Transcript Electrical Safety - HCC Learning Web

Thermostats, Pressure Switches, and Other
Electric Control Devices
Electricity for Refrigeration, Heating and Air
Conditioning 7th Edition
Chapter 12 Thermostats, Pressure Switches, and Other Electric Control Devices
Thermostats, Pressure Switches, and Other
Electric Control Devices
Upon completion of this chapter the student will be able to:
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Explain the purpose of a transformer in a control circuit
Size a transformer for a control circuit
Troubleshoot and replace a transformer in a residential air
conditioning control circuit
Explain the basic function of a line and low-voltage thermostat in a
control system
Identify the common types of thermostats used in the industry
Draw schematic diagrams using line and low-voltage thermostats and
operating and safety controls
Install line and low-voltage thermostats on heating, cooling, and
refrigeration equipment
Correctly set the heating anticipators and cooling anticipators, if
adjustable, on a residential low-voltage control system
Thermostats, Pressure Switches, and Other
Electric Control Devices
Upon completion of this chapter the student will be able to:
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Explain the modes of operation and be able to correctly set or
program a clock thermostat
Explain the function and operation of pressure switches
Install and correctly set the pressure switches in control systems
used as operating and safety controls
Troubleshoot pressure switches
Understand, install, and troubleshoot the following controls in control
systems used in the industry:
1) humidistat
2) oil safety switches
3) time-delay relays
4) time clocks
5) solenoid valves
Key Terms
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Anticipators
Clock Thermostat
Differential
High-Pressure Switch
Humidistat
Line Voltage Thermostat
Low-Pressure Switch
Low-Voltage Thermostat
Oil Safety Switches
Pressure Switches
Range
Snap Action
Solenoid Valve
Staging Thermostat
Thermostat Controlling Element
Time Clock
Time-Delay Relay
Transformer
Transformers
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The transformers is a heating or cooling system provides the low-voltage power
source for the control circuit.
Most residential and small commercial installation use a 24-volt control system.
The transformer for a residential unit is used to convert line voltage to 24volts.
Some commercial and industrial high-voltage equipment uses transformers that drop
the line voltage from 240 or 120 volts.
Operation
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Transformers are stationary inductive devices that transfer electric energy from one
circuit to another by induction.
The transformer has two windings, primary and secondary.
An alternating voltage is applied to the primary winding of a transformer and induces
a current in the secondary winding.
Sizing Transformers
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Transformers are like many other electrical components.
They are not 100% efficient.
There is a loss between the primary and secondary windings.
This loss is considered when sizing transformers for a certain job.
Transformers are rated by their primary voltage, secondary voltage
and voltamperes (VA).
• System equipment must be considered in transformer sizing along
with the transformer rating.
Thermostats
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The temperature in any structure, regardless of its age, location or design can be
maintained at comfortable levels with a thermostat.
Thermostats are designed and built in many different forms and sizes to meet the
applications required in the industry
Applications
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The basic function of a thermostat is to respond to a temperature change by opening
and closing a set of electric contacts.
Many different types of thermostats are used in the industry to perform a variety of
switching actions.
A heating thermostat closes on a decrease in temperature and opens on an increase
in temperature.
Controlling Elements and Types of Thermostats
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Two types of thermostat controlling elements are commonly used.
The controlling element of a thermostat is the part that moves when a change in
temperature is sensed.
The bimetal thermostat is commonly used to control the temperature of air in an airconditioning or heating application.
Remote Bulb Thermostat
• The power element, which is the bulb, and the diaphragm are
connected with a section of small tubing.
• The bulb is filled with liquid and gas and then is sealed.
• The pressure exerted by the diaphragm on the mechanical linkage
will open and close a set of contacts.
• As the bulb temperature changes, so will the pressure exerted on
the diaphragm.
• If the temperature of the bulb increases, so will the pressure.
Bimetal Thermostat
• The heart of most types of thermostats is a bimetal element.
• The element get its name from the fact that it uses a bimetal to
cause the movement that open and closes a set of contacts.
