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Performing Infrared Inspections of Motor Control Centers
Whether it’s your first infrared inspection or you’re a veteran with hundreds of surveys under your belt, it is important to realize that in order to successfully identify and analyze thermal anomalies, it is beneficial to understand the operation of the equipment under inspection. This paper will provide guidelines for inspecting the motor control center (MCC), identifying key components and potential problem areas, illustrating both common and not-so-common thermal anomalies.
The Motor Control Center
A Motor Control Center, or MCC, is a modular cabinet system for
powering and controlling motors in a factory. MCCs are quite common
in factories having heavy machinery. Typically, an MCC cabinet
consists of a metal enclosure with doors providing access. Although
the contents may vary, normally the MCC contains a motor starter,
circuit breaker and possibly a power transformer.
The MCC enclosure protects personnel from contact with current
carrying devices, and it protects the components from various environmental
conditions. It is important that the enclosure is mounted
to assure accessibility so that qualified personnel (such as a trained
thermographer) can open the panel under load.
Figure 1: Motor Control Centers are
common areas of interest for the infrared
inspector. The numerous incoming
3-phase power wires are indicative of
the many electrical connections inside
that need periodic inspection.
There are different classes and types of MCCs, but generally speaking,
an MCC looks like a row of fi le cabinets with each cabinet representing
an MCC section. The drawers of the fi le cabinet represent
the plug-in units that contain the motor control components. Three
phase power is distributed within the MCC by bus bars, large metal
current carrying bars. The horizontal bus provides three-phase power
distribution from the main power supply. Vertical bus in each section is connected from it to individual MCCs. Bracing and isolation barriers
are provided to protect against fault conditions. The plug-in units
of an MCC have power stabs on the back to allow it to be plugged
into the vertical power bus bars of the structure.
Beginning Your MCC
Infrared Inspection
Before opening the panel or door on a motor controller, prescan the
enclosure to assure a safe opening condition. If excessive heat appears
on the surface of the door, extra care should be taken when
opening it. The thermographer or escort may decide to note the
condition as unacceptable and not take
a chance on opening it under load. Once
the unit is open, begin with both an infrared
and a visual inspection to assure no
dangerous conditions exist.
Be systematic while conducting the infrared
inspection. Remember the system
must be under load to conduct the inspection.
Work from left to right or follow
the circuit through carefully, inspecting all of the components. Look
for abnormal thermal patterns caused by high-resistance connections,
overloads, or load imbalances. In three-phase systems this can
be accomplished by comparing phases. Adjust the level and span
on the infrared system to optimize the image. Proper adjustment will
identify primary and secondary anomalies.
Figure 2: The bus stabs at the back
of the MCC are where the incoming
connections to the main horizontal bus
occur. These are important IR inspection
points and are often overlooked or
misdiagnosed. The thermal image to
the right reveals a hot spot indicating a
potential problem.
The bus stabs and the connections to the main are important inspection
points that are often overlooked or misdiagnosed. The incoming
connection to the main horizontal bus is usually located behind a
cover or panel that is not hinged. These are typically bolted connections
and may have parallel feeders.
The bus stab connections on the back of the plug-in units are more
difficult to inspect. The thermographer does not have direct view of
the connection, and the fi rst indication of a problem can be seen on the incoming conductors feeding the breaker or fused disconnect.
Remember, even small temperature rises identifi ed at this point could
mean serious problems.
Motor Starters and
Motor Controllers
The purpose of the motor starter is to protect the motor, personnel,
and associated equipment. Over 90% of the motors used are
AC induction motors, and motor starters are used to start and stop
them. A more generic term would identify this piece of equipment as
a motor controller. A controller may include several functions, such as
starting, stopping, overcurrent protection,
overload protection, reversing, and braking.
The motor starter is selected to match
the voltage and horsepower of the system.
Other factors used to select the starter include:
motor speed, torque, full load current
(FLC), service factor (SF), and time rating
(10 or 20 seconds).
