reading a schematic- the nitty gritty

Reading Control Schematics
1. THINGS WE NEED TO KNOW: a) What are schematics?
Graphic representation of circuitry and components of a working
system.
b) WHAT IS THE PURPOSE OF THE SYSTEM? To pump chlorinated water from a well to a storage tank automatically.
c) COMPONENT APPLICATION Function together in a logical manner to produce predetermined end
result.
d) HOW DO THE COMPONENTS OPERATE? Reference to individual component description.
e) WHAT DO VERTICAL AND HORIZONTAL LINES MEAN? Vertical lines are power wiring that supply voltages and currents for
operation of components and circuitry.
Horizontal lines (rungs of the ladder) are current paths (from left to
the right) from one supply line through interconnecting wiring to
active components and finally terminating at the other supply line.
Hence, a "ladder" schematic.
f) WHY USE BOTH A.C. & D.C. SUPPLY VOLTAGES? A.C. is used at line voltage for power devices requiring relatively
large amounts of current which is available directly from power lines.
A.C. current is easily transmitted from one voltage to another without
excessive loss. ( The controls all operate at 24V AC and 24V DC for
personal safety.
D.C. is desirable for control devices that are electronically
sensitive and require more very small operating signals and currents.
A.C. circuits used in small signal and current application are
susceptible to many difficulties.
g) WHAT ARE THE LIGHTS ANS SWITCHES FOR? Lights are indicators of system status. Lights show if motors are
running, voltages are present, alarms are activated. Helpful in
troubleshooting.
Selector switches are used for selecting modes of operation and for
quickly testing the system. Test switches are a must for
troubleshooting. These switched allow bypassing of the pilot devices
without disturbing any wiring, to isolate any trouble. Alarms signal
an undesirable condition of the system which, if not corrected, could
lead to improper operation, hazardous conditions, partial or complete
breakdown of the system. Low chlorine could be hazardous to health,
low well level could burn out pump, etc. An overload light for a pump
motor shows that the pump has been overloaded and should be corrected.
Critical circuits are tied into the alarm circuits to shut equipment
down to prevent hazards damage or hazards.
h) WHAT ARE DOTTED LINES ON SCHEMATICS? Dotted lines show equipment, components, or wiring that is external of
the control panel, but are still considered part of the system: pilot
devices, motors, flow switches, etc.
i) WHAT ARE THE NUMBERS BY LINES AND COMPONENTS? Numbers under horizontal lines indicate numbered wires in the control
panel. All wires that have the same number are physically and
electrically connected together in the panel, not necessarily as shown
on the schematic, due to spacing of connected components and their
physical location from each other.
Numbers to the right of ladder refer to other line numbers that have
more associated circuitry.
Numbers above horizontal lines and placed next to components indicate
the component's pin or terminal number, as pins on relay sockets.
Squares with a number indicate terminal strip numbers.
Numbers to the left of the vertical lines are line numbers used for
reference from relays to their controlled contacts.
2. PARTS OF A CONTROL SYSTEM a) Pilot devices: float switches, flow switches, pressure switches,
thermostats, probes.
b) Relays c) Power Source. d) Power devices (motor starters) e) Indicators f) Alarms CONTROL SYSTEM COMPONENTS PILOT DEVICES
Detects a pressure, level, flow, temperature set point and generates a
signal that is sent back to the control panel. Float switches, pressure
switches, thermostats, flow switches, and well probes are all pilot
devices.
FLOAT SWITCH
Generally of plastic outer construction, housing a module containing a
mercury switch that closes or open when the float is tilted from its normal
posture. Hermetically sealed and extremely rugged. Used for monitoring or
control of liquid levels in tanks or sumps.
FLOW SWITCHES
A device for regulating or monitoring flow of fluids or gases within a pipe
or conduit. A popular type uses a flat vane, which when placed in the flow
stream, is depressed to actuate a switch. Others incorporate transducers
which generate an electric signal proportional to flow rate.
PRESSURE SWITCHES
Of various construction. Some use mercury switches, others use micro
switches or other means of contact closure. Mercury type generally are tilted by a bourdon tube actuator (spiral hollow
tube which displaces its end when subjected to a change of pressure).
