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IndustryArena Forum > CNC Electronics > Spindles / VFD > VFD to run a air compressor. Proper hookup
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  1. #1
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    VFD to run a air compressor. Proper hookup

    My son just purchased a used Champion Air compressor and wants to derate the motor and run with a VFD. It's a 10 HP Baldor and he needs to lower to a maximum of 40 amp load due to his current panel can not support a larger current load..He purchased a 7.5KW cheap VFD from China. See attached pictures. I have a few questions I hope someone can help with.
    The motor is a standard dual voltage 240/480 3 phase. I will wire for with 240 single to the VFD with 240 3phase output.

    1- Being the VFD On/Off has to be controlled by the pressure switch how do I wire it to the VFD. I assume I will need to power it somehow? please advise
    2- What would be a good ramp up time to allow it to come up to speed
    3- Is there a limit as to the lowest Hz to run for testing but not burn the motor up.
    4- What other parameters do I need to set to ensure proper operation.

    Any help here would be great

    Thanks
    Steve in Maine

  2. #2
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    Re: VFD to run a air compressor. Proper hookup

    No, this won't work at all. The motor used in a spindle which the vfd can control is totally different. It's a synchronous motor. It spins at the speed of the frequency sent to it. That's why they are great for our purposes there. The motor in an air compressor is probably just a series wound motor or ac induction motor with a starter. Totally different.

    You can't really derate it like that, because it needs to draw enough current on startup to get the motor going. You can replace it with a smaller motor.

    You could *try* using an SCR type motor speed controller, which is sort of like PWM, or just PWM with a very large transistor. However this would reduce the torque your motor produces on startup. You might get away with it a bit, but it might stall when the air presssure is higher or it might overheat more. Motor efficiency can be reduced by such measures, and overheating can, ironically, result, even though you tried to reduce motor power.

    Sorry, it's not so simple as that. You won't get the power output from your compressor anyway, you might as well get a smaller cheaper compressor instead. There's no point trying to reduce the power of an expensive compressor.

  3. #3
    Community Moderator Jim Dawson's Avatar
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    Re: VFD to run a air compressor. Proper hookup

    Quote Originally Posted by Barefootboy4 View Post
    My son just purchased a used Champion Air compressor and wants to derate the motor and run with a VFD. It's a 10 HP Baldor and he needs to lower to a maximum of 40 amp load due to his current panel can not support a larger current load..He purchased a 7.5KW cheap VFD from China. See attached pictures. I have a few questions I hope someone can help with.
    The motor is a standard dual voltage 240/480 3 phase. I will wire for with 240 single to the VFD with 240 3phase output.

    1- Being the VFD On/Off has to be controlled by the pressure switch how do I wire it to the VFD. I assume I will need to power it somehow? please advise
    2- What would be a good ramp up time to allow it to come up to speed
    3- Is there a limit as to the lowest Hz to run for testing but not burn the motor up.
    4- What other parameters do I need to set to ensure proper operation.

    Any help here would be great

    Thanks
    Steve in Maine
    What you are proposing is wrong on a number of levels. Look at the documentation for your VFD and look at the derating specification for single phase operation. Given that this is a cheap Chinese VFD, the specifications are suspect right out of the box.

    Yes, a VFD will run an air compressor just fine. Since you are limited to a 40 amp load, 7.5 HP is about as much compressor that you can reasonably run. I run my 5HP compressor on a 10HP (7.5KW) rated Chinese VFD

    The really proper way to do this would be to buy a 5HP motor and change the motor pulley to run the pump at half the current speed. If you insist on running the 10 HP motor, then doing a pulley change to reduce to pump speed is the proper way to do this. The load on the motor is pretty much linear with regard to the pump speed.

    Having said that, to answer your questions:
    1) Disconnect the existing wiring from the pressure switch, connect the VFD Run and Common control terminals to the pressure switch, I have no idea how the VFD terminals are actually labeled, but connect per the VFD documentation
    2) I use 10 seconds on mine, seems to work fine, the start load never goes over the max run load. In the case of my 5HP compressor, the max load on the single phase side is 25 amps, and is fed with a 40 amp breaker.
    3) That motor should run all day at 20 Hz, but you really want to run it at 60 Hz if possible.
    4) Set the VFD parameters per the motor nameplate. But reduce the Max Amps output to about 25
    Set for Coast to Stop rather than controlled breaking
    Jim Dawson
    Sandy, Oregon, USA

  4. #4
    Community Moderator Jim Dawson's Avatar
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    Re: VFD to run a air compressor. Proper hookup

    Quote Originally Posted by Adoug View Post
    No, this won't work at all. The motor used in a spindle which the vfd can control is totally different. It's a synchronous motor. It spins at the speed of the frequency sent to it. That's why they are great for our purposes there. The motor in an air compressor is probably just a series wound motor or ac induction motor with a starter. Totally different.

