I. Technical Background In recent years, the speed control application of AC asynchronous motors has developed rapidly. Compared with AC asynchronous motors, synchronous motors have inherent advantages. Asynchronous motors, due to the need for excitation, must draw lagging reactive current from the power supply, resulting in a very low power factor under no-load conditions. Synchronous motors, on the other hand, can change the input power factor by adjusting the DC excitation current of the rotor, allowing them to either lag or lead. When COSθ = 1, armature copper losses are minimized, saving on the capacity of transformer and frequency converter devices. Because the rotor of a synchronous motor has independent excitation, it can operate at extremely low power supply frequencies. Therefore, under the same conditions, the speed control range of a synchronous motor is wider than that of an asynchronous motor. Asynchronous motors require increasing slip to increase torque, while synchronous motors only need to increase the power angle to increase torque. Synchronous motors have a stronger tolerance to torque disturbances and can make faster dynamic responses. Due to the advantages of AC synchronous motors in reliability and maintenance, power factor, motor size and moment of inertia, control accuracy, and field weakening ratio, for large-capacity motors, countries worldwide have largely shifted towards using synchronous motors. For example, in industrial applications, 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 motor drives. In the domestic high-power AC synchronous motor drive sector, AC-AC frequency conversion speed regulation drives are the main type. These converter and frequency conversion drive devices have high power, generally ranging from hundreds to thousands of kilowatts. They are widely used in cement, mining and sewer, shipbuilding, metallurgy, chemical, oil and gas, power, pulp and paper, water supply and sewage treatment, coal, non-ferrous metals, and especially in the transmission and speed regulation of large mine hoists. High-capacity, low-speed, high-overload, fast-response, and four-quadrant operation transmission applications are mainly used in mine hoists and main rolling mills in steel plants. These applications place particularly stringent control requirements on frequency converters, which ordinary asynchronous motors and their frequency converters cannot meet. High-power synchronous motors are commonly used in these systems. Currently, my country's high-voltage, high-power synchronous motor control systems primarily employ AC-AC control frequency converters, along with AC-DC-AC methods. AC-AC frequency converters, due to their control principle limitations, have restricted functionality and application scope. The structure of an AC-AC frequency converter transforms grid frequency AC into adjustable frequency AC, belonging to direct frequency conversion circuits, and is widely used in high-power AC motor speed control transmission systems. Changing the switching frequency changes the output frequency; changing the conduction angle of the AC circuit changes the AC output voltage amplitude. As the output frequency increases, the number of grid voltage segments in one cycle of the output voltage decreases, resulting in severe waveform distortion. Voltage waveform distortion and the resulting current waveform distortion and torque pulsation are the main factors limiting the increase in output frequency. The relationship between output waveform distortion and the upper limit of the output frequency is difficult to define precisely. For example, when using a 6-pulse three-phase bridge circuit, the upper limit of the output frequency should not exceed 1/3 to 1/2 of the grid frequency. When the grid frequency is 50Hz, the upper limit of the output frequency of the AC-AC frequency converter circuit is approximately 20Hz. Another type of frequency converter is the AC-DC-AC high-voltage frequency converter. This type of frequency converter currently primarily drives high-voltage synchronous motors using V/F control. This method is suitable for applications where the speed ratio requirement is not high and the dynamic response is low. Its control method adopts the control strategy of an asynchronous motor. During startup: the high-voltage synchronous motor first undergoes asynchronous frequency conversion startup. When the speed approaches the synchronous speed, excitation current is applied to the rotor to bring the system into synchronous speed operation. The disadvantages of this control method are slow response, small speed ratio, inability to utilize the advantages of synchronous motors, inability to achieve four-quadrant operation, low output torque of the high-voltage synchronous motor, large starting current, and a tendency to lose synchronization. This type of frequency converter can only be used in applications with relatively light loads and small