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  1. #1
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    VFD Myth or Fact

    I don't know.

    If a VFD has to output 100% of duty for weeks during high demand, it has been always taught to me that it is good practice to bypass drive during that time.

    I want to know why? Why? Does it hurt the drive duty life. The Motor(s). The energy argument I will buy. I will accept that it will cost some consumption to run the fans 100% through the drive. But I want to know about if it harms anything otherwise.

  2. #2
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    A properly designed VFD will be just fine running full throttle all the time. Bypass may give higher overall efficiency, but the highest efficiency motor designs (PMSMs and SRMs) generally cannot connect directly to the mains anyways.

  3. #3
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    I don't see the point in operating the drives in bypass. If the towers are in an industrial/process application in which the cooling systems are always close to or at full load that still doesn't account for changes in the conditions outdoors. If the plant operates 24/7 then the tower will be more efficient and capable of producing cooler condenser water when the OA temp/humidity drops, this is where the drive saves you money.

    If it's a comfort cooling application then I can't see how the system would be at full load for weeks at a time if everything were properly sized. I mean, even when things start-up in the morning after a long hot night the tower should still be able to give you cooler water than your setpoint (assuming your setpoint is in the 80-85 degree range.)
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  4. #4
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    Quote Originally Posted by Dowadudda View Post
    I don't know.

    If a VFD has to output 100% of duty for weeks during high demand, it has been always taught to me that it is good practice to bypass drive during that time.

    I want to know why? Why? Does it hurt the drive duty life. The Motor(s). The energy argument I will buy. I will accept that it will cost some consumption to run the fans 100% through the drive. But I want to know about if it harms anything otherwise.
    Hmmmm,

    I don't know of any reason a relatively modern VFD design (let's say since 2000) would be damaged by running 100%. Even for extended periods.

    I've been through 3 of the factory training courses and am certified for 3 of the major players in the VFD world ... can't recall a one that has anything in their classes or their literature about any negatives to running 100% speed for long periods.

    And as a practical matter, I have a number of installations at various sites which do run 100% all the time, or most of the time, as a matter of routine. Have been doing so for years. Hasn't been a problem.
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  5. #5
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    in refrigeration, we have to have a bypass arrangement. Whether it is a condenser, suction, ect. So. I have seen this practice of using the bypass in addition to emergency duty, obviously, but then use it if say the drive has met some time threshold of 100% PID, go ahead and bypass for a period of time. I have asked why a million times. And I have gotten answers from duty life issues to energy losses.

    Take a air cooled refrigeration condenser for example on a parallel rack system. Float the Saturated Condensing Temp off of Design TD. If properly maintained, in MI climate. you might see design ambient conditions for only several days for only several hours.

    So Programming an intentional bypass sequence for something that is a myth, as well as perhaps loosing a portion of the window when we can float when we start coming down off of design ambient is just pointless. Too many technicians not controls savvy may mistake this also. not worth it to add this layer of thing. It's been a long accepted practice in my world, and I never found anyone with really good reason to give me to have this. In a given application, given the hours of run time over 365 days, you might be at 100% PID 5%.

    Now maybe an alarm can be tied to say if we stay at 100% for too long, then something might be wrong, like a plugged condenser. I will buy that. But not force byass.

    Thanks gentleman.

  6. #6
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    http://www.springfieldelectric.com/i...%20FlyerPM.pdf

    Something to consider as long as everything meets the required spec's that the VFD OEM'S ask for but there always seems to be something that someone overlooked.
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  7. #7
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    As for running a VFD in bypass over auto mode selection the only thing that I can find is that they are more concerned about the inverter going off line I would have to say less chance of failure or something unforeseen to go wrong during the peroid of concern.
    Arguing with your Boss is like wrestling with a pig in
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    It is not exactly cheating, I prefer to consider it
    creative problem solving.

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  8. #8
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    I would think as well one would want the added protection the VFD offers, single phase protection, over amperage etc, unless of course you have the same built in protection in bypass mode, which would be extremely uncommon.
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  9. #9
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    The only argument I have ever heard about this is, to bypass the drive, if and when you meet a monitored condition of the drive being at 100% for a period of time, so logic would be: If at 100% for more than 30 minutes, go to bypass, and stay for a delay. The idea is, if your asking for 100% for longer than 1/2 hour, it might be that condition for awhile, so bypass to save the energy of the drive not in play. As someone has said, energy losses.

    And this is a tightly held stand on the part of some major grocery chains energy management SOP.

    And I have always though it was pure bull$hit. If you do the math, and graph the amount of time in a 365 day window, your at 100%, is so little, less than 2% of the time if all is well, maintained, ect, what is the point to bypass. The cost savings versus all the good reasons to have the drive be in play. So I was just checking to see if I am nuts cause I just needed to find out what other guys thought. And the other thing about this is. I am not a drive guru. It's a weak spot for me. I apply them, program them to do what I need them to do, but the actual ins and out of a drive, for a question like I asked. I just don't know drives that well.

  10. #10
    Join Date
    Dec 2012
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    By running in bypass you reduce the chance that a catastrophic motor failure will damage the drive but those are pretty rare events. Perhaps it makes sense if connected to an unstable grid and you have overloads set just above the RLA. You save a few percent on energy costs and may extend the life. But wouldn't the same condensation concerns for idle motors also apply to VFD's?

  11. #11
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    Quote Originally Posted by Dowadudda View Post
    ... for awhile... I just don't know drives that well.
    dunknowd either, but use'em on H 24/7 to a couple weeks - small chillers:

    ~~~they say:

    The drive exposes the motor to millions of impulses,
    thus requiring effective maintenance and installation
    of both the motor and drive [7]. Line and load
    reactors, line traps and MOV Surge Arrestors [8] are
    protective devices that are needed even though their
    payback is not immediately apparent.

    at about some different vfd...

    http://www.whitelegg.com/products/fi...quency_Dri.pdf

  12. #12
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    We have fans that consistently run at 80-90Hz 24/7... I don't think it's a problem with the drive's capabilities causing this question. About the only real reason I can see would be energy consumption - personally, I look at bypass starters as a backup feature only, and on non-critical equipment that can handle being down for a time, wouldn't even bother asking for bypasses to be installed. The efficiency gains are not really as apparent as you might think, and even a vfd at 100% may be able to run your motor at a more efficient voltage than an across-the-line starter would.

  13. #13
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    Quote Originally Posted by CraziFuzzy View Post
    We have fans that consistently run at 80-90Hz 24/7... I don't think it's a problem with the drive's capabilities causing this question. About the only real reason I can see would be energy consumption - personally, I look at bypass starters as a backup feature only, and on non-critical equipment that can handle being down for a time, wouldn't even bother asking for bypasses to be installed. The efficiency gains are not really as apparent as you might think, and even a vfd at 100% may be able to run your motor at a more efficient voltage than an across-the-line starter would.
    I agree, if you have motors with vfds some small energy slip will come with 100% output as apposed to the bypass function. The small cost is offset when the vfd is operating at 80% to 90% for a lot longer period of time. I really never understood the bypass function, other than a sales pitch. It doubles the cost of a drive, 80 to 90 percent of the installs could not be sent to 100% full output without repercussions. I have one lead lag drive situation for a large AHU, no bypass, a separate drive installed in the same panel, I like it.

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