Emerson manual Redundant Motors - Alternate Start, Rosemount 848L, Reference Manual

Reference Manual

00809-0100-4696, Rev AA September 2004

Rosemount 848L

Redundant Motors - Alternate Start

Two motors drive two pumps in a redundant configuration. The control valve that follows the pumps will just waste the energy of the second pump, possibly damaging the valve, if both pumps on at the same time. There are times when neither pump is required. When a pump is required, the pump that was not in use last time should be started to equalize the wear on the pumps. The push button controls are Stop (input 1) and Start (input 2). An auxiliary contact is required from each starter (inputs 3 and 4).

	3	4		2				P
+	--/----	/-----		^	-----------------			( )--	+ EQ1
	P	L				+-----	+	L
+	-------	/------------				Set	------	( )--	+ EQ2
	P	L
+	-------	------------				Reset
						+-----	+
		1		2		L	Out2	Out1
+	---------	--	+--	--	+--	/---	/----	( )--	+ OUT1_EQ
				3
			+--	--	+
		1		2		L	Out1	Out2
+	---------	--	+--	--	+--	---	/----	( )--	+ OUT2_EQ
				4
			+--	--	+

Rung P is equation 1. It generates the pulse that will toggle the latch. The pulse comes from the RISE function.

•The expression is AND(NOT(IN(3)),NOT(IN(4)),RISE(IN(2)));

Rung L is equation 2. It latches on when L was false and unlatches when L was true.

•The expression is RS(AND(EQ(1),NOT(EQ(2)),AND(EQ(1),EQ(2)));

Rung Out1 is output equation 1. This is basic motor control with additional contacts.

•The expression is AND(IN(1),OR(IN(2),IN(3)),NOT(EQ(2)),NOT(OUT(2)));

Rung Out2 is output equation 2. This is also basic motor control with additional contacts.

•The expression is AND(IN(1),OR(IN(2),IN(4)),EQ(2),NOT(OUT(1)));

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