load variations. With the development and application of microelectronics technology, computers are used for control, enabling AC variable frequency control systems to evolve from analog to hybrid analog-digital, and further to fully digital systems. This allows for the software-based implementation of control schemes and strategies. In a fully digital control system, the control mode and parameters can be changed simply by modifying the software, greatly improving the system's versatility and flexibility, simplifying the hardware structure, and allowing the use of control algorithms based on modern control theory to enhance system performance. More importantly, with the development of modern communication, remote control, bus, and automation technologies, a fully digital approach is the inevitable trend for future development. Therefore, the application of fully digital AC-DC-AC high-power synchronous motor vector controllers is a future direction. However, research and application in this area are still lacking in my country. Advances in variable frequency control methods are mainly reflected in the shift from early static control to high-dynamic-performance four-quadrant vector control. This control method can effectively address the dynamic response and speed ratio requirements of high-voltage synchronous motors, enabling the system to output constant torque. II. Principle of the Fully Digital Vector Control Solution Given the shortcomings of existing technologies and the technical objectives to be achieved, the high-voltage synchronous motor vector control frequency converter we aim to implement consists of the following technical features and units: A three-phase high-voltage input phase-shifting isolation transformer, after phase-shifting isolation and step-down to multiple AC outputs, is input to the IGBT power unit with energy feedback for rectification and filtering into DC, which is then inverted by the IGBT and output to the synchronous motor; the signal from the motor's position and speed sensors is transmitted via high-speed serial encoding to the FPGA on the main control board for decoding and processing, and then sent to the DSP for data processing; current feedback is sampled by a Hall sensor, uploaded to the signal board, filtered by the analog signal processing circuit, and then uploaded to the AD sampling and processing unit on the DSP main control board; the main control board communicates with the host computer's human-machine interface in real time and reports various operating parameters and fault statuses of the system; the input/output signal unit board communicates with the main controller and handles the control functions of external input/output signals; the system principle is shown in Figure 1. [align=center]Figure 1 System Principle[/align] 1. AC-DC-AC Unit Series Multilevel Method Currently, most synchronous motor frequency converters in China use AC-AC converters. Compared with AC-DC-AC converters, they have disadvantages: complex driving thyristors; low output frequency range, only reaching 1/3 of the grid frequency; low power factor and serious harmonic pollution. In some control applications, the principle of AC-AC converters limits their high-speed application and slow dynamic response. The purpose of phase shifting in the AC-DC-AC method is to increase the pulse number of the rectifier equipment and reduce high-order harmonics on the grid side. The rectifier transformer uses secondary-side extended delta phase shifting. The AC-DC-AC method has a wide frequency speed regulation range. The power conversion circuit uses a multilevel converter (see Figure 2). Each stage of the power module uses an H-bridge IGBT drive method. Due to the large number of output levels and the increased steps in the output waveform, the modulation wave can be made closer to a sine wave, reducing voltage jumps and thus reducing harmonics. Another advantage is that the output voltage dv/dt is smaller, resulting in less impact on the load motor. Examples include some rolling mills, hoists, and winches. If an AC-AC converter is used, a deceleration mechanism must be added. However, AC-DC-AC converters allow for arbitrary frequency adjustment within a permissible range. This solves the aforementioned problem. 