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Troubleshooting Electric Control Devices
Electricity for Refrigeration, Heating and Air
Conditioning 7th Edition
Chapter 14 Troubleshooting Electric Control Devices
Troubleshooting Electric Control Devices
Upon completion of this chapter the student will be able to:
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Use extreme caution when working with any electrical circuit.
All electrical circuits should be adequately protected from over
current according to the NEC.
When making electrical connections to electrical devices,
make sure the power is off.
Make sure all electrical circuits are properly grounded.
Make sure all electrical connections are tight.
Do not come into contact with any bare conductors or
component parts.
Make sure all switches supplying electrical power to the
electrical circuit or the equipment being repaired has been
correctly labeled to prevent someone from turning the switch
on.
Key Terms
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Contactor
Ignition module
Motor
Pressure switch
Relays
Thermostats
Transformers
Electric Motors
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Motors are the most important loads in any heating, cooling, or refrigeration system.
They are used almost exclusively to cause the rotating motion of fans, compressors, pumps, and dampers.
Many different types of electric motors are used in the industry.
However, the type of motor used will have no effect on diagnosing the condition of the motor but will have a great
effect on the selection of a replacement motor if needed.
Electric Motors
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Open-type electric motors will fail in three different areas: windings, centrifugal switch, and bearings.
The windings can be checked for opens, shorts, and grounds.
One important element in the diagnosis of an electric motor winding is to know the type of motor because of the
winding layout.
The condition of the centrifugal switch can best be determined by visual.
Electric Motors
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The only part of a sealed motor, such as a hermetic compressor, that can be checked is the windings because
there are no internal parts other than the bearings.
The winding can be checked as in any other motor for opens, shorts, and grounds.
Diagnosing bearing failure in a hermetic motor is often difficult because no visual inspection or feeling of the
rotation can be accomplished.
Bearing failure on hermetic motors must be diagnosed through amperage readings of the motors along with
running characteristics.
Contactors and Relays
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Contactors and relays are used on most heating, cooling, and refrigeration equipment for the operation of loads in
the system.
Contactors and relays are similar in their operation because both contain sets of contacts and a coil used to open
or close the contacts.
The contactor is larger and capable of carrying more amperage than the relay.
Contacts
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The contacts of a relay or contactor must make good direct contact when energized for the device to function
properly.
One problem often encountered with contactors and relays is the contacts’ inability to make a good contact.
The contacts can be burned, pitted, or stuck together.
A set of burned or pitted contacts can cause a voltage drop across the contacts.
Contacts
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There are several methods of checking a set of contacts to determine if they are burned or pitted enough to
warrant changing the device.
The easiest method is to make a visual inspection.
A resistance check can also determine the condition of a set of contacts.
The device must be energized to check normally open contacts.
Normally closed contacts must be checked with the device de-energized.
Voltage Test of a set of Contacts
Coil
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The coil of a relay or contactor is used to close the contacts by creating a magnetic field that will pull the plunger
into the magnetic field.
If the coil of a relay or contactor is faulty, the device will not close the contacts.
A contactor or relay coil should be checked for opens, shorts, or a measurable resistance.
If a coil is shorted, the resistance will be 0 ohms and the coil should be replaced.
Mechanical Linkage
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The mechanical linkage of a contactor or relay can cause malfunctions in many different forms, such as sticking
contacts, contacts that will not close due to excess friction, contacts that do not make good direct contact, and
misalignment of contacts.
The best method for detecting a faulty mechanical linkage is by visual inspection.
Mechanical Linkage
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A mechanical linkage problem can cause a contactor or relay to stick open or closed or cause misalignment of the
contacts.
If a contactor or serviceable relay has a mechanical linkage problem, it should be replaced (unless it can be easily
repaired).
A sticky armature can cause a relay, contactor coil, or transformer to burn out.
Overloads
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Most major loads used in heating, cooling, and refrigeration equipment have some type of overload protection.
Overloads are often overlooked as being a problem in the system but they may be faulty.
A faulty overload can cause the equipment to run without protection or not operate at all.
The high cost of the major loads in a system makes it necessary to protect all major loads.
Fuse
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The fuse is the easiest type of overload to check because of its simplicity.
A fuse can easily be checked with an ohmmeter in most cases.
If a 0-ohm resistance is shown, the fuse is good. No continuity indicates a bad fuse.
Circuit Breaker
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The circuit breaker is another type of overload device used by some equipment manufacturers and in many
electric panels that are commonly used in the industry today.
The circuit breaker is a device that will trip or open on an overload and must be manually reset.
