1. Introduction Due to the advantages of AC synchronous motors (hereinafter referred to as synchronous motors) in terms of reliability and maintenance, power factor, motor size and moment of inertia, control accuracy, and field weakening ratio, countries worldwide have largely adopted synchronous motors for large-capacity motors. For example, high-power air compressors, water pumps, high-power hoists in the coal and non-ferrous metals industries, and large-capacity rolling mills in steel plants all use synchronous motors for drive. Speed ​​regulation of AC synchronous motors is a major challenge in the field of electrical drives. China began research on AC synchronous motor speed regulation technology in the 1970s, and by the early 1980s, a prototype of an AC-AC variable frequency synchronous motor had been successfully developed. However, high-power AC variable frequency speed control devices only began to develop in the late 1990s. Currently, there are very few successful examples of domestically produced high-power variable frequency devices applied to synchronous motors; foreign brands dominate the market. Although the application of variable frequency speed control for large synchronous motors in China is still in its early stages, it is widely used abroad. Their long-term operational experience shows that the application of high-voltage, high-power variable frequency speed control systems offers good economic benefits and guarantees reliability. Therefore, the future market prospects for high-voltage synchronous motor variable frequency speed control devices in my country are enormous. Since 1999, the Electrical Institute of the Shanghai Power Equipment Complete Set Design Institute has conducted extensive research on domestic and international high-voltage variable frequency devices, collected and compared data, and established a general technical solution, adopting a unit series multi-level approach. After several years of unremitting efforts, a 1250kW/6kV high-voltage variable frequency device prototype was successfully developed at the end of 2002, and passed the product technical appraisal hosted by the Shanghai Municipal Economic Commission in July 2003. It officially went into operation in September 2003, and the product received numerous honors upon its launch. At the end of 2005, our company successfully developed and produced the largest domestically produced high-voltage variable frequency device, with a capacity of 5000kW, which was successfully put into operation at Ningxia Duowei Pharmaceutical Co., Ltd. In 2006, our company began developing a 9600kW/10kV high-voltage variable frequency device, and in early 2007, it successfully applied for and obtained a special project from the National High Technology Research and Development Program (863 Program). The company has successfully developed a high-voltage variable frequency speed control device for MAXF synchronous motors, which was successfully put into operation on the synchronous motor of the air compressor at Henan Xinlianxin Fertilizer Co., Ltd. in October 2007. 2. Analysis of the difficulties in variable frequency speed control technology of synchronous motors The main difference between synchronous motors and ordinary asynchronous motors is that when a synchronous motor is running, the angle between the armature voltage vector and the rotor magnetic pole position must be within a certain range, otherwise the system will lose synchronization. Therefore, when the synchronous motor is variable frequency speed control, this angle must be controlled to change within the allowable range at all times. This is the main difference between variable frequency of synchronous motors and variable frequency of asynchronous motors. The following will briefly introduce the difficulties encountered in the variable frequency speed control process of synchronous motors and the corresponding solutions of the MAXF variable frequency device: (1) The starting and excitation process of synchronous motors There are usually two ways to start a synchronous motor: one is to excite first and start synchronously; the other is to start asynchronously and then excite in the same polarity. For the variable frequency start of synchronous motors, the excitation is first energized and then started synchronously, but the rotor position is often incorrectly judged, resulting in the failure of motor start-up. For synchronous motor variable frequency speed control retrofitting, asynchronous starting and polarity-based excitation are easily adopted. Therefore, the MAXF frequency converter performs asynchronous soft starting of the synchronous motor to achieve the rated starting torque, starting the synchronous motor to approximately 8Hz before polarity-based excitation. The specific excitation magnitude and frequency can be determined according to different application scenarios. At this point, after a small amount of damped oscillation, the angle between the rotor magnetic field and the stator magnetic field of the motor reliably attracts the stator magnetic poles, and the synchronous motor enters synchronous operation. The frequency converter gradually accelerates to the given frequency according to the preset acceleration. At this time, the angle between the synchronous motor armature voltage vector and the rotor magnetic pole position gradually increases to a constant value, and the motor rotor magnetic poles gradually accelerate to the desired speed under the attraction of the stator magnetic field, completing the synchronous motor starting process. For operating conditions requiring heavy-load starting, to achieve a greater starting torque, the output voltage of the frequency converter and the excitation current of the synchronous motor can be appropriately increased. (2) During the steady-state speed regulation and excitation regulation process of the synchronous motor, in order to solve the coordination between the frequency converter and the synchronous motor, the frequency