FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 3 4 8 change in resistance. Advanced SCR controls are used to automatically limit the current and eliminate the excessive current that can damage the heating elements. A small amount of phase angle firing is used on startup and then automatically transfers to burst firing mode (also known as zero cross) with power regulation (Fig. 4). During burst firing, each zone’s on/off times may be managed through distributive load management algorithms using predictive load management (PLM). Case 1: An advanced SCR power supply utilizing hybrid firing replaced an existing analog SCR with phase around 0.95, but when used below this level, significant reductions to PF can occur with levels below 0.5. The VRT control method causes high harmonic distortion on the network, and unfortunately, harmonics increase the heat losses in the system. The heat must be dissipated by liquid cooling. Conventional air cooling is not sufficient. This harmonic disturbance can impact other sensitive electronic equipment used in the factory and may not conform to the latest guidance on electrical noise. Filters and PF correction systems can be utilized to overcome some of these issues, but this results in additional capital costs and energy waste. ADVANCED SCR POWER SUPPLIES Microprocessor control in the SCR’s power supplies has developed at a pace over the past twenty years. It enables a tight relationship between the required energy input to the heating element and the control setpoint. SCRs with phase angle firing have been typically used in power supplies. PF at 1 is only possible when running at 100% output. Below this level, significant harmonic distortion can be created due to the ability to chop the electrical waveformat any point in the cycle (Fig. 2). Looking at other firing modes, such as burst firing mode, the PF is always unity (1). However, inrush currents cannot be controlled during startup of the transformer and changes in physical properties can’t be managed in certain heating elements when running purely in burst mode. Accurate, repeatable, and energy-efficient closedloop power control can only be provided by a hybrid combination of firing modes (Fig. 3)[4]. Typical heating element materials used in vacuum furnaces are MoSi2 (molybdenum disilicide) and graphite. The physical properties of these materials change between hot and cold temperatures with significant 6 Fig. 2 — Firing mode waveforms. Fig. 3 — Schematic of an advanced SCR power supply. Fig. 4 — Hybrid firing pattern.
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