Abstract : This paper starts with the "frequency setting cannot be adjusted" fault phenomenon of the Yaskawa 616P5 frequency converter and analyzes the essence of this fault phenomenon based on the principle of dual-edge modulation digital PWM. Keywords : Fault phenomenon, dual-edge modulation 1 Introduction During operation, frequency converters may encounter various problems, requiring maintenance and repair; and after a long period of shutdown, frequency converters may also malfunction due to various reasons. This paper will introduce a fault that occurred when a frequency converter was restarted after a 4-month shutdown. 2 Fault Phenomenon A Yaskawa 616P5 frequency converter driving a submersible sewage pump was found to be operating at a frequency of 50Hz and a current of 78A during the entire operation from startup. According to the process requirements, the pump should operate within a range below 50Hz. Obviously, the frequency conversion function of the frequency converter is out of control. 3 Fault Analysis and Detection The frequency converter can operate at a 50Hz power frequency and output a voltage of 380V, and the pump is running. These phenomena indicate that the power module output is normal, but the control circuit is malfunctioning. The 616P5 is a general-purpose frequency converter. Its core control circuit component is a dedicated large-scale integrated circuit L7300526A containing a CPU to generate pulse width modulation signals. This frequency converter is usually in remote transmission control and receives 4-20mA current signals from the control terminals. According to the working principle of the general-purpose frequency converter, the "frequency setting cannot be adjusted" fault phenomenon may come from two unit circuits: (1) A/D converter (2) PWM modulation signal. Following the troubleshooting approach of starting with the easy and then moving to the difficult, in order to eliminate the hidden dangers of the A/D conversion circuit, the rejection method was used for testing. That is, firstly, the relevant cables of the control terminals were disconnected, and the frequency setting was input by the keyboard (i.e., digital operator). The screen display fault phenomenon still existed. The second step was to use the comparison method for testing. That is, the MODEL100 signal generator was used to input 4-2mA and 0-10V analog signals from the control terminals FI-FC and FV-FC respectively. The screen display fault phenomenon still existed. Inputting digital <parameter setting value> via the keyboard sends the signal to the CPU system through an encoding scanning program. Analog signals input from the control terminals are converted to digital signals (A/D) and processed by logic circuits before entering the CPU system. The A/D conversion circuit can be confirmed to be functioning correctly using rejection and comparison methods. The L7300526A chip uses a digital double-edge modulation carrier method to generate pulse width modulation signals, driving a three-phase inverter composed of transistor power modules. The carrier frequency is equal to the product of the output frequency and the carrier multiple. For each value of the carrier multiple, the chip's internal decoder stores a corresponding set of 6 values ​​(6 values ​​represent an adjustable time interval used to modulate the pulse edge). Each value is stored digitally, and its corresponding pulse modulation width is determined by the counting rate of the corresponding value. [b][align=center]For more details, please click: Discussion on the "Frequency Setting Unadjustable" Fault of the 616P5 Inverter[/align][/b]