2. Energy Feedback-Based Power Units: Ordinary high-voltage frequency converters cannot be directly used in speed control systems requiring rapid start-up, braking, and frequent forward and reverse rotation, such as high-speed elevators, mining hoists, rolling mills, large gantry planers, winding mechanism tension systems, and machine tool spindle drive systems. This is because such systems require the motor to operate in four quadrants. When the motor decelerates, brakes, or lowers a potential energy load, it enters a regenerative power generation state. Due to the irreversible energy transfer of the diode-uncontrolled rectifier, the generated regenerative energy is transferred to the DC-side filter capacitor, producing a pump-up voltage. Since fully controlled devices such as GTRs and IGBTs have low withstand voltage, excessively high pump-up voltages may damage switching devices, electrolytic capacitors, and even motor insulation, thus threatening the safe operation of the system. This limits the application range of ordinary high-voltage frequency converters. Energy feedback-based systems solve these problems and achieve true energy saving rather than wasting energy. The power unit with energy feedback has three phases of the secondary step-down winding of the phase-shifting isolation transformer as input. The IGBT control signal is a PWM signal transmitted via optical fiber to control its conduction and cutoff. The output is connected in series with the unit and then sent to the motor. The principle is shown in Figure 2. [align=center] Figure 2 Schematic diagram of energy feedback unit[/align] 3. Digital vector control method The purpose of vector control is to improve torque control performance, but the final implementation is still the control of stator current. Since the physical quantities (voltage, current, electromotive force, magnetomotive force) on the stator side are all AC quantities, their spatial vectors rotate at synchronous speed in space, which is inconvenient for adjustment, control and calculation. Therefore, it is necessary to use coordinate transformation to transform each physical quantity from the stationary coordinate system to the synchronous rotating coordinate system. Observing from the synchronous rotating coordinate system, the spatial vectors of the motor become stationary vectors, and the spatial vectors on the synchronous coordinate system become DC quantities. According to the different forms of the torque formula, the relationship between the torque and the components of the controlled vector can be found, and the values ​​of each component of the controlled vector required for torque control—the DC setpoint—can be calculated in real time. By controlling these given values ​​in real time, the control performance of a DC motor can be achieved. Since these DC given values ​​do not exist physically and are fictitious, an inverse coordinate transformation process must be performed to return from the rotating coordinate system to the stationary coordinate system, transforming the aforementioned DC given values ​​into actual AC given values. The AC values ​​are then controlled in the three-phase stator coordinate system so that their actual values ​​are equal to the given values. Vector transformation control methods require the use of stationary and rotating coordinate systems, as well as vector transformations between these systems. Vector control of an AC synchronous motor involves transforming the three-phase stator stationary coordinate system (Ia, Ib, Ic) into two-phase stationary coordinate systems (α and β) (Clarke transformation, also called three-phase to two-phase transformation), and then transforming these two-phase stationary coordinate systems into a synchronous rotating field-oriented coordinate system (Park transformation). This is equivalent to DC currents Iq and Id in the synchronous rotating coordinate system (Id is equivalent to the excitation current of a DC motor; Iq is equivalent to the armature current proportional to the torque). Then, mimicking the control method of a DC motor, the control quantities of the DC motor are obtained. Through corresponding inverse coordinate transformations (Park inverse transformation) (Clarke inverse transformation), the synchronous motor is controlled. Essentially, it equates the AC motor to a DC motor, independently controlling the speed and magnetic field components. By controlling the rotor flux linkage and then decomposing the stator current to obtain the torque and magnetic field components, orthogonal decoupling control is achieved through coordinate transformation. Example Figure 3 shows the physical model of a two-pole synchronous motor. The stator three-phase winding axes A, B, and C are stationary. The three-phase voltages UA, UB, and UC, and the three-phase currents iA, iB, and iC are all balanced. The rotor rotates at a synchronous speed w1. The excitation winding on the rotor is supplied with an excitation voltage Uf, through which an excitation current If flows. The axis along the excitation pole is the d-axis, and the axis orthogonal to the d-axis is the q-axis. The dq coordinate system also rotates in space at a synchronous speed w1. The angle q between the d-axis and the A-axis is a variable. [align=center]Figure 3 Physical Model of Synchronous Motor[/align] Static Electrical Equations of Three-Phase Stator Windings: Transformation Equations from Three-Phase Stator Coordinate System to Two-Phase Static Coordinate System (Clarke Transformation) Transformation Equations from Static Coordinate System to Rotor Synchronous Rotating