Military Embedded Systems

Smart contactors provide intelligent avionics overcurrent protection


October 31, 2018

Karl Kitts

TE Connectivity

Smart contactors provide intelligent avionics overcurrent protection

Think carefully when you answer this question: What's one of the fastest, most precise ways to prevent overcurrents from damaging the power circuits used aboard today's more-electrical and all-electrical aircraft? A thoughtful answer would be "smart power contactor," a circuit-switching device that not only handles high voltages, but also incorporates the intelligence to sense a variety of abnormal electrical events and respond faster and more accurately than conventional circuit breakers.

Considering different high-power-circuit protection technologies

A common way to protect high-power circuits from overcurrents is to use bimetal-based circuit breakers that rely on thermal trip elements. While effective to some degree, thermal breakers respond slowly and have limited accuracy in the set trip point(s), because it takes time for the bimetal strip to respond to heat generated by the overcurrent. Bimetal breakers also cannot be easily tested while in service to verify performance. Fortunately, electrical systems engineers have a smarter option – power contactors – that incorporate integrated electronic current sensing technology.

Incorporating electronic sensing into the contactor provides the intelligence to monitor and respond to overcurrents quickly and accurately. In fact, these smart connectors provide at least twice the accuracy in trip setting over mechanical circuit breakers. They can also be exercised through built-in tests to simulate fault events to ensure they will perform as expected if a system fault occurs.

To detect and respond to overcurrent conditions, a smart contactor can employ one of three different technologies:
1. A precision resistor that is used as a shunt allowing the voltage across it to be measured. Drawbacks of this method: Resistor solutions generate heat and require isolating low-voltage from high-voltage circuits.
2. A simple toroid-style current transformer (CT) that is placed around conductors. The resulting magnetic field created by the feed-through current creates a secondary current in the CT in a typical 500:1 current-to-CT-current ratio. Variations in the secondary current provide a trip point. Drawbacks: CTs are accurate and simple to apply, but they are heavy.

Hall effect sensors use a flux ring or collector surrounding the contactor’s bus bar or output feeder. The sensor, which is a linear transducer that responds to electron flow, is combined with threshold detection. Modern Hall effect sensors are programmable for output voltage and linearity and can also pro-vide bidirectional current sensing and AC sensing. Hall effect current sensors have been developed with math-function circuitry to emulate the I2T [the energy available as a result of the current flow] flow delay function that closely matches the curve of a thermal circuit breaker.

There are numerous advantages of Hall effect sensors, including isolation between primary and secondary circuits, the ability to work with direct or alternating current, high accuracy, high dynamic performance, high overload capacities, high reliability, and built-in ambient-temperature compensation, unlike with conventional thermal breakers.

From responsive protection to insightful prediction

Hall effect sensors protect wiring more accurately than bimetal circuit breakers by enabling overcurrent conditions to be calibrated to a given set point compared to a breaker's time/duration method. Similar to circuit breakers, smart contactor trip times can be adjusted according to the severity of the fault. Lengthy trip times may suffice for near-normal current levels, while massive faults require trip times of less than 0.015 second. The level of fault protection for smart contactors – thanks to electronic sensing – can be adjusted by the user or application to tailor protection for each individual load. These adjustments are easily accomplished through connector pin programming, DIP [dual in-line package] switches, external resistor additions, or software coding. The user simply reconfigures the smart contactor as the application warrants.

While sensing overcurrents is generally the prime task required of a smart contactor, other faults can be sensed, including loss of phase and phase rotation, differential feeder fault, ground fault, and arc fault detection.

Data about the state of the contactor itself can be gathered and analyzed using microcontroller-based electronics. This information can go beyond basic trip circuit response history. Real-time operation can be monitored to identify trends and changes, enabling intelligent prediction of problems and proactive responses to maintain the health of the contactor.

Information on running currents, temperature, and number of cycles can be used to predict the life cycle of the contactor. For example, operating the contactor at lower current levels can significantly increase the number of switching cycles.

The collected data can also be used to monitor the system. For example, current draw during contact pickup reflects 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.

Moreover, combined information from multiple smart contactors and other sensors can support “big data” predictive analytics of conditions throughout the system.


Smart contactors: smarter operation for power management

Compared to thermal magnetic breakers, smart contactors use sophisticated sensing and electronic technologies. But don't think that smart contactors are too complex to be practical. Engineers can apply custom-designed, application-specific power panels that integrate relays and smart contactors into an advanced yet uncomplicated plug-and-play solution.

Just as aircraft power systems have evolved, so too has contactor design. Today's smart contactors and intelligent power distribution panels enable overcurrent protection, support fault management, and will monitor and analyze the health of the power system. By providing higher reliability with higher capability for status sensing and overcurrent protection, smart contactors are a smart move for more-electrical and all-electrical aircraft.