1 Introduction Brushless synchronous motors, due to the absence of slip rings, carbon brushes, and other potentially spark-generating components, are widely used in explosive environments such as chemical plants and coal mines. Furthermore, the elimination of easily damaged components like carbon brushes significantly improves the overall reliability of the motor, making it suitable for applications requiring continuous, long-term, high-reliability operation. In recent years, with the shift towards energy conservation, a large number of brushless synchronous motors are facing frequency conversion retrofitting. Due to the inherent technical characteristics of brushless synchronous motors, problems encountered during frequency conversion speed regulation, such as field weakening excitation, reverse phase operation, and inverter-exciter coordinated control, have consistently hindered their application in frequency conversion. This paper, based on the structure and principle of brushless synchronous motors, provides a detailed analysis of various problems encountered in frequency conversion operation and proposes appropriate solutions based on theoretical analysis and simulation results, enabling frequency converters to drive brushless synchronous motors reliably and economically. 2 Structure and Power Frequency Operation of Brushless Synchronous Motors 2.1 Structure of Brushless Synchronous Motors The structure of a brushless synchronous motor is shown in Figure 1. [align=center]Figure 1. Structure of a brushless synchronous motor[/align] In the figure: 1 is a sliding bearing, 2 is the brushless synchronous motor winding, 3 is a cooler, 4 is a rotating rectifier, and 5 is an excitation generator. 2.2 Excitation System Structure of a Brushless Synchronous Motor The excitation system structure of a brushless synchronous motor is shown in Figure 2, in which the excitation generator rotates coaxially with the synchronous motor. [align=center]Figure 2. Excitation System Structure of a Brushless Synchronous Motor[/align] The rotating rectifier is responsible for the demagnetization and excitation logic during the motor starting process, and its internal structure is shown in Figure 3. When the motor starts, the rotating rectifier controls the demagnetizing thyristor t4 to connect the demagnetizing resistor RF to the rotor excitation winding of the brushless synchronous motor to provide a larger starting torque and reduce the voltage at the excitation winding terminals. At this time, rectifier thyristors t1 to t3 are cut off. When the motor reaches subsynchronous speed and meets the quasi-angle condition, the controller triggers rectifier thyristors t1 to t3 to rectify the armature voltage of the excitation generator and apply it to the excitation winding of the synchronous motor, providing a continuous excitation current for the synchronous motor, while simultaneously turning off the demagnetizing thyristor t4. At this time, the rotating rectifier is equivalent to a three-phase diode uncontrolled rectifier. 2.3 Steady-State Operation of Brushless Synchronous Motors During steady-state operation at power frequency, the exciter supplies an appropriate excitation current to the stator winding of the exciter generator, inducing a three-phase AC voltage at the rotor armature winding end of the generator. This voltage is rectified into DC voltage by a rotating rectifier (equivalent to a diode rectifier) ​​and applied to the rotor winding of the brushless synchronous motor, providing a continuous excitation current. Based on the physical characteristics of the exciter generator, its output armature voltage is approximately proportional to the product of the motor speed and the exciter generator's excitation current. Therefore, the exciter can adjust the stator excitation current of the exciter generator by adjusting the thyristor firing angle, thereby regulating the rotor excitation current of the brushless synchronous motor. 2.4 Power Frequency Starting and Excitation Process of Brushless Synchronous Motors The power frequency starting and excitation process of a brushless synchronous motor is shown in Figure 4. [align=center]Figure 3 Rotating rectifier structure diagram 4 Brushless synchronous motor power frequency starting process[/align] During power frequency starting, the high-voltage circuit breaker closes first. The demagnetizing circuit of the rotating rectifier connects the demagnetizing resistor to the excitation winding of the synchronous motor based on the induced voltage on the excitation winding, and the synchronous motor gradually accelerates. After the high-voltage circuit breaker closes, the exciter triggers the thyristor to supply a certain excitation current to the stator excitation winding of the exciter generator. As the motor speed increases, the voltage of the rotor armature winding of the exciter generator gradually increases. When it exceeds the minimum operating voltage of the rotating rectifier, the rotating rectifier controller powered by it is energized. The rotating rectifier monitors the induced voltage on the excitation winding of the synchronous motor. When its period is greater than the preset value (indicating that the synchronous motor has reached subsynchronous speed) and reaches the reverse zero crossing point, it triggers the rectifier thyristor, turns off the demagnetizing thyristor, and applies the rectified rotor armature voltage of the exciter generator to the excitation winding of the synchronous motor, completing the excitation. After a brief settling process, the motor enters a stable synchronous operating state, and the motor starting process is complete. 