• A bimetal is a combination of two pieces of metal.
• The metals are welded together.
• Each metal has a different coefficient of expansion
• If the temperature of these two metals is increased, one will become
longer than the other because of the different expansion qualities.
Bimetal Thermostat
Bimetal Thermostat
Line Voltage Thermostat
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The line voltage thermostat is designed to operate on line voltage.
This type of thermostat is used to open or close the voltage supply to a load in the
system.
The line voltage thermostat lacks many good qualities that are obtained with lowvoltage thermostats.
Line Voltage Thermostat
Low-Voltage Thermostat
• The low-voltage thermostat is used to control systems with a 24-volt
supply.
• The low-voltage thermostat is used on all residential heating and airconditioning systems and many commercial and industrial systems.
• The low-voltage thermostat can be used for heating operation,
cooling operation, automatic operation of fans, manual operation of
fans, and automatic changeover from heating to cooling.
• The difference between a line voltage and a low-voltage thermostat
is in the sizing of the bimetal elements.
Anticipators
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Anticipators used to give a more evenly controlled temperature range.
There are two types of anticipators: heating and cooling.
Heating Anticipator
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A thermostat that has no means of heat anticipation will allow a wide swing from the
desired temperature, especially on forced-warm-air systems.
The temperature difference between the opening of the thermostat and the time when
the warm air is no longer being delivered to the room is called overshoot.
Cooling Anticipator
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The cooling anticipator operates somewhat differently from the heating anticipator.
This type of anticipator is also known as an off-cycle anticipator.
The cooling anticipator adds heat to the bimetal on the off cycle of the equipment
because of its parallel connection in the circuit.
When the thermostat contacts are close, the current takes the flow of least
resistance, which is through the contacts rather than the cooling anticipator.
On the “off” cycle the current passes through the anticipator and the contactor coil.
Thermostat Installation
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The installation of a thermostat is simple because all the thermostats are marked with
identifiable letters, although the letters used are not consistent from manufacturer to
manufacturer.
When installing a thermostat, choose a location that will be most suitable for
maintaining the correct temperature in the desired area.
Thermostat installed in applications that require control of the temperature of some
enclosed areas should be installed where the correct temperature can be sensed by
the thermostat without interference from some other sources of heat or cold.
Staging Thermostats
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A staging thermostat is designed to operate equipment at different times with respect
to the needs of the structure.
The staging thermostat has more than one contact and opens and closes at different
times with regard to the conditions of the area being controlled.
Staging Thermostats
Staging System
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Many heating and cooling systems are operated in stages because the load in some
structures fluctuates a great deal.
A heating or cooling system that has been designed to operate on two different
capacity levels is a staged system.
Staging systems offer many advantages because of their efficient operation.
The staging thermostat permits the equipment to operate at half of its capacity.
Staging System
Operation and Types
• A staging Thermostat is designed to be used on a system that has
two stages of heating, cooling or both.
• The staging thermostat operates on the differential in temperature
between stages.
• Staging thermostat can be obtained in a variety of stage
configurations.
• Common staging thermostats used in the industry are the one-stage
heating thermostat with two-stage cooling, the two-stage heating
thermostat with one-stage cooling, and the two-stage cooling
thermostat with the two-stage heating.
Heat Pump
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A heat pump is a refrigeration system that heats or cools by reversing the refrigerant
cycle for the heating operation and then operating conventionally for cooling.
Most heat pumps use a staging thermostat to operate a set of supplementary
heaters.
The first-stag thermostat operates the compressor.
The second stage operates the supplementary heat when the first stage cannot
handle the load.
Clock Thermostats
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A clock thermostat is used to control temperature of a structure, allowing the
customer to lower or raise the temperature control point of at least one period in 24
hours.
With energy conservation and high utility bill playing a large part in air-conditioning
and heating systems, clock and programmable thermostats are becoming
increasingly popular.
Pressure Switches
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A pressure switch is a device that open or closes a set of contacts when a certain
pressure is applied to the diaphragm of the switch.