Magnetic Motor Starter Trouble
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  1. #1
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    Default Magnetic Motor Starter Trouble

    I know this might not quite fall within the realm of this forum, but I figured the most electrical knowledge is hanging out here.

    Background info: The mag starter in question is on a Saylor Beall VT735-80 air compressor. 5hp and 3 phase. I just acquired the compressor after it sat for probably 15 years as a backup unit in a shop I used to work for. It was in a clean, dry, climate controlled environment during that time. It installed it in my home shop after some TLC. I'm running it off a 40hp North America Phase Converter unit, which is also running other equipment flawlessly. The mag starter is a Gould I-T-E piece, NEMA size 1.

    Now you'll have to forgive my lack of expertise, but my understanding is that these work by energizing an electromagnetic coil which causes it to move and thereby brings a set of movable contacts to a set of fixed contacts which allows current to pass through to the load. The issue is that this automatic sequence is not occurring. I have tried reseting it the overload portion of the mag starter. The Start and Stop buttons have no effect either.

    However, I can manually push the contacts together and the compressor starts and runs great. As soon as I release the contacts, the compressor shuts off. So it seems to me that the coil is not working correctly. But I'm not sure how the start and stop buttons come in to play... I've noticed many mag starters don't have them.

    In the final picture, you can see the coil surfaces seem a bit rusty.

    Also, I do have correct voltage coming into L1,L2,L3, I checked that with my meter. Any advice on troubleshooting this starter would be appreciated.

    Thanks.








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    The coil of the mag needs to be connected to two 'real' lines of incoming power and not to the generated leg. A typical compressor is turned ON with a maintained switch and further ON/OFF control is achieved through the pressure switch which controls the mag. Your pictures appear to show a momentary START/STOP setup..no?

    Stuart

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    Quote Originally Posted by CutterComp View Post
    Any advice on troubleshooting this starter would be appreciated.
    WRT keeping the control circuit on the utility legs when power is from RPC: your taps are the white wire out to the pressure switch from L1 and the tiny red one now at L2 (top of 2nd photo). If your generated leg is one of L1 or L2, move the control circuit tap from that leg to L3.

    WRT troubleshooting otherwise: look for 230vac between whatever leg that red power tap wire is on (L2 presently as per above) and the screw to the RIGHT of the silkscreened "Z" behind the stop button. If you aren't getting power to the Z terminal then either the contacts on the pressure switch are not closed or there's a fault in the wire run out and back to that switch, OR the normally closed contacts behind the stop button, between points marked 3 and Z, are not closed. Note: closed, dirty contacts may behave as open. The top surface is clear plastic for ease of inspection.

    If you have 230v between L2 and the Z screw, then de-power and measure resistance between the Z screw and the one with the oxide residue located a couple of inches lower, just below the return spring visible in the bottom photo. These two should span the coil. If it's open then either the coil winding is open or the module contacts are dirty/bad and you should open the contactor up to inspect/clean them or exchange for a new coil module. If the coil is good then suspicion moves to the complex of overload relays.

    Note for when you get this going: the start button has been effectively disconnected, and the stop button will only momentarily stop the motor (if the pressure switch is busy saying "on").

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    the coil is 220V, so you need two legs to the A1 and A2 terminals. The small red wire from L2 runs down the side to one of these terminals. The second should be the white wire on L1, going to the pressure switch and returning on the black wire. The black wire should terminate on the second coil terminal, A1 or A2. The push buttons should be out of the circuit.

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    As just compensation for the information you receive, you should do a class on how to post pictures because these are the best I have ever seen.

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    Quote Originally Posted by roadman View Post
    WRT keeping the control circuit on the utility legs when power is from RPC: your taps are the white wire out to the pressure switch from L1 and the tiny red one now at L2 (top of 2nd photo). If your generated leg is one of L1 or L2, move the control circuit tap from that leg to L3.

    WRT troubleshooting otherwise: look for 230vac between whatever leg that red power tap wire is on (L2 presently as per above) and the screw to the RIGHT of the silkscreened "Z" behind the stop button. If you aren't getting power to the Z terminal then either the contacts on the pressure switch are not closed or there's a fault in the wire run out and back to that switch, OR the normally closed contacts behind the stop button, between points marked 3 and Z, are not closed. Note: closed, dirty contacts may behave as open. The top surface is clear plastic for ease of inspection.

    If you have 230v between L2 and the Z screw, then de-power and measure resistance between the Z screw and the one with the oxide residue located a couple of inches lower, just below the return spring visible in the bottom photo. These two should span the coil. If it's open then either the coil winding is open or the module contacts are dirty/bad and you should open the contactor up to inspect/clean them or exchange for a new coil module. If the coil is good then suspicion moves to the complex of overload relays.

    Note for when you get this going: the start button has been effectively disconnected, and the stop button will only momentarily stop the motor (if the pressure switch is busy saying "on").
    Thanks for the detailed response. Yesterday, after reading the post from atomarc above, I swapped the red wire from L3 to L2 and white wire from L2 to L3. The red wire is the generated leg. I realize this isn't quite what you are recommending ("If your generated leg is one of L1 or L2, move the control circuit tap from that leg to L3") but I think it may have accomplished the same thing? Correct me if I'm wrong.

