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How to calculate energy used in a variable speed drive

  1. Oct 4, 2012 #1
    1. The problem statement, all variables and given/known data

    We'll be running a vsd water pump. We'll be seeing the energy savings of using a vsd, instead of using an AC motor on full load and using a tap to reduce water flow. Sorry for the lack of info.

    3. The attempt at a solution

    I'm sure it will have something to do with mass flow rates or something. But I'm not sure how I'll calculate the energu
  2. jcsd
  3. Oct 5, 2012 #2


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    I thought some form of bladder tank was used with normal pumps to match the flow rate from the pump to the outlet? eg pump runs until the tank is full then switches off. When you turn on the tap the tank delivers water without running the pump. That continues until the tank is nearly empty then the pump kicks in again. That way the pump is never restricted or loaded up with a partially open tap. I suppose there must be some additional energy consumed inflating the bladder but no idea how to calculate that.
  4. Oct 5, 2012 #3
    Thanks for your reply, but I think you misunderstood.

    We're just running a lab experiment to show how a variable speed drive saves money.

    We want to run an AC pump at full throttle, then put a restriction in the pipe to reduce mass flow rate. Then we'll run the VSD at a lower speed with no restriction to get the same mass flow rate.
  5. Oct 5, 2012 #4
    In a math model, the energy consumed is the area under the curve of a function, in this case of unit energy per time. For a full throttle motor, the area under the curve is rectangular. For a variable drive, that unit energy per time can vary, resulting in a differing amount of power consumed

    Hope this helps
  6. Oct 6, 2012 #5


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    I believe the power loss in a restriction/tap is

    ΔP * Q

    where ΔP is the pressure drop and Q the flow rate. I believe that's the additional power that will need to be provided by the full speed motor (plus losses in the motor).
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