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Thread: Supplying motor from VFD and FVNR starter in parallel

  1. #1
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    Supplying motor from VFD and FVNR starter in parallel

    I have a 480V motor that I want to run off of a VFD but want to have the capability to start this motor across the line if something happens to the VFD. This motor is already operating from a FVNR starter in an MCC and I am going to install the VFD in an enclosure right next to the MCC.

    My proposed arrangement is to leave the FVNR starter connected to the motor leads as the motor leads are currently temrinated on a terminal block in the starter. I was then thinking that I could put an ouput contactor on the output of the VFD and from this output contactor run feeder cables over to the same terminal block where the motor leads are tied into the starter. Therefore the leads from the starter and the VFD would be connected under the same terminals connecting also the the motor feeder going to the motor. (Assuming big enough terminal block.)

    Aside from figuring out a control scheme and the possible safetly issue with backfeeding the VFD enclosure does this arrangement meet code? Are there any violations here? If so can you kinly cite?

  2. #2
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    Terminal strip/ terminals have to be rated for the number and type of wires.

    Two sources of supply will be present in the starter. That could lead to safety issues since the terminals will still be energized with the starter's disconnect locked out. Look at 430.102 and 430.103 for disconnecting rules. Depending on the AHJ, you may be able to cover this with a "Warning - two sources of supply. Open disconnect XYZ before opening" nameplate, or interlocks might be needed. Verify there is a concise and clear Lock Out Tag Out procedure on the motor since it can be energized from two sources.
    Bob Wilson

  3. #3
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    Quote Originally Posted by rcwilson View Post
    Terminal strip/ terminals have to be rated for the number and type of wires.

    Two sources of supply will be present in the starter. That could lead to safety issues since the terminals will still be energized with the starter's disconnect locked out. Look at 430.102 and 430.103 for disconnecting rules. Depending on the AHJ, you may be able to cover this with a "Warning - two sources of supply. Open disconnect XYZ before opening" nameplate, or interlocks might be needed. Verify there is a concise and clear Lock Out Tag Out procedure on the motor since it can be energized from two sources.
    For that reason (and the others I'll list below), this is usually done in the same enclosure, it's a bad idea to leave them separate. I would convert the MCC bucket to a feeder CB to feed a VFD panel that has the bypass starter in it, properly configured as below.

    You CANNOT allow the line voltage to be applied to the load terminals of the VFD, otherwise all the magic smoke escapes immediately. For this reason, VFD manufacturers highly recommend using mechanically interlocked contactors on the output of the VFD that ensure that only one can be closed at any time. Electrical interlocking is required as well, but as we all know, contacts can weld and the consequence to that is the loss of a very expensive VFD.

    So VFD bypass systems come in two "flavors": "2 contactor bypass" and "3 contactor bypass". What I described above is a 2 contactor bypass system, you just have 2 mechanically interlocked contactors, the bypass also has an OL relay of course (but there is another argument to be made on the location of that OLR see below). This allows the motor to run in the event of an inverter failure. But when that happens, you would have to shut down the entire system to remove and repair the VFD. A variant of this is to use a manual switch on the load side of the VFD that isolates the two circuits mechanically, typically a double throw safety switch or a rotary cam switch with 6 contacts.

    A 3 contactor bypass adds a line isolation contactor ahead of the VFD so that if it becomes necessary to keep the motor running while the VFD is removed for repair, you can safely do so (under all the auspices of proper Arc Flash protection while doing hot work now of course). This is often done in critical equipment where automatic bypass is necessary. So consider that before deciding on a final configuration.

    As to location of the OLR, something worth considering is what happens under OL conditions. If, for example, the motor was very hot while running under load with the VFD but the VFD then fails requiring you to switch to Bypass, the OLR in the bypass starter now "thinks" the motor is cold and may allow damaging current to flow before tripping. For that reason, many people prefer to locate the OLR down stream of the common connection, so it sees the motor current regardless of how it is powered. You would typically wire the OLR aux contacts only to the bypass contactor so the VFD is doing the protection by itself most of the time just to avoid confusion.
    Last edited by Jraef; 07-19-10 at 01:11 PM.

