1. Introduction The adoption of ultra-large synchronous motor drives instead of turbine drives in large blast furnace ironmaking blowers has become a global trend in ironmaking equipment development. This is thanks to advancements in power electronics, microelectronics, and computer technology, and the successful use of AC-DC-AC frequency converters. This has solved the soft-start problem of ultra-large synchronous motors, requiring only about 25% of the motor's power for smooth starting, thus avoiding the unbearable impact on the power grid caused by asynchronous starting of synchronous motors. The smooth starting process accelerates to quasi-synchronous speed (95% of rated speed ne) in about 200 seconds, then connects to the grid and enters synchronous operation. In May 2003, Ansteel's first electric blower was successfully connected to the grid, supplying air to the new No. 1 blast furnace. The blower's drive motor is an ultra-large synchronous motor with a rated power of 42 MW. The motor starts using a frequency converter for soft starting, and the control system uses the Siemens DISC computer control system. Siemens DISC is a core technology of Siemens frequency converters. The synchronous motor and starting frequency converter of the blower of the new No. 1 blast furnace are connected in a one-to-one configuration, while the new No. 2 and No. 3 blast furnaces, which will soon be put into operation, adopt a one-to-two configuration. That is, one frequency converter can drive two synchronous motors for time-sharing starting. The Siemens frequency converter soft starter introduced by Ansteel is a new version from Siemens, and its hardware and software technology level is significantly improved compared with similar equipment introduced by other domestic steel companies. Learning and mastering these related technologies is of extremely important practical significance for production maintenance and future development. 2 Technical Data and Composition of Frequency Converters AC-DC-AC current type frequency converter for soft starting of ultra-large synchronous motors. 2.1 Main technical data (1) Rated voltage: 2×2.9kV, 3-phase voltage fluctuation range: +10%～-10%; (2) Rated frequency: 50hz±2%; (3) DC link power: 2×4.8mw; (4) Frequency control range: 1:10; (5) Normal operating ambient temperature: +5℃～40℃; (6) Under normal ambient temperature conditions, it can be started 3 times in a row, with a 60min interval between the 4th start. 2.2 The frequency converter and its power section mainly include: (1) The rectifier on the input side and the inverter on the motor side both use a 6QC7 fully controlled three-phase bridge; (2) There are no fuses on the rectifier side and the inverter side of the frequency converter; (3) The thyristors are indirectly triggered by optical fibers, and each thyristor has a feedback signal; (4) The inverter side is equipped with lem electronic current transformers (high-precision DC current transformers in each frequency range); (5) The reactor in the DC link has a sufficiently large inductance to reduce current ripple and limit the rate of change of current; (6) Overvoltage limiters are installed at both the input and output terminals of the frequency converter; (7) The starting frequency converter transformer needs to be configured at the input and output terminals of the frequency converter to achieve an optimized match with the grid voltage and motor voltage, where: Step-down transformer (incoming line side): Resin-cast dry-type three-winding transformer, 11400kVA, 10.0kV/2×2.9kV, 50Hz, UK≤13.5%; Step-up transformer (motor side): Resin-cast dry-type three-winding transformer, 11400kVA, 2×2.9kV/10.0kV, 50Hz, UK=8.5%. The system controller uses the Simadyn D control system, a fully digital, freely configurable multi-microcomputer system specifically designed for system calculation and rapid open-loop/closed-loop control. 3. Synchronous Motor Soft-Start Principle The soft-start principle of the synchronous motor adopts AC-DC-AC frequency conversion technology. The frequency converter is current-type, meaning it has a DC reactor with a large inductance in the DC link. This reactor provides filtering and prevents sudden current changes when a short-circuit fault occurs on the inverter side. The current regulator responds quickly, shifting the thyristor firing angle of the rectifier circuit backward, thus limiting the current to a safe range. Because the power supply uses a three-phase bridge rectifier circuit, the inverter output current has a large harmonic component, which causes additional heat generation and torque pulsation in the motor. Furthermore, the frequency converter generates a large common-mode voltage, which affects the motor's insulation. To address these issues, the system employs 12-pulse rectification technology. DC pulsation technology is also used during soft-start. Due to the applied excitation current in the rotor of a synchronous motor, an induced electromotive force (EMF) is generated in the stator when the rotor rotates. When this EMF acts in the reverse direction on the thyristors on the inverter side, the thyristors turn off, and this EMF can be used to achieve natural commutation of the inverter thyristors. However, when the motor speed is very low (e.g., below 