don't mean to be nettlesome, but you promised a theory discussion on this back in august of 01 when you posted the diagram and I can't find that it ever happened, but I get no returns on searches I'm sure ought to yield something, so maybe I'm just not searching right.
In any event, after dwelling on the drawing - which is a little more complete than the diagram inside the cover of the contactor, it seems to me that the point of the 3-wire switch is that if the tool/machine happens to be turned off from a remote location or not at the last switch toward the machine, it will not start when re-energized even if it had been running.
I have to say that the pattern confused me at first, esp because there was nothing left of the original switch which got crushed by a forklift during moving.
To file under almost arcane knowledge, is there some other functional advantage I'm not considering to this momentary NC / NO paired switch?
Archi, it seems you're lacking in understanding some of our drawing conventions. What you need to keep in mind is that things are shown in their 'unenergised' state. Since that state can change when power is applied, it can be hard to follow.
For a conventional 3-wire switch set-up: Your 'stop' buttons are in series, while the 'start' buttons are in parallel. All of these buttons work together with a set of contacts on the coil to control power to the coil.
Also, the buttons are all -usually- 'momentary.' That is, the contacts change position only while your finger is pressing down on them.
When you press the 'start' button, the coil closes another set of contacts, that keeps the coil energized after you release the button. The machine will remain running until this secondary circuit is interrupted. That's what the stop buttons do- they kill power to this second path as well as to the path through the 'start' button.
You need only open the circuit for a moment for the coil to drop out. Then, you need to press 'start' to get things going again.
The advantage to this arrangement is that you can use a low voltage, low current circuit to control a larger machine - or several machines. You can easily add in other controls.
To add to what John said above, the way that the 3 wire Start/Stop works is this:
Effectively, what happens is, you run a 3 wire control cable from the supplying DOL starter box to a remote Start/Stop station, this could be on the other side of a machine, but it would also need to be within sight of both the original starter box and the operating parts of the machine in question.
What happens from there, is the remote Start circuit is wired in parallel with the original Start circuit, this also encompasses the "hold-in" contact on the motor contactor, this ensures that power is continuously supplied to the coil of the contactor.
Start contacts are always wired NO (Normally Open), this to ensure, with a motor or equipment that vibrates a lot, if the Start wire breaks, the equipment cannot be started until that equipment has been repaired and re-commissioned.
On the Stop side, the switches are wired in Series, this comes from the control circuit side of the thermal overload, so you have a continuous loop of switches from the start of the Stop Circuit back to where it ends at the Stop circuit.
Stop contacts are ALWAYS wired NC (Normally Closed), this is to ensure that if a wire breaks in the Stop circuit, the machine will just stop, as you've lost power to the control circuit. E-Stops, Guard limit switches and light curtains all use this method, the effective result is the same.
The "beauty" of using a contactor control system with motors, is the fact that on loss of power, the contactor will drop out, as in if the "hold-in contact" can't be maintained through lack of voltage, it will need to be manually reset. This is what is known as a "No-Volt Release". It also stops motors from suddenly re-starting on the restoration of the supply, in process control and some other things, this is VERY important.
The aux side switch is pulled in when the contactor energizes. This could also be a N/O relay point if this was a smaller relay. You can have any number of start switches, stop switches or overloads and interlocks. You just wrap the last stop switch to the center "run" line.
sorry, thought i responded yesterday. i appreciate the attention to the question. I was really trying to confirm my assumption that the switch was designed to prevent restart if the loss and restoration of power was remote from the machine. I figured that was the principal function but I hadn't head the term "no-volt release".
The only question I have about Greg's diagram is the aux side switch. couldn't you also just pick up the same on the main switch for the motor. Currently, this machine taps the leg that would be the very bottom line in Greg's drawing (is there a color or labeling convention that would help to discuss this picture better)at the upstream side of the contactor which if you go back to scott's original drawing which Greg also linked above : https://www.electrical-contractor.n...Stop_3_wire_AC_motor_con.html#Post148638 is upstream of the motor overloads, so the load for running the contactor is not added to the motor overloads.
What I'm wondering is, couldn't you just pick up the same leg downstream of the contactor but still upstream of the overloads, i.e. at the screw terminal on the contactor. I can't easily get to the contactor to check how this one is wired, appears to have a side terminal for the switch, but no input for that side terminal, so maybe the crossover is internal? Since all is well, I can't really justify the time to pull the contactor box further apart and use a continuity test to reverse engineer what it is up to so though I'd just ask.
I suppose one downside to my simple approach adds the contactor load to the main switch capacity, but while I can see the contactor load possibly affecting overload throw if the overloads are conservative (i.e. allow very little overcurrent) I would think that the contactor would usually have more slack in rating.
And while on the subject of overloads. I assume the meaning of scott's diagram with 3 overloads in series is that these are actually auxilary contacts on the motor overloads so that if an overload trips on any leg, it has the effect of opening the stop switch as well.
The reference to the overload in Greg's drawing is as a single which could be read as a current restrictor on the contactor coil itself.
It doesn't really matter where each switch or overload is in the series (stop) or parallel (start) part actually is as long as the serial switches are electrically in front of the parallel switches. They still work the same. If any serial switch opens (bottom row on my picture) the contactor drops. As long as that stop/overload switch is open, none of the start switches are powered so you can't start it again.
The aux switch is just used to hold the contactor in, unless any of the other stop or overloads opens. The start switches are momentary contact and are only there to start the sequence bypassing the side switch until the contactor is energized.