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Thread: Tandem, or dual, air compressor questions please?

  1. #1
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    Default Tandem, or dual, air compressor questions please?

    I'd like to ask for a pressure switch brand recommendation. I both motors are 240 and 5hp. They both pull similar current at about 27-28 amps (I can't recall the exact value but these aren't box store pretend 5hp). The one that came origination my IR unit is fine but I installed an off brand on a second compressor and it didn't last a year and it was sized as required.
    I'd also like to know if anyone has replaced the motor start switch with a potential relay? My start switch contacts are corroded and a replacement is over $40.00 where a relay is much cheaper.
    Thanks for the info.

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing

  2. #2
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    IMHO... You should be switching the motors though a motor starter contactor that is controlled (for automatic) from the pressure switches and in that case the "rating" other then voltage is not material... for tandem there is usually there is a control realy on the mix also. I assume you have motor protection contactors etc?

    These are rock-solid and what I use. Direct switching of single phase 240 is limited to 3hp but not relevent with contactor control. .https://www.zoro.com/square-d-pressu...63/i/G0964162/

    more:

    https://www.zoro.com/square-d-pressu...hoCt4kQAvD_BwE

    What were you using before?
    Last edited by lakeside53; 09-09-2017 at 11:59 PM.

  3. #3
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    When I initially made the two compressor/motor setup I used a single pressure switch. It energized a contactor as you describe. The issue I was concerned about was having both run simultaneously every time the air pressure went down enough to call for more. I generally don't need a lot of air volume. Then the contactor failed and I figured I'd be better off using a setup where I have a main unit satisfy my air needs and the second unit come in when I needed a lot of volume such when my blast cabinet was being used. For this reason I went with two pressure switches. Now I'm thinking I should go back to a contactor type system BUT I am also thinking of using a contactor for each motor. I'd still use a single pressure switch to lower the coils of the contractors. Obviously I could still configure it with two pressure switches so the separate contractors energize at different pressures but I'm not sure it makes sense to spend more money on another pressure switch. Using two contractors is actually less $ than one larger contactor according to the brief online shopping I did. I can get a two pole 40 amp contactor with 240 vac coils for ten buck a 60 amp contactor is significantly more than twice that. I don't know that a single 60 amp contactor would hold up with both compressors coming on at the same time anyway and two 40 amp contractors definitely would be fine.

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing

  4. #4
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    How about 1 pressure switch, controlling one motor always, and going to the second motor through a switch and possibly a relay (so you can use a cheaper pressure switch). When you need more air volume, you throw the switch to enable the second motor.

  5. #5
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    I come across tandem in older buildings where they are used for pneumatic control of the HVAC systems. These run for decades without issue. Pressure switch failure is usually old age related - cracking of the diaphragm. You also need to consider "starts per hour". A sandblaster or bead blast cabinet is many time an hour, and will likely exceed the pressure switch "starts" rating.

    At 5hp you want to get away from direct pressure switch switching and you really should have (and by code) a motor starter with overload protection (per motor). Once you have the motor starter contractor the rest is trivial. A contactor alone is not a motor starter. Look at "hp" ratings, not just current, and be sure it lists single phase at your HP. A 40 amp general purpose contactor without hp rating will likely not be sufficient for a single phase 240v 5hp motor. Plenty of relatively inexpensive "rated motor starters" on ebay.

    The usual way is separate controls, pressure switches etc. Mine (and most I see) has two switches, each with "hand", off, and "auto". The kick-in pressure of one is slightly below the other in Auto; also separate 24v control transformers and a small relay for the hand/auto logic. It's dual 7.5hp and no way would I want both motors kicking on simultaneously. Even 2 x 5hp could present an issue; 2x startup current if you switch both motors simultaneously which may have why your contactor failed if you ran both from the one.

    I have 2 other real compressors - 3hp each. I run those with pressure switches though typical motor starters also. They will outlive me; I never have to worry about the motors overloading.


    Another issue that can make a huge difference to starting current duration is whether you have unloaders and whether they are functioning correctly. Do you have them on both compressors?
    Last edited by lakeside53; 09-10-2017 at 01:09 PM.

  6. #6
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    Yes both heads have unloaders.

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing

  7. #7
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    I bit the bullet and decided to get two magnetic motor starters and two new pressure switches to control the coils on the starters. The starters are 3 phase but that's an easy work around. They are rated as 7.5 hp but have adjustable overloads.
    After reading a lot about these motors I am surprised that IR sold this unit without a starter.

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing

  8. #8
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    Good... I assume they are 7.5hp at single phase?

  9. #9
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    They were listed as 7.5 three phase. I got 3 on eekbay for 70.00. Made by Eaton.

    Galaxy S4, Slimkat
    If I wasn't married I'd quit fishing

  10. #10
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    Are they also listed for 5hp single phase?

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