# Wire pressure switch

How do you connect a 2 wire pressure switch for an air compressor to
a three phase magnetic motor contorller (siemens)? Do both wires
connect to the coil? Does one wire connect to the power and the other
connect to the coil? Three phase power is via a phase converter.
wrote: (clip) Does one wire connect to the power and the other
^^^^^^^^^^^^^^^^^ The three phase motor controller is a contactor? Then the answer is yes. And the other wire of the coil goes back to neutral. Or, if you're running the coil across three phase delta, then to either other wire of the three.
You need a little book called Ugly's Electrical References. However, if you had it, you wouldn't be asking, so here's the relevant wiring diagram:
Normally an air compressor has a Hand/Off/Auto switch. Note that in this diagram, the little red ellipse that says 'M' inside is the magnetic coil. Leaving out the fuse and the HOA switch, then the pressure switch is indeed connected in series between one hot (power) wire and one side of the coil, and the other side of the coil (which must be rated for the correct voltage - this is critical) should go to the second real hot lead. What I mean by that is that your coil should be connected in and out between the two legs which comprise your single phase power, not the 3rd leg generated by the phase converter.
Grant
diagram:
...
BTW, the three unlabeled things that look like series capacitors are the Normally-Open (when power is off) contacts of relay "M".
jw
SNIP
SNIP
Holy Explosion Batman, why would there be a "Hand" position that isn't even a constant pressure (ie..spring loaded) type?!?!?!?! Good way to either get hurt, damage something, or at a minimum pop the breakers. Should just be Off and Auto.
Off-hand, I can't even perceive of what value this particular circuit schematic would be for ANYTHING, let alone a compressor!!
Take care. THINK.
Brian Lawson, Bothwell, Ontario.
I use my Hand switch when I just want to turn it on for a minute to show someone how quiet my compressor is, or to see if the belts are flapping, or just to check things out. Does seem a little strange now that you mention it, though, but I've seen a bunch of 'em done that way.
Grant
It's NOT the normal way of doing it - you know to only use it for testing, but someone else wandering in off the street could do very bad things playing with the "Hand" setting on an air compressor.
The only practical reason I can see is if the compressor has been rigged with a constant-run unloader system, and you manually switched it over from the 'pressure switch' mode to the 'unloader' mode to do a lot of high air demand work. The motors are only rated for so many starts an hour, and short-cycling can and will burn up the windings.
Unloader mode operation is more useful in commercial applications where you pay a demand charge on the power usage, and the idling current of the unloaded compressor motor is more than offset by not making start surges all day and bumping up your demand multiplier. But it can also be useful if your power supply is marginal and you need to weld/plasma cut or run the lathe at the same time you use air - the start surge on top of the welder current will trip breakers.
They usually use a Hand-Off-Auto switch on things like water wells, pressure boost or sewer lift pumps - for when the floats fail, or you want to avoid short cycling and just let it run.
Oh, and a wiring issue for the Original Poster - When you wire the motor starter coil, you also have to loop that circuit through the overload block switch contacts - otherwise the motor won't stop if it single-phases and trips out an overload heater, and the Magic Smoke will escape from the motor.
And it's REAL expensive to get fresh Magic Smoke installed in an electric motor, it's usually cheaper to get a new one.
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If the coil voltage is the same as the incoming phase-to-phase voltage, you use a simple series circuit like this:
Line 1 In | Manual Off/Run switch (SPST toggle - night cut-off) | Pressure switch NO | Starter coil | Starter overload section cut-off NC switch | Line 2 In
Nobody has mentioned the overload section, and that is CRITICAL to hook up properly on any 3-Phase machine. Pop a fuse anywhere up the line, have a bad starter contact, loose wire - and the motor will smoke before the circuit breaker trips.
If you have a particularly expensive motor, you want an electronic overload section (or a separate voltage monitor) that is checking for voltage imbalances or a phase rotation error - It's extremely rare (since they know better) but sometimes the power utility does overnight work and goofs with the phase rotation.
Usually it's messed up by Gomer from Jethro's Handyman Service that doesn't understand the concept of Phase Rotation and just stuffs the wires together in any old order. Git 'Er Done!
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