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CONTACTORS FOR AEROSPACE GROW SMARTER AND MORE CAPABLE AEROSPACE, DEFENSE & MARINE /// CONTACTORS FOR AEROSPACE GROW SMARTER AND MORE CAPABLE Page 2 power-management perspective, this move presents challenges in designing and applying contactors. The Challenges of Higher Voltages How much energy must be absorbed during switching the contactor's main contacts? The switching energy common with opening of the main contacts may be less severe with conventional 120 VAC systems: by definition, the voltage drops to zero at regular intervals, lessening the magnitude of spikes and clearing them more quickly. At 240 VAC, however, the spacing of open contacts may not be sufficient to eliminate arcing re-strike as voltage escalates after zero current. One way around this is to use multiple contact sets in series. The downside of this approach is increased device size, weight, and heat generation. In latest aerospace power systems, frequency is no longer held at 400 Hz. It varies from 350 to 800 Hz depending on engine speed. Designers of both contactors and power panels must carefully evaluate the affects that this wide frequency range has on device life and thermal performance. The adoption of 270 VDC and 540 VDC, first into military and now commercial aviation, has forced dramatic design changes in power contactors. Existing 28 VDC designs are not suited for high-voltage switching because of their inability to generate adequate arc voltage for interruption—at least in a reasonable package size. To overcome these physical limitations, the contactor design must rely on such methods as arc splitting plates, runners, blow-out magnets and better internal switching atmospheres. A 540 VDC system is often split into positive and negative channels that must now be controlled either with 2x contactors or new 2-pole switching designs. Figure 2. Even as contactors grow more intelligent in monitoring capabilities, they continue to evolve to meet SWaP goals. Reducing Required Power One of the first areas where electronic controls were added to contactors was for economizing circuits to reduce power consumption in the coil drive. Most contactors are configured as normally open switches. The actuators within these contactors are driven by ampere turns of the magnetic coil and magnetic iron circuit. The magnetic field created by the coil is used to close the contacts. Relays, contactors, and solenoids all take considerably more power to start the actuator's motion to close the contacts than is required to hold them closed. For example, it may take 5 A to actuate a contactor, but less than 1 A to maintain the state—an 80 percent reduction in power. The economizer circuit is a method to provide 5 A only during pickup and then reduce coil power for holding contacts closed. Two common methods used to economize power consumption are multiple coils and pulse-width modulation (PWM). In early economized contactor designs, the actual transfer of power from pickup to hold windings was accomplished using mechanical limit switches. Once the actuator has transferred through most of its travel, a switch is tripped to reduce power. Limit switches have proven problematic for several reasons. The adjustment can be extremely critical for proper long-term contactor performance since the switch can be actuated too early or too late in the cycle. Since the switch turns off the high power winding coincidental to main contact closure, it may cause increased contact bouncing or chatter. With the integration of electronic coil controls, the transfer timing of the coil power is no longer tied to actuator motion and a limit switch. It becomes possible to ensure the contact sets have fully transferred and are in a stable closed position before initiating the coil transfer. By thus controlling the timing of the transfer, reliability is significantly improved. PWM uses ON-OFF coil pulses of different durations—or duty cycles—to control the average current delivered to a coil. Due to the coil inductance, the contactor is unaffected (no contact chatter or relaxation) during the coil OFF periods. PWM has the advantage of tolerating a range of voltage levels, but may cause radiated noise if not properly filtered. PWM also holds the ability to set the steady- state duty cycle based upon actual operating voltage. During low battery condition, the duty cycle ON time is increased to effectively create a constant current source for the contactor. Overload Protection A common issue with power systems is the danger of overloads. Electrical faults can occur not only in the load equipment but also within an aircraft's entire power distribution and wiring systems. This has been well studied relative to aging aircraft and the effects of long-term environmental exposure on insulation systems. Protection includes detecting undervoltages in the generator, monitoring current levels and detecting leakage current.

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