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.