Here are some “relay logic” or “ladder logic” schematics 
commonly used in AC motor control.

File 3_wire_control_01

This is the classic “3-wire” control.  The stop button is 
Normally Closed, the start button is Normally Open.  The 
round device is a relay coil, and “40” is just a number to 
identify parts of the same relay which may appear in other 
circuits.  It is just a label, and thus can be anything handy.


File 3_wire_control_02

This is a modification of the “3-wire” control above.  It 
includes a “JOG” function, which will run the motor for as 
long as the JOG button is held down.  Useful for inching, 
bumping and/or maintenance functions.  A couple new naming 
conventions: “CR” is just control relay and “MS” is motor 
starter (usually denoting a heavy duty contactor or NEMA-sized 
motor starter).

Matt Isserstedt
22 April 2006
mattofvinings ATT aol DOTTTcom