Start/Stop Control with a Hand/Off/Auto Selector Switch
7.13 Start/Stop Control Logic
Often times, a switch is desired to select between local or “Hand” mode and remote or “Auto” mode. In most cases, local control is
performed as 3-wire logic with a normally open, momentary contact Start pushbutton and a normally closed, momentary contact Stop
pushbutton, while remote control is performed as 2-wire logic with a “Run Command” contact provided by a PLC.
The MX
2
can perform both 2-wire start/stop logic and 3-wire start/stop logic. With 2-wire logic, the starter starts when a run command
is applied to the Start input. It continues to run until the run command is removed from the Start input. With 3-wire logic, the starter
starts when a start command is momentarily applied to the Start input and continues to run until an input programmed as a Stop input
goes low.
The MX
2
automatically determines whether to use 2-wire logic or 3-wire logic by the presence of a high level on a Stop input. If there is
an input programmed as a Stop input, and that input is high when the Start input goes high, then 3-wire start/stop logic is used.
Otherwise, 2-wire start/stop logic is used. This feature eliminates the need for external logic relays often used to “seal in” the momentary
Start and Stop pushbuttons, creating a 2-wire logic signal. The key is to have the Stop input be high when the Hand/Off/Auto switch is
in the Hand position, but be low when the switch is in the Auto position.
CAUTION: It is important that the Stop push button be wired in front of the Start push button, otherwise the starter could be started
when the Stop bush button is pressed and the Start button is pressed.
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7 - THEORY OF OPERATION
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