Abstract : This article elaborates on the basic principles of AC frequency converters and methods for suppressing and handling weak electrical signal interference when used in automatic control systems. It also details some techniques and maintenance methods for frequency converters in engineering applications. I. Overview A variable speed drive system includes a motor and a speed controller of some type. Early electrical speed drives included two main types: AC and DC motors. DC motor speed control changes the DC voltage applied to the DC motor by altering the excitation current, thus allowing the DC motor to run at different speeds. The earliest electronic controllers used thyristor rectifiers to control the voltage and thus the speed of the DC motor. These DC drive devices are still widely used today and require periodic brush maintenance. Currently, AC induction motors are simple, low-cost, highly reliable, and widely used worldwide. To control the speed of AC induction motors, a complex controller is needed—a frequency converter (VFD) capable of both frequency and voltage conversion. To understand how a VFD works, it is essential to first understand the working mechanism of an induction motor. The following text will describe the working principle of an AC induction motor in detail. The operation of an asynchronous induction motor is similar to that of a transformer. When the stator (fixed external windings) is connected to a three-phase power supply, a magnetic field rotating at the set power supply frequency is established. This electromagnetic field passes through the air gap between the stator and rotor, inducing a current in the rotor windings. Thus, when the current and the changing magnetic field interact, a force (torque) is generated on the rotor, causing it to rotate. If the number of pole pairs in the windings increases, the rotational frequency of the magnetic field will decrease (i.e., two poles = 50/60Hz = 3000/3600rpm, while four poles = 50/60Hz = 1500/1800rpm). However, if the rotor rotates at the same speed as the rotating magnetic field, no induced magnetic field is generated, and therefore no torque is produced. Since the rotor current must be used to generate the output torque, the rotor always rotates slower than the rotating magnetic field. This difference in speed is called slip, typically around 3%. The motor speed depends on the applied power supply frequency, the winding arrangement, and to some extent, the load. Therefore, to control the motor speed, the power supply frequency must be controlled; if the frequency decreases, the voltage must be reduced, otherwise the motor's magnetic field will saturate. If the motor speed increases proportionally with the increase in frequency, and the motor magnetic field is under-excited due to low voltage, resulting in reduced torque, then both frequency and voltage must be controlled simultaneously during speed regulation. Let's analyze the basic principle of AC motor speed regulation. We know that the speed formula for a three-phase asynchronous motor is: n = 60f/p(1-s) Equation (1-1) In Equation (1-1): n1—stator rotating magnetic field speed, also called the synchronous speed of the asynchronous motor (r/min); n—actual rotor speed (r/min); f—power supply frequency (Hz); p—number of pole pairs of the motor. The ratio of the difference between the synchronous speed and the actual speed n of an asynchronous motor to the synchronous speed is called the slip of the asynchronous motor, denoted by s: s = (n1 – n) / n1 = 1 – n / n1 (1-2) n1s = n1 – n n = n1(1-s) n = (1-s) × 60f1 / p From equation (1-2), we can know the speed formula of the asynchronous motor. From the speed formula of the asynchronous motor, we can see that there are several methods for speed regulation of the asynchronous motor: (1) Pole-changing speed regulation, that is, changing the number of pole pairs p of the asynchronous motor. (2) Slip-changing speed regulation. (3) Frequency-changing speed regulation, that is, changing the power supply frequency f. From the analysis of the above three speed regulation methods, it can be seen that adjusting the speed of the three-phase asynchronous motor by changing the motor frequency is the best control method. Of course, other methods can also be used for speed regulation (such as cascade speed regulation of wound rotor asynchronous motors, speed regulation by changing stator voltage, and speed regulation by electromagnetic slip clutch). However, these are all measures taken in the past before frequency converters existed, and they are now obsolete. II. Basic Principles of Frequency Converters A frequency converter is basically composed of numerous devices, including: control circuit, external I/O circuit, drive circuit, detection and protection circuit, bridge rectifier circuit, intermediate DC circuit, and inverter circuit. During operation, the rectifier circuit first converts the input three-phase AC to DC. The connection between the rectifier and the inverter is called the DC circuit. Then, the control circuit issues speed control commands based on user-set data and I/O board information, directing the drive circuit to control the inverter's output conversion, and finally converting the DC current into three-phase AC with variable frequency and voltage. Figure 2-2 below shows the complete main circuit diagram of an AC-DC AC commercial frequency converter. [align=center] Figure 2-2[/align] III. Rectification and Inverter Main Circuit of a Frequency Converter In any AC-DC-AC frequency converter, to achieve AC-to-DC conversion and DC-to-variable frequency AC conversion, high-power diode rectification or controllable diode rectification is required in the input circuit. The output uses a high-power controllable switching transistor IGBT inverter module. The three-phase power supply is rectified by a full-wave rectifier and then supplied to the capacitor in the DC circuit. The capacitor reduces voltage fluctuations and acts as a filter and buffer (especially in single-phase applications), and can continue to provide energy in the event of a short-term power outage. The