Motor overamping and Ohm's Law
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  1. #1

    Motor overamping and Ohm's Law

    Hello everyone,

    I realize that when a motor is restricted in some way the amps will go up (for example, an excessively dirty blower wheel). According to Ohm's Law, as electrical resistance goes up, amperage will go down. When the blower wheel is dirty (or whatever other restriction is present), this creates resistance to what the motor is trying to do. Why then does amperage increase?

  2. #2
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    Quote Originally Posted by HVAC_Austin View Post
    Hello everyone,

    I realize that when a motor is restricted in some way the amps will go up (for example, an excessively dirty blower wheel). According to Ohm's Law, as electrical resistance goes up, amperage will go down. When the blower wheel is dirty (or whatever other restriction is present), this creates resistance to what the motor is trying to do. Why then does amperage increase?
    Assuming a PSC motor, it doesn't increase, it drops.

  3. #3
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    Quote Originally Posted by HVAC_Austin View Post
    Hello everyone,

    I realize that when a motor is restricted in some way the amps will go up (for example, an excessively dirty blower wheel). According to Ohm's Law, as electrical resistance goes up, amperage will go down. When the blower wheel is dirty (or whatever other restriction is present), this creates resistance to what the motor is trying to do. Why then does amperage increase?
    Couple different things here.

    #1. I don't see Ohms law playing into this question as much as fan affinity laws do.

    As a typical centrifugal type blower moves more air, it does more work and, therefore the motor will draw a higher amperage. When you restrict the inlet or the outlet, or otherwise reduce the amount of air the blower moves, you reduce the work done and, thus the amperage draw of the motor.

    A propeller type fan behaves opposite of this when restricted.

  4. #4
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    Quote Originally Posted by jpsmith1cm View Post
    Couple different things here.

    #1. I don't see Ohms law playing into this question as much as fan affinity laws do.

    As a typical centrifugal type blower moves more air, it does more work and, therefore the motor will draw a higher amperage. When you restrict the inlet or the outlet, or otherwise reduce the amount of air the blower moves, you reduce the work done and, thus the amperage draw of the motor.

    A propeller type fan behaves opposite of this when restricted.
    I think you pretty well covered it.

    To test it get an old belt drive blower and an adjustable sheave. Move it in then out and watch the amps.

  5. #5
    Quote Originally Posted by jpsmith1cm View Post
    Couple different things here.

    #1. I don't see Ohms law playing into this question as much as fan affinity laws do.

    As a typical centrifugal type blower moves more air, it does more work and, therefore the motor will draw a higher amperage. When you restrict the inlet or the outlet, or otherwise reduce the amount of air the blower moves, you reduce the work done and, thus the amperage draw of the motor.

    A propeller type fan behaves opposite of this when restricted.
    Based on your post, it sounds like you are saying this:
    Centrifugal blower: dirty = lower amps (b/c of less air being moved)
    Propeller fan: dirty = higher amps

    Why do propeller type fans behave differently?

  6. #6
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    Quote Originally Posted by HVAC_Austin View Post

    Based on your post, it sounds like you are saying this:
    Centrifugal blower: dirty = lower amps (b/c of less air being moved)
    Propeller fan: dirty = higher amps

    Why do propeller type fans behave differently?
    You've pretty much it. A dirty prop fan won't really choke airflow as bad until it is EXTREMELY dirty, though. This is more about "restrictions" like dirty filters and coils.


    As hvacmedic said, though, this requires a "normal" motor.

    ECM motors will tend to behave differently. If my thoughts are correct, this is because the motor controls and runs faster in order to try to maintain the same airflow.


    As to your last question, I don't really know. I can tell you, though, from experience, that it happens exactly the way that I described.

  7. #7
    Quote Originally Posted by jpsmith1cm View Post
    You've pretty much it. A dirty prop fan won't really choke airflow as bad until it is EXTREMELY dirty, though. This is more about "restrictions" like dirty filters and coils.


    As hvacmedic said, though, this requires a "normal" motor.