converter will also adjust the current excitation current and change the corresponding output voltage value when the motor speed changes (not a simple constant V/F control). When operating above a certain set frequency point, the frequency converter collects the power factor of the synchronous motor and controls the excitation current of the synchronous motor in real time through the built-in PID controller to achieve constant power factor regulation. When the power factor is 0.90 (leading), the frequency converter sends a 4-20mA command to the excitation regulator of the synchronous motor to regulate the excitation current. When operating below this frequency range, the excitation current is adjusted by the frequency converter according to the current operating conditions by outputting a 4-20mA signal to the excitation regulator, which uses frequency conversion to adjust the excitation current. The switching of the adjustment mode is automatically completed by the frequency converter, and the switching frequency of the adjustment mode can be set by parameters. After the inverter is installed, the reactive current of the synchronous motor only flows between the synchronous motor and the inverter and does not enter the power grid. Therefore, the adjustment of the excitation current during variable frequency speed regulation does not need to concern the reactive current of the synchronous motor. (3) Normal shutdown and fault demagnetization process of synchronous motor During normal shutdown, the inverter drives the synchronous motor to the stop speed and then stops the inverter output. During deceleration, when running above the constant power factor frequency point, the excitation current is adjusted according to the constant power factor. When running below the frequency point, the variable frequency and variable excitation current mode is used. This process does not require demagnetization. If a fault occurs during operation, if there is a problem with the external system of the inverter and an emergency shutdown is required, the high voltage side input switch qf and the inverter output switch km2 can be directly tripped, and the synchronous motor excitation device can be tripped at the same time. If there is a problem with the inverter system and an emergency shutdown is required, the inverter immediately stops output, notifies the synchronous motor excitation device to perform thyristor inverter demagnetization, and then trips the high voltage side input switch and the inverter output switch km2 through the fault signal. (4) During the speed regulation of a synchronous motor, the damping winding may experience a momentary inconsistency between the synchronous speed of the power supply and the actual speed of the motor rotor during the acceleration and deceleration process. This will induce a voltage and generate a current in the damping winding of the synchronous motor. Therefore, before the frequency conversion modification, it is necessary to check whether the screw connections inside the damping winding are secure, and it is best to weld them to reduce the internal resistance of the winding. In this way, even if a large induced current occurs during the speed regulation process, it will not generate a lot of heat and thus damage the motor damping winding. When the MAXF series synchronous machine frequency converter is running, it will take full control of the excitation regulation of the synchronous machine, including excitation, changing the excitation magnitude and de-excitation. The original excitation device is only an actuator, and the specific excitation magnitude is controlled by the MAXF frequency converter through a 4-20mA signal. 3. Modification Scheme of High Voltage Variable Frequency Device for MAXF Series Synchronous Motors 3.1 Requirements of High Voltage Variable Frequency Device for Synchronous Motors Due to the differences between synchronous and asynchronous motors, the requirements for high voltage variable frequency devices when modifying synchronous motors for variable frequency speed regulation are also different. The specific requirements are as follows: (1) It can solve the problem of synchronous motor starting and synchronizing; (2) It can solve the coordinated control of output voltage and excitation current during synchronous motor speed regulation; (3) The harmonics of output voltage and current of the variable frequency device should be as small as possible. 3.2 Implementation Scheme The variable frequency speed regulation device used for the modification of synchronous motor variable frequency speed regulation system should have a power frequency bypass circuit design. After the modification, it should not affect all operations and protections of the original power frequency mode. A conversion switch is used in the design, which has two positions: power frequency mode and variable frequency mode. When the selector switch is turned to the "Variable Frequency" position, the bypass switch qf3 of the variable frequency drive trips, qf1 and qf2 close after a 2-second delay, and the excitation system control circuit switches to variable frequency mode. When the switch is turned to the "Power Frequency" position, the bypass switches qf1 and qf2 of the variable frequency drive trip, qf3 automatically closes after a 2-second delay, and the excitation system control circuit reverts to power frequency mode. 4. Brief Introduction to the Application of Domestic High-Voltage Variable Frequency Drives in Synchronous Motors The synthetic ammonia production workshop of Henan Xinlianxin Fertilizer Co., Ltd. has five 800kW/10kV synthetic circulating air compressors operating in parallel, with three in operation and two on standby. The number of operating compressors is manually adjusted according to production volume. Due to production process requirements, the return valves of the operating air compressors frequently open, resulting in significant energy waste. Furthermore, the operators experience high