Advances in power relay design

Power relays have come a long way in the past 30 years, writes Ian Purcell of Matsushita.

In electrical terminology a "power relay" can cover a wide range of products.

At one end of the spectrum it may describe a 3A simple contact device, and at the other cover a major power breaker used to isolate a complete substation output.

Some 30 years ago the ubiquitous "Modern relay technology" handbook written by Hans Sauer of SDS Relais, described a power relay as one that was "for use with contact loads equal to, or greater than, 10A", and this is probably as accurate a definition as any other that has been made.

Power relays have a number of salient areas where technology has made inroads during the past few decades.

A typical power relay in the 1970s would have taken a fairly large amount of space to house.

Typically designed around an octal terminal base (similar in style to a thermionic valve base) such relays were, by today's standards, very inefficient.

Coil operation wattage's were expressed in high figures, generally mains voltage driven and emitting tens of degrees of heat into the surrounding ambient temperature.

Contacts were usually silver based and prone to unreliable switching action and wear.

Through the work of pioneering figures in the relay industry, such as the aforementioned Hans Sauer, the power relay has evolved through a number of stages to become a technically advanced component of modern electrical circuits.

Alongside these technical advances are the benefits of more cost effective product both in initial cost and the ongoing benefits of high reliability product in terms of service and replacement.

Although based on comparatively basic electrical principles the power relay coil has benefited from modern design principles.

Probably most significant in terms of improving efficiency is the development of the "polarised coil".

In principle polarised coils have a permanent magnetic flux that is superimposed on the energising magnetic flux caused by the excitation voltage.

With the correct arrangement of these fluxes it is possible to achieve a much higher efficiency because the relay armature actuating force is effectively enhanced by the product of the combined forces.

The DE relay highlights this by offering nominal coil consumption of only 200mW (100mW on the latching version) A further benefit of using polarised technology is the equalising effects and compensation on the pickup and dropout voltages that are experienced by the heating of the coil.

This heating effect can cause a major change, as much as 20 or 30%, when compared with a cold coil.

The reason for this change is the copper windings have a temperature coefficient that increases the resistance of the coil in proportion with a temperature rise.

The coil voltage thus has to be raised to provide a suitable coil drive current.

However the use of permanent magnets allows an opposing temperature coefficient element to be introduced that offsets such changes.

This offset is not exactly balanced so some changes in pickup and dropout voltages do occur but of a much lower level maybe 5% or lower.

An often-overlooked advantage of polarised coils, especially in power switching, is that of "gettering".

This is a technique where the permanent magnet is activated to attract (or "get") the debris circulating in the relay chamber caused by arcing across the relay contacts during switching.

In a normal relay such debris could cause a deterioration of the contact resistance or even sticking of the contacts.

In a "gettered" relay this debris is collected in an area where it cannot cause such a problem.

This radically increases the reliability of the relay.

Obviously the coils are not the only element of an electromechanical power relay and similar advances have been made in contact design.

With the ever increasing demands in legislation to support environmental aims many of the "heavy" metals traditionally used in contact manufacture have been phased out in favour of less toxic alternatives.

More complex alloys that offer equal or better performance have replaced materials such as cadmium and rhodium.

One area that has caused particular problems is in the demand for higher switching levels.

This is mainly driven (no pun intended) from the automotive market where advances in electric car design have required a similar advance in high end switching requirements.

Although usually associated with large contactor based relays much research has been concentrated in designing small, but reliable power relays to control the main battery sources or safety cutouts.

One avenue has been investigating the inclusion of blow-out magnets within the contact chamber.

These can often be highly effective in helping to quickly extinguish arcs drawn out across an opening contact pair.

This is especially prevalent in high current DC based circuits where often an arc will continue even after the actual contact gap has considerably widened.

Such a blow-out magnet would be of a permanent type for DC use but in an AC environment it can be achieved with a small iron based core wound such that the polarity matches that of the switched current to active maximum efficiency.

An alternative approach is to encapsulate a hydrogen gas filled contact chamber.

This has a similar effect during switching of dissipation the arc energy before it can cause damage to the contacts themselves.

Such an approach is taken in the EVPC relay manufactured especially for use in the electric car market where currents in excess of 400A are not unusual under normal use.

In the event that a power relay has to act as a reliable conductor for lower current levels or signal voltages it is often desirable to consider the use of a bifurcated contact.

In simple terms this is in effect a contact that has been divided to provide to independently sprung contact points.

In practice one side will make or break slightly earlier than its twin allowing inrush currents to be effectively channelled through one of the contact points.

The other twin then makes a clean contact allowing a more reliable connection to be achieved.

This technology can be clearly seen on the SFD forced contact relay that achieves highly reliable switching over a wide range of power loads Maybe the culmination of the above advances are most clearly seen in respect of the physical relay size.

The octal base type relays for example were substantial components that were limited to panel mounted applications.

Typically the volumetric size was in the order of 60cm3.

In contrast relays, such as the new DJ relay, are 16A devices in a 6cm3 package.

The obvious advantages are highlighted in such a small package: PCB mounting and component densities, ease of handling and transportation and maybe most importantly generation of ambient heat.

One slightly unusually aspect about the DJ range is that they are available only in a latching coil configuration (set and reset coils).

This relay has been designed for use within building network control applications where low power consumption is paramount.

The latching technology allows the relay to be controlled by individual control pulses (around 20ms) that set the condition of the contacts.

A unique feature on a power relay of this size is the inclusion of a test button to facilitate circuit checks.

From the above it can be seen that the power relay has made significant technical progress.

From a user's point of view it is neither practical nor useful to discuss the theory of all specific relay design criteria in detail.

However what is important is that future relay designers continue to investigate the possibilities of smaller, more reliable and more cost effective power relays.

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