    You can't really derate it like that, because it needs to draw enough current on startup to get the motor going. You can replace it with a smaller motor.

    You could *try* using an SCR type motor speed controller, which is sort of like PWM, or just PWM with a very large transistor. However this would reduce the torque your motor produces on startup. You might get away with it a bit, but it might stall when the air presssure is higher or it might overheat more. Motor efficiency can be reduced by such measures, and overheating can, ironically, result, even though you tried to reduce motor power.

    Sorry, it's not so simple as that. You won't get the power output from your compressor anyway, you might as well get a smaller cheaper compressor instead. There's no point trying to reduce the power of an expensive compressor.
    With the exception of servo spindles, normally found only on high end machines, the motors are asynchronous AC induction motors, but in the case of router spindles are just built to turn at high RPMs. All of them turn at near the synchronous frequency, be it 1 Hz or 400 Hz, however if you try to run a high speed spindle motor below about 1/2 it's rated RPM it will most likely not live long.

    In the case of any 3 phase motor and almost all 3 phase motors are standard AC induction motors, the VFD takes the place of the starter.

    VFDs run air compressors just fine, but you do need to match the VFD/motor to the available power.

    A SCR speed controller would not work at all on a 3 phase motor, but do more or less work on a series wound universal motor. VFDs actually output a 3 phase PWM wave form.

    As I said above, given the OPs power limitations, a 7.5 HP motor is about the max he can run.
    Jim Dawson
    Sandy, Oregon, USA

  5. #5
    Community Moderator Al_The_Man's Avatar
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    Re: VFD to run a air compressor. Proper hookup

    Agree with Jim on all counts.
    BTW, the HS spindle motors are also 2 pole 3ph asynchronous motors, 24krpm on 400Hz.
    They have to be ran typically at >6krpm due to possessing very low inductive reactance below this speed and frequency
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  6. #6
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    Re: VFD to run a air compressor. Proper hookup

    Quote Originally Posted by Jim Dawson View Post
    What you are proposing is wrong on a number of levels. Look at the documentation for your VFD and look at the derating specification for single phase operation. Given that this is a cheap Chinese VFD, the specifications are suspect right out of the box.

    Yes, a VFD will run an air compressor just fine. Since you are limited to a 40 amp load, 7.5 HP is about as much compressor that you can reasonably run. I run my 5HP compressor on a 10HP (7.5KW) rated Chinese VFD

    The really proper way to do this would be to buy a 5HP motor and change the motor pulley to run the pump at half the current speed. If you insist on running the 10 HP motor, then doing a pulley change to reduce to pump speed is the proper way to do this. The load on the motor is pretty much linear with regard to the pump speed.

    Having said that, to answer your questions:
    1) Disconnect the existing wiring from the pressure switch, connect the VFD Run and Common control terminals to the pressure switch, I have no idea how the VFD terminals are actually labeled, but connect per the VFD documentation
    2) I use 10 seconds on mine, seems to work fine, the start load never goes over the max run load. In the case of my 5HP compressor, the max load on the single phase side is 25 amps, and is fed with a 40 amp breaker.
    3) That motor should run all day at 20 Hz, but you really want to run it at 60 Hz if possible.
    4) Set the VFD parameters per the motor nameplate. But reduce the Max Amps output to about 25
    Set for Coast to Stop rather than controlled breaking
    Smart idea to change the pulley, I hadn't thought of that. Sounds like the most practical solution to me.

    Sorry re what I said about spindle motors, I thought that was how it was, I need to brush up on my knowledge of motor types I guess.

  7. #7
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    Re: VFD to run a air compressor. Proper hookup

    Quote Originally Posted by Jim Dawson View Post
    As I said above, given the OPs power limitations, a 7.5 HP motor is about the max he can run.
    Not if he only has 40Amps, 5Hp will be the max he can run on Single Phase with a 40Amp Breaker / supply

    So if his compressor was / is using a 10Hp motor on it he will need some monster Pulleys to reduce it so a 5Hp can drive it, they already have around a 5:1 so the monster Pulley may not even fit without it hitting the tank if he can even find or make a pulley the size needed

    This snip will give a reality check as to what Amperage is required for Single Phase use

    The VFD Drive he has will be happy with a 5Hp motor using a Single Phase Power supply
    Mactec54

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