Others actuate a switch by means of a flexible diaphragm or membrane. Sensitivities are varied as to application. Some pressure switches are
single acting (turn "ON" at a specific setting and "DROP OUT" at some value
slightly less). Others are differential types that allow individual
adjustment of "ON" or "OFF" settings.
WELL PROBES
Electrodes (usually three) are placed in a well at different levels to
insure that the well pump will not run un a dry condition. The information from the probes is used in control circuitry to either
allow the pump to run when sufficient water is available, or to lock out
the pump when the level is low in order to protect the pump from damage.
Probes operate by sensing electrical conductivity of water when immersed,
through sensitive D.C. relay circuits. THERMOSTAT
Used for controlling temperature. Typical operation is by a bi-metallic
element attached to a micro-switch or cam-operated switch. The Bi-metallic
element consists of a narrow sandwich of metals having different thermal
coefficients of expansion. When subjected to a change of temperature, the
two metals elongate or shrink at a different rate, causing the sandwich
strip to bend. This bending movement is caused to operate a switch which
turns heating or cooling on or off as required to maintain desired
temperature.
RELAYS
Electro-Mechanical device which opens and/or closes up to three sets of
contacts. Operation is by magnetic attraction of an armature to an electrically
magnetized pole piece consisting of a soft iron core wound with fine wire.
When an electric current is passed through the coil or wire, the magnetic
filed produced attracts the armature to which the contacts are attached
thus effecting switching operation. Relays are of several types. Some are
designed to operate on A.C., some on D.C. - different operating voltages
and sensitivities are available. More sensitive types employ built-in solid
state circuitry which enables the relay to operate with an extremely small
signal input, usually of the D.C. type. Relays are acted upon by pilot devices causing them to be in one of two
conditions; either "ON" or "OFF". Relays may sometimes be used to control
another "RELAY". A relay to control a motor starter in this manner is
necessary when operating a motor of relatively large current demand,
because the power handling ability of pilot device operated relays
generally is not sufficient to handle a motor. In this case, the first
relay operates a motor starter which in turn controls the motor. Relays are
also used when more than one item in a control circuit must be operated
independently at the same time. TIME DELAY RELAYS Operate to open or close a circuit after a period of time has elapsed.
Various types are available: "ON" Delay- circuit opens or closes some time after being energized. Relay
resets upon removal of power. TDOE- TIME DELAY ON ENERGIZATION. "OFF" Delay- circuit opens or closes some time after being de-energized.
Relay resets upon reapplication of power. TDOE- TIME DELAY ON DE-
ENERGIZATION. "INTERVAL OR SINGLE SHOT" - circuit opens or closes immeadiatly upon being
energized, and remains in that state for a period of time, then reverts to
its normal state. Relay resets upon removal of power. ICTO INSTANTANEOUSLY
CLOSED, TIMED OPEN.
MONITOR RELAY
Monitors three phase power input to control panels. If loss of power or
phase reversal in any one of the three incoming lines occurs, or if voltage
drops below a predetermined value in any of the three lines, the relay
actuates to disconnect the control circuitry. If any of the above would occur without the relay and while controlling
motors or pumps, the motors or pumps could be damaged beyond repair. With
the relay, if conditions are restored after any of the above occurs, the
relay allows the system to return to normal operation. A fail-safe device.
POWER SOURCE
Most common is derived from commercial power lines. Controlled devices (motors) utilize this source directly, while control
devices are generally operated at a lower voltage (for safety and other
reasons) and is accomplished by transforming line voltage to 24 volts or
less. Power sources derived from the line may be either A.C. or D.C.
depending upon the application or design. Other sources include batteries or power generators.
D.C. POWER SUPPLY
Transforms alternating current to direct current by circuitry and
components which allow current to flow in only one direction. Used for operation of D.C. relays and low level sensing circuits, which on
A.C. operation would be prone to various problems such as unwanted
inductive or capacitive coupling effects, producing faulty control
operation. D.C. supplies are overloaded and short circuit protected; that
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