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    115V input card, PF755 burning up

    Has any one seen this or heard of this.

    History: existing AB MCC originally fitted with 1336 drives from Allen Bradley. Retrofitted 3 years ago with PF 755 drives. Same control scheme. 480 to 115V, 200va control transformer. Transformer supplies 115V power for inputs, fans, and contactor. The contactor supplies 480V to the input of the drive. The drive enable and the contactor coil are paralleled. There are two items in the field that can interrupt the contactor and enable, an early break field disconnect and an "enable" from the control cabinet. This makes a possibility of the 115V can be applied to the rest of the 115V input card without the drive being powered up. One month after commissioning 4 separate drives had the drive enable inputs burn up ( meaning 115 was present at the card terminals but the bit was not "high" according to the drive. The 115V input cards were replaced and all was fine, Fast forward 3 years and this time two of the inputs are now scorched ( I can see two scorched components on the card). The card was replaced and it immediately blew the control transformer secondary fuse. To make sure where the control wiring was compromised, I inserted a 100 watt incandescent light bulb in series with the control fuse and by using a clamp on ammeter the "short" was traced to the the 115V input card. None of the field wiring is compromised.

    I only bring this up because of another experience with kinetix 6500 drives where the opposite needed to happen ( the drive needed to be powered by 24 volts and "happy" before the 480V was allowed to connected.

    I was able to reprogram the drive to be operated by the HIM module for the production gods.

    I do have access to the drive while it is running, but I cannot turn it off for another three months. Also during the afore mentioned outage a cleaning crew will have it locked out for most of the day and I will only be able to turn the power on and off for around an hour before the production gods will demand it is running.

    #2
    This is a very common failure. Did you read the installation section of the manual where it specifically says never feed it from a contactor? This was OK on the 1336s but wipes out a Powerflex. Some drives tolerate it but many do not.

    The problem is the precharge circuit. When the VFD first powers up, a small rectifier circuit charges the DC link capacitors then shuts down. To shorten drive startup to seconds, the precharge circuit is only designed to operate infrequently. So all it takes is trying to start and stop the drive a couple times in a row and the precharge burns up.

    Whenever you convert from starters to drives or from a contactor feeding a drive either make absolutely sure it can tolerate this or bypass the contactor. If you must do this I know that Schneider Altivar 630s and ABB ACH550s can handle it. They use the front end to throttle the current on startup. Powerflex and most Siemens drives are not.

    Sent from my SM-T350 using Tapatalk

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      #3
      This is why in our plant when we upgrade to drives, we take the contactor out of the circuit. The way I see it the drive acts as your contactor at that point. Saves money, bucket space, and makes troubleshooting easier.

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        #4
        Originally posted by paulengr View Post
        This is a very common failure. Did you read the installation section of the manual where it specifically says never feed it from a contactor? This was OK on the 1336s but wipes out a Powerflex. Some drives tolerate it but many do not.

        The problem is the precharge circuit. When the VFD first powers up, a small rectifier circuit charges the DC link capacitors then shuts down. To shorten drive startup to seconds, the precharge circuit is only designed to operate infrequently. So all it takes is trying to start and stop the drive a couple times in a row and the precharge burns up.
        Paulengr,
        The original PF 755 drive is still running but the 115V input card is history. The typical cycling of the contactor is once every three months. Yes the 1336's were brutes, they used if I remember correctly, a SCR to do the precharge duties.

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