Solid-State Switching Devices for Resistive Loads General data ■ Design Maximum load integral One of the purposes of specifying the maximum load integral ( I І t ) is to determine the rating of the short-circuit protection. Only a large power semiconductor with a correspondingly high I І t va- lue can be given appropriate protection against destruction from a short-circuit by means of a protective device matched to the application. However, the devices are also characterized by the optimum matching of the thyristors ( I І t value) with the rated cur- rents. The rated currents specified on the devices according to EN 60947-4-3 were confirmed by extensive testing.
There is no typical design of a load feeder with solid-state relays or solid-state contactors; instead, the great variety of connection methods and control voltages offers universal application oppor- tunities. SIRIUS solid-state relays and solid-state contactors can be installed in fuseless or fused feeders, as required. There are special versions with which it is even possible to achieve short- circuit strength in a fuseless design. Connection methods All SIRIUS solid-state switching devices are characterized by the great variance of connection methods. You can choose between the following connection methods: You can find more information on the Internet at:
http://www.siemens.com/cd/is_schalten/html_76/schalt.htm Screw connection ■ Integration The screw connection system is the standard among industrial controls. Open terminals and a plus-minus screw are just two features of this technology. Two conductors of up to 6 mmІ can be connected in just one terminal. As a result, loads of up to 50 A can be connected. Notes on integration in the load feeders The SIRIUS solid-state switching devices are very easy to inte- grate into the load feeders thanks to their industrial connection method and design. Spring-type terminal connection system This innovative technology manages without any screw connec- tion.
This means that very high vibration resistance is achieved. Two conductors of up to 2.5 mmІ can be connected to each ter- minal. As a result, loads of up to 20 A can be dealt with. Particular attention must however be paid to the circumstances of the installation and ambient conditions, as the performance of the solid-state switching devices is largely dependent on these. Depending on the version, certain restrictions must be obser- ved. Detailed information, for example in relation to solid-state contactors about the minimum spacing and to solid-state relays about the choice of heat sink, is given in the technical specifica- tions and the product data sheets.
Ring terminal lug connection The ring terminal lug connection is equipped with an M5 screw. Ring terminal lugs of up to 25 mmІ can be connected. In this way it is possible to connect even high powers with current strengths of up to 90 A safely. Finger-safety is provided in this case too with a special cover. Despite the rugged power semiconductors that are used, solid- state switching devices respond more sensitively to short-cir- cuits in the load feeder. Consequently, special precautions have to be taken against destruction, depending on the type of design. ■ Function Siemens generally recommends using SITOR semiconductor protection fuses.
These fuses also provide protection against destruction in the event of a short-circuit even when the solid- state contactors and solid-state relays are fully utilized. Switching functions In order to guarantee an optimized control method for different loads, the functionality of our solid-state switching devices can be adapted accordingly. Alternatively, if there is lower loading, protection can also be pro- vided by standard fuses or miniature circuit breakers. This pro- tection is achieved by overdimensioning the solid-state swit- ching devices accordingly.
The technical specifications and the product data sheets contain details both about the solid-state fuse protection itself and about use of the devices with conven- tional protection equipment. The "zero-point switching" method has proved to be ideal for re- sistive loads, i. e. where the power semiconductor is activated at zero voltage. For inductive loads, on the other hand, for example in the case of valves, it is better to go with "instantaneous switching". By dis- tributing the ON point over the entire sine curve of the mains vol- tage, disturbances are reduced to a minimum.
The solid-state switching devices for resistive loads are suitable for interference-free operation in industrial networks without further measures. If they are used in public networks, it may be necessary for conducted interference to be reduced by means of filters. This does not include the special solid-state contactors of type 3RF23..-.CA.. "Low Noise". These comply with the class B limit values up to a rated current of 16 A. If other versions are used, and at currents of over 16 A, standard filters can be used in order to comply with the limit values.
The decisive factors when it comes to selecting the filters are essentially the current loading and the other parameters (operational voltage, design type, etc.) in the load feeder. Performance characteristics The performance of the solid-state switching devices is substan- tially determined by the type of power semiconductors used and the internal design. In the case of the SIRIUS solid-state contac- tors and solid-state relays, only thyristors are used in place of less powerful Triacs. Two of the most important features of thyristors are the blocking voltage and the maximum load integral: Blocking voltage Suitable filters can be ordered from EPCOS AG.
Thyristors with a high blocking voltage can also be operated without difficulty in networks with high interference voltages. Se- parate protective measures, such as a protective circuit with a varistor, are not necessary in most cases. You can find more information on the Internet at: http://www.epcos.com For example, thyristors with 800 V blocking voltage are fitted in the devices for operation in networks up to 230 V. Thyristors with up to 1600 V are used for power systems with higher voltages. 7 Siemens · 2009