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Straight out the book. I copied this to help those who have not had formal training or professional knowledge when it comes to the scope of motor control logic. Read the text to learn the purpose of the stop button.
"""Three-wire control has three wires leading from the control device to the starter or contactor. The circuit uses a momentary contact OFF pushbutton (NC) wired in series with a momentary contact ON pushbutton (NO) wired in parallel to a set of contacts which form a holding circuit interlock (memory).
When the normally open ON pushbutton is pressed, current flows through the normally closed OFF pushbutton from L1, through the momentarily closed ON pushbutton, through the magnetic coil (relay), and on to L2. This causes the magnetic coil to energize. When energized, the auxiliary holding circuit interlock contacts (memory) close, sealing the path through to the coil circuit even if the start pushbutton is released.
Pressing the OFF pushbutton (NC) opens the circuit to the magnetic coil, causing the contactor to de-energize. A power failure also de-energizes the contactor. The interlock contacts (memory) reopen when the contactor de-energizes. This opens both current paths to the coil, through the ON pushbutton and the interlock."""
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