labor intensity, and regulating valve malfunctions are frequent. For this reason, the company decided to upgrade the synchronous motor of its No. 2 synthetic circulating air compressor with frequency conversion speed regulation. After extensive investigation and research, the company ultimately selected a high-voltage frequency conversion speed regulation device for synchronous motors jointly developed and manufactured by the Shanghai Power Equipment Complete Set Design Institute (hereinafter referred to as Shanghai Complete Set Institute) and Shanghai Keda Electromechanical Control Co., Ltd., product model: MAXF1250-10000/1250. The main reason is that the company has previously used the high-voltage frequency conversion speed regulation device for asynchronous motors produced by Shanghai Complete Set Institute, and the device is reliable in operation, features online power unit replacement, uses long-life non-polar power capacitors, and has perfect automatic voltage balancing technology. 4.1 Economic Benefits of Frequency Conversion Upgrade Since the frequency converter for the synchronous motor of the company's No. 2 synthetic circulating air compressor was put into operation on October 27, 2007, it has been operating normally, all parts of the frequency converter are functioning normally, and the appearance of the synchronous motor frequency conversion speed regulation cabinet is shown. (1) Operating parameters • The inverter load is about 540kw, the excitation current is about 135a, and the power factor is about 0.90. • The frequency generally operates at about 41hz, the input current is about 32a, and the output current is about 45a. • The excitation current and power factor are relatively stable. (2) Process operation After the inverter is put into operation, the bypasses of several circulating machines have been closed, and the main bypass regulating valve is also closed. When the production operator needs to adjust the circulation volume according to the temperature of the production system and the synthesis tower, he only needs to adjust the inverter frequency on the DCS by mouse. The adjustment is convenient and accurate. There is no need to adjust the bypass regulating valve, which can greatly reduce the maintenance of the bypass regulating valve. (3) Energy saving statistics The power consumption before and after the inverter is put into operation is as follows: ● Before the inverter is put into operation, from September 15 to October 26, the average daily power consumption is 52529.7 kWh. System parameters: Makeup gas flow rate: 76487 m³/h; System pressure: 20 MPa; System differential pressure: 1.90 MPa; Circulating hydrogen: 57.9%. ●After the frequency converter was put into operation, from October 28th to November 28th, the average daily power consumption was 49444.4 kWh. Source: Power Transmission and Distribution Equipment Network. System parameters: Makeup gas flow rate: 77046 m³/h; System pressure: 20.7 MPa; System differential pressure: 1.95 MPa; Circulating hydrogen: 57.6%. Since the company's four circulating compressors operate in parallel, sharing the gas compression load, the power saving calculation can only compare the total power consumption of the four compressors. Based on the comparison of operating data before and after the frequency converter was put into operation, the average daily power saving was 3085.3 kWh. Annual electricity savings of 1,079,855 kWh (calculated based on 350 operating days per year); annual electricity cost savings of 358,511.86 yuan (calculated at an electricity price of 0.332 yuan/kWh); cost recovery in approximately two and a half years. 4.2 Comprehensive Benefits of Variable Frequency Drive Upgrade This variable frequency speed control upgrade not only saved the company a significant amount of electricity but also greatly improved the regulation performance of the parallel operation system of multiple air compressors. Practice has verified that the effects of variable frequency speed control in a multi-machine parallel air compressor system are as follows: (1) It can achieve soft start, greatly reducing the starting current (less than 10% of the rated current), avoiding the impact of insulation aging caused by large starting current and mechanical shock caused by large electric torque on the motor life, reducing the amount of motor maintenance work, and saving maintenance costs; (2) Using variable frequency speed control avoids frequent loading and unloading of equipment, extending the life of the air compressor; (3) After using variable frequency speed control, the operating speed of the air compressor is reduced; (4) It ensures stable air supply pressure and improves air supply quality; (5) It saves a lot of electricity, saving users a lot of electricity expenses; (6) Automatic control, simple and efficient, reliable and self-protected, no need for special personnel to watch over it. 5. Conclusion Speed ​​control of synchronous motors is a major problem in the electrical drive industry. The application of high-voltage variable frequency speed control has been a research topic of variable frequency manufacturers in China in recent years. Through this frequency conversion upgrade, it was found that the frequency conversion speed regulation of the air compressor synchronous motor not only meets the normal frequency conversion speed regulation operation of the synchronous motor, but also enables the reciprocating air compressor system to operate at constant pressure, achieving significant energy savings. Furthermore, the MAXF series synchronous high-voltage frequency converter smoothly regulates the synchronous motor. Its unique features include its starting method and its excitation regulation method. It uses asynchronous starting followed by direct polarity excitation, which is highly reliable and eliminates the possibility of starting failure. Moreover, it provides stable excitation regulation for the synchronous motor, achieving constant power factor regulation.