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Reference Manual
00809-0100-4696, Rev AA
September 2004
E-7
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)));