    ECM motors will tend to behave differently. If my thoughts are correct, this is because the motor controls and runs faster in order to try to maintain the same airflow.


    As to your last question, I don't really know. I can tell you, though, from experience, that it happens exactly the way that I described.
    Thank you, jp. This is the sort of information one can't find in books. Appreciate it.

  8. #8
    Quote Originally Posted by jpsmith1cm View Post
    Couple different things here.

    #1. I don't see Ohms law playing into this question as much as fan affinity laws do.

    As a typical centrifugal type blower moves more air, it does more work and, therefore the motor will draw a higher amperage. When you restrict the inlet or the outlet, or otherwise reduce the amount of air the blower moves, you reduce the work done and, thus the amperage draw of the motor.

    A propeller type fan behaves opposite of this when restricted.
    Based on the above, I'm guessing this is how faulty belts would affect these types of fans:
    Centrifugal-type: belt too loose = higher amps; belt too tight = lower amps
    Propeller-type: belt too loose = lower amps; belt too tight = higher amps

    Sound right?

  9. #9
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    Quote Originally Posted by HVAC_Austin View Post
    Based on the above, I'm guessing this is how faulty belts would affect these types of fans:
    Centrifugal-type: belt too loose = higher amps; belt too tight = lower amps
    Propeller-type: belt too loose = lower amps; belt too tight = higher amps

    Sound right?
    Hows about this...

    Centrifugal fan. Belts too loose, low amps (low output RPM, high slippage, rapid belt wear, low airflow, low work) belts too tight, approximately normal amps, maybe a bit high, rapid belt wear, rapid bearing wear due to high axial load.

    Prop fan. Belts too loose similar to centrifugal fan. Belts too tight, similar to centrifugal fans.

    Again, a slightly different set of conditions to a restriction to airflow.

    Think dirty coil, plugged or collapsed duct work, etc.


    But you are THINKING, and that is very important to UNDERSTANDING which is pivotal to LEARNING.

    Keep it up, keep asking questions like this. This is one that blows the minds of experienced techs.


  10. #10
    Quote Originally Posted by jpsmith1cm View Post
    But you are THINKING, and that is very important to UNDERSTANDING which is pivotal to LEARNING.

    Keep it up, keep asking questions like this. This is one that blows the minds of experienced techs.

    Thanks again, jp. It helps to at least have a basic grasp of this information so when I'm doing an inspection I'm not just jotting down numbers. I love to learn this stuff. Thanks for the help.

  11. #11
    Quote Originally Posted by jpsmith1cm View Post
    As to your last question, I don't really know. I can tell you, though, from experience, that it happens exactly the way that I described.
    Found an explanation as to why the amps go down when a restriction to air flow exists with a centrifugal-type fan: http://hvacrfundamentals.********.co...fan-motor.html

  12. #12
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    keep in mind a "dirty" squirrel cage can effect amps depending on the level of dirty.

    If it's blocking the tines, then you have less air movement, aka less work, and less amps due to the lesser work.

    If you have a heavy ammount of dirty, the blower will feel the weight of the dirt, and work harder even though it's moving less air.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

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    Ohms law does not pertain to inductive loads, only resistive loads....

    The easiest way to keep it straight in your head is this;

    Get a vacuum of the Shop-Vac type. Now start the vacuum and listen to the motor speed, then plug the suction hose with your hand... What happens? The motor speeds up.. Why? The motor load drops, which causes a decrease in amperage, or more accurately, wattage.

    Now go get a box fan and do the same experiment. You will obviously need a piece of cardboard or something, start the fan and listen to the speed, then block it off... What happened? The motor slows down, why? the restriction of the air on the blades causes the motor to use more wattage.

    IMHO you need to pick up an Ugly's Electrical Handbook and keep it by the toilet to read during "quiet time"

    http://www.uglys.net/

    There is some really good information in there and will come in handy if ever doing some electrical work sizing conduits, conductors, starters etc...

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

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