PID control of AC motors
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How can a PID controller be used to control motors powered by 220V or 380V AC? I use it to regulate the opening degree of fans; a baffle is used as the valve, and the pipe diameter is 500 mm, which is used to control the volume of air flow
The opening degree of the damper is controlled using a 4-20MA signal generated by a PID controller; an electric valve without a controller is used for this purpose, and all that’s needed is a servo controller suitable for electric valves! If DCS is used, a position-proportional algorithm can be used in place of a servo controller! Both methods require an opening feedback from the damper! The best way to control the air volume of a fan is by using an inverter! It can save a lot of electrical energy! **There is a policy: for up-conversion converters, a portion of the project funds can be obtained! This post was last edited by liuei on 2009-3-1 15:08]
One dual-output electrical valve positioner + two pressure switches; if these items are available in the spare parts warehouse, this project can also be carried out!
Some of the questions raised are about the fact that PID control cannot manage the motor. There are two methods available to control the air volume. 1. Use an electric actuator to drive the fan inlet damper via a linkage; the air flow rate is controlled by adjusting the opening degree of the damper. A PID module is used in the closed-loop system to control the output of the damper, and feedback may not be necessary (feedback is used to check whether the output of the actuator is normal). 2. Eliminate the damper and use an inverter to control the fan motor; the air flow rate of the fan is controlled by adjusting the output of the inverter, with a PID module used in the closed-loop system to control the output of the inverter. Both of these are ordinary automatic control loops; the difference lies in the method of control. Suggestion: Use frequency converters for low-pressure fans as part of energy-saving initiatives ； The initial load on the inverter of the high-pressure fan is too high; it is more appropriate to use an actuator to control the opening degree of the damper.
You are referring to automatic PID control for digital signals, which is technically feasible. But it is rarely used in this way, as frequent starting and stopping of the motor can affect the lifespan of the contactor and the motor. Generally, variable frequency is used, or the flow rate at the fan inlet is controlled.
I have also encountered such cases: digital control is suitable for variable-frequency PID control, but the response to adjustments is too rapid, which can easily cause fluctuations in the pipeline system. This is especially true when it comes to flow rate regulation. It is recommended to set upper and lower limit values for protection.
Opening feedback is essential; without it, the servo controller of the electric valve cannot function! A valve positioner cannot function either without a feedback rod!
Use the pressure in the pipeline to control the fan via the frequency converter, with flow control valves in place