Today, with the growing acceptance of national energy conservation policies, the application of AC motor speed control technology has entered a development stage dominated by variable frequency speed control. Its application, especially in the speed control of low-voltage, small- and medium-capacity motors, has been recognized as the best energy-saving equipment on the demand side of electricity, representing the future direction of electrical drive development. Under this belief, the application of low-voltage frequency converters in the speed control of large and medium-capacity motors is no longer uncommon, with the maximum power range of a single unit reaching 220–560kW. However, negative effects are gradually emerging. 1. The Harm of Frequency Converter Harmonies As is well known, the input side of low-voltage frequency converters, whether VVVF control, vector control, or direct torque control, is a rectifier circuit with nonlinear characteristics. The resulting high-order harmonics distort the current and voltage waveforms of the input power supply. The output side of the frequency converter is a pulse-width modulated AC current composed of a series of rectangular waves, approaching a three-phase sine wave. Both the output voltage and current waveforms contain high-order harmonics. Under variable frequency speed control (VFD) conditions for AC motors, not only do the power electronic components inside the VFD generate heat loss and the fan consumes electrical energy, but the harmonic currents and harmonic magnetomotive forces generated on the VFD output side can also cause the motor to produce a pulsating rotating magnetic field, reducing output torque. This results in copper losses, iron losses, and stray losses in the power cable and the stator and rotor of the motor, leading to decreased motor efficiency, motor heating, power cable heating, and increased line losses. The effects of these harmonics are particularly significant for low-voltage, large- and medium-capacity motors, and in severe cases, can significantly shorten the lifespan of the motor and power cables. The operating efficiency of VFDs is only indicated in a few instruction manuals; for example, Emerson VFD manuals state an operating efficiency of ≥93% for general-purpose VFDs with power ratings of 45 kW and below, and ≥95% for general-purpose VFDs with power ratings of 55 kW and above. Typically, one to three cooling fans are installed at the top of the VFD to dissipate the heat generated by the internal power electronic components. In high-power variable frequency control cabinets, it is also necessary to consider installing cooling fans at the cabinet's exhaust vents. When the inverter's output power cable exceeds 80m, the motor power is large, or the power supply voltage waveform distortion is severe, or the power transformer capacity is large, the inverter also needs to be equipped with input, output, and DC reactors. This not only increases the voltage drop of the power line (the input reactor voltage drop is about 4.4V; the output reactor voltage drop is about 9V), reducing the motor's output torque, but also acts as a significant heat source within the variable frequency control cabinet. Simultaneously, the inverter's failure rate will increase exponentially with rising ambient temperature; for every 10°C increase in ambient temperature, the inverter's lifespan is halved. 2. Comparison of Variable Frequency Speed ​​Regulation and Power Frequency Electric Drive Data shows that compared to power frequency electric drive, variable frequency speed regulation increases the total motor loss by about 30%, current by about 10%, and temperature rise by about 20% due to the presence of harmonics. When the motor operates below its rated speed under variable frequency conditions, the reduced speed of the self-cooling fan decreases the cooling airflow, leading to a decrease in cooling efficiency and thus exacerbating the motor's overheating. Meanwhile, due to the influence of harmonics, the voltage distribution on the motor windings becomes very uneven. Furthermore, the high voltage change rate of the variable frequency rectangular wave makes inter-turn short circuits in the windings highly likely. In addition, the noise generated by harmonics also damages the insulation of the motor windings. With the widespread use of frequency converters, harmonic pollution is becoming increasingly severe, not only seriously affecting power quality and reducing power transmission and utilization efficiency, but also increasingly exposing the overheating of motors and power transformers. Even under rated current, the temperature rise can exceed the allowable level, endangering the safe and reliable operation of electrical equipment. When selecting output power cables for frequency converters, if the specified cross-sectional area is chosen based on the power frequency supply conditions, the presence of harmonic current components in the cable will generate additional copper losses and stray losses, significantly increasing heat generation. If a larger cross-sectional area cable is selected, the cable's capacitance to ground increases, leading to a larger leakage current to ground. For each increase in cross-sectional area, the frequency converter's output current should be reduced by 5% of its rated capacity. 3. Quantification, Pollution, and Control of Inverter Harmonics While the impact of the aforementioned harmonics may not be very significant for variable frequency drive systems with power ratings of 200 kW and below, its impact on inverters with power ratings of 220 kW and above should not be underestimated. For example, the fundamental current of Mitsubishi's A240 series inverters at a rated voltage of 400V is as follows: 355A at a rated power of 220 kW; 403A at a rated power of 250 kW; and 450A at a rated power of 280 kW. It is evident that the proportion of harmonic current is substantial, inevitably radiating into space through the inverter's input power lines, output cables, and grounding wires, interfering with electrical equipment connected to the same power supply along the lines. In particular, spatial radiation has varying degrees of impact on nearby communication equipment, computer monitoring and control systems, and signal lines, and in severe cases, can render the affected equipment inoperable. Currently, although active or passive filters can compensate for harmonic currents generated by high-voltage or low-voltage frequency converters, and general-purpose series of small-power products have entered the market, they are ultimately only a temporary solution and not a fundamental one. Moreover, the filters themselves consume a certain amount of electrical energy and also introduce new problems. The root cause lies in the negative effects resulting from the rapid voltage or current changes caused by the high-speed switching of the filter and the inverter within the frequency converter. 