VFD for duplex compressor
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    Default VFD for duplex compressor

    Hi all,

    I have a duplex compressor with two 5hp 3phase motors on it. The compressor can support both continuous run (head unloaders) or start/stop operation. I have single phase input.

    Is there any advantage or disadvantage to running both motors from a single properly sized VFD (20HP) versus having a smaller VFD (10HP) for each motor? Under the single-VFD they would always be started and stopped at the same time.

    The only downside with a single VFD that I have come up with is the requirement to have overload protection on each motor for a single VFD, versus leveraging the built-in protection from the individual units. I think this requirement offsets the slight premium one pays when buying two units.

    Regarding continuous run - when using a VFD is this feature necessary and/or desirable when frequent cycling would occur? The conventional wisdom is that you should have fewer than 5 or 6 motor starts per hour - does this apply to a VFD which is ramping up slowly?

    Additionally, if using the unloaders on a single VFD, the load on one pump would load or unload before the other, would this cause any issues?

    Thanks,
    Don

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    Are you running both motors at the same time, often they will alternate or be staged as to one starting and then the second based on demand airflow/tank pressure. You would need to run the single 20Hp VFD in a V/Hz setting and have each motor with separate fusing/breaker. You would probably be better off with two separate VFDs, they do make native single phase input 5 Hp VFD's that run around $500-650 a piece.
    ODE-3-320153-1042, 230V 1 Phase input, 230V 3 Phase output
    Yaskawa CIMR-VUBA0018FAA, 5 HP, 200-240V, VFD

    Your power line requirements will be much higher for the 20Hp VFD even if it is derated to 10 Hp.Cost at the end of the day will be about the same, and with two single VFDs you can use the compressors either as single or dual running. Loading/unloading should be about the same if both motors start at the same time, hard to predict if a single VFD would fault if the start-up was staggered (typically they are to all start/stop at the same time to prevent a fault). Usual start-up time for compressors on a VFD is around 3 seconds, and they should stop without VFD braking otherwise you can see an overload or over voltage fault if trying to control the braking with a VFD.

    Start/stops should not be an issue for 3 phase motors running off of VFDs, the issue is more common with single phase motors. You do not want to power a VFD on/off frequently, as this can damage the units from higher inrush current.

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    What is your goal in using a VFD on a compressor at all? It’s not a good application for a VFD, especially if it’s a reciprocal type. The only reason to use one on a compressor is if you only have single phase power and you have 3 phase motors, where the cost of changing the motor is higher than the cost of a VFD. Since you already have single phase motors, you gain nothing but you add significant complexity and risk of failure to a very simple machine.

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    Quote Originally Posted by mksj View Post
    Are you running both motors at the same time, often they will alternate or be staged as to one starting and then the second based on demand airflow/tank pressure.
    My expectation is that I will run both motors together nearly all the time and not worry about staggering or alternately starting the motors. In my anticipated usage I expect most often to run it full out when in use, and in cases where I am not using as much air it would run for shorter duration (or I could dial down the VFD for a slower fill with less power requirement). But mostly I expect it to be chugging full blast.


    Quote Originally Posted by mksj View Post
    Your power line requirements will be much higher for the 20Hp VFD even if it is derated to 10 Hp.
    ...
    Loading/unloading should be about the same if both motors start at the same time, hard to predict if a single VFD would fault if the start-up was staggered (typically they are to all start/stop at the same time to prevent a fault).
    Are you saying the wiring or the actual current draw from the unit? I did not thing there was a significant derating inefficiency? Is it not true that power from 2x5Hp VFD = 20HP derated, +/-?

    I was assuming that the unloaders would never be triggered at *exactly* the same time due to differences in pilot valve stickiness, etc, etc. I would expect it to be close, but not identical.


    Quote Originally Posted by Jraef View Post
    What is your goal in using a VFD on a compressor at all? It’s not a good application for a VFD, especially if it’s a reciprocal type. The only reason to use one on a compressor is if you only have single phase power and you have 3 phase motors, where the cost of changing the motor is higher than the cost of
    a VFD. Since you already have single phase motors, you gain nothing but you add significant complexity and risk of failure to a very simple machine.
    My compressor currently has two 5hp 3-phase motors on each pump and I have only single phase 100A service. My expectation is that I will need nearly 15HP worth of air from the compressor (but I'm not sure in the end due to the duty cycle of the powered machine + storage + psi difference). But I figured I'll try out what I have and to leave room to grow without having to rebuy motors/VFDs. If the 2x5HP doesn't deliver enough, I could either upgrade to 2x7.5HP motors, or just upgrade a single motor to 10HP.

    My thinking is 10HP of air works, a single VFD might suffice and I'll be done. If I need more, I could swap in a larger motor or two and get an additional VFD.

    I considered an RPC, but after some reading I was optimistic that the VFD will allow me to soft-start the larger motors and potentially control the HP used, in times when I don't need as much air.

    In the end, the VFD option appears to be the least in cost and the highest in flexibility/features (specifically soft-start), but most options are pretty close in price (upgrading to single phase motors, a large RPC or vfds).

