FEATURE SWITCHES & RELAYS DEALING WITH HIGH INRUSH CURRENTS
Richard Thornton Managing Director at Panasonic Electric Works UK explores the advantages of zero cross switching power relays for handling inrush protection design requirements
ED light systems have become an indispensable part of modern buildings and houses. Compared to conventional systems of the past, such as incandescent, halogen or fluorescent lamps, this type of lighting offers significant improvement in terms of energy efficiency and options for configuring the lights. However for lighting switching systems, and especially for relays, this results in higher loading of the switching contacts due to very high inrush currents involved. The LED lighting system generally requires a DC power source which can be a separate DC power supply or by a DC voltage unit integrated into the lamp socket. A smoothing capacitor is generally used after the rectifier and it acts like a short circuit since it’s in uncharged state when the power supply is turned on. As a result inrush currents of up to 600A may flow if the appropriate number of LED lamps are connected in parallel. Switching currents in these ranges are sufficient to cause thermal melting of the contact areas making the contacts catch or stick after just a few hundred switching cycles. In operation this ends the relay's reliable service life. Relay manufacturers have two preferred ways of dealing with high inrush currents of this magnitude. One approach is to change the design of the contacts. This is accomplished with a tungsten pre-make contact which closes the circuit before the actual switching contact. It is responsible for handling the inrush current. The nominal switching contact, which is made of the usual contact materials such as AgNi or AgSnO2, closes after the inrush current has subsided to the level of the rated current. This method uses the positive property of tungsten - its thermal load
L Figure 1:
The DW Series power relay from Panasonic
22 SEPTEMBER 2015 | ELECTRONICS
capacity. With a melting point significantly higher than normal contact materials tungsten offers great advantages. A much more elegant solution is to limit the amplitude of the inrush current. Zero crossing – a method that is considered by many designers to be the holy grail for extending the service life of relay contacts. It follows that the greatest potential for extending the service life of relay contacts is by selectively switching the relay contacts in the zero crossings inherent in the power supply. Implementing selective switching of the
relay contacts when the voltage crosses the zero point by means of intelligent activation reduces the inrush currents to less than one third the level compared to the most unfavourable case - switching on at the voltage maximum. Zero Crossing results in considerably less load on the contacts thus extending the service life of the switching contacts by up to 10 times as long, when compared with conventional operational life. The principle of the elements needed to
achieve Zero Crossing and how they interact are now described in detail. First an optocoupler detects the zero crossings for the power supply voltage in its input circuit. It transfers them via infrared radiation (optical signal) to a phototransistor on the output circuit and generates a corresponding signal, which is recorded by a digital input in a custom µ-controller . A trigger edge has been implemented to evaluate this signal in the µ-controller. It creates the basis for accurate time-controlled switching of the relay coil(s). In addition to the information about the time of the zero crossing it should be noted that electromechanical relays have an
operating and release time in the single- digit millisecond range due to mechanical inertia. These values can be documented by measurements in the relay manufacturer's laboratory and made available to users as reference values. The respective mean value is saved in the µ-controller's EPROM memory. However a further addition to the basic circuit in the form of load current measurement is recommended. The reasoning is initial operating and release times for the same relay type may differ by up to 2ms and are furthermore subject to additional drift over the service life of the relay due to effects such as contact erosion and the resulting decrease in the contact operating times as well as contact migration. The load current measurements makes it
possible to record and analyse the inrush current during the switching process by means of the current induced in the secondary circuit using an analogue input on the µ-controller. If the inrush current is above the target value defined in the development configuration the switching time is automatically adjusted by the software. This creates a control loop that regulates the inrush current to the lowest possible value thereby extending the service life of the switching contacts. Zero crossing thus represents enormous added value for customers. In addition to extending the service life compared to other switching concepts, it offers another impressive advantage: smaller and more cost-effective relays can be used, such as latching DW-H, DE or DSP relays. Panasonic latching relay systems require only brief control pulses to change switching states after which the integrated magnet system holds the armature and hence the switching contacts in a defined position. Then there is no need to continue
supplying the relay coil with current. This eliminates the need for continuous activation, as required for single-side stable systems, and the corresponding coil dissipation power. Along with energy efficiency latching systems also feature increased contact forces and better shock and vibration characteristics.
Panasonic Electric Works UK
www.panasonic-electric-works.co.uk Info.pewuk@
eu.panasonic.com
/ ELECTRONICS
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