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CONTACTORS FOR AEROSPACE GROW SMARTER AND MORE CAPABLE AEROSPACE, DEFENSE & MARINE /// CONTACTORS FOR AEROSPACE GROW SMARTER AND MORE CAPABLE Page 4 Phase Faults. To protect motors, fans, and other devices using three-phase power, phases must remain synchronized to ensure the proper delivery of power. Phase faults stress the operated devices, shortening their lifetime, causing improper operation, and even bringing catastrophic failure. The two main phase faults are loss of phase and phase rotation. Both result in uneven, unbalanced delivery in power. When one of the phases is lost, delivered power is diminished since only two phases are delivering power. Phase rotation occurs when the phases are not properly synchronized at 120 degrees of separation. The same techniques used to monitor current for overloads can be used to detect phase problems. By sensing and comparing current levels on each phase, any difference can be detected. Leakage Current Fault Protection Sensing leakage currents and protecting against differential faults involves multiple current sensors along a length of wiring. Outputs of the sensors are compared to detect faults. Ground fault detection is a specialized protection scheme using only one common sensor to ensure all passed current is also returned from the load without leakage. This detection means has become commonplace on aircraft fuel pump applications to reduce risk for fuel vapor ignition. Differential feeder fault protection is common in the aerospace industry. This is usually a high threshold protection to validate no current leakage on large-diameter power feeders. A typical setup includes a sensor at or even within the power generator and a second one at the main line contactor. If the sensed currents are different, a fault has occurred. Ground faults can be monitored in two ways. One way is to check for current in the ground plane. The second is to use the information provided by the phase sensors. The sum of all three phases should be zero. If it does not sum to zero, a fault exists in the wiring or load. Arc Fault Detection Arc fault detection is becoming more commonplace in circuit breakers and secondary solid- state power controllers (SSPCs). It has been demonstrated that existing protective devices are ineffective against sputtering arc faults. While current levels may not increase enough to trigger a hard fault, arc faults can generate unacceptable heat levels. Parallel arcing faults may ultimately progress to full overcurrent faults, while series arcs resulting from broken conductors or loose device terminals could generate tremendous heat well under traditional overcurrent trip curves. Detecting arc faults and even locating distances to a wiring fault is an emerging area for smart contactors. Beyond Electromechanical Contactors While solid-state relays are common, the application of power semiconductors to contactors is relatively new. MOSFETs can replace the power contacts, with the obvious advantage in improved reliability relative to no moving parts. Solid state power devices can extend the switching life of a contactor. Power contacts are subject to wear from both the mechanical mating and the effects of arcing. As contacts wear, the increased resistance across the connection means increased heat generation and end of life failures. Solid-state relays require additional thermal management versus hard contact designs. While the absence of mechanical parts makes solid-state designs very reliable, the main failure mechanism now becomes heat. The devices must be protected from overheating. Beyond heat sink thermal management, multiple power transistors can be applied in parallel to keep currents well below the maximum rated levels. For aerospace applications, transistors are de-rated at 15 to 20 percent of datasheet current- carrying rating in order to manage thermal performance effectively. From Sensing to Prediction Microcontroller-based electronic control allows more information about the state of the contactor to be gathered and analyzed. This information can be used to go beyond basic trip circuits in response to faults. It is one thing to sense a fault and shut down a component. More useful is to monitor operation in real time to identify trends and changes. This allows intelligent prediction of problems and flexible responses. Current and voltage levels can provide real-time insight into the health of the contactor and of the overall electrical system. Information on running currents, temperature, and number of cycles can be used to predict the life cycle of the contactor. Operating the contactor at lower current levels can significantly increase the number of switching cycles. For example, a contactor can offer 50,000 switching cycles at its rated current. Operating it at 25 percent of rated current can double the cycle life to 100,000. The collected data can also be used to monitor the system. For example, current draw during contact pickup reflect inrush currents to motors or pumps, yielding insight into bearing wear. The same information can indicate the need for lubrication or other maintenance. Changes over time in sensor data can also indicate faults in the wiring system. Comparing initial operation at commissioning with changes over time is fundamental to understanding and predicting problems. While the output from a single contactor can yield useful data, information from multiple contactors and from other sensors in the wiring system can be combined into "big picture" analysis and prediction since it allows comparison of conditions throughout the system. Integrated Assemblies: Power Distribution Panels As contactors become more sophisticated, they also become more complex. Many users are opting for custom-designed, application-

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