Motors may be damaged or their life significantly reduced if they operate continuously
at a current above full load current. Motors
are designed to handle in-rush or locked
rotor currents without much temperature
increase, providing there is a limited duration and a limited number of
starts. Overcurrents up to locked rotor current are generally caused
by mechanical overloading of the motor. The National Electric Code
(NEC) describes overcurrent protection for this situation as “motor
running overcurrent (overload) protection.” This can be shortened to
overload protection. Overcurrents caused by short circuits or ground
faults are dramatically higher than those caused by mechanical overloads
or excessive starts. The NEC describes this type of overcurrent
protection as “motor branch-circuit short-circuit and ground-fault
protection.” This can be shortened to overcurrent protection.
The four common varieties of motor starters are: across-the-line,
the reversing starter, the multispeed starter, and the reduced voltage
starter. Motor starters are generally comprised of the same types of
components. These include a breaker or fused disconnect, contactor
and overloads. There may also be additional components, including
control circuitry and a transformer.
Understanding the thermal patterns of this equipment is critical to
a successful inspection. Also correctly identifying the source of the
anomaly can make recommendations more valuable.
Overcurrent
Protection
NEC requires overcurrent
protection and a means to
disconnect the motor and
controller from line voltage.
Fused disconnects or
thermal magnetic circuit
breakers are typically used
for overcurrent protection
and to provide a disconnect
for the circuit. A circuit
breaker is defi ned in NEMA
standards as a device designed
to open and close
a circuit by non-automatic
means and to open the circuit
automatically on a predetermined
overcurrent without injury to itself when properly applied
within its rating. If we look at a cutaway of a breaker, we can identify
potential connection problems.
Figure 3: Cutaway of a molded case circuit
breaker showing numerous infrared inspection
points (courtesy of Cutler-Hammer
University).
The line side and load side lugs are the most common source of abnormal
heating, but many breakers have a second set of bolted connections
on the back of the breaker. Heat from this connection can
be misdiagnosed as the main lug. There are also internal contacts
where current fl ow is interrupted by exercising the component. These
contacts experience arcing each time the breaker is opened. An arc
is a discharge of electric current jumping across an air gap between
two contacts. Arcs are formed when the contacts of a circuit breaker
are opened under a load. Arcing under normal loading is very small
compared to an arc formed from a short circuit interruption. Arcing
produces additional heat and can damage the contact surfaces.
Damaged contacts can cause resistive heating. Thermal patterns
from these poor connections appear as diffuse heating on the surface
of the breaker. In addition, there are several types of breakers
that have internal coils used for circuit protection. These coils have
heat associated with them and can appear to be an internal heating
problem, when in fact, it is a normal condition.
Figure 4: Line-side and
load-side lugs are the
most common source
of abnormal heating,
but many breakers
have a second set of
bolted connections on
the back of the breaker.
Heat from this connection
can be misdiagnosed
as the main lug.
Figure 5: Damaged
breaker contacts
can cause resistive
heating. Thermal
patterns from these
poor connections
appear as diffuse
heating on the surface
of the breaker.
Fused Disconnects
Fused disconnects are used to provide over-current protection for
motor in the same manner as a breaker. Instead of opening contacts,
fuses fail opening the circuit. When overcurrent protection is provided
by fuses, a disconnect switch is required for manual opening of the
circuit. The disconnect switch and fuse block are typically one assembly.
The hinge and blade connections on the switch are a typical source
of overheating. High resistance from overuse or underuse is usually
the cause. Fuse clips are also a weak connection point for some
disconnect designs. Different types or manufacturers of fuses of the
same amperage may produce different thermal signatures.
While different size or amperage fuses will also have a different thermal
pattern, fuse bodies may appear warmer than the rest of the circuit
due to conductor size.
Contactors
Starters are made from two building
blocks, contactors and overload
protection. Contactors control the
electric current fl ow to the motor.
Their function is to repeatedly establish
and interrupt an electrical power
circuit. A contactor can stand on its
own as a power control device, or as
part of a starter.
Contactors operate electromechanically
and use a small control current
to open and close the circuit.
The electromechanical components
do the work, not the human hand,
as is the case with a knife blade switch or a manual controller. The
sequence of operation of a contactor is as follows: fi rst, a control
current is applied to the coil; next, current flow into the coil creates
a magnetic fi eld which magnetizes the E-frame making it an electromagnet;
fi nally, the electromagnet draws the armature towards it,
closing the contacts.
A contactor has a life expectancy. If the contactor contacts are frequently
opened and closed, it will shorten the life of the unit. As the
contacts are exercised, an electrical arc is created between the contacts.
