Question on moving from single ph. to 3 ph. on compressor cutoff switch
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  1. #1
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    Default Question on moving from single ph. to 3 ph. on compressor cutoff switch

    On my Dresser model 5000 air compressor I'm swapping out my single phase 220 volt 3 h.p. 1740 rpm motor to a 220 volt 3 phase 5 h.p. 1800 rpm motor using a VFD to control speed.

    Are most of the newer contactors designed to handle both single and three phase power?

    I'm only doing this because doing this will allow me to bump up the speed on the compressor to the maximum rated 770 rpm, even if only briefly as needed. I already have the 3 phase motor. (currently 5" motor drive and 18" driven compressor pulleys)

    Thanks for your insight on this.

    Dan
    Last edited by DanLinsch; 09-21-2019 at 01:42 PM. Reason: Contactor, not pressure switch

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    For what purpose do you want a contactor? To answer your question directly, lots of single phase contactors are still made.

    You should have a means of disconnect (either plug connected or a disconnect switch) to remove power from the input of the VFD. The output of the VFD should not have a contactor between it and the motor. The VFD should have the pressure switch wired to a low voltage control input on the VFD. No need for a contactor anywhere.

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    I used a slightly over-rated breaker that I use as a on/off switch. The breaker is operated by rotating a square bar that connects to a door interlock handle. When the black knob is turned clockwise the breaker closes and the square bar inside the bottom of the knob will not come out which holds the door closed.
    I was going to use a contactor with push buttons and indicator lights. I liked the door-lock/power-on at the same time.

    The black pieces of plastic on the bottom adapt a Fuji handle with a ABB breaker.
    dsc_0801.jpg

    dsc_1009.jpg

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    Yes you could follow the door inter lock with a contactor operated by a front panel switch. Contactors are not single phase or three phase but rather have some number of poles with two required as minimum for single phase and 3 as a minimum for three phase.

    If you are going to use an off and an on momentary push button switches you will (assuming three phase incoming power, you did not state) need a three pole contactor with an additional auxiliary NO contact to seal the circuit.

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    Thanks, guys. A little bit more thinking on this and I would have realized the truth in what you are saying, Bill. With the VFD, it will be 3 phase out to the motor, so I bought one of these:

    3HP Single 1 Phase or 5HP 3 Phase Magnetic Starter Motor Control 208-240V | eBay

    It looks fairly easy to connect, giving simple photos for each lead including the pressure cutoff switch.

    Dan

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    As I read this, I am wondering why you want/need a contactor with a VFD. There should be no operating switches between the VFD and the motor, the VFD does everything. The pressure switch is used to control the VFD. You are just making it difficult and possibly problematic.

    Tom

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    Send the starter back.

    Pressure switch controls the vfd via run command.

    Needs to be programmed to run whenever the input is active and stop when not.

    Vfd output directly to motor.

    A second pressure switch could be added to set to a lower pressure.

    Clever wiring or programming depending on vfd would allow higher motor RPM on low pressure where hp not needed for more air then at some point normal rpm and full hp.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Tom,

    What I understand after questioning the same thing is that he wants to use it to kill incoming power to the VFD when done for the day instead of using the door switch on the cabinet or going to the breaker panel.

    Bill

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    The magnetic starter idea can be part of a safety campaign as it goes for other tools. If the power is interrupted from a tool and then re-applied it can be dangerous to have a motor start. So the magnetic starter will prevent a re-start, the push button for "run" must be pushed.

    For a VFD if the power is interrupted the motor should not re-start. The VFD will fault out.

    BTW, A magnetic starter looks better with red and green lamps.

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    I think I'm about ready to set the contactor aside and run the pressure switch enable/disable directly to the VFD. I kept being forced to hold down the contactor manually while pressing "run" on the VFD. After a few seconds I can let up one the contactor and it will run until the pressure switch to the contactor cuts the motor off. The contactor is between the VFD and the motor.

    I emailed the seller of the contactor and finally talked to him on the phone. He said I should check the output of my VFD with a multimeter. This is 3 phase out, so I don't think I can trust just any old multimeter to get proper readings, but to humor him I tested the output of the VFD for him while o the phone and all 3 legs came in under 200 volts. He said that until I can bring those up to 208 or 220 the contactor will not operate properly. Apparently most installations of their contactors use 3 phase either from the local power supplier or from an RPC. I did check the normal running current today at 17 amps on one leg, which is the maximum acceptable on the overload relay on the magnetic contactor, so I probably should not have even tried that.

    So if I take out the contactor, will I need to boost the ramp up time during startup? If there is some kind of overload, the VFD simply errors out and there is no harm done.

    Thanks,

    Dan

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    I started using this digital pressure switch with a VFD. It just needs 12-24V which my VFD can provide. The mechanical pressure switch and contactor are gone.
    Festo is another brand that does the same thing. They can come with different options. For my needs on a compressor it was simply to program a wide hysteresis.

    dsc_0996.jpg

    dsc_0746.jpg

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    Interesting. So my old analog pressure switch will not work with the VFD, and I need to go to that digital version?

    Thanks,

    Dan

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    Your old pressure switch should work.

    My original pressure switch was bouncing around at turn off making my compressor motor do the two-step and three-step.
    So I bought three SMC switches at about $15 each, because they were in surplus. I also bought a couple of Festo's for $12.50.

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    My old switch did in fact work, however, it was about 40 years old, and I wanted to be able to get more precise with the hi/low split as well as the upper limit, so I bought the same switch you showed above. I know I could have probably tweaked the old one, but digital will allow quick adjustments as needed without the trial & error fiddling around on the old one.

    Thanks,

    Dan


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