  4. #4
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    I'll second Jref's suggestion: VFD with a bypass. No need to reinvent the wheel when you can buy one of these for just a little more money.

    But if you really insist on leaving the existing starter as a bypass, I would use a manual transfer switch to the motor input. Not ideal, but safer than paralleling two outputs that might both get turned on at the same time.

  5. #5
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    Quote Originally Posted by Jraef View Post
    A variant of this is to use a manual switch on the load side of the VFD that isolates the two circuits mechanically, typically a double throw safety switch or a rotary cam switch with 6 contacts.
    Are you saying that the manual switch will be sort of like a transfer switch between the VFD output and the bypass contactor in the drive enclosure with the ouput being to the motor?

    The motors I am considering are 150hp motors so this may make the contactor size in the drive enclosure rather large. But I gues its doable. The transer switch doesnt sound like a bad idea

    Another twist to this is the fact that the motor is a high efficiency motor that has been giving us nusiance trips on starting (previous post) In order to be able to start this across the line we have to push the code limitations of the breaker size for the starter. If we put the bypass contactor in the drive will this possibly create breaker mismatching with the fact that the VFD feeder breaker feeding the drive enclosure may not be large enough for across the line starting, or vise versa if we upsize the feeder breaker to max limits will we violate or comprimise vfd overcurrent protection. I would think that we would have one feeder breaker feeding the drive enclosure which would be a thermal magnetic breaker which could be set to the max. This breaker would serve as the drive OCPD as well as short circuit protection for across the line starter. I guess technically the NEC only requires drive OCPD to be a minimum of 125% or drive rated current so I guess it doesn't matter how large breaker is but i'm not sure. I guess drive will protect itself from overcurrent and for short circuit in a drive its already too late you cannot protect the drive from these regardless.

  6. #6
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    Quote Originally Posted by Jraef View Post


    A 3 contactor bypass adds a line isolation contactor ahead of the VFD so that if it becomes necessary to keep the motor running while the VFD is removed for repair, you can safely do so (under all the auspices of proper Arc Flash protection while doing hot work now of course). This is often done in critical equipment where automatic bypass is necessary. So consider that before deciding on a final configuration.

    As to location of the OLR, something worth considering is what happens under OL conditions. If, for example, the motor was very hot while running under load with the VFD but the VFD then fails requiring you to switch to Bypass, the OLR in the bypass starter now "thinks" the motor is cold and may allow damaging current to flow before tripping. For that reason, many people prefer to locate the OLR down stream of the common connection, so it sees the motor current regardless of how it is powered. You would typically wire the OLR aux contacts only to the bypass contactor so the VFD is doing the protection by itself most of the time just to avoid confusion.
    I've always placed the OLR just in the bypass starter. Will have to rethink this now.

    What type of circuit breaker do you use for the VFD/Bypass, Thermal Magnetic or Motor Circuit Protection (instaneous only) type? Again, I've used MCP's since this is the preferred protection when in bypass.

  7. #7
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    Quote Originally Posted by Jraef View Post
    For that reason (and the others I'll list below), this is usually done in the same enclosure, it's a bad idea to leave them separate. I would convert the MCC bucket to a feeder CB to feed a VFD panel that has the bypass starter in it, properly configured as below.

    You CANNOT allow the line voltage to be applied to the load terminals of the VFD, otherwise all the magic smoke escapes immediately. For this reason, VFD manufacturers highly recommend using mechanically interlocked contactors on the output of the VFD that ensure that only one can be closed at any time. Electrical interlocking is required as well, but as we all know, contacts can weld and the consequence to that is the loss of a very expensive VFD.

    So VFD bypass systems come in two "flavors": "2 contactor bypass" and "3 contactor bypass". What I described above is a 2 contactor bypass system, you just have 2 mechanically interlocked contactors, the bypass also has an OL relay of course (but there is another argument to be made on the location of that OLR see below). This allows the motor to run in the event of an inverter failure. But when that happens, you would have to shut down the entire system to remove and repair the VFD. A variant of this is to use a manual switch on the load side of the VFD that isolates the two circuits mechanically, typically a double throw safety switch or a rotary cam switch with 6 contacts.

