Hình 3 cho thấy các mối quan hệ cho một góc bắn α = 45 °. Khi α = 0 ° đầu ra là tối đa, tức là cung cấp điện áp được áp dụng cho tải không bị gián đoạn. Ở đầu bên kia, khi α = 180 ° điện áp liên tục bị chặn bởi thyristor thông qua nửa chu kỳ. | 4 Additional power unit functions Input signal attentuation This function is used to reduce the maximum power output of the power unit and so to adjust the power output to match the output signal from the preceding controller. The setting is made by first of all setting the controller to provide a 100 output level to the power unit for instance 10V or 20 mA . The power output from the power unit is now reduced usually by a potentiometer on the power unit until it produces the maximum power that is actually required. Soft start The soft start function ensures that the power unit does not suddenly apply a high power to the load when the system is switched on. The power applied to the load is gradually increased from 0 to the set value. This function provides increased operational safety with transformer loads since the transformer is gradually magnetized and can then be operated at full power the inrush effect is avoided. For thyristor power units in phase-angle control the soft start is implemented by starting with the load switched to the supply at a phase angle completely cut back to a 180 and then gradually advancing the phase angle. If the thyristor power unit is configured for burst-firing mode then the first half-cycle of the sinewaves are also initially cut back and then advanced to a firing angle of 0 . The following bursts are then switched through completely see . The IGBT power unit alters its output level by varying the amplitude of the current or voltage. In this case the soft start is implemented so that if there is a step change of the set level at the input to the power unit the corresponding amplitude is gradually increased from 0 to the output level that corresponds to the set value. Current limiting Current limiting is indispensible for heater elements that have a hot resistance Rh that is several Rh 15 imoQ Igmor than tho rA d roQÌQtgrìPO D foi If h QQ l anthal-. i IITCT o omontc M hcro h -- 15 Limeslai gLzIli 1 11 ili i z Lz
