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  1. #1
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    One VFD for multiple motors

    I have this kernel of an idea for a good way to save on energy for part load conditions, and I would like some input from controls guys here to see if it's a reasonable idea.
    First, I already know that a single VFD can run multiple motors, no problem. I'm wondering about the actual implementation of such a setup, what sort of motor protections I need to consider, and so forth.

    The example I want to start with is a package RTU with 4 condenser fan motors. First of all, what's the best way to protect against a catastrophic motor failure, such as a dead short? In this scenario, the VFD is a single point of failure that will wipe out the other 3 working fans. I believe I am required to have overload protection for each motor on the output of the VFD, but how well does this protection work when the VFD is running at (say) 25% speed, and might not be providing enough current/voltage to the shorted motor to trip the overload? If the overload trips and kicks the VFD offline, I assume it should be able to restart the other 3 fans and keep things working properly.

    Next, I'm wondering about minimum speed conditions. Most motors probably shouldn't be run below 20% speed, but 20% on all 4 fans might be too high. Would it be intelligent to use only 1 or 2 fans, and use a contactor to disable the other fans in order to get a reduced minimum speed? I suppose this could also result in the undesirable reverse fan rotation of the disconnected fans, which would negate the energy savings from disconnecting them in the first place.

    Finally, is it worth it to provide contactors and overloads for each fan, or should I just go for four separate VFDs that are sized appropriately, which would eliminate the need for external protection? I'm also wondering about VFD bypass schemes in case of VFD failure. I suppose multiple VFDs would fix that issue, instead of having yet another contactor to start the fans across the line in case of VFD failure.

    I don't want to totally reinvent the wheel here - just looking for input from more experienced guys that know what they are doing.

  2. #2
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    When I purchase VFD's for this the drive manufacturer includes a contactor for each motor to be controlled. I've seen this done a good bit on Fan wall units.

  3. #3
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    Quote Originally Posted by controlgeek View Post
    When I purchase VFD's for this the drive manufacturer includes a contactor for each motor to be controlled. I've seen this done a good bit on Fan wall units.
    This. I have seen a 16 motor fan wall section run off four variable frequency drives. Four motors per drive, each with it's own contactor.

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  4. #4
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    Quote Originally Posted by kontrolphreak View Post
    This. I have seen a 16 motor fan wall section run off four variable frequency drives. Four motors per drive, each with it's own contactor.
    So does each contactor have an overload built in, or do you use CTs or separate overloads?
    To prevent reverse airflow, I'm guessing shutters have to be used on the fan outlets?

  5. #5
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    I suppose another consideration is the ~3% or so energy loss from the VFD itself. Maybe better to soft-start and bypass on 3 motors, and VFD on one, thus reducing the amount of energy lost as heat from the VFD.

  6. #6
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    Each motor with it's own contactor and motor overload. Then fuse protection as well at the lower levels. Dead short the vfd 'should' protect it but then you loose all motor functions until downstream problem is isolated/corrected and vfd is reset.

    Depending on the size of the VFD, by the time you add all those accessories w/labor it very well could be more cost efficient to just buy individual VFD's...
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  7. #7
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    Just trolling here (always looking to learn...), but should there be a concern with adding additional load to an already "running" VFD?

    I'm thinking of a similar application on a chiller, either York or Carrier (I forget), where the OEM has one drive for two screw compressors. Their unit controls logic starts one compressor (with its' contactor closed and the other one open), and as load increases, the VFD ramps up, eventually to full speed. When load exceeds the one compressor, the VFD stops, then the other compressor contactor closes, and the VFD starts again, this time with both compressors. And speed is varied to match the load. I realize this a bigger load application, compared to condenser fans, but I would think that bigger VFD involved would pretty much cancel that out. Leaving me to wonder if the same type control strategy would need to be used in the case of a single VFD on multiple fan motors.

  8. #8
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    I wasn't thinking of adding loads to an operating VFD, but rather starting all 4 in parallel. However, in researching the costs of all the bits and pieces to make this work, I've come to the conclusion that my best strategy is to use the VFD on one fan, and control the other 3 with their own individual soft starters. That way, I can select soft starters that have built in bypass contactors as well as overload sensors, and it would reduce control panel clutter as well.

    I could even do the same with the compressors and blower fan(s) and get the tight, fine-grained capacity control that I want without having to have a VFD on each motor or compressor.

  9. #9
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    Quote Originally Posted by rjk_cmh View Post
    I wasn't thinking of adding loads to an operating VFD, but rather starting all 4 in parallel. However, in researching the costs of all the bits and pieces to make this work, I've come to the conclusion that my best strategy is to use the VFD on one fan, and control the other 3 with their own individual soft starters. That way, I can select soft starters that have built in bypass contactors as well as overload sensors, and it would reduce control panel clutter as well.

    I could even do the same with the compressors and blower fan(s) and get the tight, fine-grained capacity control that I want without having to have a VFD on each motor or compressor.
    I see. I must have missed that the first time, I see it now.

    Some OEMs do what you're thinking, or planning, on larger air-cooled chillers. Minus the soft starters on the other fan motors. With quite good results.

  10. #10
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    The final bit of the puzzle will be the brain that ties it together. Wonder how well the new Innotech Omni would do it? They haven't released it quite yet, but they seems to think it's the best thing since sliced bread.

  11. #11
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    Quote Originally Posted by Nuclrchiller View Post
    Just trolling here (always looking to learn...), but should there be a concern with adding additional load to an already "running" VFD?
    It would trip or burnout the inherently extremely fragile power electronics portion of the drive.
    When you start a normal motor across the line, the difference in magnetic field speed and the rotor speed is absorbed as a slip and it draws a very large amount of current until the full speed is reached.

    VSD varies both the voltage and frequency gradually up to speed, so if you apply a load on an already running VFD, it would be like doing a neutral drop in a car.

  12. #12
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    I don't know much of anything about vfd's. Would it be possible to have fussable links to each motor? In case once does burn up it doesn't take out the others in operation or cause vfd to fault . maybe cost effective too, can't imagine it would be that expensive. What you think?

  13. #13
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    Quote Originally Posted by canusayinsanity View Post
    I don't know much of anything about vfd's. Would it be possible to have fussable links to each motor? In case once does burn up it doesn't take out the others in operation or cause vfd to fault . maybe cost effective too, can't imagine it would be that expensive. What you think?
    Maybe, but it could still cause problems. Example: one motor in a group shorts one phase to ground, and blows one of the fuses and trips the drive. However, the other 2 fuses don't blow, and when the VFD tries to restart, it will have 2 legs on a motor that isn't spinning, causing an overload condition. If you want to benefit from the VFD's ability to auto-reset on a fault, you would have to use a starter and an overload so that all the legs get broken on a motor with a fault. That way the remaining motors would continue to work and the aux contact on the tripped overload could trigger an alarm.

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