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Thread: air compressor VFD control

  1. #16
    Join Date
    Feb 2003
    Location
    Doylestown, PA
    Posts
    7,541
    Quote Originally Posted by Tai Fu View Post
    The thing is, because with a VFD (especially one that is a bit undersized) the VFD can't supply the necessary current during starting since the cylinder pushes back quite hard with a few rotation. It works with 3 phase line voltage as the motor can have a large inrush current that can overcome this, but the VFD can't.

    Why is it that air compressors don't just use a solenoid to unload the cylinders during motor start to make the motor start without any load?
    I guess if someone really wanted to they could make such a thing. Just have a time delay relay in the circuit past the pressure switch. The time delay relay controls the solenoid. The solenoid is normally open, vented to the atmosphere and stays open until after the compressor motor starts then closes. It'd probably only take a couple seconds delay. It'd basically be an unloader with delayed closing, should reduce the starting amp surge as well.
    Last edited by Curt Harms; 09-17-2021 at 8:35 AM.

  2. #17
    Join Date
    Oct 2007
    Location
    New Hill, NC
    Posts
    2,568
    Air compressors require a significant amount of start up torque from the motors. I would never try to run one from a VFD due to the current inrush demands.

    I have a 10hp IR compressor that I operate off of a 30hp RPC. For most of the year startup is not a problem, but during cold winter days when the lubricant is thick it won't spin up. To solve the problem, I built an unloader for the compressor head. Basically it is an electrically operated valve (normally open) on a delay timer. The timer is set for 8 seconds.

    Works like a charm. Let me know if you want any part numbers for the components.

  3. #18
    Join Date
    Mar 2013
    Location
    Taipei, Taiwan
    Posts
    537
    Update: I bought the RPC from guy but the pony motor for some reason is frozen shut and when I try to start it up the breaker would trip. It's been sitting in storage for like 8 years and I can see serious rusting in and around the motor. He gave me a static phase converter to try instead. The static phase converter worked on the 5HP motor my bandsaw has. However what I ended up doing is connecting that output to my milling machine and lo and behold the milling machine starts up too! So now the bandsaw is acting as a rotary phase converter...

    I just need a motor or something and I am good, I think.
    Typhoon Guitars

  4. #19
    Join Date
    Mar 2013
    Location
    Taipei, Taiwan
    Posts
    537
    Further update:

    I got the compressor running, even under load.

    What I did was, use the static phase converter to start the bandsaw, then try to start the compressor. The compressor would start but when under pressure it would trip the breaker or not spin up correctly. So I moved the circuit around a bit, and what finally worked (though it's still a bit slow) is as follows:

    Start the band saw (5HP motor) using the static phase converter.

    Once the motor on the bandsaw is spinning, start the milling machine (has a 2HP dahlander motor, it has a bunch of relays controlling various things so the bandsaw motor must be spinning in order for the power to feed to the motor on the mill).

    Once both machine's motor is spinning, start the air compressor. This time it will start, though it is a second or two slower than 3 phase line. So it looks like the combined generation from 5+2HP motor is enough to get it going.

    I am going to spring for a 5HP motor. Someone has it cheap. I will use it as the idler motor, and start other machines as needed to give me more idling power.

    Seems RPC really is the way to go... if you have spare 3 phase motor sitting around you can just get a static phase converter and use that to make a RPC.

    I don't use the air compressor that often, but when I do I need it to work. So I don't mind the milling machine spinning around anyways.

    Weird thing is when an idler motor is spinning the other motors connected to the circuit doesn't really see any difference between a 3 phase line or converted... So does that mean I can for example connect an engine to a 3 phase motor, spin it at exactly 3600/1800 RPM in order to actually generate 3 phase power?
    Typhoon Guitars

  5. #20
    Join Date
    Dec 2006
    Location
    Toronto Ontario
    Posts
    11,236
    No a motor cannot be used as an alternator as it does not have a rotating magnet as the rotor.

    Regards, Rod.

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