Abstract : This paper introduces a frequency inverter speed control system using a microcontroller with PID control, including the inverter control method and control principle, which realizes constant-pressure operation of a centrifugal fan on an inlet duct steady-flow test rig. Keywords : PID regulator; steady-flow test rig; frequency inverter 1 Introduction The design of the engine intake duct plays a key role in the engine's combustion efficiency. Before solving the problem of measuring the actual in-cylinder gas flow, it is a practical method to test the designed intake duct using an engine intake duct steady-flow test rig and obtain comparable parameters such as the cylinder head gas flow rate and vortex coefficient. A simplified diagram of the intake duct test rig is shown in Figure I. The airflow test bench simulates the actual operating conditions of an engine. A centrifugal fan creates a stable negative pressure on the cylinder side of the cylinder head under test, ensuring a constant difference between the pressure measured at the pressure sensor and atmospheric pressure. Since the gas flow rate varies significantly during measurement, and based on the characteristics of the centrifugal fan, the negative pressure provided by the fan is inversely proportional to the flow rate and directly proportional to the rotational speed. Therefore, the test bench uses a Fuji inverter to adjust the fan speed to achieve the required test conditions during the measurement process. [b][align=center]For more details, please click: Application of Inverters in Airflow Stabilization Test Bench[/align][/b]