2.5 Power Frequency Shutdown Process of Brushless Synchronous Motor During power frequency shutdown, the high-voltage circuit breaker is disconnected, and at the same time, the exciter adjusts the firing angle of the thyristors to the active inverter region, rapidly reducing the stator excitation current of the exciter generator to zero. The rotor armature winding voltage of the exciter generator drops rapidly. When it is lower than the minimum operating voltage of the rotating rectifier, the rotating rectifier control circuit is de-energized, its rectifier thyristors are cut off, and the freewheeling diode connects the demagnetizing resistor to the excitation winding of the synchronous motor, rapidly reducing the excitation current of the synchronous motor to zero. The synchronous motor gradually comes to a stop under the action of load and resistance torque, etc. 3 Variable Frequency Operation of Brushless Synchronous Motor 3.1 The characteristics of the exciter generator during speed regulation operation are different from the slip ring direct excitation of the brushed synchronous motor. The excitation current of the brushless synchronous motor is generated by the rotating exciter generator. Since the voltage generated by the exciter generator is proportional to the product of the motor speed and the stator excitation current of the exciter generator, when the motor speed is significantly lower than its rated speed, the voltage generated by the exciter generator is lower. Even if the exciter outputs the maximum excitation current to the exciter generator, the excitation current of the brushless synchronous motor will still be less than its rated value. At very low speeds during initial startup, the brushless synchronous motor will not receive any excitation current. When the synchronous motor starts with no excitation current under variable frequency conditions, its stator armature winding will absorb a large inductive reactive current from the frequency converter (typically about 2 to 3 times the motor's rated current). This current flows only between the frequency converter and the synchronous motor, without being injected into the power grid, but it will cause short-term heating of the frequency converter and the stator armature winding of the synchronous motor. Therefore, when the motor speed is low, the current should be applied to the stator excitation winding of the exciter generator as large as possible to minimize the motor's starting current. 3.2 Excitation and Stepping Process During Variable Frequency Start-up The variable frequency start-up process of the brushless synchronous motor is shown in Figure 5. [align=center]Figure 5. Variable Frequency Start-up Process of Brushless Synchronous Motor[/align] After the high-voltage circuit breaker closes, the variable frequency drive (VFD) is powered on. Upon receiving the "start" command, the VFD starts outputting voltage to the stator armature winding of the synchronous motor from 0.5 Hz, and gradually increases the frequency and amplitude of the output voltage according to the preset acceleration time and V/F curve, thus starting the synchronous motor under no-load conditions. Simultaneously with the VFD outputting voltage to the stator armature winding of the synchronous motor, it instructs the exciter to begin outputting a strong excitation current to the stator excitation winding of the exciter generator. This current is greater than the rated excitation current of the exciter generator but less than the maximum short-time excitation current of the exciter generator. At this time, the rotor armature winding current of the exciter generator is approximately zero. After the VFD starts, the synchronous motor, relying on its salient pole torque and rotor residual magnetism, enters synchronous operation after a brief asynchronous acceleration and synchronizing process (approximately 1-2 seconds). Since the synchronous motor has no excitation current at this time and operates solely based on salient pole torque and rotor residual magnetism, its stator current is relatively large, approximately 2 to 3 times the motor's rated current. As the motor accelerates, the voltage induced in the rotor armature winding of the excitation generator gradually increases. When this voltage exceeds the minimum operating voltage of the rotating rectifier, the rotating rectifier control power supply is energized. Because the synchronous motor is operating in synchronous mode at this time, the slight oscillation of its rotor angle induces a low-frequency voltage in the excitation winding, the period of which satisfies the slip frequency criterion. Upon detecting this voltage, the rotating rectifier immediately triggers the rectifier thyristors, injecting excitation current