    Thanks for the comments,
    Don


    Don

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    What type of compressors are they? Reciprocals can be problematic when reducing speed; most of them are mechanically oiled, so reducing the speed reduces the oil flow / splash lube capacity and your compressor burns up. In addition, at a lower speed the flywheel effect has less energy storing capability, so you end up with pulsating torque requirements that is very hard on the DC bus capacitors in the VFD as well as causing winding movement inside of the motor stator leading to premature failure. All in all, it's not a good fit.

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    The difference between the unloaders timing would not be significant, too slow a ramp up can cause an overload error on the VFD.

    There is no derating based on the input, so if you have a 20Hp VFD, the input rating is for 20Hp be it single phase or 3 phase, the former would be a much higher current. What you are drawing/running the output at does not effect the input rating and wiring/fuse requirements. There is a performance window for most compressor pumps, typically commercial 5Hp air pumps are often also rated at 7.5Hp, but not higher or lower. If you slow down the motor/pump, you run into cooling and lubrication issues, and the motors will be producing less Hp, so no real advantage to slowing them down when you are talking about needing all the air you can get. If you over speed the pump you will greatly diminish its service life and possibly seize it up.

    Unclear as to adding or switching out the motors to larger ones, you should be able to calculate the air flow requirements and design/install the system according to your needs. It is way too expensive to find out after the mater of the fact that you need to redo everything.

    If you have single phase and you plan on running the motors most of the time, the most cost effective solution would be to use two single phase 5 Hp motors, you can pick up something like the a pair of Baldor L1430T or Leeson 131537.00 motors for less than the VFD's.

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    Quote Originally Posted by Jraef View Post
    What type of compressors are they? Reciprocals can be problematic when reducing speed; most of them are mechanically oiled, so reducing the speed reduces the oil flow / splash lube capacity and your compressor burns up. In addition, at a lower speed the flywheel effect has less energy storing capability, so you end up with pulsating torque requirements that is very hard on the DC bus capacitors in the VFD as well as causing winding movement inside of the motor stator leading to premature failure. All in all, it's not a good fit.
    They are Quincy QR-350s, with an RPM range of 400-940. Oil pressure-lubed. The 5HP runs at 400RPM, I wouldn't want to slow below that, I was thinking if I were running at higher RPM with a larger motor falling back to 400. Is that in the range you were referencing, or are we talking much lower then that?

    Quote Originally Posted by mksj View Post
    There is no derating based on the input, so if you have a 20Hp VFD, the input rating is for 20Hp be it single phase or 3 phase, the former would be a much higher current. What you are drawing/running the output at does not effect the input rating and wiring/fuse requirements.
    I'm a little unclear on this, I think the word "rating" is confusing me - are you referring to the max loading/max current draw that the VFD can handle? I understand that everything comes through the single phase, and the 1 phase current = 1.73*3 phase current, and my wiring would need to support the 1-ph current.


    Quote Originally Posted by mksj View Post
    Unclear as to adding or switching out the motors to larger ones, you should be able to calculate the air flow requirements and design/install the system according to your needs. It is way too expensive to find out after the mater of the fact that you need to redo everything.

    If you have single phase and you plan on running the motors most of the time, the most cost effective solution would be to use two single phase 5 Hp motors, you can pick up something like the a pair of Baldor L1430T or Leeson 131537.00 motors for less than the VFD's.
    This is to run a steam hammer converted to air, which requires 50cfm @ 90psi at full tilt. I have 440 gallons (59cf) of storage @ 175psi. At full blast I'm thinking that represents 1 minute of air, with a refilling at 30cfm (with 10hp total). The issue is that the air consumption is going to vary a lot, as well as the time between allotted to refills. Most likely the 30cfm will support a large portion of my usage, but it would be nice to handle the extremes also.

    Regarding switching to 1-single phase, a 20hp VFD I was looking at: Fuji Electric
    - FRN0060C2S-2U - $649.60
    which is similar to the price of a single motor. Is this not appropriate for my usage? This is for a single VFD for both motors.

    Thanks again for all the comments.

    Don

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    QR-350 pump will handle a range of 5-15Hp, CFM of 15.6-36.6 per pump, so you have a wider power window on this model. You would want to maintain the RPM to Hp per the manufactures recommendations. I would not use two different size motors with a single VFD.

    With VFDs the input wiring typically is required to handle the full load of the VFD independent of the output load, I would assume that if there was an internal failure it could pull the full amperage of the unit (not the derated). The breaker is sized at ~125% of the rated input. I have run VFDs on smaller than spec. breaker size, but the wiring must be rated to the breaker. A VFD can also provide close to 180% of its rated output for up to 1 minute, so this would need to be factored in. I haven't used the Fuji VFD's but they seem to be pushed hard these days by a few sellers. You are still going to be boxed in by the size of the VFD and would not be able to go to a larger motor which seems to be more optimum for the CFM required. The input current on the 20Hp Fuji would be 80A three phase or 57.6A with a DCR choke which I would use, converting to single phase would mean ~100A breaker.

    If you had other 3 phase machines in your shop than maybe something like a PhasePerfect would be a better choice in the long run.

