1 air conditioning relays and contactors. 2 relays the relay is an electrical device consisting of a...
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1
Air Conditioning
Relays and Contactors
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Relays
• The relay is an electrical device consisting of a coil and a set of contacts.
• The coil and the contacts are isolated (protected) from each other.
• It is designed to be useable where one circuit must control another.
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Schematic Symbols of a relay
Relay Coil
Normally Open Contact Normally Closed Contact
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The relay coil
• Coil: spool of wire wrapped around an iron core.
• Coil is energized- a magnetic field generated.
• This field pulls in an armature that closes or opens a set of contacts.
• When a relay is shown in a schematic it is always shown in the de-energized mode.
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The relay coil
• Every coil has a voltage rating. This rating is the voltage that you may apply to the coil.
• These ratings are normally 24 volts, but can be 120 volts, 208/230 volts etc.
• Coil will not work if wrong voltage is applied.
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The relay coil
• The coils rating is normally stamped on the side of the relay.
• The coil is a load in a schematic circuit.
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The contacts
• Relay contacts are the switching device of the relay.
• “Normally” refers to the position of the contacts when the coil is de-energized.
• Relay contacts come in two varieties.– Normally Open– Normally Closed
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Relay contacts
• Relay contacts are attached to the armature that is moved by the magnetic force of the coil.
• Relay contacts have metal points that come in contact with each other. These points complete the circuit.
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Relay Contacts
• Relay contacts will be labeled on a schematic to correspond to the coil that controls them.
• Ex. Fan Relay–Coil labeled IFR–Contacts labeled IFR1, IFR2,
IFR3 etc.
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Normally open contacts
• Normally open contacts are shown on a schematic diagram as an open set of contacts.
• When the relay coil is energized, the contacts will close, allowing electricity to flow through them.
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Normally closed contacts
• Normally closed contacts are shown on a schematic diagram as a closed set of contacts.
• When the relay coil is energized, the contacts will open, not allowing electricity to flow through them.
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Contact ratings
• The contacts are rated with the number of amps they can control.
• The contacts are also rated with the type of load they can control. This would either be “inductive” like a motor, or “resistive” like a heater.
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Contact ratings
• With inductive loads, the amperage spikes when it is starting.
• With resistive loads, the amperage remains constant.
• Make sure the contact ratings exceed the maximum amperage that the load is rated for.
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• Running load amperage (RLA)– Similar to full load amperage (FLA)– Amperage drawn by the motor while
operating
• Locked rotor amperage (LRA)– Amperage drawn by motor on startup– Five to seven times greater than RLA or
FLA
• Both LRA and RLA must be considered when choosing a control device
MOTOR AMPERAGE
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Relays
• It is important to realize that the coil and contacts are completely isolated electrically.
• A technician may de-energize and remove the wires from the relay contacts, for the purpose of ohmming them out, even if the coil is energized.
• This is a good way to determine whether or not a relay is working properly.
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Relays
• Example:
• A relay coil is energized with 24 volts.• The technician isolates the contacts and
reads 0 ohms across the N.C. contacts, and infinite ohms across the N.O. contacts
• He determines that the contacts are not switching positions.
• It may be that the coil is broken or the contacts are stuck.
17Relays - Example 1
C1
C1
C1
SW1
NL1120 V
18Relays - Example 2
C1
C1
C1
SW1
NL1120 V
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MARS Relay
Consists of a coil and 2 sets of isolated contacts.Each set has a common, normally open and normally closed contact.
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MARS Relay
Coil
Normally Open
NormallyClosed
Common
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Mars Relay - Schematic
1 4
3 62 5
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ContactorsSame as switching relays except:• Larger in size• Contacts can handle more amps• Contacts are visible • Larger ones can be re-built• Usually definite purpose- N.O.
contacts used to energize a large motor
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Contactors
• Contactor coils are rated by voltage.
• Contactor contacts are rated by amperage.
• These ratings are normally written on the side of the contactor.
• Do not install a contactor that goes over either of these ratings.
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HOLDING COILSTATIONARY CONTACTS
STATIONARY CONTACTS
MOVABLE CONTACTS AND
ARMATURE
COIL CONNECTIONS
26L1 L2
3A25A
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Contactor
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Contactor
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Contactor - Schematic
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