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Troubleshooting Electric Control Devices

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Title: Troubleshooting Electric Control Devices


1
Troubleshooting Electric Control Devices
  • Electricity for Refrigeration, Heating and Air
    Conditioning 7th Edition

Chapter 14 Troubleshooting Electric Control
Devices
2
Troubleshooting Electric Control Devices
  • Upon completion of this chapter the student will
    be able to
  • 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.

3
Key Terms
  • Contactor
  • Ignition module
  • Motor
  • Pressure switch
  • Relays
  • Thermostats
  • Transformers

4
Electric Motors
  • 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.

5
Electric Motors
  • 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.

6
Electric Motors
  • 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.

7
Contactors and Relays
  • 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.

8
Contacts
  • 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.

9
Contacts
  • 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.

10
Voltage Test of a set of Contacts
11
Coil
  • 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.

12
Mechanical Linkage
  • 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.

13
Mechanical Linkage
  • 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.

14
Overloads
  • 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.

15
Fuse
  • 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.

16
Circuit Breaker
  • 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.

17
Voltage Check of a Bad Fuse
18
Line Voltage Overload
  • 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.

19
Pilot Duty Overload
  • 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
    magnetismall three are in common use in the
    industry today.

20
Pilot Duty Overload
  • 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.

21
Internal Overloads
  • 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.

22
Thermostats
  • There are two basic types of thermostats used in
    the industry today the line voltage thermostat
    and the low-voltage 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 voltsusually 24 voltsis 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.

23
Line Voltage Thermostat
  • 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.

24
Voltage Check of Open Thermostat
25
Low Voltage Thermostat
  • 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.

26
Pressure Switches
  • 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.

27
Troubleshooting Chart for Thermostat
28
Pressure Switch Operation
29
Transformers
  • 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.

30
Electric Heating Controls
  • 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.

31
Diagram of an Electric Furnace
32
Voltage Test of a Set of Contacts
33
Safety Shutoff Troubleshooting Chart
34
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