Emerson manual WRITING 848L EQUATIONS Basic Motor Control, Rosemount 848L, Reference Manual

Rosemount 848L

Reference Manual

00809-0100-4696, Rev AA September 2004

Timed Switch

The pumps have a known life time within an acceptable risk, so one pump is allowed to run until that time expires. The other pump automatically takes over at that time.

Switch on Failure

If a process condition can be sensed that says the running pump failed, then the other pump is started regardless of the starting protocol.

WRITING 848L

EQUATIONS

Basic Motor Control

It is not easy to convert a functional diagram to an 848L equation because all of the functions must be nested in the proper order. One way to begin the process is to draw the functional diagram in ladder logic. The following is a basic two button and auxiliary contact motor control that is drawn with channel numbers.

	1		2		Out1
+--	--	+--	--	+----------------------	( )--+ OUT1_EQ
			3
		+--	--	+

Input 1 is from a normally closed stop button, which does not have to be inverted in the equation. This is true for all stop buttons in the following examples.

Input 2 is from the normally open start button and Input 3 is from the contactor’s auxiliary contact.

The ladder coil shown is at Out1, which is the value of output equation 1. Wires from output 1 will switch power from the control voltage to the contactor’s solenoid or pilot relay.

Since the 848L uses RPN, begin from the lowest line in the ladder diagram and work upwards.

The first expression is OR(IN(2),IN(3)) from “2 or 3” in the ladder diagram. This is one term in an AND function, so build the function around it.

The top and final expression is AND(IN(1),OR(IN(2),IN(3))) from “1 and (2 or 3)” in the ladder diagram.

Enter the expression as the contents of parameter OUT1_EQ in the logic transducer block. Do not forget to append the terminating semicolon.

Connect the buttons, auxiliary contact and contactor (a small relay will do) in order to test the operation.

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