voltage applied to the capacitor is uncontrollable and depends on the peak value of the AC power supply voltage. The control circuit converts the DC voltage to AC using pulse width modulation (PWM) technology. The ideal waveform is established by the output IGBT switching at a fixed frequency. By changing the switching time of the IGBT, the ideal current can be obtained. The output voltage is still a series of square wave pulses, which the inductance of the motor windings transforms into a sinusoidal motor current. Because the inverter uses PWM control technology, high-order harmonics are generated in the input and output circuits, interfering with the power supply system, load, and other nearby electrical and instrumentation control equipment. In engineering applications, inverter harmonic interference problems are frequently encountered. The following will detail the mechanism of harmonic generation, propagation path, and effective methods for suppressing interference. IV. Harmonic Generation Mechanism of Frequency Converter The main circuit of a general frequency converter is usually an AC-DC-AC conversion circuit. When the frequency converter is working, it introduces a non-sinusoidal current from the power supply. This is because the input rectifier of the frequency converter generates harmonics when converting the AC voltage and current of the power supply into DC voltage and current. (1) Input rectifier: Introduces current from each phase of the three-phase power supply. The introduced current is non-sinusoidal, unlike the standard AC sine wave, and is distorted during the rectifier conversion process. Distortion means that the power supply current waveform contains harmonics, because the distorted waveform can be decomposed into a set of sinusoidal components, which are called harmonic components. Since the power supply is a symmetrical standard three-phase power supply, and the rectifier section is an open-loop uncontrollable system, the number of harmonics is constant, so the harmonic components generated by the rectifier section are very few. (2) Output inverter: The DC current is filtered by the filter capacitor and inverted into AC with variable frequency and voltage by the high-power transistor switching element IGBT. In the inverter output circuit, the output current is a pulse waveform modulated by the PWM carrier signal. For GTR high-power inverter elements, the PWM carrier frequency is 2-3kHz, while the PWM carrier frequency of IGBT high-power inverter elements can reach up to 15kHz. Similarly, the output circuit current can also be decomposed into a fundamental wave containing only a sine wave and other harmonics. The main harmonic orders are the first harmonic (fundamental, 50Hz), the fifth harmonic (250Hz), the seventh harmonic (350Hz), the eleventh harmonic (550Hz), and the thirteenth harmonic (650Hz). From the working mechanism of the inverter, we can see that most of the harmonics and interference in the frequency converter are generated in the inverter. High-order harmonic currents have direct interference to the load (when acting on a motor, the motor heats up and makes an unpleasant noise; when acting on an analog instrument, the instrument's pointer and reading jump erratically; these phenomena are all caused by interference). In addition, high-order harmonic currents can also radiate into space through cables, interfering with nearby electrical equipment. V. Methods for Suppressing Harmonic Interference in Engineering The core of the frequency converter is to generate PWM or SPWM control signals of a certain width and polarity through high-speed power electronic switches. These pulse signals with very steep edges will generate strong electromagnetic radiation, especially in the output circuit. They will spread their energy in various ways, interfering with the normal operation of surrounding equipment and seriously exceeding the limit value requirements of electromagnetic compatibility standards. Therefore, frequency converter manufacturers have manufactured some special equipment for users to suppress the electromagnetic interference generated by the frequency converter in order to meet the quality inspection standards and ensure the safe operation of the frequency converter and other equipment. The propagation paths of harmonics are roughly of two types: conduction and radiation. The main way to solve conduction interference is to filter out or isolate the high-frequency current conducted in the circuit. The solution to radiation interference is to shield the radiation source or the line being interfered with. The specific commonly used methods are as follows: (1) The power supply of the frequency converter system is independent of the power supply of other equipment, or an isolation transformer is installed on the input side of the frequency converter and other electrical equipment to cut off the harmonic current. (2) The cable between the motor and the frequency converter should be laid in steel pipe or armored cable, and should be laid separately from other low-voltage signal cables in different cable trenches to avoid radiation interference. (3) Shielded cables should be used for signal lines, and they should be staggered from the main circuit control lines of the frequency converter by a certain distance (at least 20cm) to cut off radiation interference. (4) The frequency converter should use a dedicated grounding wire, and a thick, short wire should be used for grounding. The grounding wires of other electrical equipment nearby must be separated from the frequency converter wiring and short wires should be used. This can effectively suppress the radiation interference of current harmonics to nearby equipment. Interference problems caused by incorrect grounding frequently occur when using frequency converters. Therefore, it's crucial to first distinguish between "safety grounding" and "electromagnetic interference grounding," especially in high-frequency regions. Due to the "skin effect," the connection point will exhibit high impedance, leading to poor grounding and amplifying external interference signals, making the system more sensitive to external influences. Therefore, low-impedance connections are necessary to prevent electromagnetic interference grounding. Specifically, the following two points should be noted: 1) When using the chassis as a common ground, remove all paint or other coatings from the connection points to ensure a low-impedance connection. 