Download Chapter 5 Control Logic

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
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 L1 and L2.
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 L2.
Magnetic motor starter
coils are connected to L2 indirectly through NC overload contacts.
Control devices are connected between L1 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 L2 (neutral conductor)
and the other lead moved through the circuit starting with L1 (hot
conductor).
When a magnetic motor starter overload contact is used in a
circuit, the DMM lead connected to L2 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.