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Fan Current to Speed (Percentage)

  1. Jun 21, 2017 #1
    Hi All,

    I have a VFD running supply fans and I currently have two values I can see. One is the VFD actual speed percentage from the VFD. The other is a calculated Fan Speed based off of VFD current. (I control off the calculation). The calculation is (Current^.33)*25.54=%. Does anyone know this equation or how I might get it. I am thinking the .33 is based off the VFD model and manufacturer and the 25.54 is a conversion.

  2. jcsd
  3. Jun 21, 2017 #2


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    Staff: Mentor

    Welcome to PF!

    I don't see how speed can be a function of amperage unless several other included variables are fixed and ignored (or rolled into the equation). Can you explain in more detail what the fan is doing/how the fan is controlled?
  4. Jun 21, 2017 #3
    Thanks for the welcome. :-)

    The fan(s) are supplying a common air plenum for my building. The fan speed is ultimately based off of a plenum pressure set point (.6"wc).

    However, I am only concerned on how the ALC logic is calculating this particular fan speed. It may not even be fan speed but it is the only thing that makes sense as some numbers are over 60 {55.49}, which to me, eliminates frequency.

    Attached Files:

    Last edited: Jun 21, 2017
  5. Jun 21, 2017 #4
    Updated control logic added. :-) upload_2017-6-21_14-24-26.jpg
  6. Jun 21, 2017 #5


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    Science Advisor

    Motor Current ∝ Load Torque
    Fan Torque ∝ somewhere between Speed2 and Speed3

    It seems to be calculating speed based upon fan loading. Perhaps to detect cavitation or "stalling". I notice that the Damper position is also involved, but it's unclear to what purpose.
  7. Jun 21, 2017 #6
    The value of 'VFD_FBK' is 100 when VFD Current 'AI' equals approximately 62.5. This may suggest 100% fan output at 62.5 amps motor loading. Where is the 'VFD_FBK' signal used?

    Not necessarily. Although it doesn't appear to be the case here (VFD frequency 'ANI' is scaled from 0 to 60 Hz to 'AV' = 0 to 100% speed) it isn't uncommon to operate a three-phase AC motor faster than base frequency.
  8. Jun 22, 2017 #7


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    So, @Berg88; can you read the FLA off the motor for us? I think this is a good possibility....

    ....which doesn't mean I think it is a good calculation/control feedback.

    See the fan affinity laws:

    They show that power is a cube function of both rpm and airflow (which means rpm and airflow are proportional). So your cube root function is likely trying to match either airflow or rpm (or frequency in Hz or %) to fan power. There are several problems with this:

    1. Fan power is typically not proportional to amperage for a VFD. The voltage of VFDs is usually varied in proportion to the frequency, so power is a square function of amperage.

    2. The affinity laws only apply (in their basic form) to systems where the fan is changing but nothing else is. That means if you are controlling to static pressure because you have a VAV system and VAV boxes downstream are varying airflow, the affinity laws do not apply in their basic form.

    All that said, I'm having trouble reading the diagram due to the resolution, but it looks to me like that point and calculation are just triggering a fan failure alarm and action. Is there anything else that that calculation (VFD_FBK point) are used for?

    [edit] Or maybe I'm reading the block diagram wrong and that calculation is only used to open the fan isolation damper (DMPR_ON) and the fan/VFD fail acts from the current directly (if >3 = "on"?)?.
    Last edited: Jun 22, 2017
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