ower control is a critical aspect of the electrification trend, expanding demand for improvements, upgrades and
modernisation throughout infrastructures from solar and wind generators to points of use. Equipment targeting residential applications faces intense cost pressure, with demands for miniaturisation to allow unobtrusive and fashionable styles. On the other hand, reliability and fault handling are dominant requirements in industrial electrification, while advancements that improve efficiency are wanted everywhere.
When it comes to handling high currents and voltages, electromechanical switches including contactors and relays are chosen for their high ratings and ability to safely isolate inactive loads. Contactors tend to have larger electromagnetic coils than relays as well as spring-loaded contacts for breaking the circuit, and have been preferred in applications that involve controlling extremely high currents. Contactors also tended to contain built-in weld detection that can protect the systemif the main contacts become fused and fail to open when required. This is typically implemented with a set of auxiliary contacts that mirror the main contact structure. With their auxiliary contact mechanism, higher
coil rating, and spring loading, contactors tend to be physically larger than ordinary relays and can support lowermaximum switching frequency. Interconnection typically relies on screw terminals, which require manual assembly. In today’s electrifying world, new DC-switching
Figure 1. High-current relays now incorporate auxiliary contacts for fault detection
applications include large inverters for utility- grade photovoltaic generators and battery- energy storage systems, high-speed electric vehicle chargers and domestic wallboxes, and uninterruptible power supplies (UPS). These are disrupting the old order, demanding high current capability and safety features of contactors, smaller size, and lower power consumption, in a device compatible with high-volume production techniques. Similar pressures apply to more traditional AC loads such as lighting, HVAC and FA, where power density is increasing and demand for smaller and compact devices becomes a necessity.
New developments among relays allow current ratings in the 50A-300A range, which lets these devices offer an alternative to contactors in many domestic, industrial, and utility-grade applications. The latest devices also feature weld detection with fault signalling thereby permitting comparable systemprotection. The auxiliary contact mechanism shown in Figure 1 ensures safe insulation, with a withstand voltage of 2.5kV or minimum contact gap of 0.5mm, even after the coil is de-excited when the main contacts have a welding failure. Among the inherent advantages of relays,
smaller component dimensions permit more compact and lower-profile enclosure sizes. This can be important for consumer markets that desire fashionable and minimalist styles to industrial and utility applications challenged to install more and smarter infrastructure within existing space constraints. Contrasting the properties of a typical contactor with a comparable relay reveals more than 66% weight saving, as well as more than 60% less height and 85% lower volume. The relays’
smaller size and typically lower weight permit PCB mounting of high- power switches that
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can simplify and streamline equipment assembly. PCB-mounted components are compatible with automated production processes, allowing circuits to be assembled at high speed using inline insertion equipment. Subsequent automated soldering lets equipment vendors design-out traditional bulky and expensive components such as busbars and screw terminals that need to be manually fastened. In addition to permitting faster and more efficient assembly, soldered connections save human errors such as incomplete screw tightening or applying incorrect torque, permitting more consistent production quality.
Moving established design practices and production flows away from traditionalmanually installed contactors to PCB-mount relay assemblies involves literally going back to the drawing package to create a new circuit-board design. PCB design guidelines include ensuring adequate copper thickness to carry the intended current, noting that enlarging the terminal surface area can help boost heat dissipation. A heatsink or insulated metal substrate can help protect the board in applications that demand extremely high current. In addition, for high-capacity PCB relays,
implementing a holding-voltage circuit or PWM drive circuit to minimise power consumption can effectively ease thermalmanagement and can potentially reduce the drive power to 25%. Equipment vendors can recoup the investment to redesign their PCBs through greater saleability, delivering smaller, lightweight PCB-based assemblies that fulfil market desires. The production area may also need to be
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