TE - White Papers

CONTACTORS FOR AEROSPACE GROW SMARTER AND MORE CAPABLE AEROSPACE, DEFENSE & MARINE /// CONTACTORS FOR AEROSPACE GROW SMARTER AND MORE CAPABLE Page 3 Many existing applications still rely on thermal trip elements using bimetal-based circuit breakers. While effective, these breakers have limited accuracy in the set trip point(s). They also cannot be in-service performance tested periodically. To overcome these deficiencies in overload protection, one option for the electrical systems engineer is electronic sensing integrated into the power contactors. Electronic sensing provides more reliable sensing of overcurrents. These circuits generally provide at least twice the accuracy in trip settings over mechanical circuit breakers. Electronic sensing devices can also be exercised through built-in tests to simulate fault events to insure they will perform as expected if a system fault occurs. A method to monitor running current through the contactor accurately is the first requirement for electronic overload protection. The simplest method is to use a precision resistor as a shunt and simply measure the voltage across it. The method is very accurate, but can generate considerable heat in high-current contactors. It also is less than desirable for mixing control circuits and 120 V/240 V sense lines for overall systems integrity. Care must be taken to isolate low-voltage circuits from high-voltage ones. Optical isolators are commonly used for this purpose. A second method for monitoring current is a current transformer (CT). CTs are simple toroids placed around conductors. The magnetic field created by the feed through current in turn creates a secondary current in the CT. The current is proportional, but is much lower. A typical ratio of current to CT current is 500:1. CTs are simple to apply and accurate, but also heavy. Hall-effect sensors are another common method of measuring the magnetic field created by the current. Hall-effect elements change a voltage output level based upon exposure to a magnetic field. This field is most commonly expressed across the Hall-effect sensor using a flux ring or collector surrounding the contactor's bus bar or output feeder. Not only are modern Hall-effect sensors programmable for output voltage and linearity, they also can allow bidirectional current sensing and AC sensing. TE advanced its Hall- effect current sensor with the addition of the math function circuitry to have to emulate the I 2 T trip function of a thermal circuit breaker. Using the Hall-effect technology the device trip times are not affected by ambient temperatures like conventional thermal breakers. This device has an optional reverse current trip function that gives additional protection. Figure 3. Hall-effect sensors are flexible and accurate. Advantages of the Hall-effect sensor are: Isolation between primary and secondary circuits Works with direct or alternating current High accuracy High dynamic performance High overload capacities High reliability Regardless of sensor type, supporting electronics is used to collect information from these sensors and make decisions on system configuration. In certain instances, the integrated electronics only communicates running conditions to other aircraft systems. This information can be very useful in decision-making for load shedding if a power source is lost. Aircraft loads are prioritized for criticality so that noncritical convenience loads are depowered in order to maintain flight-essential and other critical loads. In addition to communicating circuit conditions, contactors with integrated sensing electronics can react independently to overload fault conditions. Overcurrent conditions can be measured and integrated versus time/duration to provide wiring protection much more accurately than bimetal circuit breakers. Similar to circuit breakers, trip times can be adjusted according to the severity of the fault. Lengthy trip times suffice for current levels just about normal, while massive faults require trip times of less than 0.05 second. The level of fault protection for smart contactors—i.e., those with electronic sensing—can even be adjusted by the user or application to tailor protection for each individual load. Such adjustments can be accomplished through connector pin programming, DIP switches, external resistor additions, or software coding. This allows the smart contactor to be reconfigured if the application warrants changes. Additional Fault Detection and Protection While sensing overcurrents is generally the prime task required of a smart contactor, other faults can be sensed. These include: Loss of phase and phase rotation Differential feeder fault Ground fault Arc fault detection

• Cover