Transcript Electrical Safety - HCC Learning Web

Contactor, Relays and Overloads
Electricity for Refrigeration, Heating and Air
Conditioning 7th Edition
Chapter 11 Contactors, Relays and Overloads
Contactor, Relays and Overloads
Upon completion of this chapter the student will be able to:
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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.
Electrical Safety
Upon completion of this chapter the student will be able to:
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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 pushbutton stations.
Troubleshoot magnetic starter and push-button stations.
Key Terms
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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
Contactors
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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.
Contactors
Coils
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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.
Contacts
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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.
Contacts
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The contactor contacts must be in good condition to ensure that proper voltage
reaches the load.
Relays
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Relays are used to open and close a circuit to allow the automatic control of a device
or circuit.
Relay Applications
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Relays can be used to control indoor fan motors, condenser fan motors, damper
motors, starting capacitors, and control lockouts.
Overloads
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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.
Over Load Protection
Fuses
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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.
Line Break and Pilot Duty Overloads
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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.
Line Break Overload
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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.
Internal Compressor Overload
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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.
Internal Compressor Overload
Pilot Duty Overload
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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
Current Overload
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Works similarly to the line break overload excepts that a pilot duty set of contacts is
opened rather than the line.
Magnetic Overload
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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.
Magnetic Overload