    A 3 contactor bypass adds a line isolation contactor ahead of the VFD so that if it becomes necessary to keep the motor running while the VFD is removed for repair, you can safely do so (under all the auspices of proper Arc Flash protection while doing hot work now of course). This is often done in critical equipment where automatic bypass is necessary. So consider that before deciding on a final configuration.

    As to location of the OLR, something worth considering is what happens under OL conditions. If, for example, the motor was very hot while running under load with the VFD but the VFD then fails requiring you to switch to Bypass, the OLR in the bypass starter now "thinks" the motor is cold and may allow damaging current to flow before tripping. For that reason, many people prefer to locate the OLR down stream of the common connection, so it sees the motor current regardless of how it is powered. You would typically wire the OLR aux contacts only to the bypass contactor so the VFD is doing the protection by itself most of the time just to avoid confusion.

    I have been going back and forth on a design for this. The thing that adds to the difficulty is that there are two motors (primary and backup) that I want to operate from this VFD so including contactors for both motors from the VFD and the bypass contactor in the existing enclosure makes things very tight.

    These two starters for both pumps are already fred from two starter buckets. I dont know if I can cram everything into the drive enclosure where I already have the drive located only controlling one of these motors. The goal is to add the second motor to the drive as well as the bypass.

    My thought was to maybe keep the existing starters and put some sort of disconnect switch after each of the two contactors in the VFD conclosure for the two motors connected to the drive. This will provide a place for someone to lock out the indivual motor and not effect the other motor weather running off of the drive or from its starter across the line. I will also put a warning label on the MCC starter door and VFD enclosure warning of the other source feeding these enclosures. What do others think of this from both a practical and safety standpoint?

  8. #8
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    Quote Originally Posted by Jraef View Post
    For that reason (and the others I'll list below), this is usually done in the same enclosure, it's a bad idea to leave them separate. I would convert the MCC bucket to a feeder CB to feed a VFD panel that has the bypass starter in it, properly configured as below.

    You CANNOT allow the line voltage to be applied to the load terminals of the VFD, otherwise all the magic smoke escapes immediately. For this reason, VFD manufacturers highly recommend using mechanically interlocked contactors on the output of the VFD that ensure that only one can be closed at any time. Electrical interlocking is required as well, but as we all know, contacts can weld and the consequence to that is the loss of a very expensive VFD.

    So VFD bypass systems come in two "flavors": "2 contactor bypass" and "3 contactor bypass". What I described above is a 2 contactor bypass system, you just have 2 mechanically interlocked contactors, the bypass also has an OL relay of course (but there is another argument to be made on the location of that OLR see below). This allows the motor to run in the event of an inverter failure. But when that happens, you would have to shut down the entire system to remove and repair the VFD. A variant of this is to use a manual switch on the load side of the VFD that isolates the two circuits mechanically, typically a double throw safety switch or a rotary cam switch with 6 contacts.

    A 3 contactor bypass adds a line isolation contactor ahead of the VFD so that if it becomes necessary to keep the motor running while the VFD is removed for repair, you can safely do so (under all the auspices of proper Arc Flash protection while doing hot work now of course). This is often done in critical equipment where automatic bypass is necessary. So consider that before deciding on a final configuration.

    As to location of the OLR, something worth considering is what happens under OL conditions. If, for example, the motor was very hot while running under load with the VFD but the VFD then fails requiring you to switch to Bypass, the OLR in the bypass starter now "thinks" the motor is cold and may allow damaging current to flow before tripping. For that reason, many people prefer to locate the OLR down stream of the common connection, so it sees the motor current regardless of how it is powered. You would typically wire the OLR aux contacts only to the bypass contactor so the VFD is doing the protection by itself most of the time just to avoid confusion.
    Quote Originally Posted by Jraef View Post
    For that reason (and the others I'll list below), this is usually done in the same enclosure, it's a bad idea to leave them separate. I would convert the MCC bucket to a feeder CB to feed a VFD panel that has the bypass starter in it, properly configured as below.

    You CANNOT allow the line voltage to be applied to the load terminals of the VFD, otherwise all the magic smoke escapes immediately. For this reason, VFD manufacturers highly recommend using mechanically interlocked contactors on the output of the VFD that ensure that only one can be closed at any time. Electrical interlocking is required as well, but as we all know, contacts can weld and the consequence to that is the loss of a very expensive VFD.