into the rotor excitation winding of the synchronous motor. Since the motor speed is still relatively low at this time, the armature voltage of the excitation generator is low, resulting in a low excitation current output to the synchronous motor. After excitation is applied, the stator armature current of the synchronous motor will decrease. As the motor accelerates, the voltage output by the excitation generator gradually increases, and the excitation current received by the synchronous motor also gradually increases, until the stator armature current of the synchronous motor gradually decreases below the rated current. Once the motor speed reaches the minimum operating speed (lower limit of the speed regulation range), the frequency converter instructs the exciter to adjust the stator excitation current of the exciter generator to the maximum continuous operating excitation current (or rated excitation current) of the exciter generator. The motor starting process is complete, and the motor can run under load at this speed, or accelerate to the desired speed according to process requirements. Since the above motor starting process is relatively short (typically about 30 seconds), and the winding temperatures of the exciter generator and synchronous motor are usually not high before starting, this process will not cause overheating of the exciter generator and synchronous motor. 3.3 Speed ​​Regulation Range For brushless synchronous motors, because the voltage output by the exciter generator is lower at low speeds, the excitation current obtained by the synchronous motor is smaller, and its maximum output torque (out-of-step torque) is smaller. Therefore, it is necessary to determine its minimum operating frequency based on the motor's excitation-torque characteristics. Generally, in order to improve the speed regulation range of the motor, at low speeds, the exciter outputs the maximum continuous operating excitation current of the exciter generator to the stator excitation winding of the exciter generator. At this time, the rotor armature winding of the exciter generator will output the maximum induced voltage at that speed (still less than the rated armature voltage at its rated speed), and the synchronous motor can also obtain the maximum excitation current at that speed. Based on the output voltage of the exciter generator at its maximum continuous operating excitation current, the resistance of the rotor excitation winding of the synchronous motor, and the excitation-torque characteristic curve of the synchronous motor, the maximum output torque (out-of-step torque) of the synchronous motor at each speed can be calculated. Generally, the minimum operating speed of the synchronous motor can be determined according to the principle that the maximum output torque of the synchronous motor is not less than 1.3 times the peak load torque under that speed condition (typically 60% to 70% of the rated speed of the motor). 3.4 Excitation Regulation (1) When the synchronous motor is running at the lowest speed, it will absorb a certain amount of inductive reactive current from the frequency converter, and the power factor will lag. As the speed increases, its power factor will gradually increase until unity power factor (pf=1). In order to reduce losses and improve system efficiency, the exciter should output its maximum continuous operating excitation current to the stator excitation winding of the excitation generator before the power factor reaches the unity power factor. (2) When the speed of the synchronous motor increases further, it will send an inductive reactive current to the frequency converter, and the power factor will lead. At this time, in order to reduce losses and improve system efficiency, the frequency converter will communicate with the exciter according to its output power factor to reduce the excitation current output by the exciter to the excitation generator, so that the synchronous motor can operate at the unity power factor. 3.5 Frequency converter shutdown process After receiving the "stop immediately" command, the frequency converter will stop outputting voltage to the stator armature winding of the synchronous motor and notify the exciter to demagnetize. The excitation current of the excitation generator will decrease rapidly, and the excitation current of the synchronous motor will decrease rapidly through the demagnetizing resistor. The motor will gradually stop under the action of load and resistance torque. 3.6 Loss-of-Step Protection When the frequency converter detects a loss of step in the synchronous motor, it immediately stops outputting voltage to the stator armature winding of the synchronous motor, simultaneously notifying the exciter to demagnetize, and reporting the fault. 4 Conclusion This paper, based on the structure and principle of brushless synchronous motors, analyzes in detail the problems that may be encountered when brushless synchronous motors are operated by frequency converters, and proposes a frequency converter operation mode for brushless synchronous motors. Under this mode, the frequency converter can drive the brushless synchronous motor to operate with reliable and economical speed regulation.