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Thread: VFD (Solving DC bus issue)

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    VFD (Solving DC bus issue)

    I am currently working a project that uses a 40HP 460VAC motor to drive a reciprocating pump. I have done several of these in the past without issue, however my boss decided to through me a curve ball and add a VFD to the mix. The motor SF is 1.15 so I selected a 50HP VFD (GS4-4050). The issue I am having is once the motor gets moving (over 50hz) I get a DC Bus Overvolt alarm. I believe I have traced this to the fact it is a reciprocating pump and the motor regeneration is causing the over volt. Now from what I have read is that there is 3 ways to solve this 1. Add mass (not sure I could sell this one) 2. Using a breaking resistor 3. Buying a drive that monitors shaft load and can back of on excitation to avoid over generation. Option 2 being the most economical I would love to be able to go that route but the Brakes for the drive I currently have are only rated to 10% duty cycle and I am not sure how to determine if my current load would exceed that. Anyone know how to calculate this? Alternatively anyone know of a good VFD that monitors shaft load?

    Thanks for the help,
    Paul

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    Does the drive have a sensorless vector mode?
    Bob

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    Bob,

    Yes it does.

    Regards,
    Paul

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    Try it if it is off. Turn it off if it is on.
    Bob

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    Quote Originally Posted by pguerra75 View Post
    I am currently working a project that uses a 40HP 460VAC motor to drive a reciprocating pump. I have done several of these in the past without issue, however my boss decided to through me a curve ball and add a VFD to the mix. The motor SF is 1.15 so I selected a 50HP VFD (GS4-4050). The issue I am having is once the motor gets moving (over 50hz) I get a DC Bus Overvolt alarm. I believe I have traced this to the fact it is a reciprocating pump and the motor regeneration is causing the over volt. Now from what I have read is that there is 3 ways to solve this 1. Add mass (not sure I could sell this one) 2. Using a breaking resistor 3. Buying a drive that monitors shaft load and can back of on excitation to avoid over generation. Option 2 being the most economical I would love to be able to go that route but the Brakes for the drive I currently have are only rated to 10% duty cycle and I am not sure how to determine if my current load would exceed that. Anyone know how to calculate this? Alternatively anyone know of a good VFD that monitors shaft load?

    Thanks for the help,
    Paul
    Regeneration would cause a rise in DC liknk voltage but I don't see why a reciprocating as opposed to centrifugal pump would cause that. Perhaps the decelerating rate is too great. A flywheel could exacerbate the problem. Slowing the decel rate should be the simplest fix. Restive braking may be an answer but needs more hardware and more cost

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    Extending it would certainly help but if ACTL starters worked for the rest, why does it need a deceleration rate at all? Coast to stop seems appropriate.
    Tom
    TBLO

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    Quote Originally Posted by ptonsparky View Post
    Extending it would certainly help but if ACTL starters worked for the rest, why does it need a deceleration rate at all? Coast to stop seems appropriate.
    Well, yes if it always ran at fixed speed. But then, why use a VSD unless you want to vary the speed? And that could be up or down.

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    Quote Originally Posted by Besoeker3 View Post
    Well, yes if it always ran at fixed speed. But then, why use a VSD unless you want to vary the speed? And that could be up or down.
    The OP was surprised with the VFD where the fixed speed worked before. Changing the speed could certainly be advantageous but is deceleration that important for this setup? More of a question for the OP, than you.
    Tom
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    Quote Originally Posted by ptonsparky View Post
    The OP was surprised with the VFD where the fixed speed worked before. Changing the speed could certainly be advantageous but is deceleration that important for this setup? More of a question for the OP, than you.
    Agree. The rationale for including a VFD has not been explained.

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    When you say 'reciprocating pump' do you mean something like a 'pumpjack' where the beam is rising and falling pretty slowly?

    My understanding is that unless these are very well balanced they commonly regenerate over part of the pumping cycle. When connected across the line the motor draws power for part of the cycle (say lifting the beam) and then regenerates to the grid on the other half.

    If this is the case, then on a VFD you are regularly going to be regenerating a significant portion of your power consumption.

    You can always use a larger braking resistor; resistors are separately rated in ohms and watts, and you simply need a resistor of the proper resistance (based on drive to give proper braking performance) and proper wattage (to handle the continuous regeneration). However once you consider the energy lost to this resistor it is almost certainly not the cheapest approach.

    In addition to the options you are considering, you can also look at the following:
    1) line regeneration modules connected to the VFD, which will dump the regenerated energy back to the grid
    2) additional DC link energy storage, so that the regenerated energy increases the DC bus voltage in a tolerable fashion

    -Jon

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