Contactor, Relays and Overloads

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Contactor, Relays and Overloads

• Electricity for Refrigeration, Heating and Air
  Conditioning 7th Edition

Chapter 11 Contactors, Relays and Overloads
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Contactor, Relays and Overloads

• Upon completion of this chapter the student will
  be able to

• Explain the parts and operation of contactors and
  relays.
• Explain the application of contactors and relays
  in control systems
• Correctly install a contactor or relay in a
  control system
• Draw a simple schematic wiring diagram using
  contactors and/or relays to control load in a
  control system.
• Understand the types and application of overloads
• Troubleshoot contactor and relays.
• Identify the common types of overload used to
  protect loads.
• Explain the operation of the common overloads.
• Determine the best type of overload for a
  specific application.
• Draw schematic wiring diagrams using the proper
  overload to protect loads.

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Electrical Safety

• Upon completion of this chapter the student will
  be able to

• Troubleshoot common types of overloads.
• Explain the operation of a magnetic starter.
• Size the overload devices to be used in a
  magnetic starter for motor protection.
• Wire a magnetic starter using switches,
  thermostats, and push-button stations.
• Troubleshoot magnetic starter and push-button
  stations.

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Key Terms

• Coil
• Contactor
• Contacts
• Current Overload
• Fuse
• Inductive Load
• Internal Compressor Overload
• Line Break Overload
• Magnetic Overload
• Magnetic Starter
• Mechanical Linkage
• Overload
• Pilot Duty Overload
• Push-Button Station
• Relay
• Resistance Load

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Contactors

• A contactor is used to control an electric load
  in a control systems.
• Contactors make or break a set of contacts that
  control the voltage applied to some load in
  cooling systems.

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Contactors
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Coils

• Coil Characteristics depends on the type of wire
  and the manner in which it is wound.
• Coils are typically designed to operate on 24
  volts, 120 volts, 208/240 volts and occasionally
  480 volts.
• The coil is identified by the voltage marked on
  it.

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Contacts

• The contacts of a contractor make a complete
  circuit when the contactor is energized, allowing
  voltage to flow to the controlled load.
• Contractors are rated by the ampere draw they can
  carry.
• There are two types of loads that a contractor
  can control an inductive load, and a resistive
  load.
• Contacts are made of silver and cadmium which
  resists sticking.
• The chemical composition of contacts is such that
  they operate at cool temperatures of up to 125
  of their current-carrying capacity.

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Contacts

• The contactor contacts must be in good condition
  to ensure that proper voltage reaches the load.

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Relays

• Relays are used to open and close a circuit to
  allow the automatic control of a device or
  circuit.

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Relay Applications

• Relays can be used to control indoor fan motors,
  condenser fan motors, damper motors, starting
  capacitors, and control lockouts.

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Overloads

• An overload is an electrical device that protects
  a load from a high ampere draw by breaking a set
  of contacts.
• The simplest form of overload protection is the
  fuse.
• Fuses can be used to protect wires and
  non-inductive loads, but they provide inadequate
  protection for inductive loads.
• A load that is purely resistive in nature with no
  coils to cause induction is called resistive or
  non-inductive load.

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Over Load Protection
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Fuses

• Fuses consists of two ends or conductors with a
  piece of wire that will melt and break the
  circuit if the current passing through it exceeds
  the amperage rating of the fuse.

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Line Break and Pilot Duty Overloads

• Overloads can be divided into two basic groups
  Line break and Pilot duty.
• The line break overload breaks the power to a
  motor.
• A pilot duty overload breaks an auxiliary set of
  contacts connected in the control circuit.

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Line Break Overload

• One of the most common types of line voltage
  overloads is the metal disc mounted between two
  contacts.
• This is called a bimetal line break overload.

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Internal Compressor Overload

• The most popular line break overload for use in
  small central residential system is an internal
  compressor overload.
• The internal compressor overload is a small
  device inserted into the motor windings.
• This overload can sense the current draw of the
  motor, as well as the winding temperature, more
  effectively than external overloads.

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Internal Compressor Overload
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Pilot Duty Overload

• The pilot duty overload breaks the control
  circuit when an overload occurs, which would
  cause a contractor to be de-energized.
• This type of overload is common on larger systems
  and still exists on smaller systems currently in
  the field.
• Two basic pilot duty overloads are being used in
  the industry today
• The current overload
• Magnetic overload

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Current Overload

• Works similarly to the line break overload
  excepts that a pilot duty set of contacts is
  opened rather than the line.

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Magnetic Overload

• Consists of a movable metal core in a tube filled
  with silicone or oil.
• Surrounding the metal tube is a coil of wire.
• When the current increases, so does the magnetic
  field of the coil.
• The overload operates by the magnetic field
  created by the coil.
• The device is designed to create a magnetic field
  that is strong enough to pull the coil up,
  opening the pilot contacts on overload.

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Magnetic Overload