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Thread: Switching the output of a VFD under load..... instant destruction... or not?

  1. #1
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    Default Switching the output of a VFD under load..... instant destruction... or not?

    So...

    "Everyone knows" the VFD will be instantly destroyed if you switch the output lines under load..... (not that I demand that you do that......I do not.) For things like that, it is important to know why the thing is true, assuming it is true, so that you know if you can break the "rules", and when you can break the "rules" (assuming they are valid rules).

    Now, I happen to disagree, in the case of essentially any modern VFD. But the solemn warning is trotted out with regularity, so it may as well be hashed out.

    If you think it is true, what is your reasoning for claiming that? Lets see the list.
    1601

    Keep eye on ball.
    Hashim Khan

  2. #2
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    Well, A VFD creates DC and then generates AC from the DC bus by some clever method of switching. My Monarch lathe has a DC motor which I power with a DC drive which is basically the first half of a VFD in that it has a DC output but AC is not generated from it.

    I've been running that DC drive permanently enabled and switching the output though the original contactors for maybe 5 years now and it's yet to go pop.
    Peter - novice home machinist, modern motorcycle enthusiast.

    Denford Viceroy 280 Synchro (11 x 24)
    Herbert 0V adapted to R8 by 'Sir John'.
    Monarch 10EE 1942

  3. #3
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    Do you mean reversing the motor as in power tapping on a lathe? I've done that for years running production parts with no apparent issues. Occasionally there'll be a squeal of the motor drive belt.

  4. #4
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    The manual to my TECO CV7300 says to not switch the output. Doesn't have details as to why.

    Sent from my SM-G950U1 using Tapatalk

  5. #5
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    Quote Originally Posted by DR View Post
    Do you mean reversing the motor as in power tapping on a lathe? I've done that for years running production parts with no apparent issues. Occasionally there'll be a squeal of the motor drive belt.
    Nope, I mean a disconnecting switch or contactor.
    1601

    Keep eye on ball.
    Hashim Khan

  6. #6
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    I've built and run several irrigation pump systems with 2 or 3 100hp motors on a constant pressure water conduit. The 3 pump system consist of 2 ways to supply power to the motor, one busbar supplied by 400v main voltage with 3 contactors and thermal protection for each motor and a second busbar connected to the output of a 125hp VFD with mechanical and electrical interlocked (3) contactors to each motor as well. Using a PLC to control the system and the PID controller of the VFD, the system tries to keep the conduit's pressure constant. Starting initially with one pump, if the targeted pressure isn't reached and the motor is at maximum speed, the VFD receives an freewheel stop command and the the VFD bus contactor cuts out and the mains bus contactor switches on keep the motor running. Now we can connect another motor to the VFD and try to get the target pressure. If it's reached at a frequency below maximum, fine, the system carries on with the 2 pumps, if not, it repeats the process with the third motor.
    If the water demand falls, one of the mains connected pump is stopped and the VFD continues with its job.

    Using this method, there is no need for other soft starters or star/delta systems to get the stopped motors running, thus avoiding pump damage.

    The freewheel command instantly disconnects the output stage and the most important it the interlocking system to avoid powering the VFD's motor output.
    Helder Ferreira
    Setúbal, Portugal

  7. #7
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    I've done similar on a much smaller scale, 3 hp 3-speed lathe motor, interlock on the speed selector (Frankenstein switch) puts it into coast while changing speeds, after which the vfd matches speed before going full power - works pretty well.

    Dave H. (the other one)
    Rules are for the obedience of fools, and the guidance of wise men.

    Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

  8. #8
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    Jerry knows more about electronics than most of us.
    So not sure why he as asking this here.
    I know about electricity and 3 phase, and a basic understanding
    of electronics. So my educated guess is that the IGBTs degrade
    in some way from the spark made when a set of contacts on
    the output side are broken. I guess the back EMF from the spark
    erodes some part of the silicon junctions inside the chip more
    and more until ultimate failure at some point.
    But this is just how I understand it. Maybe Jerry knows the real
    story and is just gathering public perception on what has been
    passed around from the salesmen who deal in dribble and try
    to look smart in front of their customers.

    -Doozer
    DZER

  9. #9
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    Because it says Never switch or open the output leads of the VFD while its powered up running a load. Jerry knows that for sure. My best guess the drive is internally monitoring the load and when it suddenly sees the output go low it raises the voltage to correct. Poof and its smoke time. I remember the time the electricians on the job (I was the HVAC guy) wired the input power to output of the VFD, poof and it was smoke time.
    Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

  10. #10
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    My Danfoss VLT 500 series VFD that I use to provide fixed 50Hz 3 phase for the machines in the workshop specifically says in the specs 'switching on output....unlimited'. I happily direct-start the lathe, surface grinder and/or milling machine on it. The Hardinge gets chopped from high speed to low speed on the dual-speed motor to slow it down quicker than the brake can. The milling machine gets switched from forward to reverse to slow it down fast because I haven't built a DC injection brake for the 'off' position yet.

    In terms of less industrial VFDs, thyristors were used originally. These require clever circuits to turn them off once they're turned on, but are susceptible to being turned on by things they don't expect (like fast rising pulses reflected back from the load). This has undesirable effects when the thyristor connecting the positive bus to the output terminal is suddenly joined by the thyristor connecting the negative bus to the same terminal. or vice-versa. Another common form of suicide was from the current rising too fast once the thyristor had turned on. This burned out the initially small conductive area of the device before the whole area was conductive, this localized short circuit limits the usefulness of the rest of the device... GTO thyristors can turn themselves off, without additional tricks, but they are even more delicate than the plain type. Both types can switch incredible amounts of voltage and current, but they're a bit like a glass sledgehammer. IGBTs came in in the late '80s to mid '90s and these were much nicer devices. The first couple of generations still had issues with getting stuck on when they should be turned off. The third generation of IGBTs that came in at about the turn of the millennium got around the original problems and IGBTs are now pretty bullet proof, electrically.

    What this means is that a VFD of current design should be able to cope with downstream switching, If it can't, then by definition, it isn't a current design. Lower spec VFDs may well trip when subject to the load trying to send power back to them (decelerating too fast or reversing) but they shouldn't die. If they've got the ability to use a braking resistor or are clever enough to control their outputs to avoid too high an intermediate bus voltage, then they'll happily cope with most of the abuse you can throw at them.

    What all this waffle means is that if the manual for a particular VFD says that switching of the load is not permitted, then it's an antique and shouldn't be used. If it doesn't specifically warn against the practice, JFDI.
    Last edited by Mark Rand; 06-06-2019 at 07:17 PM.

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