1. Introduction Variable frequency drive (VFD) technology arose to meet the need for stepless speed regulation of AC motors. Starting in the late 1960s, power electronic devices evolved from SCRs (Signal Transistors), GTOs (Gate Turn-Off Thyristors), BJTs (Bipolar Junction Transistors), MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), SITs (Stationary Induction Transistors), SITHs (Stationary Induction Thyristors), MGTs (MOS Controlled Transistors), and MCTs (MOS Controlled Thyristors) to today's IGBTs (Insulated Gate Bipolar Transistors) and HVIGBTs (High Voltage Insulated Gate Bipolar Transistors). This advancement in devices has spurred the continuous development of power conversion technology. From the 1970s onwards, research on pulse width modulation variable voltage variable frequency (PWM-VVVF) speed regulation attracted significant attention. In the 1980s, the optimization of PWM modes, a core aspect of VFD technology, attracted considerable interest, leading to the development of numerous optimization modes, among which the saddle-shaped wave PWM mode demonstrated the best performance. Starting in the late 1980s, VVVF frequency converters from developed countries such as the US, Japan, Germany, and the UK were introduced to the market and widely used. 2. Efficiency Analysis of Variable Frequency Speed ​​Control Systems 2.1 Efficiency and Losses of Frequency Converters Frequency converter efficiency refers to its conversion efficiency. Regarding the two types of frequency converters: AC-AC frequency converters, although highly efficient, have limited frequency range and applications. Currently, the most commonly used frequency converters are AC-DC-AC type. Their working principle is to first convert the industrial frequency AC power to DC power through a rectifier, and then use an inverter to convert it back to AC power of the required frequency. Therefore, the losses of a frequency converter consist of three parts: rectification loss accounts for approximately 40%, inverter loss accounts for approximately 50%, and control circuit loss accounts for 10%. The first two losses vary with the capacity, load, and topology of the frequency converter, while the control circuit loss does not change with the capacity or load of the frequency converter. The frequency converter adopts modern power electronic technology such as high-power self-turn-off switching devices. Its rectification loss and inverter loss are smaller than those in traditional electronic technology. According to the data provided in reference [1], the efficiency of the frequency converter is 86.4% to 96% when it is running at rated conditions, and it can be improved as the power of the frequency converter increases. 2.2 Changes in motor efficiency after frequency conversion speed regulation After frequency conversion speed regulation, the various losses and efficiencies of the motor change. According to the theory of motors, the losses of the motor can be divided into several types, such as core loss (including hysteresis loss and eddy current loss), bearing friction loss, wind resistance loss, stator winding copper loss, rotor winding copper loss, and stray loss. The expression for hysteresis loss in the iron core is: [img=100,48]http://www.cechinamag.com/images/Article/7aea2d9b-6b5a-4fae-80dd-6a8ed7744ede/221_new.gif[/img] This shows that hysteresis loss Pn is proportional to the alternating frequency f of the magnetic flux and proportional to the α power of the amplitude Bm of the magnetic flux density. For general silicon steel sheets, when Bm = 0.8～1.6W/m2, α = 2. According to the theory of fans and pumps, the relationship between the flow rate Q and the required motor shaft power P and speed n is: Q∝n; P∝n3; P∝Q3. After frequency conversion speed regulation, the reduction rate of hysteresis loss is slower than the reduction rate of the active power of the motor, and the proportion of loss increases. The eddy current loss expression is: Pe∝af²; where a=（Bm）²d²/rw; Bm is the amplitude of the magnetic flux density; d is the core thickness; and rw is the equivalent resistance of the eddy current loop. Bearing friction loss: Pz∝f¹.5 Wind resistance loss: Pf∝f³. The magnitude of stator winding copper loss and rotor winding copper loss is not directly related to the power supply frequency f, but higher harmonics and pulsating currents increase the copper loss of the motor. Stray losses and additional losses: Regardless of the type of frequency converter, after frequency conversion, in addition to the fundamental frequency, harmonics are generated. Many of these additional higher harmonics have torque directions opposite to the fundamental frequency torque direction. In addition, higher harmonics also increase eddy current losses. In summary, after frequency conversion speed regulation, the proportion of hysteresis loss, eddy current loss, bearing friction loss, stator and rotor copper loss, and stray losses in the power of the motor all increase. Relevant literature indicates that after frequency conversion speed regulation, the motor current increases by 10%, and the temperature rise increases by 20%. 3. Rational Selection of Variable Frequency Drive (VFD) Control Method The control method is crucial in determining the performance of a VFD. Currently, there are many brands of low-voltage general-purpose VFDs on the market, including approximately 50 types from Europe, America, Japan, and China. When selecting a VFD, do not assume that a higher grade is always better. In fact, as long as the load characteristics are considered and the requirements are met, it is sufficient to ensure appropriate use and cost-effectiveness. The parameters in the attached table are for reference during selection. [img=570,205]http://www.cechinamag.com/images/Article/7aea2d9b-6b5a-4fae-80dd-6a8ed7744ede/222_new.gif[/img] 4. Selection and Related Issues of Torque Control VFDs Based on their advantages of convenient speed regulation, energy saving, and reliable operation, variable frequency drives (VFDs) have gradually replaced traditional pole-changing speed regulation, electromagnetic speed regulation, and voltage regulation speed regulation methods. Several years after the introduction of PWM flux vector control inverters, DTC (Direct Torque Control) inverters emerged in late 1998. ABB's ACS600 series was the first generation of inverters to adopt DTC technology. It could accurately control speed and torque in an open-loop manner, and its dynamic and static performance was superior to that of PWM closed-loop control. Direct torque control uses measured motor current and DC voltage as inputs to an adaptive motor model. This model generates a set of precise actual torque and flux values ​​every 25μs. Torque and flux comparators compare the actual torque and flux values ​​with the given torque and flux values ​​to determine the optimal switching position. This demonstrates that it adjusts the switching state of the inverter circuit instantly by measuring torque and flux, thereby adjusting the motor's torque and flux to achieve precise control. 