Induction furnace switch key
Crucible Change-over Switch
The crucible change-over switch, also known as the changeover switch, is used in the circuit of induction furnaces for two primary purposes.
The first application of these switches is to connect or disconnect the power feeder to the crucible of the induction furnace. In this context, often referred to as the on/off switch, the purpose is to establish or break the connection between the induction furnace coil and its power source (the output of the resonance tank).
The second application of the changeover switch is to switch the operational crucible of the induction furnace. In this case, the connection between the active coil and the furnace feeder is disconnected, and the reserved coil is connected to the feeder. The term “changeover switch” or “crucible change-over switch” is most commonly associated with this function.
In all types of crucible change-over switches used in induction furnaces, the electrical connections from the furnace feeder (output of the resonance tank) and the input connections to the induction furnace coil are permanently connected to the fixed contacts of the switch. The moving contacts of the switches, when activated, either connect or disconnect the two sides by bridging the fixed contacts.
A critical point regarding the connection or disconnection using these switches is that both during disconnection and reconnection, the furnace feeder must be completely de-energized. The change in the switch’s contact positions must occur only when there is no electrical power, as failure to do so can cause severe damage to the furnace’s electrical equipment. This operational principle of the crucible change-over switch can be compared to the operation of disconnectors used in electrical substations.
In terms of quality, the most important characteristic of crucible change-over switches is the stability and durability of the complete and seamless connection between the fixed and moving contacts. Even the smallest gap between the fixed and moving contacts can result in partial electrical discharges and, at times, severe electrical arcing between the two contacts, which in turn can cause significant damage to the switch contacts and other electrical components of the furnace. Factors contributing to this issue include high temperatures at the contacts during operation, the passage of transient currents exceeding the permissible levels, and inefficient connection mechanisms within the switches.
Therefore, manufacturers of crucible change-over switches employ various methods and mechanisms to achieve a reliable and stable connection between the moving and fixed contacts. Most of these methods are based on applying unilateral or bilateral pressure from the fixed contact to the moving contact to maintain the connection. In the case of bilateral pressure, a larger area can be allocated for the common cross-sectional surface between the two contacts.
It is worth noting that regardless of the design and pressure retention mechanism between the fixed and moving contacts of crucible change-over switches, the connection mechanism can vary in all of them. The following classification summarizes the common mechanisms used for establishing or disconnecting connections between the contacts of crucible change switches.
Common Mechanisms for Establishing Connections Between Fixed and Movable Contacts in Crucible Change Switches for Induction Furnaces
Lever
This mechanism is particularly popular among Indian furnace manufacturers.
Pneumatic
The pneumatic mechanism is used in both singl-side-pressure and dual-side-pressure switches, and it is implemented by companies like Turkish manufacturers, ABP of Germany, and some Indian producers.
Hydraulic
This mechanism is primarily employed in high-current single-side-pressure switches by certain Chinese manufacturers.
Motorized
The connection mechanism powered by an electric motor is similar to the hydraulic mechanism and is favored by some Chinese furnace manufacturers.
Magnetic (Absorptive)
The magnetic mechanism for connecting the fixed and moving contacts operates similarly to the mechanism used in electrical contactors. It establishes the connection by sending a signal to the switch’s coil, which pulls the two contacts together. The majority of crucible change-over switches used in induction furnaces, manufactured by the German company Otto Junker, utilize this type of mechanism.