2) Use short, flat wires to connect different grounding points (as flat wires have better high-frequency, low-impedance characteristics). Regularly check all grounding points to prevent them from becoming loose or detached. The frequency converter is installed in a control cabinet, which shields it from radiated energy and prevents external electromagnetic interference. During installation, ensure the chassis, cable shielding, and motor casing are reliably connected. (5) Add an input reactor, generally a passive reactor should be used, the purpose of which is to allow only specific frequency signals to pass through and block interference signal sources. It also helps to absorb the surge voltage generated when heavy-duty equipment is put into operation and the voltage spike of the main power supply. A 2% impedance input reactor is usually sufficient to absorb voltage peaks and can prevent harmful shutdown of the frequency converter in most applications. It can also protect the DC circuit capacitors inside the frequency converter from overheating and reducing their lifespan due to absorbing surge leakage voltage, and increase the power supply impedance. A 4% impedance input reactor is most suitable for reducing the harmonic frequency current generated by the frequency converter, and can also prevent interference signals from propagating through the ground wire and being reflected back to the interference source. When installing the input reactor, it should be as close as possible to the frequency converter and share the same base plate with it. If the distance between the two exceeds the standard specified in the frequency converter instruction manual, a flat cable should be used for connection. (6) The output reactor is installed at the output terminal (U, V & (W) This allows the motor to operate with long cables. Connecting a reactor in series at the inverter output solves the problems of motor overheating and noise pollution. Using an output reactor eliminates the need for a shielded cable between the inverter and the motor. This not only reduces costs but also effectively suppresses interference generated by the inverter, a major advantage of using a reactor. The output reactor's inductance compensation compensates for the phase-to-phase and phase-to-ground distributed capacitance of the motor cable. As the motor cable length increases, the total distributed capacitance of the motor cable also increases. This distributed capacitance, along with residual voltage peaks at the inverter output (due to IGBT switching), causes peak currents to flow back to the inverter. These peak currents can cause harmful tripping of the inverter, which can be avoided by installing an output reactor. The reactor is installed at the inverter output to allow operation with long cables, compensating for the distributed capacitance in the cable. (7) Other measures. 1) Use shielded cables to weaken electromagnetic induction and electrostatic induction. 2) Connect capacitors of less than 100n between the positive and negative terminals of the DC side of the inverter circuit and the ground wire. This method is particularly suitable for electrical equipment with a power consumption of less than 7.5KW. 3) Separate control lines, signal lines and power lines. The distance should usually be more than 200mm. If the control line must cross the power line, the crossing angle should be as close to 90 degrees as possible. VI. Insulation test during inverter maintenance Insulation test has been performed on the inverter at the factory. New inverters should not be tested for insulation. However, after long-term operation, if insulation test is required during maintenance, follow the steps below. Otherwise, the internal components of the inverter may be damaged. First, connect the main circuits of the inverter according to Figure 3-1. If the wires are connected incorrectly, the inverter will be damaged. [align=center] Figure 3-1[/align] (1) Use a DC 500V megohmmeter. Test under the condition of disconnecting the main power supply. (2) Disconnect all control circuit connections to prevent experimental voltage from entering the control circuits. (3) Connect the main circuit terminals with a common line as shown in Figure 1-1. (4) Apply the megohmmeter voltage only between the main circuit common connection line and ground (terminal). (5) A megohmmeter reading ≥ 5M ohms is considered normal and qualified. VII. Conclusion With the continuous development of the national economy, the application of AC variable frequency speed control devices and frequency converters in all walks of life has been increasing dramatically, and they can be said to have been applied to everyone's daily life (such as in household appliances: air conditioners, refrigerators, washing machines, etc.). Meanwhile, high-priced, maintenance-intensive DC speed control devices have been gradually phased out by the tide of time and technology. AC variable frequency speed control can be seen as a testament to the progress of humankind. However, behind this great progress lie a series of technical challenges (such as: improving the efficiency of frequency converters, suppressing power grid interference, suppressing analog signal interference in automatic control, enabling continuous overload operation for extended periods, and standardizing the parameters, communication interfaces, and communication protocols of frequency converters from various manufacturers), all of which await our consideration and solutions. Author Biographies: Wang Lanna, Assistant Engineer, Engineering Construction Department, Tianjin Alkali Plant; National Registered Second-Class Construction Engineer; Lian Shouying, Assistant Engineer, Tianjin Keyuanjia Fire Protection Engineering Co., Ltd.