Should I get a VFD or a single phase motor?
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    Default Should I get a VFD or a single phase motor?

    My compressor has a 5HP 3-phase motor. All I have is single phase 240V. The pump and motor are both 100% duty cycle. Anyway, the motor is in good shape, just needs new bearings. Should I buy a VFD and run the motor I have? My other option would be getting a 5HP 1750RPM 1PH motor and a new pulley (current motor has a 1-1/4" shaft). I like the idea of being able to program a VFD to soft start or vary the speed. The spec sheet for the pump lists different operating RPM depending on what HP motor it was packaged with - 3HP, 5HP, or 7-1/2HP, so I wouldn't be hurting anything by running the pump a bit slower or a bit faster. I think the RPM range was from around 400 to 800-ish. Swapping to a 1PH motor would be more simple, but I'd loose the extra bit of control that comes with using a VFD and I doubt I'd find a motor built as well as the one I have for a reasonable price. Thoughts?

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    Given the choice I'd go a VFD everytime, ........compared to 3 phase, single phase motors are terrible things

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    Limy is not wrong, but there is a cost factor. If you leave the compressor on 24 and 7, so is the VFD, and the power draw. There will also be a small efficiency loss. Power conversion is never for free. I suggest that you shut off the air at the tank when not in use as well as power to the VFD.

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    Quote Originally Posted by 52 Ford View Post
    My compressor has a 5HP 3-phase motor. All I have is single phase 240V. The pump and motor are both 100% duty cycle. Anyway, the motor is in good shape, just needs new bearings. Should I buy a VFD and run the motor I have? My other option would be getting a 5HP 1750RPM 1PH motor and a new pulley (current motor has a 1-1/4" shaft). I like the idea of being able to program a VFD to soft start or vary the speed. The spec sheet for the pump lists different operating RPM depending on what HP motor it was packaged with - 3HP, 5HP, or 7-1/2HP, so I wouldn't be hurting anything by running the pump a bit slower or a bit faster. I think the RPM range was from around 400 to 800-ish. Swapping to a 1PH motor would be more simple, but I'd loose the extra bit of control that comes with using a VFD and I doubt I'd find a motor built as well as the one I have for a reasonable price. Thoughts?
    NO idea what "control" you seek with a VFD for a basic tank-equipped air compressor that its unloader, switches, pressure-regulation doesn't already do for you?

    From what it seems you have? I'd "personally" run it off a 2 HP Reliance RPM III 180 Volt DC motor & KB-Penta DC Drive set to slow-ramp it!

    Because I can. And DC drives last longer than VFD do.



    IF you go with a 1-P motor? Wise to up it to 7 1/2 HP "nominal".

    Because single-phase motors do not deliver the goods as well as 3-P motors do when run close to their rated max.... and then... 1-P have "failure modes" with finite lives (as to how they are started) that a 3-P motor doesn't need nor even own.

    Compressors do a LOT of "starts", too. Nature of their service.

    If you are to task a VFD with that along with 1-P to 3-P conversion? EG: probably a 10 HP-rated VFD?

    Wise it be a well-proven and well-regarded VFD, conservatively rated, and conservatively "provisioned" or configured as to ramp-up.

    AND NOT an over-stressed "El Cheapo".

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    The fundamental issue with single phase motors on compressors is starting load. Single phase motor start windings are always overdriven so the number of starts per hour has to be restricted to let them cool down. There are also run time issues including minimum time between starts. The conventional compressor on a tank system starting up against considerable pressure works the motor pretty hard.

    I'd be very dubious of any modern, affordable, commodity single phase motor of nominally correct size standing the strain for any length of time.

    Correct way to do things with a single phase motor is to run up with the unloading vent valves open, preferably the in cylinder head type, then pump up a large vented down manifold or small intermediate tank to pressure and charging the main reservoir via a non return valve once the manifold or intermediate tank is up to pressure. Not common these days. My old Atlas Copco KE14, a 16 cfm Vee twin, was so set-up with a large collector manifold around 16" long by 4 inches diameter. Even that made a single phase motor work. Had to send the first motor back, despite being compressor duty and theoretically overpowered because it couldn't cope. Obtained a "very expensive when new" industrial Brook of 50% oversize which sorted it but even that was working hard to get it going.

    Being able to vary the speed of the compressor can be very useful when you have a fairly high demand load as you can match output to load close enough to let the compressor keep running with the tank pressure fluctuation within the switching range. Generally a better way of going about things than switching on and off every couple or five minutes.

    Any decent brand inverter will be up to the job of running things. Consider investigating the Eaton DE1 series display-less boxes. These are touted as pretty much drop in replacements for standard contactor switch controls to give soft start control and basic switch or calibrated knob speed controls. I believe they can be set-up to be controlled by a simple main power line switch, running up and shutting down automatically as needed.

