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Thread: Old Inverter to new VFD

  1. #1
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    Old Inverter to new VFD

    I have an old Reliance inverter that is being upgraded to a new VFD. I am familiar with the functions of a VFD and have troubleshot and changed parameters in them etc. but I have never installed one myself from scratch. I plan to demo the components use the existing inverter enclosure and mount the drive inside the cabinet.
    What I have....
    480V 40HP 48A (52FLA) motor on a blower for an air handler controlled by a thermostat. The motor has a fused disconnect and the inverter is being fed from a 100A breaker and is protected with 200A fuses in the enclosure.
    My thoughts are that I can install an Allen Bradley PowerFlex 750 480V 60HP VFD 20F11ND077AA0NNNNN in place of the old inverter and other than a means of locking disconnect on the outside of the enclosure door and a 24v power supply for the thermostat control what else will I need? I know the NEC requires a way of protecting the motor (fused disconnect) and the drive (100A breaker) and obvious grounding, wire size etc.
    Do I need overload protection between the drive and the 100A breaker or is overload protection already built into the drive? Should I protect the VFD with a fused disconnect, if so what size fuses? Do I need a a soft start or starter/bypass starter?
    I know the VFD acts as a soft start by ramping the frequency and Im assuming I can control the speed by adjusting HZ and creating a set point parameter in the VFD that it will ramp the motor up to once the thermostat tells it to turn on. Can I wire the thermostat to a set of contacts on the drive for start/stop of the motor?

    Is there anything Im missing? Should just be able to install the VFD with minimal components from what Im thinking. Any help is welcome.

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    What is the thermostat doing? Is it just on-off? If so, why have a VFD? What is going to tell the VFD what speed to run at? Start there before spending time on the rest of the issues.
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    Quote Originally Posted by travmad804 View Post
    I have an old Reliance inverter that is being upgraded to a new VFD. I am familiar with the functions of a VFD and have troubleshot and changed parameters in them etc. but I have never installed one myself from scratch. I plan to demo the components use the existing inverter enclosure and mount the drive inside the cabinet.
    What I have....
    480V 40HP 48A (52FLA) motor on a blower for an air handler controlled by a thermostat. The motor has a fused disconnect and the inverter is being fed from a 100A breaker and is protected with 200A fuses in the enclosure.
    My thoughts are that I can install an Allen Bradley PowerFlex 750 480V 60HP VFD 20F11ND077AA0NNNNN in place of the old inverter and other than a means of locking disconnect on the outside of the enclosure door and a 24v power supply for the thermostat control what else will I need? I know the NEC requires a way of protecting the motor (fused disconnect) and the drive (100A breaker) and obvious grounding, wire size etc.
    Do I need overload protection between the drive and the 100A breaker or is overload protection already built into the drive? Should I protect the VFD with a fused disconnect, if so what size fuses? Do I need a a soft start or starter/bypass starter?
    I know the VFD acts as a soft start by ramping the frequency and Im assuming I can control the speed by adjusting HZ and creating a set point parameter in the VFD that it will ramp the motor up to once the thermostat tells it to turn on. Can I wire the thermostat to a set of contacts on the drive for start/stop of the motor?

    Is there anything Im missing? Should just be able to install the VFD with minimal components from what Im thinking. Any help is welcome.
    If all you need is soft starting then a soft starter may be a less expensive option than a VFD. So per what Jraef asked, what will determine motor speed, or is there just an on-off control and it runs full speed?

    Supply conductors are per code supposed to be sized per the input rating of the drive - since you are only supplying a 40 HP motor but have a 60 HP drive, you sort of are supposed to use same conductors you would for 60 HP motor - or even higher capacity if that is what drive calls for. Reality is it won't draw anything significantly more than what a 40 HP motor draws.

    Supply side overcurrent protection technically should be in accordance with drive instructions as well - most want rather expensive semiconductor protection fuses - that will maybe yield less spectacular failure mode when the front end rectifier of the drive fails, but won't do anything to prevent such failure.

    Overload protection is built into drive, maximum settings still protect the drive, proper settings in accordance with your motor protect the motor as well as load side conductors.

    You do not want any switches, fuses, etc in the load side conductors. If you must put a disconnect at the motor, good idea to have one with an aux contact to interrupt control circuit so the drive output is disabled when you open that disconnect. Opening the circuit under load isn't as bad of a thing for the drive as closing the disconnect (with motor load) into an active drive output.
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    Disconnect goes on line side of drive. Fuses or AB allows breakers follow VFD manual requirements but they aren't typical breakers or fuses.