5%ne), the stator EMF is too low to turn off the thyristors and achieve natural commutation. To solve this problem, DC pulsation technology is used. In other words, during the initial startup phase of the motor, when the motor speed is below 5%ne, and the inverter thyristors require commutation, the inverter current is reduced to zero, temporarily turning off all the inverter thyristors. Then, pulses are applied to the thyristors that should be turned on according to the triggering sequence. When the DC current is restored, the current flows through the newly turned-on thyristors in the triggering sequence, thus achieving commutation from one phase to another. Because the inverter thyristors conduct sequentially, the DC current flows sequentially through the corresponding windings of the motor stator, generating a composite magnetic field. This continuous change in winding current will inevitably generate a rotating magnetic field in the motor, driving the rotor to rotate. The rotor speed is determined by the inverter's triggering cycle. When the motor speed reaches above 5%ne, and the electromotive force generated by the motor stator is large enough, the inverter thyristors use natural commutation. The starting torque generated by the motor rotor will cause the motor to continue to increase its speed until it reaches 95%ne, at which point the motor will be connected to the grid and brought into synchronization (if the grid connection conditions are met). The inverter then exits the system, thus achieving soft starting of the synchronous motor. Using soft-start technology, the motor must be started and connected to the grid under no-load conditions. The required power is only about 9000 kW, far less than that of asynchronous start and rated power (42000 kW), so the impact on the power grid is minimal. According to relevant data, if the motor loses synchronization and the switching fails due to grid voltage drop at the moment of synchronous switching, the frequency converter is capable of "catching up" with the motor again at any speed, accelerating it to synchronization until the switching is successful. During normal operation, if excitation is present, and the motor loses synchronization for more than 200 ms due to short-term overload or 10 kV voltage loss, the frequency converter will receive a signal from the excitation controller and automatically restart, capable of "catching up" with the motor rotor at any speed, accelerating it to synchronization until it switches back to the grid. These two points are essential for the safe and continuous blast furnace air supply. 4. Soft Starter Control 4.1 Soft Start Control Principle and Process The soft-start Simadyn D digital control system applies vector principles and uses open-loop and closed-loop control to control the soft-start process. The purpose of using vector control is mainly to improve the dynamic performance of the frequency converter. Based on the dynamic mathematical model of an AC motor, the stator current of the AC motor is decomposed into magnetic field components (current) and torque components (current) using coordinate transformation, and controlled separately. This mimics the control method of a naturally decoupled DC motor, controlling the magnetic field components and torque components separately to achieve dynamic performance similar to DC motor speed regulation. In vector control, the actual values ​​of the magnetic field current and torque current can be calculated by transformation based on the measured actual values ​​of the motor stator voltage and current. The actual values ​​of the magnetic field current and torque current are compared and adjusted with their corresponding setpoints. Open-loop control includes: control when the motor speed is ≤5% of the rated speed; control of opening and closing short circuits; and logic control between pressure, temperature, and various protection interlocks. Closed-loop control includes: current control and speed control; the system is designed as a speed loop control with current closed-loop control, i.e., a dual closed-loop system; by controlling the rectifier on the power supply side, the motor flows with the corresponding current to obtain the torque required to maintain the motor torque. Square wave current flows into the motor stator through the inverter. A magnetic field current flows through the motor rotor. Due to the rotor's rotation, a spatially varying magnetic field is generated, inducing an electromotive force (EMF) in the motor stator. At low speeds, the excitation current remains constant, and the stator voltage is only proportional to the speed. To determine the sequence of stator currents (the triggering sequence of the inverter thyristors), the stator voltage is measured (absolute value, phase angle), and then trigger pulses are generated for the inverter, enabling natural commutation. The commutation voltage is provided by the synchronous motor. Between 0 and 5% of rated speed, the motor voltage is very low, preventing natural commutation. To ensure reliable commutation, DC pulsation technology is employed. The DC link current is periodically reduced to zero, triggering the inverter thyristors periodically according to a set value, driving the rotor to rotate. When the motor