Troubleshooting Electric Control Devices

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Troubleshooting Electric Control Devices

• Electricity for Refrigeration, Heating and Air
  Conditioning 7th Edition

Chapter 14 Troubleshooting Electric Control
Devices
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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.

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Key Terms

• Contactor
• Ignition module
• Motor
• Pressure switch
• Relays
• Thermostats
• Transformers

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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.

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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.

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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.

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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.

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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.

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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.

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Voltage Test of a set of Contacts
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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.

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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.

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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.

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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.

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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.

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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.

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Voltage Check of a Bad Fuse
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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.

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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.

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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.

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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.

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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.

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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.

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Voltage Check of Open Thermostat
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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.

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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.

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Troubleshooting Chart for Thermostat
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Pressure Switch Operation
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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.

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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.

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Diagram of an Electric Furnace
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Voltage Test of a Set of Contacts
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Safety Shutoff Troubleshooting Chart
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