Coordinate System (Park Transformation) Transformation of the α and β two-phase static coordinate systems into synchronous rotating field-oriented coordinate systems d and q is shown in Figure 4 [align=center]Figure 4 Coordinate Transformation Diagram[/align] The synchronous motor adopts an improved space vector field-oriented control strategy. The control system adopts a dual closed-loop structure of speed loop and current loop. The current loop uses a PI regulator, which is simple to implement and can achieve good current tracking performance. The speed loop uses a PI regulator, which can effectively limit the overshoot of the dynamic response and speed up the response speed. The system adopts a dual closed-loop speed control system of speed and current. The key to the fully digital system is the digitization of the current loop, which is to realize the analog current loop in the hybrid digital-analog frequency converter system in a digital way. Its core is to improve the processing speed of the current loop to reach or approach the response speed of the analog current loop. Based on the current level of microprocessors, DSPs, and A/D devices, the hardware requirements can be met; the other aspect lies in the optimization of control strategies and control software. Good system hardware and software design is the guarantee for the practical application of the developed system. While meeting performance requirements, it is essential to fully utilize hardware resources, improve integration, reduce hardware costs, and achieve the goal of productization. The decoupling of the vector control system involves subtracting the speed feedback from the speed setpoint ω to obtain the speed error. This speed error is then regulated by a PI controller to output the torque current setpoint iq. The excitation current setpoint id is adjusted according to the dynamic needs of the system, with values ​​derived from experience based on different motors and loads. The motor's three-phase current feedback ia, ic, and ib are sampled by sensors and then subjected to Clarke transformation based on the rotor position electrical angle θ. The transformed values ​​are ialpha and ibeta. ialpha and ibeta are then transformed by a Parker transform to output id and iq. The error between the id and iq values ​​and the setpoints iqref and idref is calculated and regulated by a PI controller to output Vq and Vd. The voltage vector and rotor position electrical angle θ undergo inverse Parker and inverse Clarke transformations to output the motor stator three-phase voltages Va, Vb, and Vc. These three-phase voltages Va, Vb, and Vc are used as comparison values ​​for PWM (Pulse Width Modulation), and the output PWM waveform is sent to the inverter to drive the motor rotation. The control principle block diagram of the entire system is shown in Figure 5 above. [align=center] Figure 5 Vector Control Principle Diagram[/align] The excitation current of the synchronous motor in this scheme is If, which operates according to the fixed excitation current setting. For the given rotor excitation current If of the synchronous motor, the various parameters of the motor are measured and the required rated excitation current is calculated by testing the no-load characteristic and short-circuit same-type characteristics of the synchronous motor. At this time, the power factor of the system can be adjusted by adjusting the demagnetizing current Idref on the stator side according to the rated excitation current If. The power factor angle δ=arctan(iq)/(id) can be adjusted by controlling Idref to make the system operate with the power factor leading or lagging. 1) The hardware control of the main system is shown in Figure 5. A DSP (Digital Signal Processor) acts as the main CPU, while programmable logic devices (PLDs) handle algorithm calculations, waveform generation, and signal processing. An analog-to-digital (AD) converter samples and processes current and voltage feedback signals, transmitting them to the DSP. Communication between the unit and the main CPU uses high-speed fiber optic serial communication. Unit status information is serially encoded by the PLD and transmitted to the main controller's receiving board via fiber optic cable. The receiving board performs serial-to-parallel decoding and transmits the data to the main CPU. The main CPU adjusts the system's control state based on the unit status information. Signals from the speed and position sensors are serially encoded by the PLD on the sensor board and transmitted via high-speed serial transmission to the PLD on the main controller board. The PLD processes the speed and position signals from the sensors. A variable M/T (Mechanical/Transmitter) speed measurement method is used, achieving high-precision speed measurement. The PLD calculates the effective values ​​of speed and position, detects sensor malfunctions, and reports to the main CPU. Simultaneously, the