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Thread: VFD Application?

  1. #1
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    VFD Application?

    In an industrial plant there is an existing conveyor transporting solid material at a constant rate 24/7, 350 days per year. Motor is 100 HP, 460 v, IEEE 841 design. Based on the quantity of material required by the process, the motor load is only approx. 45% of FLA. The system was designed like this because on rare occasions for fairly short periods the conveyor needs to transport twice as much material. So now the possibility of installing a VFD has been raised, along with the question of how power consumption would, or would not, vary. So, how would the total input power change between the two arrangements given that the amount and rate of material transported will be the same.

  2. #2
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    They are very efficient and improve pf
    at 45% pf is low

    how do they double the speed now?
    or speed is constant and they just load it up more?



  3. #3
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    Quote Originally Posted by Ingenieur View Post
    They are very efficient and improve pf
    at 45% pf is low

    how do they double the speed now?
    They don't - the existing arrangement is constant speed - when they need to double throughout, they just put twice as much on the belt.

  4. #4
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    Quote Originally Posted by Fnewman View Post
    They don't - the existing arrangement is constant speed - when they need to double throughout, they just put twice as much on the belt.
    That is where I see a problem. To maintain desired speed you still need to run at full voltage and frequency, little will change when it comes to energy usage, you can always put on capacitors to correct power factor.

    You might be better off running say a 60 or 75 HP motor and possibly run it into it's service factor during those infrequent periods of higher loading.

  5. #5
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    If you're not varying the speed, why would you want a VFD?

  6. #6
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    Quote Originally Posted by Cow View Post
    If you're not varying the speed, why would you want a VFD?
    For starting, and maybe also to run the motor more efficiently at full speed but partial load?

    Sent from my XT1585 using Tapatalk

  7. #7
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    Quote Originally Posted by GoldDigger View Post
    For starting, and maybe also to run the motor more efficiently at full speed but partial load?

    Sent from my XT1585 using Tapatalk
    Motor efficiency is probably nearly unchanged though if still running at same frequency and voltage isn't it? Power factor improvement on the supply side of the drive is about all that you are gaining, and you could have done that with much less expensive capacitors.

    Now if they want to put more material on the conveyor and control the volume delivered to whatever this feeds by varying the speed of the conveyor it may be more efficient that way, but the overall process may or may not allow that.

  8. #8
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    Quote Originally Posted by Cow View Post
    If you're not varying the speed, why would you want a VFD?
    If it ain't broke, don't fix it?
    Si hoc legere scis nimium eruditionis habes.

  9. #9
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    Quote Originally Posted by GoldDigger View Post
    For starting, and maybe also to run the motor more efficiently at full speed but partial load?

    Sent from my XT1585 using Tapatalk
    the VFD does not change how much energy is used to move the load other than by changing the PF to near unity on the line side. but that does not amount to much.

    as for starting, it may well prolong the life of the motor and the conveyor. but you can get most of the same benefits with a soft starter.
    Bob

  10. #10
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    Quote Originally Posted by Fnewman View Post
    In an industrial plant there is an existing conveyor transporting solid material at a constant rate 24/7, 350 days per year. Motor is 100 HP, 460 v, IEEE 841 design. Based on the quantity of material required by the process, the motor load is only approx. 45% of FLA. The system was designed like this because on rare occasions for fairly short periods the conveyor needs to transport twice as much material. So now the possibility of installing a VFD has been raised, along with the question of how power consumption would, or would not, vary. So, how would the total input power change between the two arrangements given that the amount and rate of material transported will be the same.
    I thought about this for a bit. The conveyor can clearly transport the heavier load you sometimes put on it. I suppose you could put the same load on it for normal operation and run it at half speed with a VSD to achieve the required material. But then you would have higher motor current than you do now and that means greater losses.

    So, I'm not sure that installing a VFD would be the right way to go for this application. Sure, 45% of FLC is very lightly loaded and the motor would be fairly low on its efficiency curve. Loading the motor more heavily, as you do on occasion, will move it up the efficiency curve but with greater losses. Then you'd have to add in the losses in the VFD which are typically around 2-3% at rated load a couple of kW.

    It wouldn't get my vote...........
    Si hoc legere scis nimium eruditionis habes.

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