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  1. #1
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    Variable Speed vs. Multiple Speed Motor Question

    I am trying to understand the difference between how furnaces use a variable speed motor versus a multiple speed motor (e.g. 2 speed, 3 speed, 4 speed, etc.) The model I have is a Trane XV80. I would greatly appreciate if someone could answer/confirm the following questions/concepts:

    1) A furnace with a multiple speed motor (let's use 2 speed) simply has an AC step type motor that has a set number of fixed speeds - two speeds for this example. If the control input to the motor indicates speed 1, then the motor spins at a fixed RPM for speed 1. If the control input to the motor indicates speed 2, then the motor spins at a different fixed RPM that is set for speed 2. The multi-speed motor does not care about actual airflow, CFM, etc., rather it just spins at the fixed RPM settings for the different speeds. Is this correct?

    2) A furnace with a variable speed motor has what is a DC motor with has a motor controller that coverts the AC to DC, and the control can vary the speed to almost any level within the design range between the lower and upper RPMs for the motor. The motor controller can change speeds based on the inputs to the motor controller. Typically, in a furnace the variable speed motor will adjust the actual RPM based on control inputs to keep a constant airflow, CFM, etc. that has been set by the DIP switches. Is this correct?

    3) The furnace control board controls the inputs to indicate speeds (RPMs) to both types of motors. In the case of the variable speed motor, there will typically be additional sensors/inputs that indicate the actual airflow connected to the control board, and they will provide the feedback to the control board that can be used to adjust the speed of the variable speed motor. Is this correct?

    4) The thermostat (non-communicating type) does not play a role in directly controlling the fan speed of either type of motor. Rather, the thermostat indicates fan is required based on the fan setting. In the case of changing motor speeds, this is controlled by whether the thermostat is calling for 1st stage, 2nd stage, 3rd stage, etc. heating or cooling. The multiple speed motor simply changes the speed to one of the fixed speeds based on the level (stages) called for. The variable speed motor also changes speed based on the level (stages) called for, but the control board adjust the speed to maintain the airflow (CFM) that has been set for that stage. Is this correct?

    5) If the above is not correct, then how does the thermostat control the fan speed?

    6) The advantages of a variable speed motor are it is quieter, consistent airflow based on DIP switch settings, more electrically efficient for moving a set volume of airflow, and more balanced heating/cooling. The disadvantage are initial cost, and potential repair cost if required. Is that correct?

    Thanks for taking the time to review/answer my questions.

  2. #2
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    I'd like clarification on this also...v.s. multi speed & whatever else
    this is called.

    you are in the right place OP to findout this info...
    sales speak or true difference??

    best of luck!
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  3. #3
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    The control board on a VS furnace does not control the speed to maintain set CFM. The board only tells the motor/module what CFM to move. The motor's module controls speed. No additional sensors are required for the VS blower to know what CFM is being moved.
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  4. #4
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    so what does multiple/2staged do that differes from vs?
    this hasn't been explained clearly to me by 'multiple" (LOL)
    hvac companies.
    I realize it is a stupid/uninformed query...but I just don't get it.
    and I subscribe to the school of thought that a stupid question
    is the one unasked.

    tia beenthere!
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  5. #5
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    All VS blower gas furnaces are either 2 stage, 3 stage, or modulating. The control board tells the motor to move X CFM. The blower speed is determined by the motor's module based on its programming and feed back from the motor on work being done.

    2 stage furnaces also come in standard multi speed blower versions. Where the blower simply is set to work off of either of 2 terminals on the furnace control board. Which simply energizes either the first stage blower speed tap, or the second stage speed tap. And the blower just spins at that RPM, no matter how much air it is or isn't moving.
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  6. #6
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    Quote Originally Posted by beenthere View Post
    All VS blower gas furnaces are either 2 stage, 3 stage, or modulating. The control board tells the motor to move X CFM. The blower speed is determined by the motor's module based on its programming and feed back from the motor on work being done.

    2 stage furnaces also come in standard multi speed blower versions. Where the blower simply is set to work off of either of 2 terminals on the furnace control board. Which simply energizes either the first stage blower speed tap, or the second stage speed tap. And the blower just spins at that RPM, no matter how much air it is or isn't moving.
    So, are most of my original thoughts are correct except that there are no external sensors providing airflow or other feedback to control the variable speed motor rather it is using the motor controller itself to get the feedback and adjust motor speed?

    Also, so I am clear then there is nothing external such as a thermostat that would control the speed of a variable speed motor, correct? This is all done via the thermostat calling for 1st stage, 2nd stage, etc. and the furnace control board uses the airflow (CFM) settings to call for the different requirements for the variable speed motor which the motor controller then manages.

  7. #7
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    Quote Originally Posted by cpeek View Post
    So, are most of my original thoughts are correct except that there are no external sensors providing airflow or other feedback to control the variable speed motor rather it is using the motor controller itself to get the feedback and adjust motor speed?

    Some modulating furnaces use a sensor to tell the board if the air temp rise is correct. Which will then tell the motor to move more or less CFM.

    Also, so I am clear then there is nothing external such as a thermostat that would control the speed of a variable speed motor, correct? This is all done via the thermostat calling for 1st stage, 2nd stage, etc. and the furnace control board uses the airflow (CFM) settings to call for the different requirements for the variable speed motor which the motor controller then manages.
    Carrier Infinity/Bryant Evolution control can control the CFM of the blower. Along with a few other proprietary communicating thermostats. A regular 24 volt thermostat can't control blower speed in heating mode. Some thermostats in cooling mode can slow the blower if the indoor humidity is too high(done by sending or not sending a signal to the board, not directly to the blower module).
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  8. #8
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    Quote Originally Posted by beenthere View Post
    Carrier Infinity/Bryant Evolution control can control the CFM of the blower. Along with a few other proprietary communicating thermostats. A regular 24 volt thermostat can't control blower speed in heating mode. Some thermostats in cooling mode can slow the blower if the indoor humidity is too high(done by sending or not sending a signal to the board, not directly to the blower module).
    OK. That makes sense. I understand that the proprietary communicating thermostats are a whole different scenario as the manufacturers can do anything they want.

    On the regular thermostats that can slow the blower in cooling mode, how do they do that - e.g. are they just switching 24VAC onto an additional wire/terminal (not G, Y1, Y2, W1, W2), or are they doing something like varying the voltage on the fan wire or 1st/2nd stage cooling wire (16VAC calls for a certain speed, 18VAC a different speed, 20 VAC another speed, etc.)?

  9. #9
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    Switching 24 on or off on an additional wire.
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  10. #10
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    Quote Originally Posted by beenthere View Post
    Switching 24 on or off on an additional wire.
    Thank you very much for answering all of my questions. I really appreciate what the forum does to help explain things.

  11. #11
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    Your Welcome.
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  12. #12
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    and thanks for me also!
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  13. #13
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    Quote Originally Posted by energy_rater_La View Post
    and thanks for me also!
    I gotta send you a bill. ROFL.
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