Abstract: In April 2007, I entered Guangzhou Haoda Electromechanical Co., Ltd. in Guangdong Province for pre-graduation comprehensive practice, working on frequency converters. This paper introduces the system structure and operation mode of frequency converter control in the spindle drive of a CNC lathe, and briefly describes the basic application of a sensorless vector frequency converter. Keywords: Vector control, Frequency converter, CNC lathe Introduction CNC lathes are typical mechatronics products, automated equipment integrating machine tools, computers, motors and their drives, automatic control, and detection technologies. Spindle motion is a crucial aspect of CNC lathes, used to complete cutting tasks, and its power accounts for approximately 70%–80% of the entire lathe's power. Basic control includes the forward, reverse, and stop of the spindle, with automatic gear shifting and stepless speed regulation. Currently, in CNC lathes, the spindle control device typically uses an AC frequency converter to control the AC spindle motor. To meet the requirements of CNC lathes for spindle drive, the following performance characteristics must be met: (1) wide speed range and high speed stability; (2) small speed fluctuation of the motor under intermittent load; (3) short acceleration and deceleration time; (4) strong overload capacity; (5) low noise, low vibration and long service life. This article introduces the system structure and operation mode of frequency conversion control in the spindle drive of CNC lathes, and elaborates on the basic application of sensorless vector frequency converters. 1. Explanation of Vector Control of Frequency Converter In the 1970s, Siemens engineer F. Blaschke first proposed the vector control theory of asynchronous motors to solve the torque control problem of AC motors. The basic principle of vector control is to measure and control the stator current vector of the asynchronous motor, and control the excitation current and torque current of the asynchronous motor according to the principle of magnetic field orientation, so as to achieve the purpose of controlling the torque of the asynchronous motor. Specifically, vector control decomposes the stator current vector of an asynchronous motor into a current component that generates the magnetic field (excitation current) and a current component that generates torque (torque current), controlling them separately while simultaneously controlling the amplitude and phase of the two components, i.e., controlling the stator current vector. This control method is called vector control. Vector control includes slip frequency-based vector control, sensorless vector control, and vector control with a speed sensor. This allows a three-phase asynchronous motor to be controlled as an equivalent DC motor, thus achieving the same static and dynamic performance as a DC speed control system. Vector control algorithms have been widely used in inverters from large international companies such as Siemens, AB, GE, and Fuji. General-purpose inverters using vector control can not only match the speed range of DC motors but also control the torque generated by asynchronous motors. Because vector control relies on accurate parameters of the controlled asynchronous motor, some general-purpose inverters require accurate input of these parameters, while others require the use of speed sensors and encoders. Currently, new vector control general-purpose frequency converters already possess automatic detection, automatic identification, and adaptive functions for asynchronous motor parameters. These frequency converters can automatically identify the parameters of the asynchronous motor before it begins normal operation, and adjust relevant parameters in the control algorithm based on the identification results, thus enabling effective vector control of ordinary asynchronous motors. [b]2 System Structure and Operation Mode of CNC Lathe Spindle Frequency Conversion[/b] 2.1 Basic Principle of Spindle Frequency Conversion Control According to asynchronous motor theory, the formula for the spindle motor speed is: n = (60f/p) × (1-s) where P—number of pole pairs of the motor, s—slip rate, f—frequency of the power supply, and n—motor speed. From the above formula, it can be seen that the motor speed is approximately proportional to the frequency. Changing the frequency can smoothly adjust the motor speed. For frequency converters, the frequency adjustment range is very wide, adjustable arbitrarily between 0 and 400Hz (or even higher frequencies). Therefore, the spindle motor speed can be adjusted within a relatively wide range. Of course, after the speed is increased, the impact on its bearings and windings should also be considered to prevent excessive wear and overheating of the motor. This can generally be limited by setting the maximum frequency. Figure 2-1 shows the application of frequency converter in CNC lathe. The connection between frequency converter and CNC device usually includes: (1) forward and reverse signals from CNC device to frequency converter; (2) speed or frequency signals from CNC device to frequency converter; (3) fault and other status signals from frequency converter to CNC device. Therefore, all operations and feedback on frequency converter can be programmed and displayed on CNC panel. 2.2 System configuration of spindle frequency conversion control CNC lathes that do not use frequency converter for speed transmission generally use time controller to confirm that the motor speed reaches the command speed and start feeding. However, after using frequency converter, the machine tool can feed according to the command signal, which improves efficiency. If the workpiece has the shape shown in Figure 2-2, it can be seen from Figure 2-2 that corresponding to segment AB of the workpiece, the spindle speed is maintained at 1000 rpm, and corresponding to segment BC, the motor drives the spindle to move at a constant linear velocity, but the rotational speed varies, thus achieving high-precision cutting. In this system, the speed signal is transmitted through the analog given channel (voltage or current) from the CNC device to the frequency converter. By setting the input-output characteristic curve of the input signal and the set frequency inside the frequency converter, the CNC device can conveniently and freely control the spindle speed. This characteristic curve must cover different configurations of voltage/current signals, direct/reverse action, and single/bipolar to meet the requirements of rapid forward and reverse rotation, free speed adjustment, and variable speed cutting of CNC lathes. 