Air Compressor Wiring Befuddlement
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    Default Air Compressor Wiring Befuddlement

    I'm at a loss as to why my used 5hp, 3 phase Ingersoll Rand air compressor will not run with the wiring setup shown in the sketch below, which is the way it was supposedly wired by the previous owner. The coil is the correct voltage and tested to make sure it works. The compressor will run if I manually close the contacts. Also I'm sure the low level oil switch and pressure switch are in the closed position. Any suggestions?
    air-compressor-002.jpg

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    Have a look at the check valve that feeds the tank. Make sure the un-loader circuit is working.

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    Quote Originally Posted by sealark37 View Post
    Have a look at the check valve that feeds the tank. Make sure the un-loader circuit is working.
    The check valve is new and working correctly. Same with the unloader setup. For some reason I don't get the proper voltage across the coil terminals, which should be 240 VAC.

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    I can't follow your sketch. Where are the contacts?

    To get 240V across the coil, you need L1 to be hooked to one side, and L2 to the other side. You can wire the safety switches any way you like, but there has to be a path.

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    Quote Originally Posted by ewlsey View Post
    I can't follow your sketch. Where are the contacts?

    To get 240V across the coil, you need L1 to be hooked to one side, and L2 to the other side. You can wire the safety switches any way you like, but there has to be a path.
    No doubt the sketch could be better. I tried to show all of the wiring terminations. The power coming into the magnetic starter is marked "line". The coil, pressure switch and low oil level switch are also identified. There is also a hand/off/auto switch. The wiring going to the motor begins at the thermal overload device. There is clearly a path from L1 through the low oil level and pressure switch to one side of the coil but the other side is difficult to understand because it ties into a terminal on the bottom of the overload protection with no clear path to L2. The "contacts" are between the terminals marked L1, L2 and L3 and where it branches off to the motor. It's sort of moot if I can get the coil energized.

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    So what happens if you jump L1 and L2 to the coil? Seems like if that works you can step backward through the circuit and find where you are losing voltage. The coil should need very low amps to pull in, so you could jump it with some regular jumper wires.

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    Default Air Compressor Control Wiring

    Hopefully this will help it make more sense.

    Make sure your overload relay is reset.
    Leave one probe on L1 and test all terminals on the L2 side.
    Leave one probe on L2 and test all terminals on the L1 side.
    That should identify where the open device is located.
    240v-compressschematic.jpg

    SAF Ω

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    Quote Originally Posted by beeser View Post
    Any suggestions?
    air-compressor-002.jpg
    I suggest that you make your presentation more clear rather than have it come as bits and pieces. A drawing like that invites guesses and more questions.

    Sounds like I'm too critical. No. I would like to know what air pressure switch you have there. The last one I was using gave a few thumps to the motor starter every time it shut off. I replaced it with a SMC digital pressure gauge/switch.

    All your high voltage wiring is something that I replaced. Had a oil pressure switch contact welded together for who knows how long. Since I always looked at the
    oil level glass it was never a issue, just a disappointment. Replaced my controls with 24v.

    And maybe I'm wrong but here is a guess. From you drawing it looks like the auxiliary set of contacts on the starter are shorted together.

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    Quote Originally Posted by ewlsey View Post
    So what happens if you jump L1 and L2 to the coil? Seems like if that works you can step backward through the circuit and find where you are losing voltage. The coil should need very low amps to pull in, so you could jump it with some regular jumper wires.
    I jumped L1 and L2 to the coil to make sure the coil was functioning.

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    Quote Originally Posted by SAF View Post
    Hopefully this will help it make more sense.

    Make sure your overload relay is reset.
    Leave one probe on L1 and test all terminals on the L2 side.
    Leave one probe on L2 and test all terminals on the L1 side.
    That should identify where the open device is located.
    240v-compressschematic.jpg

    SAF Ω
    Thanks SAF! It does help. I think your sketch and the wiring on mine are functionally the same except branched a little different at the pressure switch. I'll go through the checks you suggested and think more on it today. Hopefully I can get this worked out.

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    Quote Originally Posted by rons View Post
    I suggest that you make your presentation more clear rather than have it come as bits and pieces. A drawing like that invites guesses and more questions.

    Sounds like I'm too critical. No. I would like to know what air pressure switch you have there. The last one I was using gave a few thumps to the motor starter every time it shut off. I replaced it with a SMC digital pressure gauge/switch.

    All your high voltage wiring is something that I replaced. Had a oil pressure switch contact welded together for who knows how long. Since I always looked at the
    oil level glass it was never a issue, just a disappointment. Replaced my controls with 24v.

