A. O. Smith HVACR User Manual
A. O. SmithLevel 2 A. O. SmithLevel 2
Introduction
This application guide is not intended to be a
repair manual for any of the equipment
described. It is intended to provide general
knowledge to persons working in fields related to
electric motors and to provide service information
to qualified persons.
Motors are of little use by themselves. They
are designed to be coupled to some other part and
convert electrical energy into mechanical energy.
The selection of a replacement motor for a specific application would be a very involved process
if it were necessary to start with the basics of
magnetism, electricity, and motor construction
each time a replacement was required. In order
to present replacement motor offerings in a logical, orderly manner, the motors are grouped by
type, construction, and often, the application for
which they are intended.
Motors in original applications are designed to
be operated under specific conditions. If the
equipment or installation is modified, the motor
may be overloaded. For example, if a furnace
designed for use in a home with duct work is
placed in a shop and has little or no supply duct
work, it may be close to a free blow condition (no
static pressure or resistance to air flow). The
motor may become overloaded.
A.O. Smith’s replacement motor selections are
identified by applications where possible. This
makes it easier to narrow the choices in the
selection process.
Heating, Ventilating, Air Conditioning, & Refrigeration Motors 1
32 Motor Troubleshooting Guide
� Most motor applications have service manu-
als whose procedures, especially those involving safety, should be followed. Within the
scope of this discussion, the assumption must
be made that the procedures have been followed to the point where the motor is suspected, and proper voltage (nameplate as connected
+
10%) is available to the motor.
� In addition to the electrical hazard, an
applied motor could be very hot if a start was
attempted during other test procedures.
� Always ground motor and secure so starting
torque will not cause it to move in bench test.
� Always use the proper tools (not a hammer)
to remove fans and pulleys from a motor that
is to be reinstalled. Likewise, never pound
anything onto a motor shaft. A ball bearing
can easily become brinelled or work hardened
leading to eventual failure.
� In a new installation of new equipment that
does not start, follow the established procedures to the point where a motor check is recommended. The procedure is basically the
same for a replacement motor on an existing
application except the case where the motor
burned out as soon as the power was applied.
In this case, the motor was probably hooked
up wrong.
� On a replacement motor there is slight possi-
bility that the wrong nameplate was applied.
Does the connection information physically
match the motor? Are other motors with the
same part number but different date codes
the same? Specific resistance values from the
manufacturer may be required for positive
confirmation.
A. O. SmithLevel 2
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2 Heating, Ventilating, Air Conditioning, & Refrigeration Motors
HEATING, VENTILATING, AIR CONDITIONING &
REFRIGERATION (HVAC&R)
Heating and air conditioning applications often
use the same types of motors or share the same
motors if the systems are combined. Heating systems using motors heat either air or water. Once
heated, the air or water is moved by a motor and
blower or motor and pump.
All air conditioning systems using motors within the scope of this manual use essentially the
same principles. The components may look different depending on the system capacity and
configuration.
Heating,
Ventilating,
Air Conditioning &
Refrigeration
(HVAC&R)
Motor Troubleshooting Guide 31
� If motor appears to run okay it may be okay.
Or, it may have an internal problem such as
loose or plugged cooling fan. It is possible for
a winding coil to be shorted onto itself without indicating an open condition. The motor
would be weak and probably overheat under
load. If the motor has a terminal board, connections could be loose causing intermittent
operation.
� Check for phase imbalance on three phase
systems.
� Checking a motor in its application is the
only practical method for most people to
determine performance under load. If the
motor is defective, the application provides
many clues to help determine the cause . Did
the motor fail due to a defect or old age. Or,
was its failure hastened by the application or
environment? The application is the only
place an attempt may be made to check voltage.
� On permanently installed equipment, the
voltage may vary depending upon the total
system load. It can also vary with the total
load on the power company grid.
� Mobile equipment such as rental units are
often abused through the use of inadequate
extension cords or undersized generators.
� Was the motor misapplied or has the applica-
tion load change? Dirty or blocked coils may
overload a condenser motor. Reduced static
pressure may overload a motor driving a
blower wheel.
� Is the belt driven load free to turn, and in
proper operating condition?
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Heating, Ventilating, Air Conditioning, & Refrigeration Motors 3
THE REFRIGERATION CYCLE
The refrigeration cycle involves a number of
physics principles to convert energy from one
form to another, and to transfer that energy from
one point to another.
One of the principles in the change of state.
Just as it takes energy to change ice at 32
degrees fahrenheit to water at the same temperature, energy transfer is required to change the
state of a refrigerant.
The diagram below describes the refrigeration
cycle. Simply stated, electrical energy in the case
of an electrically driven compressor, compresses
the refrigerant gas and causes it to move through
the system where it changes from a gas to a liquid and back. In addition to the changes in gas
volume, heat is added or taken away at different
points in the process.
The
Refrigeration
Cycle
▼
▼
▼
▼
▼
▼
▼
DISCHARGE LINE
SUCTION LINE
LIQUID LINE
RECEIVER
EXPANSION
VALVE
CAPILLARY TUBE
BLOWER/MOTOR
BULB
FAN/MOTOR
COMPRESSOR
EVAPORATOR
CONDENSER
HIGH PRESSURE SIDE
LOW PRESSURE SIDE
REFRIGERATION DIAGRAM
30 Motor Troubleshooting Guide
� On belt driven loads with sleeve bearing
motors, check to see that the motor is oriented properly in relation to the bearing’s oil
wick window.
