Emerson manual Intermediate Stop, Rosemount 848L, Reference Manual

Rosemount 848L

Reference Manual

00809-0100-4696, Rev AA September 2004

Intermediate Stop

A reversible motor requires two contactors. One of them swaps two of the motor wires so that it will run in the opposite direction. The contactors must never be closed at the same time, because that would place a short circuit across one of the three phases. Furthermore, motors with lots of attached or internal inertia can be damaged if the shaft does not come to rest before starting up in the other direction. Sometimes a brake is used to reduce the stopping time. The Forward/Reverse selector switch has one contact that is closed in the Forward position (input 1) and one contact that is closed in the Reverse position (input 2). The center of the three position selector is Off. The push button controls are Stop (input 3) and Start (input 4). These require the auxiliary contact on each starter (input 5 and 6). A 30 second off delay timer is used. The following ladder diagram shows one way of doing this:

	5								T
+	----	+------------------------------							( )--	+ EQ1
	6
+	----	+
	1		3		4	T		Out2	Out1
+	-------		--	+--	---	/--	+--	/----	( )--	+ OUT1_EQ
					5
				+--	--------		+
	2		3		4	T		Out1	Out2
+	-------		--	+--	---	/--	+--	/----	( )--	+ OUT2_EQ
					6
				+--	--------		+

Rung T is equation 1. It is necessary because it is needed in two equations and because the equations would be 80 characters long without the semicolon.

•The expression is TOF(OR(IN(5),IN(6)),300);

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

•The expression is AND(IN(1),IN(3),OR(AND(IN(4),NOT(EQ(1)),IN(5)),NOT(OUT(2));

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

•The expression is AND(IN(2),IN(3),OR(AND(IN(4),NOT(EQ(1)),IN(6)),NOT(OUT(1));

If a brake was required, it could be controlled by output equation 3:

•The expression is AND(EQ(1), NOT(IN(5)),NOT(IN(6))); This simulation requires two relays in addition to the switches.

E-10