Emerson manual Maximum Restarts, Rosemount 848L, Reference Manual

Reference Manual

00809-0100-4696, Rev AA September 2004

Rosemount 848L

Maximum Restarts

A large motor is too expensive to replace if it burns up because the operator wanted to give it another try, when in fact the pump was jammed. The life of the contactor is also shortened when it has to interrupt locked rotor current. In this case, it is normally possible for the motor to clear the jam on the second or third try. A counter limits the number of starts to 3, for example, within a preset time since the first attempt.

Notice that C1 in EQ1 and T2 in EQ2 are forward references that cannot have a bad status. A Bad status at channel 2 (the start button) will propagate to all of the equations and make the output Bad. A bad stop button or confirm contact will only make the output Bad. The I/O transducer block parameter OUT_1_CONFIG.FAIL_SAFE defaults to Fail False, which will stop the motor on any bad input status, or it may be set to Fail Last Good, which will not allow the stop button to turn it off. You probably don’t want to uses input devices with status for this application.

	2	C1				T1
+	--------	/------------------------				( )--	+ EQ1
	T1			+	-----+	C
+	----------------------			Count------		( )--	+ EQ2
	T2
+	----------------------			Reset
				+-----	+
	C					T2
+	-----------------------------------					( )--	+ EQ3
	1	T1				Out1
+	-- --+--	--	+----------------------			( )--	+ OUT1_EQ
		3
	+--	--	+

Rung T1 is equation 1. The pulse time must be set to the allowable on time for the locked rotor condition to prevent tripping the overloads, in this case 2.5 seconds.

•The expression is TP(AND(IN(2),NOT(EQ(2))), 25);

Rung C is equation 2, which counts the attempts to start, and holds at the count until the 30 minute timer expires.

•The expression is CTU(RISE(EQ(1)),EQ(5), 3);

Rung T2 is equation 3, the 30 minute timer:

•The expression is TON(EQ(2),18000);

Rung Out1 is output equation 1:

•The expression is AND(IN(1),OR(EQ(1),IN(3)));

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