� Excess heat is the enemy of a motor. If the
motor has been operating properly, has anything changed? Is the voltage too high or too
low? Is the ambient too high for the motor?
Are internal or external fans clean and
intact? Has foreign material gotten into the
motor? Is normal air f low around the motor
blocked, causing recirculation?
� Is there any evidence of physical damage or
overheating?
� In applications such as direct drive furnace
blowers, the motor needs system air for cooling. There is not a general rule of thumb,
but use of a replacement motor significantly
stronger than the original may result in overheating. Speed will be up and the system
may not provide enough air for cooling.
Select a close replacement and check amps
after installation.
� Check windings for continuity.
� Check for continuity between motor leads or
terminal board and frame as indication of
short to ground.
� Check capacitor if capacitor start and/or run.
� Check the overload if accessible.
� If a single phase motor with a mechanical
start switch starts but does not come up to
speed, check the switch for proper operation.
If this same type motor just hums and does
not start, check the switch and start winding.
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4 Heating, Ventilating, Air Conditioning, & Refrigeration Motors
Electrical energy is converted to mechanical
energy by the compressor motor. The compressor
draws a refrigerant gas from the evaporator, compresses it and sends it to the condenser where it
is liquefied. Refrigerant gas entering the condenser is full of heat picked up from the previous
cycle plus heat from the compression cycle. In
the condenser, heat is removed from the refrigerant and it returns to the liquid state. An expansion valve controls the flow of refrigerant
between the condenser and evaporator. The temperature of the gas entering the evaporator is
lower than that of the air to be cooled. Heat is
removed from the air to be cooled. This heat is
absorbed into the liquid refrigerant causing a
change of state back to a vapor. The cycle continues until a thermostat senses that the desired
temperature has been reached and shuts the system down.
HERMETIC COMPRESSORS
The motors and other components of the system operate in a closed system and must be compatible with each other. Compressors are designated as hermetic or semi or accessible hermetic
depending on the type of construction. The word
hermetic means an airtight seal or sealed from
outside elements. Typically, home refrigerator,
room air conditioner, home air conditioner, and
small commercially applied compressors are in a
welded shell, leading to the hermetic designation.
Larger commercial compressors use castings and
other parts, similar to an automobile engine,
which are bolted together and get the designation
accessible hermetic.
Hermetic
Compressors
Hermetic Stator/Rotor
Motor Troubleshooting Guide 29
Knowing why a motor failed may be important
in selecting the proper replacement motor, and
insuring a good service life.
Try to determine the age of the installation and
if the motor is the original. If the motor is a
replacement, how long has it been in service?
Does it appear to have been properly installed?
What is the duty cycle or service period?
Having some indication of the problem often
indicates items to check first. A bench check is
usually a confirmation of a suspected problem.
The type of motor and its construction will
dictate which items may be checked without
disassembly.
The following tips and suggestions are by no
means complete, but offer some of the basics
relating to a motor and its application. Other
modules cover some subjects in more detail, and
specific knowledge from other sources may be
needed.
� To the degree possible, check the same areas
in the application as would be done in a
bench test.
� Is there any evidence of water damage?
� Does the shaft turn freely?
� On sleeve bearing motors, check end play
(.01-.06” normal). Lack of end play could
cause motor to bind when hot.
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Heating, Ventilating, Air Conditioning, & Refrigeration Motors 5
The function of both types is essentially the
same. Due to the initial cost of the larger systems, parts may be salvaged and used in rebuild
or remanufacturer operations.
Hermetic motors are used in hermetic compressors. The sealed system contains a refrigerant
gas and lubricating oil.
Hermetic motors are functionally the same as
other types of motors except that the housing,
bearings and shaft are provided by the compressor manufacturer. These motors are often
referred to as rotor/stator kits. Single phase versions are often of the permanent split capacitor
(PSC) type which does not require a starting
switch. Systems using PSC compressor motors
often have “Dual Can” capacitors which are two
separate capacitors in one housing. One capacitor is used for the compressor and one for the fan
motor.
A.O. Smith has a selection of hermetic
rotor/stator kits for rebuild and remanufacture
operations.
28 Motor Troubleshooting Guide
Motor
Troubleshooting
Guide
Some procedures require special tools and specific product knowledge which is beyond the
scope of this module.
These tips and suggestions are offered for persons with proper qualifications and necessary
test equipment, and as information of general
interest to persons in the replacement motor
distribution channel.
There is not a single listing of motor troubleshooting procedures to be followed in a given
order. The procedures will also differ for new
and existing installations and motors that are
being bench checked. As with anything dealing
with electricity, personal safety is the prime concern.
BENCH CHECK
First, consider what symptom lead to the
motor’s removal from service. Examples: “It was
noisy,” “it wouldn’t start when the power was
applied,” “it tripped the breaker,” “it runs for several minutes then shuts off,” “it just hums when
power is applied,” “smoke came out when it was
turned on.”
WARNING!
This is not a guide for
the do-it-yourselfer.
Bench Check
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