    However, the starter seems to function correctly now. It starts when I throw the disconnect on the wall, shuts off when the pressure switch detects 150psi and starts again at 125psi. As you mentioned, if I hit the stop button, it only cuts power as long as I depress the switch. The start button has no effect at all.

    Dumb question: why must the control circuit be on the utility legs?

    Thanks

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    Quote Originally Posted by CutterComp View Post
    ........

    Dumb question: why must the control circuit be on the utility legs?

    Thanks
    This is most important on the actual RPC, because there is no generated leg until the RPC starts....... a bit hard to start the RPC using the generated leg that does not yet exist.

    In load motor use, the problem is that the generated leg is a bit variable, so when the motor load is thrown on it, the voltage drops. If it drops enough, contactors connected to it will drop out, so the motor starts, but stops before it gets going faster.

    The utility legs are more stable. So you can better count on them to reliably power the contactor as the motor starts, unless you have a really "pathological" case of voltage drop. Just hook to the utility legs and don't worry.

    If the generated leg is very solid, then the use of the utility legs only is not essential. This is hardly ever the case, since the common case is with fairly well matched load and RPC. Only a ridiculously oversized RPC might allow you to ignore this advice.

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    Quote Originally Posted by CutterComp View Post
    I swapped the red wire from L3 to L2 and white wire from L2 to L3. The red wire is the generated leg. I realize this isn't quite what you are recommending ("If your generated leg is one of L1 or L2, move the control circuit tap from that leg to L3") but I think it may have accomplished the same thing?
    Yup.

    It would have also reversed the direction of motor rotation (unless you also shuffled colors at T2 and T3). Typically the valves in the head don't care what direction the pump runs, but some oil slingers are designed to work much better in one direction than the other, and if there is a built-in oil pump it may not function at all well when run backward. In terms of external cues to get the right direction, typically you want the pump flywheel to turn such that air flows from the outboard side of the flywheel, through it, past the intercooler and then the rest of the pump. If that's not what you see/feel, you should shuffle any two of the starter output wires.

    If you want to eliminate the stop button as a momentary interrupter, move the black wire coming from the pressure switch from terminal 3 to the one to the right of the Z.

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    just to cover the bases, an AC starter draws a much higher current when the armature is open and drops down when it closes. The reason is that when the magnetic circuit is complete, the impedance of the coil is increased. The result is that the coil load my pull the voltage down, too low to close the starter. The open current draw may be much greater than you think.

    Bill

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    Quote Originally Posted by 9100 View Post
    just to cover the bases, an AC starter draws a much higher current when the armature is open and drops down when it closes. The reason is that when the magnetic circuit is complete, the impedance of the coil is increased. The result is that the coil load my pull the voltage down, too low to close the starter. The open current draw may be much greater than you think.

    Bill
    Yup, like 90 VA open, and 10 VA closed.

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    As rusty as that coil armature is, I doubt that it barely gets to the sealed in position. And will be very noisy.

    Surprised that the OP never mentioned it. That unit needs a disassembly and cleaning.

    Polish the pole faces with 400 grit paper to remove the rust for quiet reliable operation.

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    You have a typical starter setup with an auxiliary contact. The auxiliary contact is connected in parallel with the start button and the stop is in series with it. You push the start button and the starter closes, closing the start circuit. Pressing the stop button breaks the circuit and the starter opens. For some unknown reason, someone has disabled that circuit but it should be easy to reconnect it and eliminate switching the disconnect.

    Bill

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    Quote Originally Posted by 9100 View Post
    You have a typical starter setup with an auxiliary contact. The auxiliary contact is connected in parallel with the start button and the stop is in series with it. You push the start button and the starter closes, closing the start circuit. Pressing the stop button breaks the circuit and the starter opens. For some unknown reason, someone has disabled that circuit but it should be easy to reconnect it and eliminate switching the disconnect.

    Bill
    This is an air compressor that has its control through a pressure switch. Did you not read the original post? Not a typical application for a START/STOP station.

    Stuart

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    Quote Originally Posted by atomarc View Post
    This is an air compressor that has its control through a pressure switch. Did you not read the original post? Not a typical application for a START/STOP station.

    Stuart
    Of course I read the original post but didn't think it through. You are correct. Using the start and stop buttons. Would mean that when the pressure switch opened the contactor, the auxiliary contact would open and the compressor would stay off until you pressed the start button again.

    Bill

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    Thanks to everyone, this has been helpful and informative. It seems to be working correctly now and I got the pump spinning the correct direction, as confirmed by an arrow cast into the flywheel.

    Quote Originally Posted by SAF View Post
    As rusty as that coil armature is, I doubt that it barely gets to the sealed in position. And will be very noisy.

    Surprised that the OP never mentioned it. That unit needs a disassembly and cleaning.

    Polish the pole faces with 400 grit paper to remove the rust for quiet reliable operation.
    I did mention this in the OP and I would like to get the coil cleaned up. Not quite sure how the starter comes apart so I can access the coil and sand it. You'll see in the pics there is a plastic front cover (with the name FTE on it) that is removable by the metal spring clips on the sides and that provides access to the contacts. But to remove the spring loaded movable portion of the coil assembly, I'd have to play around with it a bit. I'll take a look tomorrow.


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