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  1. #14
    Join Date
    Nov 2001
    Location
    Seattle, WA
    Posts
    7,748
    Originally posted by jeffw_00
    I get it now. So there's a cfm sensor as part of the system./j
    Actually, to my knowledge their is no "cfm" sensor in any of the residential furnaces that use a VS motor. The only thing a VS motor does is attempt to maintain its preprogrammed power useage based on a external signal given to it, which usually comes from the air handler microprocessor and/or the thermostat.

    As a by-product only the VS motor tends to maintain a more constant CFM as reasons to maintain less or more CFM are created. This includes a dirty air filter. But that perprogrammed power usage which can equal a certain CFM is only within the boundries of the programming. Once these factors are exceeded then the VS motor is instructed to shut itself down or slow down to a minimun speed for it's own protection.

    They are a good thing but like everything else they must be applied properly and understood to properly trouble-shoot.
    "The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
    - Alexis de Toqueville, 1835

  2. #15
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by jeffw_00
    I'm sold on VS, and all you say, except I don't understand...

    how does lower cfm make filter MORE effective?

    /j

    If you move less air (cfm)thru any filter,it will capture more particles from the air.

    Goggle "merv" to find how filters are rated,and you'll find that they are more effective with less cfm going thru them.Same is true from Electronic Air Cleaners.

  3. #16
    Join Date
    Apr 2005
    Location
    Richmond, Virginia
    Posts
    365
    Check GE's website out. Varible speed motors are ECM's electronically communicated motors. Here's the link, http://www.geindustrial.com/cwc/prod...ecm&lang=en_US

    scroll down and read the "Benefits of Constant Air Flow". It's got a good motor curve diagram. Basically, ECMs don't ride up their curve.
    Sean Cantrell

  4. #17
    Join Date
    Jun 2005
    Location
    New England
    Posts
    906
    great links, thanks - but other than that it doesn't use sensors, it doesn't tell how it manages to maintain the constant air flow. I believe that it does, but the engineer in me is still curious 8-}
    /j

  5. #18
    Join Date
    Apr 2005
    Location
    San Luis Obispo County, CA
    Posts
    215
    Originally posted by jeffw_00
    great links, thanks - but other than that it doesn't use sensors, it doesn't tell how it manages to maintain the constant air flow. I believe that it does, but the engineer in me is still curious 8-}
    /j
    Basic, non technical explaination:

    As a filter clogs, reducing the CFM of the system, the load on the motor is reduced, so it draws less amperage. If we increase the speed of the motor, the fan will draw more air, the load will increase, and the amp draw will increase. Now, this means that the motor must be able to increase in rpm ABOVE the rpm the motor will run to with the reduced load.
    (picture how a vacuum cleaner motor increases in speed when you block the suction hose... this is because the motor is actually running under a reduced load...)

    If you partially block the suction, the motor will increase in speed, but not as much as when it is fully blocked. the partially blocked hose is moving less air. If you could increase the motor speed above what it is running partially bloked, the fan would move more air, and the flow of air could be made equal to the unblocked amount. And the load on the motor would be the same.

    So here is the trick, unrelated to HVAC question.
    Which motor has a higher load on it, a vacuum with a plugged hose or one with out a plugged hose?

    HVAC question... which blower motor has a higher load, one with a plugged filter or one with a new filter...

    its not what you might think, and motor failure occurs because of different reasons than load.

  6. #19
    Join Date
    Apr 2004
    Posts
    7,680
    Originally posted by jeffw_00
    great links, thanks - but other than that it doesn't use sensors, it doesn't tell how it manages to maintain the constant air flow. I believe that it does, but the engineer in me is still curious 8-}
    /j

    I started to answer your question first yesterday and got distracted and it was still on my screen this morning.

    The module in the ECM is programmed with software. This software has the blower curve and the specific unit in mind figured in. By monitoring the torque and the rpm's it can figure CFM because it has the physics of the particular blower in the software. The Carrier Infinity or Even the others that may offer a CFM indication simply use this feedback information to calculate the CFM and output that to the display. The Rheem RBHM uses a flashing LED to indicate the CFM. The others, may or may not indicate CFM but the motor knows.

