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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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