kể từ khi các biến thể của biến quá trình là không thể chấp nhận được trong ứng dụng như vậy. Các yếu tố kiểm soát như biến máy biến áp đã được sử dụng kể từ khi sự khởi đầu của tự động hóa, vì họ cho phép một sự biến đổi liên tục của năng lượng điện. Biến áp biến, tuy nhiên, rất tốn kém, để mặc, và chỉ cho phép điều chỉnh chậm. | 3 Closed control loops and underlying controls In this chapter we will take a look at electrical power units in a closed control loop using a furnace control system as an example. The electrical supply voltage is connected to the power unit. The controller derives the output level yR from the difference between the set value w for the furnace temperature and the actual or process value x which is acquired by a sensor inside the furnace. The output level can vary over the range 0 100 and is produced as a standard signal output . 0 10 V. The output level signal is fed to the power unit. The task of the power unit is to feed energy into the heater elements in the furnace proportional to the controller output level - For a thyristor power unit using phase-angle control this means that it alters the firing angle over the range from 180 to 0 corresponding to a controller output level of 0 100 - If the thyristor power unit is using the burst-firing mode it alters the duty cycle T from 0 100 to correspond to the controller output level of 0 100 - When using an IGBT power unit the amplitude of the load voltage is varied from 0 V to VLoad max to correspond to the controller output level of 0 100 Now let s look at the response of the electronic power unit in Fig. 11 to variations of the supply voltage using the example of a thyristor power unit operating in burst-firing mode Assume for example that the controller is regulating the thyristor power unit at an output level of yR 50 . This means that the power unit is operating with a duty cycle of 50 . the supply voltage is switched through to the load for half of the complete sinewaves of the supply voltage. The energy that the power unit is feeding to the load the furnace is say y A 5kW and is just that which is needed to keep the furnace at the required temperature for example 250 C . Now assume that the supply voltage sags by 10 from 230 V AC to 207 VAC. The thyristor power unit is still being regulated by the output .