4. Several Suggestions for the Application of Frequency Converters in Large and Medium-Capacity Electric Drive Systems As mentioned above, it is clear that the harmonic problem of frequency converters must be controlled or mitigated at its source. However, since the advent of frequency converters, the harmonic problem has not been fundamentally solved at its source, nor has it been thoroughly resolved through the integrated design concept of the motor and frequency converter. Therefore, in order to reduce the impact of the above-mentioned harmonic currents on the power grid, nearby electrical equipment, communication cables, and variable frequency speed control motors and their input and output power cables from the source, this paper proposes the following suggestions when selecting technical design schemes for power frequency electric drives or variable frequency drive systems with large and medium capacity motors: (1) For large and medium capacity electric drive application systems, high-voltage motors should be selected to drive the load in order to reduce the operating current of the motor. Table 1 is a comparison table of the technical and economic indicators of the application of the same capacity 280kW 0.4kVY355L1-4 series low-voltage three-phase asynchronous motor and 6kVY series medium-sized high-voltage three-phase asynchronous motor. Table 1 Comparison of Economic Guidance for Three-Phase Asynchronous Motors: As can be seen from Table 1, if a low-voltage motor is selected, not only will the line loss, motor copper loss and iron loss increase, but the investment in power cables will also be considerable and the failure rate will be higher than that of a high-voltage motor of the same capacity. At the same time, a power transformer needs to be configured to provide low-voltage power, increasing the land area and infrastructure investment. However, if a high-voltage motor is selected, not only will the line loss, motor copper loss and iron loss decrease significantly, but the investment in high-voltage power cables and the failure rate will also be significantly reduced. Furthermore, there is no need to configure a power transformer to provide low-voltage power, increasing unnecessary infrastructure and installation costs. (2) For the speed regulation system of large and medium capacity fans and water pumps, under the condition that the speed regulation range is not wide and the speed regulation accuracy is not high, it is recommended to select a high-voltage internal feed chopper speed regulation system, which includes an AC power speed controller and a chopper internal feed speed regulation motor. Currently, the technical performance indicators of domestically produced 6-10kV YQT series internally fed speed-regulating motors and ZNT-2000 chopper internally fed AC power speed-regulating control devices are as follows: speed range of (60%-100%) of rated speed; starting current of (2.5-3.0) times the rated current; control mode with both open-loop and closed-loop control functions, digital adjustment of given parameters, and convenient realization of closed-loop control of pressure and flow. The biggest advantage of this system compared to high-voltage frequency converter speed regulation systems is that it controls the electromagnetic power of the motor rotor to achieve speed regulation. Therefore, the AC power speed regulation control device for high-voltage motors not only has lower voltage, lower power, and lower harmonic content in the motor stator current, but also requires less equipment investment, has significant energy-saving effects, relatively stable motor output torque, and significantly reduced electromagnetic interference to the power grid and nearby electrical and communication equipment. A detailed comparison of the technical and economic indicators of high-voltage frequency converters and power speed regulation systems is shown in Table 2. Table 2 Comparison of Technical and Economic Indicators of High Voltage Variable Frequency Drives and Power Speed ​​Control Systems: (3) For power transmission speed control systems in industries such as electric locomotives, ships, steel rolling, hoisting, and papermaking with large and medium capacity, there are not only speed control requirements, but also stringent requirements such as fast load dynamic response speed, high steady-state accuracy, and large low-speed torque. At present, high-performance, large-capacity high voltage variable frequency drives produced by several well-known foreign variable frequency drive manufacturers have relatively mature technologies, but the products are generally extremely expensive, and there are also some problems that are not suitable for the national conditions, such as inadequate technical data, training, and after-sales technical services; while the technology of this product in China is still in the research stage, more than 20 companies have the production capacity of high voltage variable frequency drives with large and medium capacity, and domestic brands account for more than 30% of the domestic market share. As long as the system reform, human resource mobilization, capital investment, production management, and business decision-making are on track, the technology of high-performance high voltage variable frequency drives with large and medium capacity in China will inevitably catch up. Therefore, it is recommended that users determine the preferred choice based on their specific conditions and selection principles in the above-mentioned power transmission speed control systems. 5. Conclusion In summary, for large and medium capacity electric drive systems, considering the principles of energy conservation and resource saving, whether the electric drive system is speed-regulating or non-speed-regulating, high-voltage motors should be prioritized to drive the load in order to constitute a more complete technical solution.