3. Sensorless Vector Control Inverters 3.1 Basic Selection of Spindle Inverters Currently, the simplest type of inverter is V/F control (scalar control), which is a voltage generation mode device that adjusts the voltage in a given change mode during frequency modulation. Common types include linear V/F control (for constant torque) and square V/F control (for variable torque fans and pumps). The weaknesses of scalar control are insufficient low-frequency torque (requiring torque boost) and poor speed stability (speed range 1:10). Therefore, it has been gradually phased out in lathe spindle frequency conversion applications, while vector control inverters are gradually being promoted. In simple terms, vector control, to enable a squirrel-cage induction motor to have excellent operating performance and high control performance like a DC motor, controls the magnitude, frequency, and phase of the inverter's output current to maintain the internal magnetic flux of the motor at a set value, generating the required torque. Compared to scalar control, vector control has the following advantages: (1) excellent control characteristics, comparable to the adjustment of armature current plus excitation current of DC motor; (2) adaptable to situations requiring high-speed response; (3) large speed range (1:100); (4) torque control is possible. Of course, compared to scalar control, vector control has a more complex structure, more complicated calculations, and requires the storage and frequent use of motor parameters. Vector control is divided into two types: sensorless and sensor-equipped. The difference is that the latter has a higher speed control accuracy (0.05%), while the former is 0.5%. However, in CNC lathes, the control performance of sensorless vector inverters already meets the control requirements, so sensorless vector inverters are recommended and introduced here. 3.2 Sensorless Vector Inverters Sensorless vector inverters currently have mature products from manufacturers such as Siemens, Emerson, Toshiba, Hitachi, LG, and Senlan. Summarizing the characteristics of their respective products, they all have the following features: (1) Automatic identification of motor parameters combined with manual input; (2) Strong overload capacity, such as 50% rated output current for 2 minutes and 180% rated output current for 10 seconds; (3) High output torque at low frequency, such as 150% rated torque/1HZ; (4) Complete protection (in layman's terms, it is not easy to blow up the module). Sensorless vector control inverters not only improve the characteristics of torque control, but also improve the speed controllability under various load changes in unspecified environments. Figure 3-1 shows the torque test data of a certain brand of sensorless inverter products in the low frequency and normal frequency range (motor is 5.5kW/4 poles). It can be seen from the figure that it can also generate strong torque in the low speed range. In the experiment, we compared the 2Hz vector frequency conversion control and the V/F control frequency conversion and found that the former has a stronger output torque, and the cutting force is almost the same as that of the normal frequency band (such as 30Hz or 50Hz). Figure 3-1 Torque characteristics of sensorless vector frequency converter [align=left]3.3 Motor parameter identification in vector control Since vector control focuses on the rotor flux to control the stator current of the motor, a large number of motor parameters are involved in its internal algorithm. As can be seen from the T-type equivalent circuit representation of the asynchronous motor in Figure 3-2, in addition to the conventional parameters such as the number of motor poles, rated power, and rated current, the motor also has R1 (stator resistance), X11 (stator leakage reactance), R2 (rotor resistance), X21 (rotor leakage reactance), Xm (mutual inductance reactance), and I0 (no-load current). Parameter identification is divided into two types: static identification and rotating identification. In static identification, the frequency converter can automatically measure and calculate the resistance of the top and rotor and the leakage inductance relative to the basic frequency, and write the measured parameters at the same time. In rotating identification, the frequency converter automatically measures the mutual inductance and no-load current of the motor. [/align] Figure 3-2 Equivalent circuit of asynchronous motor in steady state [align=left] In parameter identification, the following must be noted: (1) If an overcurrent or overvoltage fault occurs in rotating identification, the acceleration and deceleration time can be appropriately increased or decreased; (2) Rotating identification can only be performed under no-load conditions; (3) The parameters of the motor nameplate must be correctly entered before identification. 3.4 Functional settings of CNC lathe spindle frequency converter vector control As can be seen from Figure 1-1, the functional settings of the frequency converter used in the spindle are divided into the following parts: 1 Setting of vector control mode and motor parameters; [/align] 2 Digital input and output of switch quantity; 3 Analog input characteristic curve; 4 Setting of SR speed closed loop parameters. [b]4 Conclusion[/b] For the spindle motor of CNC lathes, the use of sensorless variable frequency speed controllers for vector control has the following significant advantages: significantly reduced maintenance costs, even maintenance-free operation; high-efficiency cutting and high machining accuracy; and strong torque output at both low and high speeds. References 1. Wang Kanfu. CNC Machine Tool Control Technology and Systems [M]. Beijing: Machinery Industry Press, 2002. 2. Du Jincheng. Electrical Variable Frequency Speed ​​Regulation Design Technology [M]. Beijing: China Electric Power Press, 2001. 3. Gao Zhongyu. Electromechanical Control Engineering. Beijing: Tsinghua University Press, 2002. 4. Liu Zhubai. Knowledge Innovation Thinking Methodology. Beijing: Machinery Industry Press, 1999. Editor: Chen Dong