Transcript Electrical Safety - HCC Learning Web

Solid-State Controls and Systems
Electricity for Refrigeration, Heating and Air
Conditioning 7th Edition
Chapter 18 Solid-State Controls and Systems
Solid-State Controls and Systems
Upon completion of this chapter the student will be able to:
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Identify and describe the operation of basic electronic system components
Identify and describe the operation of common one-function electronic
controls that are used in the industry
Troubleshoot one-function electronic controls
Describe the function and operation of an electric defrost board used in a
heat pump.
Describe the operation of an electronic motor protection module used on
motors
Troubleshoot electronic defrost modules
Troubleshoot electronic motor protection modules
Understand and trouble basic electronic control systems used in residential
conditioned air systems
Identify electronic control systems used in commercial and industrial
equipment and structures
Key Terms
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Defrost Module
Diode
Electronic Control System
Electronic Module
Motor Protection Module
One-Function Solid-State Device
Rectifier
Semiconductor
Thermistor
Transistor
Triac
Varistor
Voltage Spike
Electronic System Components
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In the past 40 year the electronic industry has seen a vast improvements and
advancements.
Originally electronic devices were constructed using vacuum tube technology.
Along with being large and bulky, the old vacuum and gas-filled tubes were made of
many delicate parts.
This has been replaced with the introduction of the solid-state devices.
These devices are smaller and more durable than the old vacuum tubes.
This reduction in size has made it possible to incorporate them into many, airconditioning, and refrigeration control circuits.
Semiconductors
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Solid state refers to a physical description of the component.
Semiconductor refers to the electrical conductivity of the materials used in solid-state
components.
The conductivity of materials plays an important part in the action of semiconductors.
Good electrical conductors are copper, silver and aluminum; insulators like plastic,
glass and mica are poor conductors.
Some of the materials used as semiconductors have a resistance almost halfway
between that of glass and copper.
Semiconductor devices are versatile.
Transistors
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The transistor is a combination of two other semiconductor devices; the thermistor
and the varistor.
The thermistor is a solid-state device whose resistance decreases with an increase in
temperature.
The varistor’s resistance decreases with an increase in voltage.
Diodes and Rectifiers
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The diode and the rectifier are the simplest of the solid-state components.
The only difference between the two is their size; they are basically the same device.
A diode is an electronic device that allows current to flow in only one direction.
Diodes are rated at less than one ampere.
A rectifier is a similar component that is rated above one ampere.
Diodes and rectifiers are composed of p-type materials bonded to n-type materials.
When these two materials are bond together the resulting component allows
electricity to flow in one direction only.
Rectifier Circuit
Transistors
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Transistors are composed of N-type and P-type materials.
Their exact composition is dependent on the type of material used in the middle.
If the N-type material is sandwiched between two P-type materials, the transistor is
referred to as a PNP transistor.
If the P-type material is sandwiched between two N-type materials then the transistor
is referred to as NPN transistor.
Both the NPN and PNP type transistors are constructed of three parts: Base, Emitter
and Collector.
Composition of a Transistor
Triacs
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A triac is an electronic control that has two diodes in a single device.
Triac are used as switches in AC circuits because they allow current to pass in both
directions when a certain current level is reached at the gate.
LEDs
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Light-emitting diode is an electronic device that gives off light when current is passed
through it.
They are often used in solid-state control boards to send signals of some type, such
as power to board, a blinking light for trouble diagnosis, and other indicators.
Thermistors
• A thermistor is a resistor that changes resistance with temperature.
• There are two types of thermistors: positive temperature and
negative temperature coefficient.
• The resistance of the positive temperature coefficient thermistor
increases as the temperature increases.
• The resistance of the negative temperature coefficient thermistor
increases as the temperature decreases.
Basic Electronic Control Fundamentals
• Electronic controls used in the heating, air-conditioning, and
refrigeration industry are built as one component.
• Equipment used in the industry is usally powered by alternating
current in the voltage range from 120 to 480 volts.
• When alternating current is used, there are often voltage spikes due
to power surges, circuit characteristics, or lighting.
• Electronic control circuits generally are direct current circuits with
their own power supply, usually an AC-to-DC diode rectifier.
Simple Electronic Temperature Control
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A simple control circuit for temperature control would have a thermistor in
the input circuit, which varies its resistance with a change in temperature.
A change in the resistance of the input circuit would cause a change in the
current to the transistor base, and the change would amplified through the
transistor.
Solid State Timers
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One of the most popular solid-state devices is the timer.
Solid-state timers are used in the same manner as the conventional time-delay relay
but with more flexibility.
Solid-state timers can delay on the closing or opening of an operating control.
Anti-Short-Cycling Devices
• One of the simplest solid-state devices in common use today is a
control that prevents a compressor from starting more than once in a
certain period of time.
• These devices can delay the starting time from 30 seconds to 5
minutes between cycles.
Solid-State Devices for Electric Motors
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A solid-state device is now used to assist the permanent split-capacitor
motor in starting.
In many cases, this device can be used instead of a potential relay and a
starting capacitor.
The device is used to drop out the starting winding of a split-phase hermetic
compressor motor.
The solid-state starting relay is actuated on a thermal basis through the use
of a semiconductor material.
For troubleshooting the device, it is only necessary to determine if the
device is dropping out the starting winding of the hermetic compressor
motor.
Solid-State Devices for Heat Pumps
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Many heat pumps use a solid-state defrost control module that initiates and
terminates the defrost cycle of a heat pump.
This defrost control keeps the outdoor coil of a heat pump clear of frost, making the
unit more efficient.
Schematic of a Heat Pump defrost Control
Solid-State Devices for Motor Protection
• Motor protection modules are another area in which solid-state
devices are used effectively.
• These types of safety control systems use thermistors to indicate the
temperature of a hermetic compressor motor.
• Heat is the main enemy of an electric hermetic motor.
• The most common overload protection used is a single-module,
three sensor system.
• The sensors effectively sense the temperature of a hermetic motor.
Sensor Trip-Out Point and Reset Point
Operation Diagram of the heat cycle of a rooftop
unit