    As for the benifits, I copied my original answer that was still parked on my screen this morning....

    ...Jeff.

    The ECM motors will push more than 1600 cfm if they have to. What they are designed for is to overcome the 3 things PSC motors cannot do.

    1. Efficiency, the PSC motor requires alot mor energy to run, they really cannot build them more efficient than they are now. Many of the higher SEER units will only acheive these numbers when matched with the VS motor because watts versus output is how efficiency is determined. Some people like to run UV lights or special filters and leave the fan on all the time. A VS fan will lower the airflow between system cycles at a significant savings over the PSC continuing to run. We are looking at maybe 100 watts versus 400-700 watts difference in this case.

    2. Airflow. As restrictions in the system change the ECM motor will change with them and keep the airflow constant. This protects your system from low load conditions caused by the changes. A PSC motor will reduce airflow as the restrcitions increase. (changing restrictions are filter building up, rooms closed off, coil dampness, etc.).

    3. comfort. Yes you can add the humidistat for all of 20 bucks right into your return duct if you dont opt for a nicer control. You can also connect a two stage thermostat and in effect, turn a single stage system into a quazi-two stage unit. When the 1st bulb calls for cooling the airflow would be reduced. This will in effect give you better humidity control and a lower sensible (temperature you read with a thermometer) output. If the demand in the home increases the second bulb will cancel the lower airflow and bring the unit up to normal capcities.


  7. #20
    Join Date
    Jun 2005
    Location
    New England
    Posts
    906
    thanks guys - so there is a sensor -it's of amperage (a proxy for torque) and is in the motor. makes sense.

    We have a humidistat next to the thermostat for the heating system. It's pretty accurate. I wonder if we can/should wire that in for the A/C.

    I think the clogged vacuum has higher torque 8-}
    /j

  8. #21
    Join Date
    Nov 2001
    Location
    Seattle, WA
    Posts
    7,748
    And when most of the VS motors first start they go through a self calibrating, self checking, self determining procedure that is mainly undetectable to us.

    Most of the shafts start backward first as a check for proper rotation. Then the module keeps tap of the first periods of rotation to see how quickly the power curve rises. Then the module actually shuts down the motor to see how long or how quickly the shafts slows downs which it equates to resistance of the air system. After all this, which takes a few seconds, the module powers the DC motor to do what the module tells it to do, which is based upon a predetermined software program installed by the manufacture of that particular air handler equipment to match its fan curve and static pressure data for that particular air moving device.

    Most manufacturers use the same VS motor but they program their own software based on their own engineering data for their specific unit. This is why interchangeability is not possible with VS blower modules unless you know how the module was programmed and can compare the different systems.

    The VS motor in no way controls static pressure in this type of application for in order to control a medium there must be a medium sensor. This is basic control application.

    You cannot control the temperature of your home properly without a thermostat located in the conditioned space! But VS equipment manufacturers have allowed our field to believe that static pressure/CFM is being controlled by their VS blower motors. It's good for sales.

    I have run across a light commercial/residential air handler/furnace that actually has a static pressure sensor and controls the static pressure in a true control application form.

    In large systems, mainly commercial, static pressure sensors are installed which signals some mechanical and/or electronic device which regulates the speed of a motor or the inlet vanes to that blower motor to maintain the static sensors set point.

    The HVAC residential market has taken a lazy way out in letting technicians/owners believe that the average residential furnace with a VS motor actually controls CFM or static pressure. One the other hand imagine attempting to explain the real workings of a VS motor to your average home owner so I can't really fault them.

    However, any HVAC technician that believes that a VS blower motor controls and maintains static pressure and/or CFM in a system that does not contain a static pressure and/or CFM measuring device along with the associated devices is mistaken.
    "The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
    - Alexis de Toqueville, 1835

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