    Clive

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    Compare cost of VFD vs 5hp single phase motor? I would expect the VFD to cost less. While in there consider motor pulley diameter increase and dropping operating pressure to get more useful air flow.
    Bill D

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    Quote Originally Posted by steve-l View Post
    Limy is not wrong, but there is a cost factor. If you leave the compressor on 24 and 7, so is the VFD, and the power draw. There will also be a small efficiency loss. Power conversion is never for free. I suggest that you shut off the air at the tank when not in use as well as power to the VFD.
    Right. I always shut the ball valves on my main compressor tank when the compressor isn't running and I cut the power when I don't need the compressor. I've had air hoses blow and had my compressor run overnight. Didn't hurt anything, just got a lot of water in the tank.

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    Quote Originally Posted by thermite View Post
    NO idea what "control" you seek with a VFD for a basic tank-equipped air compressor that its unloader, switches, pressure-regulation doesn't already do for you?

    From what it seems you have? I'd "personally" run it off a 2 HP Reliance RPM III 180 Volt DC motor & KB-Penta DC Drive set to slow-ramp it!

    Because I can. And DC drives last longer than VFD do.



    IF you go with a 1-P motor? Wise to up it to 7 1/2 HP "nominal".

    Because single-phase motors do not deliver the goods as well as 3-P motors do when run close to their rated max.... and then... 1-P have "failure modes" with finite lives (as to how they are started) that a 3-P motor doesn't need nor even own.

    Compressors do a LOT of "starts", too. Nature of their service.

    If you are to task a VFD with that along with 1-P to 3-P conversion? EG: probably a 10 HP-rated VFD?

    Wise it be a well-proven and well-regarded VFD, conservatively rated, and conservatively "provisioned" or configured as to ramp-up.

    AND NOT an over-stressed "El Cheapo".
    I do actually have a 2HP DC motor out there. I don't have a power supply for it, though. Would a 2HP DC motor really work as well as a 5HP induction motor? I assume it has to do with the torque curve. To be clear, I'm not actually considering using a DC motor - I have other plans for it.

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    VFD it is! Thanks for the input, everyone. I think I know where I can buy a second hand Yaskawa that'll work.

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    Quote Originally Posted by 52 Ford View Post
    I do actually have a 2HP DC motor out there. I don't have a power supply for it, though. Would a 2HP DC motor really work as well as a 5HP induction motor?
    Some can. More to the point, MOST can work as well as the THREE HP your mechanical compressor bits are rated to work with.

    The advantage is that ramped starts are no sweat. Depending on the power made available a straight shunt DC motor can come off rest with around 6 to 9 times its sustained reserve torque rating and sustain that for up to 90 seconds. Or so published Reliance.. from actual and extensive lab tests.

    But one need not.

    Set a(ny) basic DC Drive - KB-Penta. Minarik, etc. - to ramp it up over several seconds and it won't much care how many times it has to start in a day - even with no unloader valving.


    I assume it has to do with the torque curve.
    Starting, ramp-up, and sustained torque, yes.

    Easy comfort with constant starts from rest - stopping/braking, and reversing is a major part of why DC dominated hoist and elevator drives for scores of years.

    In compressor use, it is only the indifference to starts you would benefit from. But it helps..

    CANNOT cost-justify it with NEW DC motors. RPM III cost the very Earth.

    Might more easily do so with used-but-good.

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    Quote Originally Posted by thermite View Post
    Some can. More to the point, MOST can work as well as the THREE HP your mechanical compressor bits are rated to work with.

    The advantage is that ramped starts are no sweat. Depending on the power made available a straight shunt DC motor can come off rest with around 6 to 9 times its sustained reserve torque rating and sustain that for up to 90 seconds. Or so published Reliance.. from actual and extensive lab tests.

    But one need not.

    Set a(ny) basic DC Drive - KB-Penta. Minarik, etc. - to ramp it up over several seconds and it won't much care how many times it has to start in a day - even with no unloader valving.



    Starting, ramp-up, and sustained torque, yes.

    Easy comfort with constant starts from rest - stopping/braking, and reversing is a major part of why DC dominated hoist and elevator drives for scores of years.

    In compressor use, it is only the indifference to starts you would benefit from. But it helps..

    CANNOT cost-justify it with NEW DC motors. RPM III cost the very Earth.

    Might more easily do so with used-but-good.
    Gotcha. Definitely going to try to go second hand on the VFD. The compressor has plenty of life left in it (I took it apart for a health check), but I get by using multiple smaller compressors right now, so this one is more of a luxury than mission critical.

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    Quote Originally Posted by 52 Ford View Post
    Gotcha. Definitely going to try to go second hand on the VFD. The compressor has plenty of life left in it (I took it apart for a health check), but I get by using multiple smaller compressors right now, so this one is more of a luxury than mission critical.

    Sent from my SAMSUNG-SM-G891A using Tapatalk
    Second-hand DC Drives I've had one bad one (NOS, but with an infant-mortality issue) out of ...6 ISTR?

