Solid-State Switching Devices for Resistive Loads Solid-State Relays General data ■ Overview ■ Function Solid-state relays SIRIUS solid-state relays are suitable for surface mounting on existing cooling surfaces. Mounting is quick and easy, involving just two screws. The special technology of the power semicon- ductor ensures there is excellent thermal contact with the heat sink. Depending on the nature of the cooler, the capacity re- aches up to 88 A on resistive loads. Three-phase solid-state switching devices Two-phase controlled version A three-phase control system is not required for many three- phase current applications.
Loads in a delta circuit or star circuit which have no connection to the neutral conductor can also be safely switched on and off using just two phases. The solid-state relays are available in three different versions: • 3RF21 single-phase solid-state relays with a width of 22.5 mm • 3RF20 single-phase solid-state relays with a width of 45 mm • 3RF22 three-phase solid-state relays with a width of 45 mm. Nevertheless, the three-phase 3RF22 and 3RF24 solid-state switching devices permit all three phases to be connected to the switching device, in which case the middle phase is looped di- rectly through the device.
Compared to a three-phase controlled device, the lower power loss allows more compact installations. The 3RF21 and 3RF22 solid-state relays can be expanded with various function modules to adapt them to individual applications. Three-phase controlled version This version is used for three-phase current applications in which the system requires all phases to be switched on and off, or for loads in a star circuit with connection to the neutral conductor. Version for resistive loads, "zero-point switching" This standard version is often used for switching space heaters on and off.
Version for inductive loads, "instantaneous switching" ■ Configuration In this version the solid-state relay is specifically matched to in- ductive loads. Whether it is a matter of frequent actuation of the valves in a filling plant or starting and stopping small operating mechanisms in packet distribution systems, operation is carried out safely and noiselessly. Selecting solid-state relays When selecting solid-state relays, in addition to information about the network, the load and the ambient conditions it is also necessary to know details of the planned design.
The solid-state relays can only conform to their specific technical specifications if they are mounted with appropriate care on an adequately di- mensioned heat sink. Single-phase solid-state relay with a width of 22.5 mm With its compact design, which stays the same even at currents of up to 88 A, the 3RF21 solid-state relay is the ultimate in space- saving construction, at a width of 22.5 mm. The logical connec- tion method, with the power infeed from above and load connec- tion from below, ensures tidy installation in the control cabinet. The following procedure is recommended:
• Determine the rated current of the load and the mains voltage • Select the relay design and choose a solid-state relay with hig- her rated current than the load • Determine the thermal resistance of the proposed heat sink • Check the correct relay size with the aid of the diagrams Single-phase solid-state relay with a width of 45 mm The solid-state relays with a width of 45 mm provide for connec- tion of the power supply lead and the load from above. This makes it easy to replace existing solid-state relays in existing ar- rangements. The connection of the control cable also saves space in much the same way as the 22.5 mm design, as it is sim- ply plugged on.
You can find more information on the Internet at: http://www.siemens.com/cd/is_schalten/html_76/schalt.htm Three-phase solid-state relay with a width of 45 mm With its compact design, which stays the same even at currents of up to 55 A, the 3RF22 solid-state relay is the ultimate in space- saving construction, at a width of 45 mm. The logical connection method, with the power infeed from above and load connection from below, ensures tidy installation in the control cabinet. The three-phase solid-state relays are available with • two-phase control and • three-phase control. 8 Siemens · 2009