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Air Conditioning

Air Conditioning. Relays and Contactors. 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. Schematic Symbols of a relay.

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Air Conditioning

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  1. Air Conditioning Relays and Contactors

  2. 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.

  3. Schematic Symbols of a relay Relay Coil Normally Open Contact Normally Closed Contact

  4. 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.

  5. 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.

  6. The relay coil • The coils rating is normally stamped on the side of the relay. • The coil is a load in a schematic circuit.

  7. 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

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. MOTOR AMPERAGE • 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

  15. 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.

  16. 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.

  17. Relays - Example 1 120 V N L1 C1 C1 C1 SW1

  18. Relays - Example 2 120 V N L1 C1 C1 C1 SW1

  19. MARS Relay Consists of a coil and 2 sets of isolated contacts. Each set has a common, normally open and normally closed contact.

  20. MARS Relay Normally Open Normally Closed Coil Common

  21. Mars Relay - Schematic 4 1 3 2 6 5

  22. Contactors Same 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

  23. 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.

  24. MOVABLE CONTACTS AND ARMATURE STATIONARY CONTACTS COIL CONNECTIONS STATIONARY CONTACTS HOLDING COIL

  25. L1 L2 25A 3A

  26. Contactor

  27. Contactor

  28. Contactor - Schematic

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