Arcs produce heat, which can damage the contacts.
Contacts eventually become oxidized with a black deposit. This
black deposit may actually improve the electrical connection between
the contacts by improving the seat, but burn marks, pitting,
and corrosion indicate it is time to replace the contacts.
The following thermal patterns are associated with contactors. The
coil of the contactor is usually the warmest part of the unit. High temperatures
may indicate a breakdown of the coil. Line side and load
side lug connections may show high resistance heating from poor
connections. Heating from burned and pitted contacts may be thermally
“visible” on the body of the contactor.
Figure 6: Thermal image of a normally
operating overload on a specifi c piece
of equipment. Contactors to the left
side of the image are warm. The coil of
the contactor is usually the warmest
part of the unit. High temperatures may
indicate a breakdown of the coil.
Overload Protection
The ideal motor overload protection is a unit with current sensing
capabilities similar to the heating curve of the motor. It would open
the motor circuit when full load current is exceeded. Operation of this
device would allow the motor to operate
with harmless temporary overloads,
but open up when an overload
lasts too long.
This protection can be provided by
the use of an overload relay. The
overload relay limits the amount of
current drawn to protect the motor from overheating. It consists of a
current sensing unit and a mechanism to open the circuit. An overload
relay is renewable and can work for repeated trip and reset cycles.
Overloads, however, do not provide short circuit protection. The
melting alloy (or eutectic) overload relay consists of a heater coil, a
eutectic alloy, and a mechanical mechanism to activate a tripping device
when an overload occurs. The relay measures the temperature
of the motor by monitoring the amount of current being drawn. This
is done indirectly through a heater coil, which under overload conditions,
melts a special solder allowing a ratchet wheel to spin free and
open the contact.
A bimetallic thermal overload uses a U-shaped bimetal strip. In an
overload condition heat will cause the bimetal to deflect and open
a contact.
The solid state overload relay does not generate heat to cause a
trip. Instead, it measures current or a change in resistance. The
advantage of this method is that the overload relay doesn’t waste
energy generating heat and doesn’t add to the cooling requirements
of the panel.
Normal heating for an overload may look like a thermal anomaly.
Heat generated in the coil or bimetal may look like a connection
problem. Typical thermal problems in overloads are found in the
connections to the contactor, overload relay, or motor.
Starters
Starters are the combination of a controller, usually a contactor and
an overload relay. The above descriptions of the individual components
apply to the starter systems. Reduced voltage starters are
used in applications that involve large horsepower motors. They are
used to reduce the in-rush current and limit the torque, and thus the
mechanical stress on the load. The components of this type of starter
should be inspected as the motor steps up to speed. A separate lowvoltage
starter circuit is used to step the motor up to speed. Once at
operating speed, these components are de-energized.
Completing
Inspections
Remember that primary anomalies
are the problems that
readily stand out while secondary
anomalies may require that
primary anomalies be adjusted
into saturation to allow for the
identification of a secondary
anomaly. For example, different
fuse types and sizes will
cause different thermal signatures
as will overload relays
that are sized differently within
the same circuit. Anomalies
like this should be identified
and reported.
Figure 7: Visual inspection of equipment
is also important and may reveal
problems that go otherwise unnoticed.
Also note that when evaluating
the severity of a problem,
temperature is just one
variable. All of the parameters
involved with the severity of the
anomaly should be considered.
To improve temperature measurements, avoid low emissive
surfaces. Look for cavity radiators or highly emissive insulation on
conductors. Measure loads where component sizing, overloading,
or load imbalances are observed. Beware of the effects of wind or convection on components. Note ambient temperatures, large
thermal gradients, and the source of heating. Safety should be the
top consideration.
Conclusion
Knowing the equipment under inspection allows for the correct
identification of problems that could be misdiagnosed or
overlooked. Analyzing unfamiliar thermal patterns on a component
is easier when equipment design is reviewed. More precise repair
recommendations can also be made.
Locating temperature differences qualitatively or quantitatively is
the real benefit of infrared thermography. Knowing where to look
for these temperature differences comes from knowledge of the
equipment, and knowledge of the equipment will make a better
thermographer.
Acknowledgment
This article was written by Roy Huff of The Snell Group.
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