    So VFD bypass systems come in two "flavors": "2 contactor bypass" and "3 contactor bypass". What I described above is a 2 contactor bypass system, you just have 2 mechanically interlocked contactors, the bypass also has an OL relay of course (but there is another argument to be made on the location of that OLR see below). This allows the motor to run in the event of an inverter failure. But when that happens, you would have to shut down the entire system to remove and repair the VFD. A variant of this is to use a manual switch on the load side of the VFD that isolates the two circuits mechanically, typically a double throw safety switch or a rotary cam switch with 6 contacts.

    A 3 contactor bypass adds a line isolation contactor ahead of the VFD so that if it becomes necessary to keep the motor running while the VFD is removed for repair, you can safely do so (under all the auspices of proper Arc Flash protection while doing hot work now of course). This is often done in critical equipment where automatic bypass is necessary. So consider that before deciding on a final configuration.

    As to location of the OLR, something worth considering is what happens under OL conditions. If, for example, the motor was very hot while running under load with the VFD but the VFD then fails requiring you to switch to Bypass, the OLR in the bypass starter now "thinks" the motor is cold and may allow damaging current to flow before tripping. For that reason, many people prefer to locate the OLR down stream of the common connection, so it sees the motor current regardless of how it is powered. You would typically wire the OLR aux contacts only to the bypass contactor so the VFD is doing the protection by itself most of the time just to avoid confusion.

    I have been going back and forth on a design for this. The thing that adds to the difficulty is that there are two motors (primary and backup) that I want to operate from this VFD so including contactors for both motors from the VFD and the bypass contactor in the existing enclosure makes things very tight.

    These two starters for both pumps are already fred from two starter buckets. I dont know if I can cram everything into the drive enclosure where I already have the drive located only controlling one of these motors. The goal is to add the second motor to the drive as well as the bypass.

    My thought was to maybe keep the existing starters and put some sort of disconnect switch after each of the two contactors in the VFD conclosure for the two motors connected to the drive. This will provide a place for someone to lock out the indivual motor and not effect the other motor weather running off of the drive or from its starter across the line. I will also put a warning label on the MCC starter door and VFD enclosure warning of the other source feeding these enclosures. What do others think of this from both a practical and safety standpoint?

  9. #9
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    What if I brought the leads off of the external starter into the drive enclosure?

    If I then had two transfer swithes in the enclosure to go between the drive outputs and the external starter ouput with a neutral position which could be locked then esentially you are providing a single lockout point for maintnance personal.

    The only safety issue I see here is you will still have external power coming into the VFD enclosure while operating the pumps, but you'd have power in the cabinet anyway if you had the bypass contactor in the cabinet when you were removing the drive. I guess both of these situations would fall under an Arc-Flash condition anyway.

    Does anyone know of a good source for the type of manual transfer switch I'm describing?

  10. #10
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    So you have two motors and one VFD and you want to be able to run either motor from the VFD or from it's existing FVNR start, right?

    Complex, and sometimes the complexity is not worth what you are saving by just buying a second VFD in my experience.

    Each FVNR starter must be electrically isolated from ever being in contact with the VFD output. Each circuit must have OCPD and OL protection for both the VFD operation and the FVNR operation. You cannot legally use MCPs from the FVNR starters to protect the VFD, it will need its own TM breaker (or fuses) ahead of it. You can use 2 VFD manual transfer switches on the load side, but LOTO becomes complex and signs may not be satisfactory, you need to think that through very carefully. I think you may need a 3rd MTS on the VFD so that it can only be feeding one motor circuit at a time, or maybe a Kirk-key interlock that can only allow the two MTS switches to me in a specific set of position possibilities. Or you can do it with contactors and logic because either way, you have to figure out how to prevent someone from switching either / any of those transfer circuit devices while the VFD is running, otherwise you can damage the VFD. Relays, interlocks, aux. contacts, switches, contactors etc. etc. I would use a small PLC or PLR, but that doesn't solve the power isolation and transfer issues. Murphy will have lots of ways to get into this situation...

    I've drawn it out, I'll scan it and post it if I have time.

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