    If you must use an output side disconnect use aux contacts back to drive enable with early and late break capability series together. So as the disconnect opens the drive gets a disable command shortly before the main contacts open and vice versa enable is only after main contacts have closed and drive can't start until the disconnect closes. If you use the AB drive remove the jumper so the enable is hard wired and can't be turned off. Best not to do this but if the disconnect visibility is an issue this is the best way to solve it. This is the strategy if you must use a disconnect and operator training is suspect. I have had go do this even on starters in industrial plants occasionally.

    24 V power supply usually isn't necessary if all you are doing is dry contacts...can operate off on board power.

    Drive self protects against motor failures.

    In terms of bypass contactors be aware this reduces overall reliability long term. It's the 10 speed vs. 3 speed mountain bike concept, simpler is more reliable. Back pre 1990s VFD reliability was awful, 18 months at best between failures. Now failure rates are not much worse than a bypass contactor itself. Franklin does make them as a separate unit. ABB and Schneider make custom VFDs with a bypass contactor built in. It's simple enough to do. You use 3 contactors. There is an input and an output contactor on the VFD. There is also a separate starter. In bypass the contactors stay off and typically also an output speed command is sent to a separate output to drive say an external damper. In VFD mode the start command is sent to the VFD and the two contactors. Need to program the VFD to respond to input/output losses by shutting off but not to fault out in this configuration. I've built a couple of these as well, very simple.

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    Typically the problem with a bypass contactor system on an air handler run via a VFD is that there is often an issue of supply or makeup air flow based on the modulated blower speed using the VFD, and running at full speed with the bypass quickly becomes an air balancing issue. People think they want bypass from a reliability standpoint, then discover that they can actually never use it.

    On one job I was involved with, as soon as they put the exhaust fans onto the bypass and ran full speed while the supply fans were still modulated, the building pulled a slight vacuum and the doors were all sucked in to where nobody could leave! They panicked and called the fire dept., who solved the problem with their axes on some VERY expensive glass doors...
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    Quote Originally Posted by Jraef View Post
    Typically the problem with a bypass contactor system on an air handler run via a VFD is that there is often an issue of supply or makeup air flow based on the modulated blower speed using the VFD, and running at full speed with the bypass quickly becomes an air balancing issue. People think they want bypass from a reliability standpoint, then discover that they can actually never use it.

    On one job I was involved with, as soon as they put the exhaust fans onto the bypass and ran full speed while the supply fans were still modulated, the building pulled a slight vacuum and the doors were all sucked in to where nobody could leave! They panicked and called the fire dept., who solved the problem with their axes on some VERY expensive glass doors...
    second that often times it is easy for electrical maintenance or even engineers to overlook mechanical aspects such as air balancing. mechanical also has a habit of thinking things they change don't impact electrical.

    i am thinking the drive is to fine tune air flow after installation, set the speed and forget it.

    when all you have is a hammer, all problems look like nails

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    Quote Originally Posted by Jraef View Post
    What is the thermostat doing? Is it just on-off? If so, why have a VFD? What is going to tell the VFD what speed to run at? Start there before spending time on the rest of the issues.
    Jraef, The thermostat is just telling the drive to turn on when the set temperature is in/out of range. The drive kicks in and runs the motor about half speed. There is no speed reference from the motor to the drive. Currently the speed is adjusted by a pot in the drive

    With the new drive I should be able to do the same by creating a speed set point for the motor correct? The drive will also act as a soft start, ramping up to that half way speed?

    Right now there is a fused disconnect between the motor and the drive. I will remove that from the load side of the drive.

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    Quote Originally Posted by travmad804 View Post
    Jraef, The thermostat is just telling the drive to turn on when the set temperature is in/out of range. The drive kicks in and runs the motor about half speed. There is no speed reference from the motor to the drive. Currently the speed is adjusted by a pot in the drive

    With the new drive I should be able to do the same by creating a speed set point for the motor correct? The drive will also act as a soft start, ramping up to that half way speed?

    Right now there is a fused disconnect between the motor and the drive. I will remove that from the load side of the drive.
    Modern drives often have multiple ways possible to set speed command. Speed pot like you had before is one possibility, or you can set it directly on the drive interface that usually is standard equipment anymore.
    I live for today, I'm just a day behind.

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    If it never VARIES the speed, i.e. nobody is going to tweak that potentiometer and it runs at the same speed all of the time, you would be far better off just using another method of running the fan at half speed such as changing belt drive sheaves and use a simple soft starter to ramp it up. The value of a VFD is when you have some sort of automated way to vary the speed to fit changing conditions.
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  10. #10
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    I wonder what would have possessed someone so far back in time to have put a crazy expensive VFD on if they didn't really need one.
    Bob

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