Abstract : This paper analyzes the causes of current display errors in small frequency converters and proposes corresponding solutions. Keywords : Waveform distortion; Current error 1 Introduction During the commissioning of our plant's proposed thin-water borosilicate project, five 5.5kW washing liquid pumps were equipped with Fuji 7.5kW frequency converters for variable frequency speed control. The commissioning process went smoothly, but the output current of the frequency converter showed significant errors on the displays of the control cabinet, field control box, and VIA FB2000DCS distributed control system. When approaching the rated load, the field control box and DCS distributed control system displays showed very small currents below 3A, and the control cabinet below 5A; under no-load or low load conditions, the field control box and DCS distributed control system displays showed no current, and the current on the control cabinet was very small. We inspected and analyzed the inverter current loop: the control cabinet and field control box are equipped with 42L6 type rectifier ammeters, and the current transformer is a single-turn LMZJ1-0.5 20/S. The current signal is taken from the power supply side of the inverter. The field control box and the main control room are not far from the inverter cabinet, about 30m away. The current loop wiring is normal, and the current transformer is not overloaded. What is the reason? According to the analysis, the reasons for the current display error are as follows. [b]2 Causes of Current Display Error 2.1 Distorted Current Waveform on the Power Supply Side of the Inverter and Containing Large High-Order Harmonics[/b] Because the inverter uses an uncontrolled rectifier bridge on the power supply side and has a large filter capacitor installed in the intermediate DC circuit, the current waveform on the power input side of the inverter is an intermittent pulse with double peaks at the peak of the input voltage waveform. The current pulse width is slightly wider when the load is large and very narrow when the load is small. This distorted current waveform contains a large number of harmonics, and the peak value of the current pulse is much larger than the average value. The waveform coefficient of this distorted current is the square root of the current pulse width percentage. Since small inverters generally do not have a power factor correction DC reactor installed in the intermediate DC circuit, the effective value reaches 2 to 3 times the average value and is non-linear. When the load is small, the effective value is relatively larger than the average value. 2.2 Incorrect Ammeter Selection The 42L6 rectifier ammeter measures current based on the average current value principle. When measuring a 50Hz perfect sine wave, it is calibrated with a waveform coefficient of 1.11. Under conditions of a 50Hz perfect sine wave or when the waveform error is not too large, it can accurately or essentially reflect the effective value of the current. However, for the waveform distortion pulse current on the inverter input side, a larger error will occur. Furthermore, because the effective value and average value of this waveform-distorted current are non-linear, different current segments have different errors, making it difficult to implement corresponding compensation and correction measures in the circuit. [b][align=center]For details, please click: Solutions for Current Display Errors in Small Inverters[/align][/b]