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Thread: measuring blower motor current

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    DIY Junior Member leeelson's Avatar
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    Default measuring blower motor current

    I've noticed that my electricity consumption has increased recently and for various reasons I suspect an HVAC squirrel cage blower motor. Although it doesn't seem excessively hot, I want to find out if it's drawing more than the rated current. It has a shielded AC cable running to it. What is the best way to measure current? Can I use a clamp meter to do this or would it make more sense to do something like measure impedance with the power off? I don't want to do any dis-assembly and I do have some knowledge of electricity.

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    DIY Senior Member Murphy625's Avatar
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    Use a clamp meter on an unshielded wire.

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    Electrician ActionDave's Avatar
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    You need a clamp on meter and a spot where the sheathing is stripped off; at the breaker or at the disconnect will work. If the unit is pulling more than its rated current then the overloads should trip.

    If you power bill is way up I should think the unit is running more than it has in the past rather than drawing excessive current. It's also possible your utility has raised its rates.

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    Moderator & Master Plumber hj's Avatar
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    A clamp meter only works on a single conductor so you would have to measure the load inside one of the junction boxes at either end of the cable.
    Licensed residential and commercial plumber

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by leeelson View Post
    I've noticed that my electricity consumption has increased recently and for various reasons I suspect an HVAC squirrel cage blower motor. Although it doesn't seem excessively hot, I want to find out if it's drawing more than the rated current. It has a shielded AC cable running to it. What is the best way to measure current? Can I use a clamp meter to do this or would it make more sense to do something like measure impedance with the power off? I don't want to do any dis-assembly and I do have some knowledge of electricity.
    If it is drawing more than what it is designed to draw then it will stop drawing at all. The motor will not cause your power bill to go up simply because it will burn up before you see any change in your power consumption.

    Should it be warmer than usual and the compressor be working more then the consumption will go up.

    Just what are your plans after you take this reading?

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    Jack of all trades DonL's Avatar
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    I squirrel cage blower will normally draw less current if the fins are dirty, because it is moving less air.

    Many of the motors have a place to add bearing lubricant.

    A clamp on amp meter can tell if the compressor is operating properly, without needing to read the refrigerant pressures.

    You may be barking up the wrong tree.

    How were you going to measure the motor impedance ?

    Setting the temperature a few degrees higher can save a lot of money.


    Good Luck.
    Last edited by DonL; 09-11-2013 at 09:52 AM.
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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by DonL View Post
    I squirrel cage blower will normally draw less current if the fins are dirty, because it is moving less air.

    Many of the motors have a place to add bearing lubricant.

    A clamp on amp meter can tell if the compressor is operating properly, without needing to read the refrigerant pressures.

    You may be barking up the wrong tree.

    How were you going to measure the motor impedance ?

    Setting the temperature a few degrees higher can save a lot of money.


    Good Luck.
    by measuring the inductance first
    I am definitely not a pro plumber, but I am a pro crastinator

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    DIY Junior Member leeelson's Avatar
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    Quote Originally Posted by jwelectric View Post
    If it is drawing more than what it is designed to draw then it will stop drawing at all. The motor will not cause your power bill to go up simply because it will burn up before you see any change in your power consumption.

    Should it be warmer than usual and the compressor be working more then the consumption will go up.

    Just what are your plans after you take this reading?
    The motor (Dayton 4K259G) runs a fan that circulates air (no A/C). It often runs 8 hours/day. The fan is rated at 6.7 amps @ 115 volts=771 watts. If run for 8 hours, this is 6.164 KWH/day, a significant fraction of my 19 KWH/day average use. If it is pulling 30% more power than it should I doubt it would throw the 20 amp breaker but could account for the 10% increase I've seen in power use.

    The troubleshooting guide for the motor says to "check for high current draw". I suppose if its high, I'll replace the motor since I can't find any evidence of replacement parts.
    Last edited by leeelson; 09-12-2013 at 03:22 PM.

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    DIY Junior Member leeelson's Avatar
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    Quote Originally Posted by ActionDave View Post
    You need a clamp on meter and a spot where the sheathing is stripped off; at the breaker or at the disconnect will work. If the unit is pulling more than its rated current then the overloads should trip.

