Commercial, industrial, and telecom power panels often use banks of relays to control loads from just a few amps to the hundreds of amps and thousands of VA that pose a challenge solid-state devices. Frequently overlooked in favour of more exotic devices, the advantages that electromechanical relays offer in low duty-cycle applications include simplicity in design, the ability to withstand severe overloads, very high power-handling-to-size ratio, long life, and relatively low cost. Better yet, relays for power switching applications invariably come with safety-agency approvals that guarantee reliability within their voltage and current ranges. But what of their energy efficiency?
Due to the thickness of silicon that’s necessary to guard against avalanche breakdown and device destruction, semiconductors become increasingly less conductive as voltage levels rise. As a result, their I2R losses increase substantially with rising voltage and current levels. Alternatively, the metal-to-metal contact systems within traditional relays offer virtually loss-free switching that remains relatively stable over the relay’s operating range.
In addition to saving power by minimising conduction losses, relays require no heatsink, thus saving space in cramped control panels that must typically maintain ambient temperatures of no more than 35oC. A lower power level in the control panel means reduced load on environmental cooling systems; indeed there may be no requirement for forced-air cooling around the power switches at all, providing additional, indirect energy savings.
In the simple example of a conventional normally-open relay, the majority of the device’s power dissipation arises from the magnetising current that flows within the coil that closes the contact set. This activation current flows the entire time that the relay is closed, and - while being a relatively small value in comparison with the relay’s load current - still detracts from efficient operation.
For example, a typical 16A/250Vac double-pole, double-throw relay requires 2.5 to 4W to activate, while a single-pole 200A/400Vdc relay can require as much as 24W to sustain its load circuit. Sensitive-coil relays typically consume between 200 and 400mW at 16A and power-saving coils are available to reduce holding current in high-power devices to about 2W, but these currents are still continuously required for the switch to remain closed.
Latching relays overcome this static power consumption issue by requiring power only to change state. Functionally the electromechanical equivalent of a flip-flop, latching relays achieve bistable operation through techniques that range from mechanical cams to sophisticated magnetic circuits. But from an application viewpoint, it’s the coil drive requirements that are typically of most interest.
The simplest latching relay uses a single set/reset coil, requiring sequential reversals in dc current polarity from a circuit such as an H-bridge driver to toggle between states. Alternatively, devices that include separate set/reset coils benefit from the simplicity of single-polarity drive, but require additional connections.
In either case, having changed state, the relay maintains its position until another current pulse changes it back, so the relay holds its last position during power outages, which is useful in some applications.
The author’s company can offer a family of 2A - 200A-rated latching relays, the PCB-mounting devices switching up to 60A/250Vac. Like most latching relays, these offer single- and dual-coil operation in a variety of terminal configurations. The devices require dc drive currents at voltages from 5 – 48V, consuming 1W per coil for the minimum of 50ms that’s necessary to open or close its single form-A contacts. In practice, the manufacturer advises using 100 – 200msec pulse-widths to assure reliable switching under worst-case conditions for relays of up to 60A capacity; the broadly similar 100A, 120A, and 200A products all require at least 200msec to actuate. Notably, the 200A/440Vac-rated product comes in single-coil configurations that comprise two-terminal, polarity-reversal types, and three-terminal, centre-tapped versions where the centre terminal is negative with respect to the set/reset control terminals. With two form-A contacts that suit power-hungry contactor replacements, these compact relays require 12W and 24W to activate, respectively.
Because applications that use high-power relays typically have very low duty cycles, the power savings that latching types offer are huge. For example, assume that a conventional high-power relay with a 12W coil switches twice a day to control an HVAC system. Every day, that relay consumes 0.096 kW/h over its 8-hour on-time, which totals about 35kW/h per year assuming everyday use; under similar circumstances, a latching relay driven by two 500msec control pulses could use 3,600 times less energy.
This is an extreme example, but it serves to highlight the importance of selecting the best relays for your applications. It is also important to understand how to apply latching relays correctly to ensure reliable operation, with switching reliability statistics varying between competing manufacturers. To assist customers with their designs, for example, the author’s company can offer application engineering expertise and full laboratory testing facilities for its range of more than 7,000 specialist components.
Jeremy Lester is technical director, Switchtec