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Thread: Pressure switch control wiring

  1. #1
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    Pressure switch control wiring

    Is there any other way to operate a 3 phase air compressor pressure switch other than using a starter? Am looking for an inexspensive way to wire the unit by just using a 3 pole fused disconnect. Can't think of any way to have the motor drop out when the switch reaches the desired pressure other than de energizing a coil on a starter. Thanks

  2. #2
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    How were you planning on providing overload protection without a starter?

    I didn't think starters were terribly expensive, you figure if they can buy a compressor, they can certainly spend a few bucks properly wiring it.

  3. #3
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    Quote Originally Posted by nizak View Post
    Is there any other way to operate a 3 phase air compressor pressure switch other than using a starter? Am looking for an inexpensive way to wire the unit by just using a 3 pole fused disconnect. Can't think of any way to have the motor drop out when the switch reaches the desired pressure other than de energizing a coil on a starter. Thanks
    Sure just use a shunt trip breaker so that the pressure switch trips out the breaker everytime is reaches the set pressure, but I don't think thats what you want, and I agree with Cow without overloads this motor would not last long as the first time it dead heads into a full pressurized cylinder someone will be replacing the motor.

    You could get a two pole pressure switch (depending upon the HP rating) but I would still at least use a manual motor starter ahead of it to protect the motor.

    Keep in mind that not any pressure switch will work, as one for an air compressor will have a head bleed off line to remove the head pressure so the motor does not dead head in to a locked load.

    The best setup I like is the motor starter that has the bleed off valve built into the contactor, the reason is, I have had many who like to turn off their compressor at night and then try to turn it back on in the morning, if the system is tight and holds pressure then since it was turned off before the bleed off valve was activated it will trip on over loads or breaker when they try to turn it back on, with bleed off done with the contactor then it will bleed off when they kill the power.
    Last edited by hurk27; 11-05-11 at 12:25 AM.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

  4. #4
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    You could use the pressure switch. Most the pressure switches are rated at 25 to 30 amps.
    If the motor has thermal overload switch and not to large for the pressure switch.


    If three phase you could use a manual motor starter in conjunction with the pressure switch.

  5. #5
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    I have a customer that has a small farm mechanics shop, and he has a five HP single phase compressor that came from the factory with the pressure switch directly controlling the compressor motor. He has to replace it about once a year because it doesn't hold up. He's been wanting me to come buy and add a motor starter to it. The motor has internal overload protection, so I could use a DP contactor, which would be a lot cheaper.

  6. #6
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    Quote Originally Posted by nizak View Post
    Is there any other way to operate a 3 phase air compressor pressure switch other than using a starter? Am looking for an inexpensive way to wire the unit by just using a 3 pole fused disconnect. Can't think of any way to have the motor drop out when the switch reaches the desired pressure other than de energizing a coil on a starter. Thanks
    All the industrial compressors I've worked with (say over 15kW) "unload" the compressor rather than shut it off. There are various ways to do this depending on size, manufacturer preferences, and technology.

    Now answering the question you didn't ask ... an inverter is occasionally used to reduce output flow to manage pressure minimizing need for unloading or stopping. This requires a pressure transducer and some control device ... perhaps a PID controller, perhaps a PLC.

  7. #7
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    Quote Originally Posted by Cow View Post
    How were you planning on providing overload protection without a starter?

    I didn't think starters were terribly expensive, you figure if they can buy a compressor, they can certainly spend a few bucks properly wiring it.
    Most likely they needed a larger compressor than what is common to find in single phase and did spend more for it. Proper protection is not important.... until the motor burns out. Protection with properly sized fuses only will work, until they have to change fuses because of an overload and put in larger fuses... so it will not happen again




    Quote Originally Posted by GeorgeB View Post
    All the industrial compressors I've worked with (say over 15kW) "unload" the compressor rather than shut it off. There are various ways to do this depending on size, manufacturer preferences, and technology.

    Now answering the question you didn't ask ... an inverter is occasionally used to reduce output flow to manage pressure minimizing need for unloading or stopping. This requires a pressure transducer and some control device ... perhaps a PID controller, perhaps a PLC.
    These are probably not reciprocating compressors, the compressor in the OP probably is. Addition of a motor starter to a screw compressor probably looks like a small expense as compared to the compressor, especially over 15kW.

    I don't think you would want to unload a recriprocating compressor vs shutting it off. They wear out fast enough as it is when cycling them as needed, to let them run constantly would make that even worse. Putting a clutch on to disengage the compressor but leave motor running doesn't make much sense either as it is as easy to control as a motor contactor and probably cost about same.

  8. #8
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    Quote Originally Posted by kwired View Post
    Most likely they needed a larger compressor than what is common to find in single phase and did spend more for it. Proper protection is not important.... until the motor burns out. Protection with properly sized fuses only will work, until they have to change fuses because of an overload and put in larger fuses... so it will not happen again






    These are probably not reciprocating compressors, the compressor in the OP probably is. Addition of a motor starter to a screw compressor probably looks like a small expense as compared to the compressor, especially over 15kW.

    I don't think you would want to unload a reciprocating compressor vs shutting it off. They wear out fast enough as it is when cycling them as needed, to let them run constantly would make that even worse. Putting a clutch on to disengage the compressor but leave motor running doesn't make much sense either as it is as easy to control as a motor contactor and probably cost about same.
    Actually quite a few reciprocating compressors are of the unloading type, and are design for places that will have a high rate of cycling, but there are many who purchase these through surplus that don't really need them and I have had to convert them to a shut down type with an unloading contactor instead of them just shutting it off every nite.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

  9. #9
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    the company i work for loves to buy used equipment that has had the control panels yanked off the most recent was a duplex ( 2 pumps and motors ) compressor 2 5 hp single phase motors with no pressure switches or anything just a piece of flex hanging off the motors and of course its outside so everthing has to be 3R on single phase compressors with overloads on the motors i use a square d pressure switch that has the head unloader , and i wire in a 2 pole contactor mounted in a 3 r jbox and then of course discos with approiate fuses , i have set 2 of them in the shop for cont run due to cycleing issues , there is one that has a really cool control box if it sees the compressor cycle more than 3 times in ten mins it will hold in on cont. run i belive for 1 hour or something like that then go back to normal low pressure on and hi pres off , ive been tryin to talk them into a rot. screw and get rid of all of the small compressors im sure it would help the power bill i have a little emglo compressor that i can wheel around with me for blowing rope thru conduit or whatever its selsctable 120/240 SP and selectable cont run or normal on off

  10. #10
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    A motor starter and overload for a small 3 phase motor can be purchased for under $50. Considering the cost of a compressor, I don't see the attraction to being cheap.
    Bob

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