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  1. #1
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    Apr 2013
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    Confused Inductive reactance and motor performance

    I am looking at and testing different scenarios on a blower unit. I found that if I cover the return duct, I have greatly UNLOADED the motor.. the rpm rises and with this, so does the inductive reactance/back EMF which actually drops the amps to the motor. If I am looking at this correctly, the load is less, the amp draw is less, but physically the heat of the motor is rising. What is causing this heat rise??? Just curious because I'm missing something here..apparently.

  2. #2
    Join Date
    Dec 2012
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    columbus, OH
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    Not enough air moving over it to cool it.

  3. #3
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    Dec 2008
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    Ontario Canada
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    Quote Originally Posted by Core_d View Post
    Not enough air moving over it to cool it.
    +1

  4. #4
    Join Date
    Aug 2001
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    Mechanicsville, Virginia
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    When a PSC motor is underloaded/oversized the energy normally converted to torque is converted to sensible heat.
    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    efficientcomfort.net

  5. #5
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    Quote Originally Posted by penderway View Post
    When a PSC motor is underloaded/oversized the energy normally converted to torque is converted to sensible heat.
    That sounds like a good answer. My problem with it is, the energy converted to torque is simply no longer present, as watts have decreased. Not saying your wrong, could you elaborate?

  6. #6
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    Aug 2001
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    Mechanicsville, Virginia
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    The input power (watts) has decreased yes, so less work is being done, but the work being done has changed from mostly mechanical and a little heat to now less mechanical and more heat. The ratio of mechanical energy versus heat energy (efficiency) has changed.
    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    efficientcomfort.net

  7. #7
    Join Date
    Apr 2013
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    Colorado
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    Makes sense..

    Quote Originally Posted by penderway View Post
    The input power (watts) has decreased yes, so less work is being done, but the work being done has changed from mostly mechanical and a little heat to now less mechanical and more heat. The ratio of mechanical energy versus heat energy (efficiency) has changed.
    Maybe what I need is a phasor diagram which shows the relative magnitudes and angles of the currents in and voltages across the run & auxiliary windings. Preferably under full load and no load conditions. I DO NOT doubt your explanation at all and appreciate your input. I want to move a bit deeper and gain a deeper understanding of what's going on.

  8. #8
    Join Date
    Aug 2001
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    Mechanicsville, Virginia
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    Quote Originally Posted by taggs56 View Post
    Maybe what I need is a phasor diagram which shows the relative magnitudes and angles of the currents in and voltages across the run & auxiliary windings. Preferably under full load and no load conditions. I DO NOT doubt your explanation at all and appreciate your input. I want to move a bit deeper and gain a deeper understanding of what's going on.
    I can relate to that. Unfortunately I'm lost at "angles of the currents". BTW, Core_d and LKJoel are correct. The temperature rise occurs when more heat is produced than dissipated. But I'm sure you already knew that and are looking for a deeper meaning.

    Also, should you encounter a troll in this forum do us a favor and don't feed them.
    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    efficientcomfort.net

  9. #9
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    Dec 2012
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    Feel free to post any good links you come across on the subject.

  10. #10
    Join Date
    Apr 2013
    Location
    Colorado
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    I've got some ideas

    Quote Originally Posted by penderway View Post
    I can relate to that. Unfortunately I'm lost at "angles of the currents". BTW, Core_d and LKJoel are correct. The temperature rise occurs when more heat is produced than dissipated. But I'm sure you already knew that and are looking for a deeper meaning.

    Also, should you encounter a troll in this forum do us a favor and don't feed them.
    Tomorrow, I will be in the lab. I can try to put together a test bed and make some measurements.

  11. #11
    Join Date
    Aug 2012
    Location
    Gulf Coast
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    Well, I thought matter/energy cant be destroyed, which means the heat could rise because energy has to go somewhere, and it is not going down the duct in the form of cfm.

    Sent from my SCH-I500 using Tapatalk 2

  12. #12
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    Apr 2013
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    Colorado
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    Core_D, Will do.. Thanks for the insight on this. I have a couple of projects I'm working on concerning phase shifts provided by the run and start caps and how that is happening, too. So will be glad to share what I find out.

  13. #13
    Join Date
    Nov 2006
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    Quote Originally Posted by taggs56 View Post
    Core_D, Will do.. Thanks for the insight on this. I have a couple of projects I'm working on concerning phase shifts provided by the run and start caps and how that is happening, too. So will be glad to share what I find out.
    How that is happening is the addition of capacitive reactance to the otherwise inductive character of the motor windings.

    In the case of a PSC motor, the run cap provides a comparative difference to the voltage and current relationship of the run winding, by applying a capacitive correction reactance in the aux winding, essentially creating an artificial second phase.

    A start cap behaves in a similar manner by bringing the power factor closer to unity during the low RPM of starting the motor.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

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