The source of frequency converter interference is interference from the external power grid.

Harmonic interference in the power grid mainly interferes with the frequency converter through its power supply. The power grid contains numerous harmonic sources such as various rectifiers, AC/DC converters, electronic voltage regulators, nonlinear loads, and lighting equipment. Most frequency converter inverters use PWM technology, which generates significant coupling noise when operating in switching mode and performing high-speed switching. Therefore, the frequency converter is an electromagnetic interference source for other electronic and electrical equipment in the system. Both the input and output currents of the frequency converter contain many high-order harmonic components. Besides the lower-order harmonics that contribute to power supply losses, there are also many high-frequency harmonic components. These will transmit their energy in various ways, forming interference signals to the frequency converter itself and other equipment. Regarding the input current waveform, the input side of the frequency converter uses diode rectification and capacitor filtering circuits. Clearly, charging current only occurs in the rectifier bridge when the power supply line voltage UL is greater than the DC voltage UD across the capacitor. Therefore, the charging current always appears near the amplitude of the power supply voltage, in the form of a discontinuous shock wave. It has a strong high-order harmonic component. Relevant data indicates that the 5th and 7th harmonic components in the input current are the largest, accounting for 80% and 70% of the 50Hz fundamental frequency, respectively. Regarding the waveforms of the output voltage and current, most inverter bridges of frequency converters use SPWM modulation, and their output voltage is a series of rectangular waves with a sinusoidal duty cycle. Due to the inductive nature of the motor stator windings, the stator current is very close to a sine wave. However, the harmonic components equal to the carrier frequency are still relatively large. Furthermore, these loads cause waveform distortion of the voltage and current in the power grid, thus interfering with other equipment in the power grid. If the power supply of the frequency converter is not treated after being interfered with by a polluted AC power grid, the grid noise will interfere with the frequency converter through the power supply circuit. The interference from the power supply mainly affects the frequency converter through overvoltage, undervoltage, instantaneous power outage surges, voltage dips, and radio frequency interference. Interference from Thyristor Converters to Inverters: When there are large-capacity thyristor converters in the power supply network, the thyristors are always conducting for only a portion of each phase's half-cycle, easily causing a voltage dip and severe waveform distortion in the network. This can damage the inverter's input rectifier circuit due to large reverse recovery voltages, leading to input circuit breakdown and burnout. Interference from Power Compensation Capacitors to Inverters: Power companies have certain requirements for the power factor of electricity users. Therefore, many users use centralized capacitor compensation in substations to improve the power factor. During the transient process of capacitor switching on or off, the network voltage may exhibit very high peak values, potentially causing the inverter's rectifier diodes to break down due to excessive reverse voltage. Secondly, there is interference from the inverter itself to external sources. The inverter's rectifier bridge is a non-linear load for the power grid, and the resulting harmonics can cause harmonic interference to other electronic and electrical equipment on the same grid. A frequency converter includes a rectifier circuit and an inverter circuit. The input AC power is converted into DC voltage through the rectifier circuit and smoothing circuit. Then, the inverter converts the DC voltage into pulse voltages of different widths (called pulse width modulation voltage, PWM). Using this PWM voltage to drive the motor can achieve the purpose of adjusting the motor torque and speed.