The circuit breaker is checked by taking a voltage reading on the load side of the circuit breaker.
If line voltage is read on the load side of the circuit breaker, it is probably good.
Voltage Check of a Bad Fuse
Line Voltage Overload
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A line voltage overload installed on a load device is the easiest type of overload to check.
It is used on small hermetic compressors and motors and is connected directly to the line voltage supply.
The line voltage overload can be open, permanently closed, or open on a lower-than-rated ampere draw.
A line voltage overload has only two or three terminals to check.
An ohmmeter across the terminals will indicate whether the overload is open or closed.
Pilot Duty Overload
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A pilot duty overload has a set of contacts that will open if an overload occurs in the line voltage side of the
overload.
These overloads are arranged so that the line voltage feeds directly through them and then on to the load.
The line voltage section of a pilot duty overload can be controlled by heat, amperage, or magnetism—all three are
in common use in the industry today.
Pilot Duty Overload
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The pilot duty contacts on a pilot duty overload are easy to check by using an ohmmeter.
The pilot contacts are usually easy to distinguish from the line voltage components of the overload because of
their small size in relation to the large size of the line voltage connections.
The contacts will either be open or closed.
If the contacts are open, the overload is bad (or there is an overload in the circuit). If the contacts are closed, the
overload is good.
Internal Overloads
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Another type of overload is the internal overload used in hermetic compressors.
This type of overload is actually embedded in the windings of the hermetic compressor motor, which gives it a
faster response to overloads.
The type of internal overload currently used is hard to check because it has no external connections.
It is in series with the common terminal of the compressor motor.
This type of overload must be checked as part of the windings of the motor, which makes it extremely hard to
diagnose for troubles.
Thermostats
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There are two basic types of thermostats used in the industry today: the line voltage thermostat and the lowvoltage thermostat.
The line voltage thermostat is used to make or break line voltage to a load.
Its only function is to open or close a set of contacts on a temperature rise or fall.
The low-voltage thermostat is used when a voltage lower than 120 volts—usually 24 volts—is used to operate a
control system.
The low-voltage thermostat can have many functions.
It can stop and start a fan motor, operate a fan motor independently of other parts of the system, and do many
other functions sometimes required in control systems.
Line Voltage Thermostat
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The line voltage thermostat is easy to troubleshoot because of its simplicity.
A line voltage thermostat could have two, three, or four terminals.
The most important element of checking line voltage thermostats is to be sure the contacts are closed in the
correct temperature range.
Once it has been determined that the thermostat should be opened or closed, it can be checked with an
ohmmeter.
Voltage Check of Open Thermostat
Low Voltage Thermostat
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The low-voltage thermostat is more difficult to troubleshoot than the line voltage thermostat because of the many
functions of the low-voltage thermostat.
The low-voltage thermostat operates the heating and cooling of the system, operates the fan motor with the
heating and cooling operations, operates the fan motor independently, often operates two-stage systems, operates
damper motors, and operates a pilot function of a gas heating system.
Pressure Switches
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Pressure switches are used on heating, cooling, and refrigeration systems to start or stop some electric load in the
system when the pressure in the system dictates this action.
Pressure switches are used as safety devices or as operating controls.
A pressure switch used as a safety device will stop an electric load when the pressure in a system reaches an
unsafe condition.
A pressure switch used as a safety device can be used to protect a refrigeration system from excessive discharge
pressure or low suction pressure.
Troubleshooting Chart for Thermostat
Pressure Switch Operation
Transformers
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A transformer is a device used to raise or lower the incoming voltage by induction to a more usable voltage for the
control system. Some types of transformers are used to buck (lower) or boost (raise) the incoming voltage to an
air-conditioning unit.
A buck-and-boost transformer is used in conjunction with a voltage system that is too high or too low to supply the
correct voltage to a system.
Electric Heating Controls
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The sequencer and contactor are commonly used to control the operation of electric
resistance heaters in electric furnaces and other applications such as duct heaters or
supplementary heat for heat pumps.
Thermostats are used as operating controls and limits switches in these appliances.
Electric motors are used to move air through the heaters to the conditioned space.
Controls and devices that have been covered previously will not be covered again.
In most cases, sequencers are used to control the operation of the electrical
resistance heaters in an electric furnace and other applications.
The sequencer is an electrical switch that acts much like a time-delay relay.
The sequencer has an electric heater that heats the bimetal element, causing the
contacts of the sequencer to close.
Diagram of an Electric Furnace
Voltage Test of a Set of Contacts
Safety Shutoff Troubleshooting Chart