4.1 Selection Principles Before selection, the maximum input power required by the machine (i.e., the minimum rated power of the motor) must be determined based on the machine's requirements for speed (maximum, minimum) and torque (starting, continuous, and overload). P = n . T / 9950 (kW) Where: P—Required input power of the machine (kW) n—Mechanical speed (r/min) T—Maximum torque of the machine (Nm) Then, select the number of poles and rated power of the motor. The number of poles of the motor determines the synchronous speed. The synchronous speed of the motor should cover the entire speed range as much as possible to maximize the continuous load capacity. To fully utilize the equipment's potential and avoid waste, the motor can be allowed to exceed the synchronous speed for short periods, but it must be less than the maximum allowable speed of the motor. The torque is the maximum torque of the equipment under conditions such as starting, continuous operation, overload, or maximum speed. Finally, determine the parameters and model of the frequency converter based on the inverter's output power and rated current being slightly greater than the motor's power and rated current. It should be noted that the rated capacity and parameters of the frequency converter are specified for a certain altitude and ambient temperature, generally referring to altitudes below 1000m and temperatures below 40℃ or 25℃. If the operating environment exceeds these specifications, the resulting derating factor should be considered before determining the model based on the frequency converter parameters. 4.2 Selection of Inverter Capacity The selection of general-purpose inverters includes two aspects: inverter type and capacity selection. The general principle is to first ensure reliable fulfillment of process requirements, and then save costs as much as possible. Based on control functions, general-purpose inverters can be divided into three types: ordinary function u/f control inverters, high-performance u/f control inverters with torque control functions (also known as trip-free inverters), and vector control high-performance inverters. The selection of inverter type should be based on load requirements. For fan, pump, and similar applications with square torque (TL∝n²), the load torque is relatively small at low speeds, and ordinary function inverters are usually selected. For constant torque loads or machinery with high static speed requirements, high-performance inverters with torque control functions are more ideal. This is because such inverters have high low-speed torque, high static mechanical stiffness, are not afraid of load impacts, and have excavator-like characteristics. Examples include Fuji Electric's FRENIC5000G11/P11 and Sanken's SAMCO-L series. There are also examples of using ordinary type inverters. To achieve constant torque speed regulation with a large speed ratio, the capacity of the frequency converter is often increased. For production machinery requiring high precision, good dynamic performance, and fast response (such as papermaking machinery and rolling mills), vector control high-function general-purpose frequency converters should be used. Yaskawa's VS-616G5 series and Siemens' 6SE7 series frequency converters belong to this category. Most frequency converter capacities can be expressed from three perspectives: rated current, available motor power, and rated capacity. The latter two are often given by frequency converter manufacturers based on standard motors produced in their country or company, or decrease with the frequency converter's output voltage, making it difficult to accurately express the frequency converter's capability. When selecting a frequency converter, only the rated current is a key quantity reflecting the load capacity of the semiconductor frequency converter. Ensuring the load current does not exceed the frequency converter's rated current is a fundamental principle for selecting a frequency converter. It is important to emphasize that before determining the inverter capacity, the process conditions of the equipment and the parameters of the motors should be carefully understood. For example, the rated current of submersible pumps and wound-rotor motors is greater than that of ordinary squirrel-cage induction motors. Roller motors commonly used in the metallurgical industry not only have much larger rated currents, but they also allow for short-term stall operation, and most roller conveyor drives are multi-motor drives. Under fault-free conditions, the total load current should not exceed the rated current of the inverter. The frequency converter supplies pulsating current to the motor. Under rated operating conditions, the current supplied by the frequency converter is greater than that supplied by the mains power grid. Therefore, the current or power rating of the frequency converter should be one level higher than that of the motor, generally: Pnv ≥ 1.1Pn Where: PNV—rated power of the frequency converter, kW; PN—rated power of the motor, kW . 5. Issues to be considered in frequency conversion speed regulation design 5.1 Load matching issues Maximum energy saving for pumps and motors depends on matching the selected model and capacity with the actual load, including the matching between the pump and the motor. Avoid "oversized motor for undersized load"; generally, the design margin should be controlled within 10%. In China's industrial system design, there is often a tendency to increase the load by adding more components at each stage, preferring larger rather than smaller. It can be said that coefficients are added when specifying flow rate in the process, then again when selecting pumps and motors, resulting in extremely low actual operating efficiency of pumps and motors in some industrial systems. If variable frequency drive (VFD) speed regulation is added to the design of pump loads to achieve energy saving, it actually increases the number of VFDs, nominally increasing new energy losses and investment. Many current analyses of the energy-saving benefits of VFD speed regulation often neglect the efficiency of the VFDs, resulting in distorted theoretical calculations. In industrial production, pump loads are not constant due to variations in production load and seasonal changes; sometimes the range of variation is quite large. Some power plants also experience significant diurnal load variations due to peak-shaving needs. Moreover, after one year of use, the output flow rate of pumps will decrease compared to the rated flow rate. The effect of VFD speed regulation on pump loads with large flow variations is obvious, and the larger the load variation range, the better the energy-saving effect. 