main CPU can dynamically adjust the system according to speed measurement requirements. Speed ​​measurement method and time; the current detection element in this system is a Hall effect current transformer made based on the magnetic field compensation principle to meet the requirements of real-time current monitoring. The three-phase current and voltage signals of the motor are processed by the signal conditioning circuit and converted into analog voltage signals, which are then input to the AD conversion chip on the main control board. This AD chip can sample, hold, and convert the three-phase voltage and current signals instantaneously, thus ensuring the accurate reproduction of the transient three-phase voltage and current waveforms of the motor. After sampling, the AD conversion chip uploads the three-phase data. The main control CPU communicates with the host system using RS232 mode, receiving the parameter settings given by the host computer in real time and reporting the operating status and various data of the entire system. The external I/O inputs and outputs of the system are isolated and transmitted to the I/O port of the main control CPU. The main control CPU performs corresponding execution control according to the control requirements. The AT25128 is a serial EEPROM that communicates with the main control CPU using SPI mode. The EEPROM mainly serves to save the various setting parameter values ​​of the host system and store some operating status information of the system. The main control CPU uses a DSP. It is the TMS320LF2407A from TI's C2000 series. DSP is a high-speed microprocessor, whose biggest feature is its fast computing speed, which is at least an order of magnitude faster than the current 16/32-bit microprocessors and single-chip microcomputers. This high computing power of DSP can meet the high requirements of real-time control of current loop. It can simultaneously identify the rotor position and speed of the motor to achieve sensorless vector control requirements. It can also adopt advanced modern control strategies to obtain higher control performance and more complete functions. The principle block diagram of the entire hardware is shown in Figure 6. [align=center] Figure 6 Main controller structure diagram[/align] 2) Implementation principle of programmable logic device The communication between the unit module and the main controller adopts the fiber optic serial high-speed communication mode with a communication rate of 4MHz, which can meet the real-time control requirements. Each module communicates with the main control board in full-duplex mode. The PWM waveform signal issued by the programmable logic device is encoded and converted from parallel to serial, and sent to the unit module through fiber optic drive. At the same time, the programmable logic device receives the serial encoding of the unit, performs serial-to-parallel conversion, and uploads the unit's status information and fault signal to the main control DSP in interrupt mode, as shown in Figure 7. [align=center]Figure 7 Unit Communication Processing Module[/align] Speed ​​measurement of sensor signals: The serial input encoding is decoded to output the motor rotor position signal data. Speed ​​is measured based on the sensor's speed pulse signal using a variable M/T speed measurement method. The rotor speed is calculated based on the measured M and T values. Due to the use of programmable logic device hardware logic speed measurement, the speed measurement range is wide and the accuracy is high, meeting the system accuracy requirements. PWM signal generation: Based on the data output from the DSP, the programmable logic device generates data using a high-speed clock and performs phase shifting on the PWM waveform in series with the unit, outputting it to the fiber optic transmission module. Since the system adopts a fully digital control method, all control strategies are implemented by software programming. Therefore, the software design determines the overall system performance. The control strategy adopts a dual closed-loop system of speed and current, where the speed loop uses PI regulation and the current loop uses PI regulation. The algorithm is implemented by DSP digital signal processor software programming. The input of the speed loop is the difference between the speed feedback and the speed setpoint, and the output serves as the setpoint for the current loop. The output of the current loop controls the PWM waveform generator, which in turn controls the switching of power switching devices in the inverter to achieve motor speed regulation. The entire software processing system adopts a foreground/background processing mode, and the program's interrupt services use nested processing to ensure real-time signal processing. There are four interrupt sources: system protection interrupt, on-chip current loop timing processing interrupt, speed loop timing processing interrupt, and