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Soft start
Soft start protects the power controller and heater from being damaged by high inrush current during startup. Soft start is required for variable-resistance heater elements such as: Tungsten Soft start is available for single-phase and three-phase, three-leg models only, and requires phase angle control. Why is soft start required? Some heater elements show large resistance-value changes with temperature (i.e., a high hot-to-cold ratio). If full power is applied when the element is cold, there will be an inrush of high current, which will blow semiconductor fuses and degrade the heater. For example, suppose that we have a 230V tungsten heater. At operating temperature, the resistance of the heater element is 2.3 ohms. Ohm's Law tells us that Current = Voltage/Resistance, therefore the current through the element is 100 Amps (230/2.3). When the heater is shut off, the resistance of the element drops to 1/15 its hot value, or 0.153 ohms. If full power is applied, the current will be about 1500 Amps (230/0.153). Soft start prevents this large inrush current, which lengthens heater life and prevents blown semiconductor fuses. Note: Some heater elements also require current limiting. Soft start allows the heater element to warm up before full load voltage is applied to the heater. Soft start allows only a small amount of current through the heater. The current is gradually increased until the requested power level is achieved. You can field-select a soft-start duration between 0.1 and 120 seconds.
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