Equipment for local and volumetric PWHT (heat treatment of welded joints)

Сontactor control circuits using features

Сontactor control circuits using features

Not all specialists of heating and local heat treatment of welds understand the intricacies of building a heating control system in heat treatment units. In this connection, some unscrupulous marketers give incorrect information, misleading when choosing a suitable units for heat treatment of welded joints.

Particularly, there is an opinion that the contactor heating control circuit, unlike the semiconductor (thyristor) circuit, does not allow to obtain an even and smooth direct heating line, forming a "ladder". It's not like that at all!

The regulation accuracy and the shape of the heating curve, depends on the temperature controller, which gives a control signal to the power contactor or thyristor. Modern “smart” temperature controllers used in our PWHT units monitor and control the heating process based on a complex self-adjusting PID-algorithm. This means that the controller takes into account the properties of a particular heated product in specific conditions and adjusts all the components of the control law (proportional, integral and differential) to the current task. It makes possible to use the same controller for post welded heat treatment of welded joints, even for furnace heating of whole product.

In addition, when you choose one or another power unit for heat treatment, it is necessary to take into account the maintainability of the system and the cost of reconditioning. The power contactor is a fairly simple product. In the case of its “sticking” or seizing, it is enough just to disassemble it, clean and assemble it back. For such simple operations, the simplest set of locksmith tools is enough: a screwdriver, a wrench and a brush. The most difficult thing that can happen to a contactor is damage of contact pair. This contact pair changes very easily even in the most difficult working conditions and is quite inexpensive.

The thyristor unit consists of several components. It includes the thyristor key directly, various boards (coordination, control, etc.). Such system cannot work without forced cooling; therefore, a cooler and a cooling fan must be present. If any of these components fails, the entire assembly ceases to work. For the diagnosis and repair of the corresponding complex electronic unit, complex diagnostic equipment (oscilloscope), special skills of an electronic engineer, etc. are required. Typically, the entire thyristor assembly must be replaced. The cost of that operation is several times higher than the cost of a contactor.

Any of such heat treatment unit with any of the power element types described above can be supplied by our company. However, taking into account the fact of frequent use of equipment for heat treatment in difficult conditions of elevated temperatures, high dust content or a great distance from "civilization", we most often offer a contactor control scheme for the operation control of heating mats for heat treatment. However, the final choice always remains with the Customer and we would be glad to offer you a heat treatment unit taking into account your specific requirements.

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