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