done so by a contractor.

Comments

  • JUGHNE
    JUGHNE Member Posts: 11,266
    edited July 2016
    I had my but kicked on a service call, the cond fan motor was shutting down on overload. Replacement motor did the same. A salesman who kept all old specs of ancient equipment, of which this was one, helped out. He found that the original motor speed was 825 or so and the replacements were both 1075. This had a huge blade, the high speed was moving too much air and overloaded the motor.
    The unit was on a roof and this fan cycling was not noticed......but there were 2 dead 5 ton compressors lying next to the unit. :*

    You could measure the amp draw and see if it is near or above the nameplate rating.
    SWEISolid_Fuel_Man
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,261
    You might be able to get a fan with a different blade pitch so it moves the right amount of air without overloading the motor. Or if you are very skilled, you might be able to bend the blades on the current fan till you get a suitable pitch.
  • aircooled81
    aircooled81 Member Posts: 205
    Proper, not sure if anything is proper these days.
    Safe, as long as the motor doesn't over amp, no danger here.
    Effcient, well theres a few ways to look at that. It may have been efficient to get a motor that was on the shelf, rather than back-order one or spend time trying to hunt one down at the lower less common rpm.
    The condenser will likely get more air across it, this would allow for a bit more cooling, lower head pressure, compressor doesn't work as hard, so that might be effcient.?
    The safest bet is to take an amp clamp, measure the amperage of the motor (like mentioned above) and see if the fan is over amping.
  • Techman
    Techman Member Posts: 2,144
    Put the proper motor in. I go out to my truck and get it,and cap and rain shield and a can of Never-Seize. Done and over w/.
  • John Mills_5
    John Mills_5 Member Posts: 952
    Yea, you'll shut down that 1075 in no time unless you go to a lot larger motor. Then you'll probably tear up the fan blade at some point.
  • JUGHNE
    JUGHNE Member Posts: 11,266
    Run time to OL shut down for my situation took a while depending on amb temp. The compressor probably would go off on its OL limit. Fan would cool quicker than compressor.
    (There might have been 3 compressor carcasses on the roof.....typical "beer joint/bar" equipment of mismatched components)
  • John Mills_5
    John Mills_5 Member Posts: 952
    I remember a friend of mine changing a fan motor in a store I ran back in 1988. A Carrier round, the first ones made from 1966. He put the motor in, it ran 1 minute and quit. Cooled down, ran another minute. That's when we thought we better check RPM!
  • John Mills_5
    John Mills_5 Member Posts: 952
    Oh, another story. Customer down the street, retired guy, his rental, old Lennox. Would go off on high head for no reason at least apparently. So finally he pulled up a lawn chair and sat 3 HOURS til the fan motor finally quit and the pressure switch tripped. Turns out the motor was replaced a year or so prior by someone else, we were too expensive. They put a 1075 on not an 825.

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