codewritinfool wrote:
A N.O. pushbutton applies line to the transformer primary, whos secondary connects through a N.C. switch to the contactor coil, which causes the contactor to engage. That's your energize button. A separate line from the load side of the contactor also feeds the primary. The N.C. switch would be your release button.
This would work but the usual way it is done is to have the control transformer energized all the time - usually because there are other things (other contactors/starters) being operated at the same time. Consider the traditional "ladder diagram" i.e. two parallel vertical lines corresponding to the rails and horizontal stuff the rungs. The top rung would be the secondary of the control transformer so the left rail is "hot" 24 volts and the right hand rail is neutral 24 volts. For the second (and only in this application) rung starting at the left rail you connect one pole of a normally closed momentary operation switch (push button). This is the "stop" or "off" button (often with a red mushroom head). Moving to the right, the remaining pole of the stop switch is connected to a momentary normally open pushbutton switch (start). Moving still further to the right, the other pole of the start switch is connected to the contactor coil and the other pole of the contactor coil is connected to the right rail. Finally auxialliary NO contacts on the starter/contactor are connected across the start switch. When you push the start button the contactor energizes closing the auxiallary contacts so the circuit stays "latched" after you release the start button and until you push the stop button. If power goes down it stays unlatched until you push the start button. The nice thing about this arrangement is that you can put other low voltage, NC low current contacts in series with the stop push button to unlatch the circuit if, say, temperature or pressure go too high or liquid level gets too low or too high. Or you can have a separate rung for each element. The possibilities are limitless.
A generic configuration might have a contactor in each of three rungs with the "hot" side of each coil tied to the common pole of a three position toggle switch (Home Depot item). One of the other 2 poles goes directly to the left rail and the other to the left rail through an external NO contact which could, for example, be in a temperature contoller (I use this arrangement frequently). In center position the contactor (and its load) are off. Flipped up the contactor is on. Flipped down the contactor is controlled by the external controller. I usually lable the switch positions "ON - OFF - AUTO" ( because the external contact is usually not a manual control but rather something like a level switch, pressure switch or PID controller) (
http://www.pbase.com/agamid/image/124858766). For master/safety off you can use a simple, inexpensive, low voltage relay wired in a latching circuit i.e., starting at the left rail, NC stop button, NO start button, relay coil, right rail, NO contact on relay across start button. Picture this as the second rung on the ladder (controll transformer secondary is the top rung). Just below this second rung the left rail is broken so that the first two rungs and the last three (with the load contactors in them) are separate. The upper left rail is connected to the lower left rail through another NO contact on the relay. So push start and control relay latches energizing the lower ladder which now turns loads on or off per the manual setting of switches or the automatic control of some other device. In an emergency you push the stop button unlatching the relay and powering down the lower ladder. All loads go off untill you push start again. Loads do not come back on after a power failure.
You can, of course, in this configuration also put other emergency stop NC contacts in series with the stop button. For example you coild mounte a float switch that opens in the event of water rising to a certain depth in the brewery. If this switch trips the whole circuit goes down and stays down until you get the water down and press start again.
codewritinfool wrote:
4500 * 3 / 240 = 56 amps. Your magnetic switch isn't gonna do the job. Neither is the typical contactor.
You can buy contactors or starters for any current load you want.
I'll note that the item described in the OP is a starter. This differs from a contactor in that it has elements designed to sense overly large or overly long inrush current for a motor - as the name suggests these things are designed for starting motors. The thermos are wired in series with the stop button. You don't need those functions for this application. A simple contactor will do. It doesn't hurt to have them but why pay for them if you don't need them.
