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Thread: 1hp VFD blowing 1 of the line fuses.

  1. #1
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    1hp VFD blowing 1 of the line fuses.

    We have a 480V Powerflex 40 1hp VFD that continuous to blow one of the fuses on the line side of the drive and then continue to operate on single phase supply. The fuse size that keeps blowing is a 6A fuse.

    This motor is a 9-lead motor which origonally was wired incorrectly, which we thought was the problem, however now we have confirmed that the motor is wired correctly and it is still blowing this same phase fuse every time.

    I would'nt suspect the fuses are too small because after the fuse blows and the drive is running on single phase supply we would expect the current on the two remaining phases to increase and thus blow the other two fuses if they were marginal.

    Anybody have any idea what can be causing the same phase fuse on the line side to keep blowing?

  2. #2
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    What is the circuit diagram of the input diode rectifiers in the VFD?

    .

  3. #3
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    It is doubtful that a fault on the load side would blow a single line side fuse.

    My guess is an intermittant ground fault of some kind on that line. Is this a grounded power system? It would be unusual for a GF to open a fuse like that if it was not a grounded power system.

    Once in a long while the case of an input rectifier can develop a flaw and create a current path from the line to the frame of the drive, which will open a fuse on a grounded power system PDQ. It could be heat related so it only happens when the rectifier is hot.

    Try powering it up and putting a heat gun on the input rectifier with the motor not running.
    Bob

  4. #4
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    Quote Originally Posted by gar View Post
    100301-1200 EST

    What is the circuit diagram of the input diode rectifiers in the VFD?

    .
    I'm not sure I follow what you mean here.

    Quote Originally Posted by petersonra View Post
    It is doubtful that a fault on the load side would blow a single line side fuse.

    My guess is an intermittant ground fault of some kind on that line. Is this a grounded power system? It would be unusual for a GF to open a fuse like that if it was not a grounded power system.

    Once in a long while the case of an input rectifier can develop a flaw and create a current path from the line to the frame of the drive, which will open a fuse on a grounded power system PDQ. It could be heat related so it only happens when the rectifier is hot.

    Try powering it up and putting a heat gun on the input rectifier with the motor not running.
    There is actually a set of 6A fuses at the drive and then another set of 6A fuses at the panel feeding the drive. The fuse that is blowing is at the panel. So it is hard to tell if the problem is on the feeder cable thus blowing the panel fuse, or the problem is at the drive, and the panel fuse is blowing first thus removing current and not causing fuse at drive to blow.

    This drive is on a solidly grounded wye system.

  5. #5
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    philly:

    Look at the circuit diagram in the manual for the drive. There should be at least 3 diodes used to rectify the AC input. Quite likely 6 diodes.

    Your new information that the fuse at the panel is blowing may imply that there is short or leakage before you get to the VFD fuses.

    .

  6. #6
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    Quote Originally Posted by philly View Post
    I'm not sure I follow what you mean here.



    There is actually a set of 6A fuses at the drive and then another set of 6A fuses at the panel feeding the drive. The fuse that is blowing is at the panel. So it is hard to tell if the problem is on the feeder cable thus blowing the panel fuse, or the problem is at the drive, and the panel fuse is blowing first thus removing current and not causing fuse at drive to blow.

    This drive is on a solidly grounded wye system.
    That scenario right there shows me the distinct probability that you have a fault on the cable going from the panel to the drive. The only possibility of the VFD making only one fuse blow consistently is if it had a bad diode or something else failed on the front end. That would cause the fuses closest to the drive to blow (assuming the fuses are the same type) because the added wire resistance would make the upstream fuse see slightly less current if the fault were at the VFD. A problem on the load side would cause the VFD to trip. The only thing you are left with is a fault on the cable run. Most likely a ground fault due to a nick in the insulation from pulling.

  7. #7
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    Quote Originally Posted by philly View Post
    Anybody have any idea what can be causing the same phase fuse on the line side to keep blowing?
    Possibly a ground fault on that phase.

  8. #8
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    Megger time.
    Tom
    TBLO

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    Quote Originally Posted by ptonsparky View Post
    Megger time.
    Don't forget to remove the conductors from the VFD first!

  10. #10
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    Quote Originally Posted by Jraef View Post
    That would cause the fuses closest to the drive to blow (assuming the fuses are the same type) because the added wire resistance would make the upstream fuse see slightly less current if the fault were at the VFD.
    Fuses in series will see the same current, so you can't really predict which one will blow first. So I think it could be the cable, or it could be the drive.

    You could try rotating all the input leads to the VFD. Then, if the same fuse blows, there is a good chance the cable is bad. If a different fuse blows, it's probably the VFD or motor.

    Steve

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