5.2 High-Order Harmonics High-order harmonics generated by VFDs can cause distortion of the power grid voltage waveform. The smaller the effective capacity of the power grid and the larger the capacity of the VFD, the more severe this effect. This pollution of the power grid can cause power capacitors, reactors, and transformers to overheat and generate electromagnetic resonance, leading to additional losses in motors and generators, and malfunctions in relays. Various countries have corresponding regulations for voltage distortion and harmonic control. China's GB12668-90 standard stipulates that the voltage distortion rate should be less than 10%, any odd harmonic should not exceed 5%, and any even harmonic should not exceed 2%. After using a frequency converter, local areas of the power grid may exceed national standards, so appropriate measures must be taken. 5.3 Motor Selection Because the carrier frequency of the frequency converter is higher, the motor windings must withstand higher impulse voltages. In addition, the motor efficiency decreases, and the cooling effect is reduced after the speed decreases; these are all factors to consider. Therefore, after frequency conversion speed regulation, damage to both the frequency converter and the motor is more common. If conditions permit, YTSP and YSG series dedicated frequency conversion speed regulation motors can be used. AC speed control drive systems composed of general-purpose frequency converters commonly use standard asynchronous motors. When using a PWM frequency converter to power an asynchronous motor, the stator current inevitably contains high-order harmonics, resulting in lower power factor and efficiency when the motor is running under no-load, and increased iron losses when running under load. Under rated load, the motor current increases by about 8%, and the temperature rise increases by about 20%. This is a problem that cannot be ignored for motors that have been working at full load or near full load for a long time. Therefore, the motor capacity should be increased appropriately to prevent excessive temperature rise and affect the service life of the motor. The heat dissipation capacity of the general standard squirrel cage asynchronous motor is based on the rated speed and self-fan cooling. When the motor is running at a constant torque load with speed regulation, its heat generation remains unchanged, but the heat dissipation capacity at low speed becomes worse. The method of adding a constant speed cooling fan or using a motor with a higher insulation level can be adopted. 5.4 External configuration of frequency converter and issues to be noted (1) Select a suitable external fuse to protect the rectifier components from damage caused by internal short circuit. After the frequency converter model is determined, if there is no fast fuse to protect the silicon components before the internal rectifier circuit of the frequency converter, a fuse and disconnect switch that meet the requirements should be configured between the frequency converter and the power supply. Air circuit breakers cannot be used to replace fuses and disconnect switches. (2) Select the input and output cables of the frequency converter. Select a three-core or four-core shielded power cable with a suitable conductor cross-section according to the power of the inverter. In particular, the power cable between the inverter and the motor must be a shielded cable and should be as short as possible to reduce electromagnetic radiation and capacitive leakage current. When the cable length exceeds the allowable output cable length of the inverter, the stray capacitance of the cable will affect the normal operation of the inverter, so an output reactor should be configured. For control cables, especially I/O signal cables, shielded cables should also be used. The length of the connecting cable between the inverter and the optional components of the inverter should not exceed 10m. (3) Install an AC reactor or EMC filter on the input side. According to the requirements of other equipment in the inverter installation site for power grid quality, if the inverter operation has affected the normal operation of these equipment, an AC reactor or EMC filter can be installed on the input side of the inverter to suppress harmonic distortion and conducted radiation caused by the switching of power components. If the neutral point of the transformer of the power grid connected to the inverter is not grounded, an EMC filter cannot be selected. When a frequency converter drives a motor with a voltage of 500V or higher, a dv/dt filter must be configured on the output side to suppress inverter output voltage spikes and voltage variations, which helps protect the motor. It also reduces capacitive leakage current, high-frequency radiation from the motor cable, high-frequency losses in the motor, and bearing current. When using a dv/dt filter, it is important to note that the voltage drop across the filter will cause a slight reduction in motor torque; the cable length between the frequency converter and the filter should not exceed 3m. 6. Conclusion The design of a variable frequency speed control system should consider the system's static and dynamic indicators, using the mechanical characteristics of the load as the main design parameters. Based on a focus on the selection of the frequency converter control method and the selection of the frequency converter itself, practical problems in the design of the variable frequency speed control system should be solved to ensure that the designed system has high static and dynamic indicators and a high performance-price ratio.