Download Relays

• Control method for machinery that creates a sequential
operation of events.
• The RELAY is the major component for this control
method.
• The main purpose of a PLC was to replace the relay.
• The relay is a mechanic device that wears out
• Changing the relay control structure required shutting down the
line (equipment) and rewiring
• PLC relays are bits inside the computer that can be turned off
and on millions of times, changes to the program can be made in
minutes without physical rewiring
• An electromagnetic device.
• A device that requires the flow of current to produce
a magnetic field.
• Magnetic field will cause mechanical movement.
• Need a potential difference to produce current.
• Need current to produce a magnetic field.
• Need a magnetic field to produce mechanical
movement.
• Three major parts of an electromagnetic relay.
• Coil
• Three major parts of an electromagnetic relay.
• Coil
• Pole (armature)
• Three major parts of an electromagnetic relay.
• Coil
• Pole
• Contacts
• Coil
• Fine wire wrapped around a steel or iron core.
• Current flows through wire to produce a magnetic
field.
• Coils will have a voltage rating. Typical ratings:
120VAC, 24VDC, 12 VDC and 6 VDC.
• Coil
• Sample data sheet of a relay coil.
• Potter/Brumfield Relay
• Pole
• Part of relay that moves.
• Electrical terminal connection to POLE is called a
common.
• Other names for a POLE:
• Armature and clapper.
• Contacts
• Part of relay that makes an electrical connection out to
a load.
• Contacts are controlled by the coil. Contacts will only
change state when the coil is energized.
• A contact can be:
• NC ~ Normally closed
• Contacts are closed when no current through coil.
• Contacts are open when current flows through coil.
• NO ~ Normally open
• Contacts are open when no current through coil.
• Contacts are closed when current flows through coil.
• Relay manufacturers use a common code to simplify the
identification of relays.
• This code uses a form letter:
• Form A: One contact that is normally opened NO.
• Form B: One contact that is normally closed NC.
• Form C: Has one pole that will provide one NO and one NC set of contacts.
• Contacts
• The electrical terminal connection of a relay contact is
called a throw.
• Contacts
• The electrical terminal connection of a relay contact is
called a throw.
• Relay contacts will have a voltage and current rating.
• Drawing symbols
Electronic Symbols
Control Symbols
• Drawing symbols
Relays on electrical prints are always
drawn in an un-energized state.
Electronic Symbols
Control Symbols
• Relay Nomenclature
• Single Pole, Single Throw ~ SPST
• One armature that has only one electrical terminal.
• Relay Nomenclature
• Single Pole, Single Throw ~ SPST
• One pole that has only one electrical terminal.
• Example:
• Relay Nomenclature
• Double Pole, Single Throw ~ DPST
• Two poles that each have only one electrical
terminal.
• Relay Nomenclature
• Double Pole, Single Throw ~ DPST
• Two armatures that each have only one electrical
terminal.
• Example:
• Relay Nomenclature
• SPST
• DPST
• SPDT
• DPDT
• Relay Nomenclature
• Break
• Number of places the a pole opens or closes an
electrical circuit.
• Single Break:
• Double Break:
• Double break reduces arcing as the contacts are separated
allowing the relay to operate with high voltages
• Relay Nomenclature
• Pole – Throw - Break
*Application for relay controls.
*When relay A is energized:
*Contact A1 will close and place
voltage across solenoid valve.
*Application for relay controls.
*When relay A is energized:
*Contact A1 will close and place
voltage across solenoid valve.
Control Drawing
Electrical Drawing
Control Drawing

Task:
◦ Switch S1 will enable control relay CR
◦ CR will activate solenoid SOL 1 and motor control relays
MCR 1 and MCR 2.
◦ If water level falls below level probe LP then only run pump
#1 and shut off pump #2.
◦ Create a control schematic.