Magnecraft® Solid State Relays - Magnecraft - #29

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Dpl_News_HR.pdf table.main {} tr.row {} td.cell {} div.block {} div.paragraph {} .font0 { font:6.00pt "Arial", sans-serif; } .font1 { font:8.00pt "Arial", sans-serif; } .font2 { font:9.00pt "Arial", sans-serif; } .font3 { font:11.00pt "Arial", sans-serif; } .font4 { font:15.00pt "Arial", sans-serif; } .font5 { font:17.00pt "Arial", sans-serif; } Application Data (continued) Magnecraft® Solid State Relays Transformers In controlling transformers, consider the characteristics of the secondary load because they reflect the effective load on the SSR. Voltage transients from secondary loads circuits, similarly, are frequently transformers and can be imposed on the SSR. Transformers present a special challenge in that, depending on the state of the transformer flux at the time of turn off, the transformer may saturate during the first half-cycle of subsequently applied voltage. This saturation can impose a very large current (10 to 100 times rated typical) on the SSR which far exceeds its half cycle surge rating. SSRs having random turn on may have a better chance of survival than a zero cross turn on device for they commonly require the transformer to support only a portion of the first half cycle of the voltage. On the other hand, a random turn on device will frequently close at the zero cross point and then the SSR must sustain the worst case saturation current. A zero cross turn on device has the advantage that it turns on in a known mode and will immediately demonstrate the worst case condition. The use of a current shunt and an oscilloscope is recommended to verify that the half cycle surge capability is not exceeded. A rule of thumb in applying an SSR to a transformer load is to select an SSR having a half cycle current surge rating greater than the maximum applied line voltage divided by the transformer primary resistance. The primary resistance is usually easily measured and can be relied on as a minimum impedance limiting the first half cycle of inrush current. The presence of some residual flux plus the saturated reactance of the primary will then further limit, in the worst case, the half cycle surge safely within the surge rating of the SSR. Switching Devices The power family of semiconductors consists of several switching devices. The most widely used of this family are metal-oxide semiconductor field effect transistors (MOSFETs), silicon controlled rectifiers (SCRs), Triac, and Alternistor Triac. In many applications these devices perform key functions and therefore it is imperative that one understand their advantages as well as their shortcomings to properly design a reliable system. Once applied correctly SSRs are an asset in meeting environmental, speed, and reliability specifications which their electromechanical counterparts could not fulfill. • MOSFET A power MOSFET is a specific type of metal oxide semiconductor field-effect transistor (MOSFET) designed to handle large amounts of power. It is a vertical structured transistor capable of sustaining high blocking voltage and high current. Power MOSFET's are used in DC switching applications. Care must be taken to ensure that there is proper polarity for all DC ports. Failure to do so can lead to permanent device damage. • Triac A TRIAC, is an electronic component approximately equivalent to two silicon-controlled rectifiers joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. This results in a bidirectional electronic switch which can conduct AC current only. The Triac is ideal for switching non-reactive loads. • Alternistor Triac The Alternistor has been specifically designed for applications that switch highly inductive AC loads. A special chip offers similar performance as two SCRs wired inverse parallel (back-to-back), providing better turn-off behavior than a standard Triac. The Alternistor Triac is an economical solution; ideal for switching inductive AC loads. • SCR The SCR (silicon-controlled rectifier) acts as a switch, conducting when its gate receives a current pulse, and continue to conduct for as long as it is forward biased. The SCR is ideal for switching all types of AC loads. Schneider ^Electric 29