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    Quincy 350s - I just added VFD to mine a couple months back so I wouldn't have to start the RPC to get air.
    https://www.amazon.com/Variable-Freq...V1R63TTYPY4R6B

    1/4 Inch Direct Acting Fast Response Aluminum Electric Solenoid Air Valve 220VAC | eBay

    I added an air solenoid to the unloader and disconnected the mechanical system. Then programed the VFD to open this solenoid when starting and drop it when at speed. solves the VFD overload at startup issue.

    Has worked great for about two months now.

    I'd just do this twice for your application
    Attached Thumbnails Attached Thumbnails quincy-running-vfd.jpg  

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    Quote Originally Posted by mksj View Post

    Start/stops should not be an issue for 3 phase motors running off of VFDs, the issue is more common with single phase motors. You do not want to power a VFD on/off frequently, as this can damage the units from higher inrush current.
    I think this misses the point, and is bad advice for the OP's situation. Soft start is about lowering peak amps for metering purposes. It is accomplished by lowering the effective voltage, so the motor draws less amps, but for a longer period of time. It therefore does not reduce motor heating. Quincy's limitation on number of starts per hour is strictly to limit motor heat generation to the amount the motor cooling fan can dissipate during the time it is running At that point the motor will run cooler if it starts once and runs continuously, controlling tank pressure by simply unloading the pump. This is simple, and the way internal combustion engine driven compressors work.

    Using the compressor to drive a power hammer is going to yield a widely varying duty cycle; working one piece there will be significant down time while the work goes back in the forge, while working several pieces in rotation can see the hammer running almost continuously. The OP claims his unloaders can be set up for continuous running; he should make use of this feature.

    Dennis

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    Quote Originally Posted by mksj View Post
    With VFDs the input wiring typically is required to handle the full load of the VFD independent of the output load, I would assume that if there was an internal failure it could pull the full amperage of the unit (not the derated). The breaker is sized at ~125% of the rated input.
    Thanks for the clarification.


    Quote Originally Posted by Karl_T View Post
    Quincy 350s - I just added VFD to mine a couple months back so I wouldn't have to start the RPC to get air.
    ...
    I added an air solenoid to the unloader and disconnected the mechanical system. Then programed the VFD to open this solenoid when starting and drop it when at speed. solves the VFD overload at startup issue.
    By mechanical system, are you referring to only the hydraulic (oil pressure) unloader or the whole valve? Are you running start/stop?

    I was wondering how long the pumps would run before the loaders kicked in, I was hoping that it was sufficiently long that one doesn't need to do what you did. Thanks for the tip, that seems like a very nice solution.


    Thanks,
    Don

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    Quote Originally Posted by Modelman View Post
    I think this misses the point, and is bad advice for the OP's situation. Soft start is about lowering peak amps for metering purposes. It is accomplished by lowering the effective voltage, so the motor draws less amps, but for a longer period of time. It therefore does not reduce motor heating.
    Dennis
    No I think you are missing what I am saying, this is powering the VFD On and OFF frequently, it has nothing to do with the cycling of the output. Most VFDs recommend when turning off a VFD that you allow the capacitors to discharge, i.e. wait about 5 minutes before powering it back up. Turning it off and on too quickly can lead to an inrush surge which can damage the unit. On the motor run frequency we are not talking about rapidly cycling the motor on off, and with a 3 second start time I cannot see that you would have any issues with the normal compressor run cycling times.

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    Quote Originally Posted by dschad View Post
    Thanks for the clarification.




    By mechanical system, are you referring to only the hydraulic (oil pressure) unloader or the whole valve? Are you running start/stop?

    I was wondering how long the pumps would run before the loaders kicked in, I was hoping that it was sufficiently long that one doesn't need to do what you did. Thanks for the tip, that seems like a very nice solution.


    Thanks,
    Don
    Thought you could see it in the pic. I undid the line before and after the hydraulic unloader. put them into the solenoid valve. For under $25, this is a no brainer.

    Yep the motor/vfd goes on/off with the pressure switch. like others' have mentioned have your hi/lo on the pressure switch settings far enough apart that the motor is not kicking in/out constantly.

    <EDIT>
    If you want to get elegant, you could add a "HARD RUN" switch. have it kick out the unloader instead of shut down the VFD/motor. Then in "NORMAL" have it shut off the VFD/motor.

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    Quote Originally Posted by Karl_T View Post
    Thought you could see it in the pic. I undid the line before and after the hydraulic unloader. put them into the solenoid valve. For under $25, this is a no brainer.

    Yep the motor/vfd goes on/off with the pressure switch. like others' have mentioned have your hi/lo on the pressure switch settings far enough apart that the motor is not kicking in/out constantly.

    <EDIT>
    If you want to get elegant, you could add a "HARD RUN" switch. have it kick out the unloader instead of shut down the VFD/motor. Then in "NORMAL" have it shut off the VFD/motor.
    Thanks for the clarification. I will do something similar in terms of the solenoid, once I figure out exactly what approach I am going to take.


    I appreciate all the comments/suggestions. I'll post my resolution once I get there.

    I finally installed the hammer today, so next step is to get the compressor into place.

    Don


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