voltage is higher, natural commutation is achieved. The trigger signal for the inverter thyristors from one phase to another is obtained from the synchronous voltage. The zero-crossing point of the synchronous motor voltage is measured and used as the trigger signal for the motor-side inverter. This ensures that the thyristor triggering of the motor-side inverter is always synchronized with the motor voltage, keeping the synchronous motor synchronized. When the actual speed of the motor is less than the set speed, the speed detector outputs a signal to the current controller. The current controller changes the firing angle of the rectifier thyristor, increasing the output DC current, increasing the motor torque, and increasing the motor speed until the electromagnetic torque of the motor balances with the load torque. When the motor speed reaches the quasi-synchronous speed, a signal is sent to the synchronizer. The synchronizer begins detection and comparison. When the synchronization condition is met, the synchronizer issues a command to close the circuit breaker, the synchronous motor is connected to the grid, the soft starter exits, and the soft start process is completed. Soft start open-loop and closed-loop control are implemented in the Simadyn D control system. All control function files are installed in eight processors. Each processor executes a function package for a specific task. The function package's function is defined by parameters and a strucg diagram. 4.2 Function Packages The Simadyn D system also includes a function package for establishing communication between the processor and peripheral devices (@-fp). (1) Module SE21.2: Interface module between processor PS16 and motor-side thyristor, used to measure actual values, detection values ​​and thyristor status; (2) Module SE48.1: Interface module between processor PM16 and power supply-side thyristor, used to measure actual values ​​and thyristor status; (3) Module SA60: Synchronization module, used to measure motor voltage and generate pulse synchronization signals after calculation; (4) Module SAV22: Trigger module, connected to thyristor via optical fiber (to isolate control equipment from high-voltage equipment), provides trigger signal to thyristor, and monitors thyristor status using optical signal; (5) P1 (PM16): Completes closed-loop speed control and corresponding phase sequence control; (6) P2, P3 (PG16): Perform closed-loop control of power supply-side thyristor; (7) P4 (PS16): Performs control of motor-side thyristor; (8) P5, P6 (PM16): Completes startup process diagnosis; (9) p7, p8 (pm16): Perform thyristor status diagnosis; (10) cs7: Communication module; (11) em11: Analog and digital mixed input/output; (12) ea12: Analog output; (13) ts12: Trigger module. 4.3 Sequential control of motor starting process (1) Detect the start-up conditions of the main machine and auxiliary equipment; (2) The DCS sends a start-up command to the S7-300 (motor running PLC); (3) The S7-300 sends a start-up command to the Simadyn D; (4) The Simadyn D closes the start-up transformer circuit breaker; (5) Detect that the circuit breaker is closed; (6) Open and close loop control; (7) Apply excitation; (8) Check whether the excitation is normal; (9) Send the start-up trigger pulse signal; (10) Apply trigger pulse to the thyristor on the power supply side; (11) Apply trigger pulse to the thyristor on the motor side; (12) The synchronizer detects synchronization; (13) After synchronization, the synchronizer sends the network access command. 5. Excitation System The synchronous motor of the electric blower uses separately excited brushless excitation. The excitation power supply is taken from the 380V low-voltage operating power supply, which is converted into adjustable AC power by a low-voltage frequency converter and input to the stator of the excitation motor. The exciter rotor is coaxial with the synchronous motor rotor, and the rotor winding is connected to the synchronous motor rotor excitation winding via diodes to provide DC excitation current to the winding. The excitation current is controlled by the Simadyn D system during soft start-up and by the S7-300 after grid connection. During startup, a large excitation current should be maintained to prevent the motor from losing steps and to generate a large voltage in the motor stator for accurate motor speed measurement, thus completing the closed-loop control for soft start-up. After grid connection, there are three excitation control methods: constant excitation current control, constant motor voltage control, and constant power factor control. To ensure the motor does not lose steps, constant power factor control should also be used for motor excitation during normal operation. 6. Conclusion Since commissioning, the blast furnace blowers have consistently started successfully on the first attempt and operated continuously. The entire startup process is identical to normal operation, with minimal electromagnetic noise and vibration from the motor. Furthermore, the system boasts comprehensive monitoring functions and a user-friendly human-machine interface, enhancing its reliability. Since commissioning, there has never been a production disruption caused by a malfunction in the motor control system.