Solid-state switching devices are used in most modern automation systems to switch electrical loads. They offer high durability where a high number of switching operations or a fast switching speed is required. Further benefits are resistance to mechanical shock or vibration, silent, maintenance free service, resistance to moisture and dust and reduction of electromagnetic noise due to arcing. When certain precautions are taken outstanding performance and durability is achieved. Development in this field is rapid and the latest trends are to incorporate intelligence in the switching devices.
While electromechanical relays can withstand high fault currents, the mechanical movement and inertia as well as the heat build-up and interference due to arcing of contacts are a severe drawback. SSRs are therefore becoming increasingly popular.
Less expensive SSRs incorporating triacs are suitable for resistive loads only (AC1 rating). SSRs with anti-parallel thyristors can also switch inductive loads (AC1 and AC3 rating) and are thus also suited for control of solenoids and transformers. Switching is initiated by applying a low DC signal (typically 4 to 32 V, 4 mA) to the trigger input, placing less demand on control outputs and enhancing safety. SSRs are normally 'zero crossover switching', only closing or opening when the AC power sine wave crosses zero. This virtually eliminates electromagnetic interference and harmonics on the power supply.
Precautions:
(1) Fuse protection: Solid-state relays can only sustain limited fault currents. Circuit breakers and similar overload protection devices are too slow to prevent damage to SSRs when short circuits occur. For this purpose ultra-rapid fuses are needed. It is common practice to install DIN-rail mounted fuse holders, making replacement of fuses easy.
(2) Heatsinks: SSRs dissipate power proportional to the load current. The resultant heat needs to be dissipated by suitable heatsinks. When the load current exceeds 4 A, heat sinking becomes essential. It is not good practice to mount SSRs directly onto the backing plate of instrument panels as this often leads to overheating. Reputable manufacturers of SSRs also offer specially designed heatsinks, combining compact size with high heat dissipation. It is advisable to study the current-versus-temperature derating curves when selecting heatsinks. It may be a good idea to put thermal strips onto heatsinks, changing colour when the rated temperature is exceeded. This makes it possible to detect overheating.
(3) Ventilation: Because solid-state relays dissipate heat; adequate ventilation needs to be provided in panels and enclosures containing SSRs. For ambient temperatures in excess of 40°C the current rating of SSRs needs to be derated. If a number of SSRs and other heat generating components are mounted in a panel, it may be necessary to provide forced ventilation. Care should be taken to ensure good airflow over the heatsinks. Therefore, the positioning of air vents is important. Obstructions and blind corners should be avoided. When using fans it is recommend that the fans blow out rather than suck in. Inlet vents should be spread over a big area, preferably on the underside of the panel. This prevents plastic foil or paper from blocking the suction inlet. Air vents on top of panels may not be a good idea because somebody may innocently place an obstruction there.
(4) Overvoltage/spikes: Solid-state relays can only block a limited voltage. If excessively high voltage or spikes occur during the off state, the solid-state relay may be damaged. Fuses will not prevent this failure. Most reputable manufacturers will incorporate some overvoltage protection in the SSR. However, there is a limitation to the energy such protective devices can absorb. In areas where thunderstorms and lightning are prevalent it may be a good idea to mount more powerful external transient suppressors in the panel to protect the equipment.
(5) On three-phase loads it is advisable to use SSRs with a rating of 400-480 V, even if a neutral is connected. When the neutral conductor fails or is absent, the voltage per phase may exceed 250 V and reach 400 V or more. SSRs with a voltage rating of 250 V (normal for single phase use) will fail in this instance.
Gefran is a leading manufacturer of industrial solid-state relays. The comprehensive range includes SSRs with matching heatsinks, three-phase SSRs, power solid-state relays with analog control and 3-phase synchronisation (master-slave) and power controllers up to 400 kW.
Latest developments in the Gefran series are intelligent solid-state relays with serial communication and internal PID control. These are suitable for operation with PLCs that have memory and speed constraints and can only handle a limited number of PID loops. Gefran is distributed and stocked in South Africa by UniTemp.
For more information contact Neil Coetzer, UniTemp, +27 (0) 11 392 5989, [email protected], [email protected]
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