    And maybe I'm wrong but here is a guess. From you drawing it looks like the auxiliary set of contacts on the starter are shorted together.
    The pressure switch is a Square D manufacture with an unloader option and is new. I'll pay more attention to the aux. contact when working more on it today.

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    I think SAF nailed it. The overload relay was open despite attempting to reset it. After bypassing the relay the compressor started right up. I don't know if the relay can be repaired or if it's a separate part from the thermal overload. Regardless, it may be time to replace the entire starter as it is supposedly limited to a 3 hp motor at the voltage being used. It was originally set up at a higher voltage. That being the case, any suggestions on a replacement? And thanks all for the comments and suggestions. They were truly appreciated.

    PS The comment regarding the aux. contact threw me off a little. If I'm not mistaken its full function was not used in this setup. Only one end of it was used a terminal for 3 wires.

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    If the starter is for a 3hp max motor, and you are using it on a 5hp compressor, the overloads will not be large enough for the initial starting load, and it sounds like they have quit! Look at the full load amps on the motor rating plate, and select an overload with the fla of the motor in the middle of the range, and set the overload to match the fla of the motor. for instance, if the FLA of the motor is 5.5A you need a 4 to 8A overload set to 5.5A.

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    The amp rating for the Baldor motor on the compressor is 13.2 at the voltage being applied. The maximum amps listed on the starter is 18. Even though the starter shows a maximum 3hp motor for this voltage (5hp for a higher voltage) is this still OK?

    I hate to admit but it turns out the overload relay is OK after all. I didn't realize the reset button had to be pushed in harder.

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    From the form of your sketch, i would guess that you have a Furnace brand starter, with the dual voltage coil.

    If your compressor was previously wired for 480V, you likely need to change out the heater elements, to match the motor FLA. If you keep running with the small ones for the higher voltage, you will damage the overload relay from excessive current and heat. If you allow that to happen, you will need to replace the starter.

    NEMA heater elements already have 25% more current capability than listed on the heater charts.
    So you should size the heaters using the FLA current directly from the charts.

    Repeated tripping with small heater elements will damage the calibration of the overload relay.

    You will not likely have problems running the starter you have, once you change the heaters.

    SAF Ω

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    Dayton Model 5X155B NEMA Size 1 Magnetic Motor Starter. For 3 HP 120V, 7-1/2 HP 208 to 230 V and 10 HP 430/460 VAC.

    Line taken from a spec. Notice the ratings and how they change. What voltage are you at? A you in the 3HP category 208 to 230?

    The Dayton example is something I used on a 5Hp motor running on a air compressor. I was in the 7-1/2HP 208 to 230 range.

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    Quote Originally Posted by beeser View Post

    PS The comment regarding the aux. contact threw me off a little. If I'm not mistaken its full function was not used in this setup. Only one end of it was used a terminal for 3 wires.
    Sorry. You might not be good there having three wires there. I don't go past two using spade connectors.

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    Quote Originally Posted by SAF View Post
    From the form of your sketch, i would guess that you have a Furnace brand starter, with the dual voltage coil.

    If your compressor was previously wired for 480V, you likely need to change out the heater elements, to match the motor FLA. If you keep running with the small ones for the higher voltage, you will damage the overload relay from excessive current and heat. If you allow that to happen, you will need to replace the starter.

    NEMA heater elements already have 25% more current capability than listed on the heater charts.
    So you should size the heaters using the FLA current directly from the charts.

    Repeated tripping with small heater elements will damage the calibration of the overload relay.

    You will not likely have problems running the starter you have, once you change the heaters.

    SAF Ω
    I don't know if this makes a difference in the scheme of things but the reason for the thermal overload tripping was probably due to the compressor seizing. The previous owner (company) apparently ran it low on oil after not fixing an oil seal leak. One or both connecting rods seized at the crankshaft bushing bending one rod and completely braking the other. Luckily both cylinders escaped damage. I was able to get the compressor fairly cheap and rebuilt it. You are correct, the starter is an older Furnas manufacture. I'll try to find another heater but it may be cheaper to just purchase another starter. The prices seem to be all over the place but I don't want to just buy cheap and create another problem.

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    After doing some more checking it looks like I need Furnas Type H37 heaters instead of the H30 now installed on the starter, that is if I'm reading things correctly. The good news if that's the case is they are readily available and somewhat inexpensive. Is this a good way to go instead of replacing the entire starter?

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    That's what I would do. And likely why the starter used, has a dual voltage coil, so it could be converted.

    SAF Ω


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