    Bought new, with warranty, are far the better deal, even so.

    Used VFD I wouldn't even go near.

    Their capacitor banks chemically age by the calendar as well as by the Power-ON-Hours.

    If VFD you must, go brand-new, with warranty, from a reputable maker AND dealer, and on a model that has good reviews.. Too new to HAVE a field-service history or older with a history of grief you do not want.

    Good VFD have 9 to 12 year published capacitor life, can go twice that.

    Cheaper ones are swapped-out entire because the caps are oddballs, physical size & rating-wise, (built to a price and space limit, purchased in HIGH volume the VFD maker's enjoy but you do not get the advantage of with "fewsies").

    By the time you NEED new caps? Most will have gone scarce ... and are usually priced to cost more than a whole new VFD.

    Keep in mind that conversion from single-phase is hard on the cap bank and the thermal balance of the incoming rectifier matrix, both especially so as to constant starting cycles.

    Wisest to purchase a NEW VFD purpose-built to be run off single-phase, too - not just ass-u-me you can de-rate any-old.

    Polyspede made their bones at that specialization, but all the "najors" have models for it too, long since.

    "RTFM" before you make your VFD selection. ALL the makers provide free downloads of brochures and tech manuals with specs and setup instructions.

    No question you can "make this work".

    The "question" is "at what REAL cost over a long time?"

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    Quote Originally Posted by thermite View Post
    Second-hand DC Drives I've had one bad one (NOS, but with an infant-mortality issue)) out of ...6 ISTR .. bought. new, with warranty were far the better deal, even so.

    Used VFD I wouldn't even go near. Their capacitor banks chemically age by the calendar as well as by the Power-ON-Hours.

    If VFD you must, go brand-new, with warranty, from a reputable maker oAND dealer, and a model that has good reviews, not to new to HAVE a field-service hisotry or has a history of grief. Good ones have 9 to 12 year published capacitot life, can go twice that.

    Cheaper ones are swapped-out entire because the caps are oddballs. physical size & rating-wise, have gone scarce ... and priced to cost more than a whole new VFD.

    Keep in mind that conversion from single-phase is hard on the cap bank and the thermals of the incoming rectifier, too.
    I guess I should give a little more info... a company is switching from one brand to another and they're swapping all of their VFDs and some other stuff like servos. Maybe they didn't like the service reps or something. No clue.

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    Quote Originally Posted by 52 Ford View Post
    I guess I should give a little more info... a company is switching from one brand to another and they're swapping all of their VFDs and some other stuff like servos. Maybe they didn't like the service reps or something. No clue.

    Sent from my SAMSUNG-SM-G891A using Tapatalk
    Second-hand VFD - for compressor-duty - off single-phase -- using goods that some already experienced user wants to see the BACK of?

    May as well try your luck with used-ladies.. in parallel?

    Some death-wishes are more fun than others!!!

    Or so the bigger kids tell me..

    Might want to "get a klew?"

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    Quote Originally Posted by thermite View Post
    Second-hand that some already experienced user wants to see the BACK of?

    May as well try your luck with parallel WIVES.

    Some death-wishes are more fun than others?

    LoL. I know one of the people doing the change outs. They are pretty new VFDs. It's a fairly large company, anyway. I'm not one to look a gift horse in the mouth - unless it has an abscess tooth or gingivitis or something, I guess. Even if it's a gift horse, hygiene still matters.

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    I would get some more details about the "used" VFD's you are considering purchasing. Yaskawa is a good brand the MTBF on their VFD's is something like 27 years, but depends on a lot of factors. If the VFD's were pulled and sitting on the shelf for any length of time, the capacitor life could be compromised. The second factor is that most likely if these VFD's are coming from an industrial setting they are all 3 phase input, so you will need to double the VFD rated output amps relative the the motor FLA. You will also need to add a DC buss choke, at least according to Yaskawa's recommendations. Yaskawa does make a single phase input 5Hp VFD which last I checked ran around $800, I have also installed their CIMR-VU2A0040FAA 3 phase input (10Hp) VFD in similar applications. If you look at other VFD's, in particular newer models they have significant deratings for single phase input and also the type of load. So check the VFD rated output under the operating conditions you are using and compare it to you motor FLA.

    I guess a lot comes down to the duty cycle of the compressor and the frequency of use, if it isn't under heavy high demand use a single phase motor is an easy swap and less expensive than a VFD. If it has high demand and/or frequent start stops with high flow use than a VFD may be a more attractive option. If the used ones does not work out you could get something like a Fuji Electric FRN0033C2S-2U for around $500. You do not need anything fancy.

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    Quote Originally Posted by 52 Ford View Post
    I'm not one to look a gift horse in the mouth - unless it has an abscess tooth or gingivitis or something, I guess. Even if it's a gift horse, hygiene still matters.
    Jerry Clower - Kiss The Mule - YouTube


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