Chapter 5 Control Logic Basic Rules of Line

Chapter 5 Control Logic Basic Rules of Line Diagrams • Simplifying Printreading • Line Diagrams — Signals, Decisions, and Action • Logic Functions • Common Control Circuits • Control Circuit Troubleshooting

No more than one load should be placed in any circuit line between L 1 and L 2.

Loads must be connected in parallel when more than one load must be connected in the line diagram.

Control relays, solenoids, and pilot lights are loads that are connected directly or indirectly to L 2.

Magnetic motor starter coils are connected to L 2 indirectly through NC overload contacts.

Control devices are connected between L 1 and the operating coil.

Two control devices may be connected in series or parallel to control a coil in a magnetic motor starter.

Each line in a line diagram should be numbered starting with the top line and reading down.

The locations of normally open contacts controlled by a device are determined by the numbers on the right side of the line diagram.

Normally closed contacts are indicated by numbers which are underlined to distinguish them from NO contacts.

Each wire in a control circuit is assigned a reference point on a line diagram to keep track of the different wires that connect the components in the circuit.

Manufacturers include terminal numbers to identify and separate the different component parts included on individual pieces of equipment.

Contacts found in different control lines that belong to the same control switch are illustrated using the dashed line or numerical cross-reference method.

Line, cross-reference, wire reference, and manufacturer’s terminal numbers are used to simplify electrical prints.

All control circuits are composed of signal, decision, and action sections.

In AND logic, the load is ON if both of the control signal’s contacts are closed.

In OR logic, the load is ON if any one of the control signal’s contacts is closed.

The decision section of any circuit may contain one or more logic functions.

In NOT logic, the load is ON only if the control signal contacts are closed.

NOR logic is an extension of NOT logic in that two or more NC contacts in series are used to control a load.

NAND logic is an extension of NOT logic in which two or more NC contacts are connected in parallel to control a load.

Auxiliary contacts are added to give circuits with pushbuttons memory.

A common start/stop control circuit is created by adding the NOT logic of a stop pushbutton to the memory logic of magnetic coil contacts.

Two stop pushbuttons connected in series and two start pushbuttons connected in parallel are used to control a motor from two locations.

Two start/stop stations are used to control two separate magnetic motor starter coils with a common emergency stop protecting the entire system.

Two motors can be started almost simultaneously from one location to prevent product separation or stretching.

A pilot light is used with a pressure switch to indicate when a device is activated.

A pilot light is used with a start/stop station to indicate when a device is activated.

NOT logic is used to indicate when a device is not operating.

A sequence control circuit does not let the first conveyor operate unless the second conveyor has started and is running.

A selector switch is used to provide a common industrial jog/run circuit.

When using the tie-down troubleshooting method, one DMM test lead should be placed (tied down) on the L 2 (neutral conductor) and the other lead moved through the circuit starting with L 1 (hot conductor).

When a magnetic motor starter overload contact is used in a circuit, the DMM lead connected to L 2 can be moved to the other side of the overload (side connected directly to the starter coil) to check if the overload is open.

A good mechanical switch indicates source voltage when open and 0 V when closed.

A good solid-state switch indicates source voltage when open and a slight voltage drop when closed.

When two pieces of wire touch because of damaged insulation, the wires produce a dead short across the circuit.

A continuity tester can be used to test for short circuits.

In a good circuit, a DMM reads total circuit resistance when all open contacts are closed. In a circuit with a short, a DMM reads near 0 Ω.