    If you power bill is way up I should think the unit is running more than it has in the past rather than drawing excessive current. It's also possible your utility has raised its rates.
    I have a good handle on how much it runs and although it is a factor, it doesn't account for the whole increase in usage (I don't understand your comment about utility rates since that doesn't affect how much power I use in a month.) The power usage is up about 10%, much more when I have all 3 similar fans running.

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    DIY Junior Member leeelson's Avatar
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    Quote Originally Posted by DonL View Post
    I squirrel cage blower will normally draw less current if the fins are dirty, because it is moving less air.

    Many of the motors have a place to add bearing lubricant.

    A clamp on amp meter can tell if the compressor is operating properly, without needing to read the refrigerant pressures.

    You may be barking up the wrong tree.

    How were you going to measure the motor impedance ?

    Setting the temperature a few degrees higher can save a lot of money.


    Good Luck.
    My first post was a little misleading: its just a motor and fan, no A/C or compressor involved. Looks like I can get a power reading by using a clamp meter where the timer switch controls the power: there are separate power wires there. Thanks for your input.

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    Moderator & Master Plumber hj's Avatar
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    IF something happened at the fan so it is moving, or trying to move, more air, that would overload the motor, but that is about the only thing that would change to power usage.
    Licensed residential and commercial plumber

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    Electrician ActionDave's Avatar
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    Quote Originally Posted by leeelson View Post
    I have a good handle on how much it runs and although it is a factor, it doesn't account for the whole increase in usage (I don't understand your comment about utility rates since that doesn't affect how much power I use in a month.) The power usage is up about 10%, much more when I have all 3 similar fans running.
    Often when someone thinks they are using more power they base it on the size of their electric bill. I see you are talking about actual usage and not just price so ignore that part of my post.

    You will be able to check amps at the switch. If you are drawing more than name plat amps a.k.a FLA then the motor would kick off on thermal or burn up at some point. It is unlikely it was pulling thirty percent less and unless it was there is no chance for it to be able to pull thirty percent more.

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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by leeelson View Post
    My first post was a little misleading: its just a motor and fan, no A/C or compressor involved. Looks like I can get a power reading by using a clamp meter where the timer switch controls the power: there are separate power wires there. Thanks for your input.
    I see you mentioned that this is just a squirrel cage blower and only an air mover, but is it controlled by a variable speed frequency inverter drive? I ask only because if it is now running at a higher RPM than it used to, it will be drawing more current than it would at a lower RPM. You did mention a shielded cable, so that's why I'm asking.
    Last edited by BobL43; 09-13-2013 at 03:47 AM.
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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by leeelson View Post
    My first post was a little misleading: its just a motor and fan, no A/C or compressor involved. Looks like I can get a power reading by using a clamp meter where the timer switch controls the power: there are separate power wires there. Thanks for your input.

    My mistake. That is why it is not good to assume.

    It may be hard to tell if it is Using more power then it did. You have nothing to compare it to.

    As far as I can tell, that motor is rated for 7.6 Full Load Amps.

    As long as it is not above that then it is working within limits.

    There are a few variables that will affect the load on the motor.


    In my area sometime they would estimate my usage, because they did not want to pay a meter reader person.

    Now we have SmartAss meters, And my bill went down.


    Good Luck.
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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by BobL43 View Post
    I see you mentioned that this is just a squirrel cage blower and only an air mover, but is it controlled by a variable speed frequency inverter drive? I ask only because if it is now running at a higher RPM than it used to, it will be drawing more current than it would at a lower RPM. You did mention a shielded cable, so that's why I'm asking.

    Good question Bob.

    I think the way they use them blowers there could be a small flex conduit, to the control box. Then there may be a thermostat if used as a attic fan.

    I remember the good old days of proper attic ventilation.

    I grew up with no air conditioning. When You went outside to play.

    People are spoiled now a days.


    Have a Great Friday the 13th.
    Last edited by DonL; 09-13-2013 at 06:04 AM.
    Theory only works perfect in a vacuum.

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