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Reference Manual
00809-0100-4696, Rev AA
September 2004
Rosemount 848L
E-10
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.