Blower CFM Rating vs. Motor Overload Situation (theoretical question)
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  1. #1
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    Blower CFM Rating vs. Motor Overload Situation (theoretical question)

    This much I understand: If a blower doesn’t get enough supply air to feed it, the fanwheel will overspeed & cause the motor to overload. This is caused by the fanwheel not getting enough air to “bite into” & provide adequate resistance against the motor; the motor then runs freely & overloads. But what about the other way around? What will happen if the return air path is too restrictive for a particular blower?

    Example, if a blower’s rating chart shows the highest SP (lowest CFM) that it is normally operated at, what will happen if there is too much restriction on the outlet/pressure side? In other words, if a blower is rated at 1000CFM at it's highest normal SP rating, what will happen if it is only allowed to deliver 700CFM? Will that cause the motor to overload as well? Or would the motor NOT overload, but the fanwheel would just sort of push up against a resistant wall of air?

    Note: I recognize it wouldn’t be efficient, but my question is strictly about motor loading – nothing about efficiency, proper coil airflow, heat rise, etc., etc..)

    I just love learning about all this stuff - Thanks again to everyone…

  2. #2
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    a fan motor will only turn at it synchronous speed, minus slip, so it wont exceed it's rated load, would draw more if the pitch of the fan was too much and it was trying to move too much air

  3. #3
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    I believe your question is redundant. Restricted return air is the same as underfeeding and the motor/blower will free wheel. = no load. Put an amp probe on a squirrel cage motor. Plug it in. Open draw will result in low amp draw. Block part (1/3) of the return path with card board. The amp draw will approach normal. The same can be said if you do not block any of the return path, but block 1/3 of the supply path. One or the other......... or an equal combination of both return and/or supply static resistance will cause the motor to approach full load amps.
    "Surprised ?! If I woke up tomorrow with my head sewn to the carpet, I wouldn't be more surprised."
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  4. #4
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    And yes the motor will over load... the only fan/motor configuration that can "try" to push more than capacity is a standard fan "blade". It, if restricted, will "over work" it's self "trying" to push more air. I'm sorry, I don't mean to be anthropomorphic.. but the squirrel cage is the only HVAC set up that will under work itself to death.
    "Surprised ?! If I woke up tomorrow with my head sewn to the carpet, I wouldn't be more surprised."
    Clark Griswold

  5. #5
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    Quote Originally Posted by lra View Post
    I believe your question is redundant. Restricted return air is the same as underfeeding and the motor/blower will free wheel. = no load. Put an amp probe on a squirrel cage motor. Plug it in. Open draw will result in low amp draw. Block part (1/3) of the return path with card board. The amp draw will approach normal. The same can be said if you do not block any of the return path, but block 1/3 of the supply path. One or the other......... or an equal combination of both return and/or supply static resistance will cause the motor to approach full load amps.
    Well stated. Less air across motor, no matter the cause, higher motor temps. This is why if you have a higher then normal SP you can overcome it with a larger HP blower motor of the same RPM.
    Government is a disease...
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  6. #6
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    Not being argumentative - just trying to clarify & understand the concept...

    Ira: my question may be redundant, but I’m not really sure.

    It would seem that if the supply side is blocked by ‘x’ percent, the wheel would be turning in a more vacuous atmosphere, and there would be very little pressure ‘buildup’ in front of the wheel - thus it would spin more freely due to reduced air friction.

    If the return side is blocked by the same ‘x’ percentage, there is no vacuum formed behind the wheel (there is plenty of air for the wheel to bite into) and the pressure ahead of the wheel would also provide resistance against motor torque (???). Of course, I could be TOTALLY wrong – it’s the reason I asked. It just seems logical…

    Roboteq: I should have explained my question more thoroughly. I’m actually referring to electrical overloading, versus overheating due to inadequate airflow over the motor itself. For example, what about situations where the drive motor is not situated in the airstream, and therefore does not rely on that airstream for cooling?

    Thank you both for your replies - it is MUCH appreciated!

  7. #7
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    OK, I'm more on track with the intent of the question. As I think about it though, it still comes down to heat energy within the motor. The more resistance, or less air, will create more electrical heat energy even if the motor is out of the air stream and not reliant on air-over for external cooling.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  8. #8
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    Yes, the heat energy of the motor is what I apparently don’t understand. If a blower that’s rated for a minimum ‘x’ SP were to run unrestricted, the charts show a ‘motor overload’ situation. Seems kind of counter-intuitive to me – there’s less work (resistance) for the motor to overcome vs. when it is actually moving more air.

    That’s what got me thinking (always a dangerous thing). How does a motor ‘overload’ when it’s producing a minimal amount of work? What about when it’s producing more than it’s rated amount of work, i.e., when the supply side is restricted? Hmmm….

    My car's engine doesn't 'overload' when at idle. Of course, I don't want to see if the connecting rods come through the sides of the block if I were to floor it in neutral. I know - apples & oranges. (I'm not actually an electrical engineer, but I play one on T.V. )

  9. #9
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    If a blower has less air from a restricted return there is less matter to push allowing the motor to speed up. The motor is not designed to speed up and so electrical heat energy is created.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  10. #10
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    Oops

    My BAD - I meant to say RETURN not SUPPLY:
    That’s what got me thinking (always a dangerous thing). How does a motor ‘overload’ when it’s producing a minimal amount of work? What about when it’s producing more than it’s rated amount of work, i.e., when the supply side is restricted? Hmmm….

    So it should have read:
    That’s what got me thinking (always a dangerous thing). How does a motor ‘overload’ when it’s producing a minimal amount of work? What about when it’s producing more than it’s rated amount of work, i.e., when the RETURN side is restricted? Hmmm….

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