external communication interrupt. Upon power-up, the software system initializes, disables interrupts, clears various flags, configures the DSP's peripheral modules and I/O ports, reads parameter information from the EEPROM, calculates the motor's position signal and electrical angle, and performs a delay to check if high voltage is applied. Entering the system main loop; System protection interruption: Detecting the fault status of unit modules and system protection interruptions. In case of overcurrent, overvoltage, PLC, or other faults, the system shuts down the IGBT output, stops, and reports the system fault information; System main flow: After the system powers on, it sets parameters for the system's RAM space and various peripheral modules, clears the RAM, then resets and initializes the external I/O and PLC, reads the initial motor rotor positioning information from the EEPROM, and checks if the high voltage is ready. If ready, it enables various interrupts and enters the main loop; otherwise, it continuously checks the high voltage readiness status until the high voltage is ready. The main flow is shown in Figure 8. [align=center] Figure 8 Main Program Flowchart[/align] 3) Current Loop and Speed ​​Loop: Interruption handling in the speed and current loops; real-time monitoring of system speed information; speed command input via HMI; calculation of current speed command based on motor operating status and acceleration/deceleration time parameters; calculation of error based on speed command and feedback, followed by PID adjustment; output of iqref; iqref value limited by set maximum and minimum torque current values; output of iqref to the current loop as torque current command; Hall effect sensors detect ia and ic two-phase current feedback values, calculating three-phase... The current feedback value is used to calculate the current rotor position electrical angle θ based on the rotor position reflected by the position and speed sensor. The values ​​ia, ib, and ic are then processed by CLARKE to output iα and iβ. iα and iβ are then processed by PARK to output iq and id. The torque and demagnetizing current given from the speed loop output are compared with the feedback value to calculate the error, and PI regulation is performed to output Vq and Vd. Vq and Vd are then processed by inverse PARK to output Vα and Vβ. Vα and Vβ are then processed by inverse CLARKE to output Va, Vb, and Vc. The duty cycle values ​​of the three-phase Va, Vb, and Vc PWM are output to the FPGA. The drive waveform is output through the FPGA's PWM waveform generation module to the fiber optic driver, and then transmitted via fiber optic cable to each power unit module to control the IGBT switching. A partial flowchart is shown in Figure 9. [align=center] Figure 9 Speed ​​Loop Flowchart[/align] III. Experimental Verification After productization, the above technical solution was verified as feasible, and technical tests and evaluations were conducted on different devices. Example 1: A 6000V, 630KW, 6-pole synchronous motor was tested and verified on this equipment. The entire system can operate at a low speed of 0.01Hz and a high speed of 1.5 times the rated speed. The low-speed torque characteristics are stable, and it can operate at a constant torque of 0Hz. The power factor of the entire system is adjustable, as shown in Figures 10 and 11. Example 2: A 6000V, 630KW, 36-pole synchronous motor for a ball mill was also tested. Figure 10 shows the waveform recorded during the experiment with the ball mill synchronous motor under load. During the ball mill load experiment, the system started under heavy load, and the output waveform of the frequency converter was very stable. The system accelerated evenly during startup and there was no inrush current. It can be seen from the figure that the current leads the voltage, and the system operates in a state of leading power factor. The original ball mill synchronous motor started using a water resistor, resulting in an inrush current exceeding three times the rated load current, which posed a significant threat to the power grid. The replacement with a synchronous motor vector control frequency converter effectively solved the startup and operation problems. Figures 12, 13, and 14. [align=center] Figure 12 Load output waveform Figure 13 Synchronous motor driving the ball mill Figure 14 Ball mill for cement plants[/align] This article cites: *Electrical Machinery*, Electronic Industry Press, Charles Kingsley Jr., and *Speed ​​Control of AC Synchronous Motors*, Science Press, Li Chongjian. About the Authors: Du Xinlin (1968-), male, Chief Engineer, is currently the Executive Vice President and Chief Engineer of Hekang Yisheng Technology Co., Ltd., responsible for the company's R&D work. Shen Shijun (1974-), male, Engineer, works in the R&D department of Hekang Yisheng Technology Co., Ltd., engaged in the R&D of high-voltage frequency converters.