Digital brushless DC servo motor systems, or as commonly known in the industry, digital permanent magnet AC servo motor systems (permanent magnet AC servo systems), have undoubtedly become the mainstream product and development trend of servo drives in the contemporary motion control field. Major international servo drive manufacturers and motor manufacturers had already developed a series of general-purpose permanent magnet AC servo motor systems in the 1990s. Rated power is generally between 50W and 20kW, and speed is generally between 1000 and 3000 r/min. The development of domestic servo systems by research institutes and universities started no later than that of foreign countries, but its entry into industrial production was later. Taking Zhuhai Yunkong Motor Co., Ltd. as a representative example, it first began producing its own designed servo motors in 2000, initially formed a series of servo system products ranging from 50W to 5kW in 2003, and began developing towards a more complete and sophisticated product series and gradual mass production in 2006. This indicates that domestic servo series products are about to enter a new stage of development, better meeting the needs of a wide range of users. However, in the process of developing domestic servo series products, it was also found that, due to the extremely wide range of fields involved in motion control systems, the requirements for servo systems are ever-changing and very diverse. Therefore, on the basis of basic series products, it is also necessary to continuously expand products with relatively special requirements according to the requirements of application systems, so as to better meet the needs of the broad market. This is also one of the important ideas for our product development [1]. This article introduces an example of a high-speed servo system applied to CNC shoe last machine. my country is the world's number one shoe manufacturing country, and the output and export volume of footwear products have always been the world's largest. The development of the shoe manufacturing industry has led to the rapid development of the shoe last industry, which is inseparable from it. Correspondingly, CNC shoe last machines, or computer shoe last machines, have also emerged. Shoe last machines are used to process shoe last models. Its motion control system includes four coordinates, as shown in Figure 1. Among them, the θ axis drives the workpiece to rotate, and the x, y and θ axes motion synthesize the contour of the shoe last model; the r axis drives the tool bar to rotate and complete the cutting process. The first three axes are typical position servo systems, and the appropriate model of the basic series can meet the requirements. The r-axis is different; there is no position control requirement. It only needs to maintain an appropriate rotation speed according to the material of the workpiece. Since the material of the shoe last model is mainly plastic, the tool needs to rotate at a high speed, at least 9000 r/min. Otherwise, it is difficult to ensure the smoothness and beauty of the machined surface. Moreover, the rotation speed should not fluctuate significantly due to changes in cutting force during the cutting process. In the basic series of AC servo systems, there are no such high-speed specifications. Therefore, in the existing CNC shoe last machines, only three axes use AC servo systems [2]. The tool holder is driven by a standard asynchronous motor. One example I saw was a common four-pole asynchronous motor, which was driven to rotate the tool holder by a 1:5 speed increase. The tool holder speed was about 7000 r/min. The disadvantages of this system are: the structure is not compact, the speed is low, and it is unstable, resulting in poor surface processing quality. If an AC servo system is used, although its position servo function is not needed, its speed loop and current loop can ensure that the speed is very stable. Even if the load changes, the impact is very small. Moreover, in multi-head shoe last machines, it can completely ensure the consistency of the shape and surface quality of multi-head processed workpieces. This will bring about a breakthrough in CNC shoe last machines. It is worthwhile to develop a high-speed servo system in addition to the basic series. Figure 1 Schematic diagram of four axes of shoe last machine [font=black] [color=black]Design structure features and key technologies [/color][/font] The servo motor that drives the CNC shoe last machine tool bar to rotate for cutting is driven by each single-head tool bar independently. Based on actual experience, its basic technical requirements are roughly determined as follows: rated speed nn=9500r/min; rated power pn=1000w. Generally speaking, high-speed permanent magnet motors will increase the requirements for rotor strength and bearings, which will cause noise and vibration problems, increase mechanical loss, core loss and temperature rise, etc. [3]. Servo motors should also consider the use of high-speed encoders. There is no fundamental impact on the servo drive. The position sensor used in the basic series servo system is a general incremental optical encoder, mainly with 2500 lines. From the perspective of pulse frequency limit and mechanical strength, the maximum speed of this type of encoder is limited to 5000r/min, which cannot meet the requirements of the high-speed servo system. The position sensor is changed to TS2620N21E11 type resolver, which has a maximum speed of 30000r/min, which can fully meet the requirements. At the same time, the matching decoding chip AU6802N1 is used, which can provide the excitation signal required by the resolver, demodulate the output signal of the resolver and convert it into different types of digital signals, including digital signals like those of incremental optical encoders, which is very convenient to use [4]. When the speed is not very high, around 10000r/min, the bearing does not need to use a special cooling method and can still use ordinary bearings with grease lubrication. Only when the load allows, a smaller bearing should be used appropriately, and the appropriate bearing housing and bearing seat fit size should be selected during assembly. The rotor of a permanent magnet motor usually adopts a surface permanent magnet structure, and the permanent magnet poles are usually glued to the surface of the rotor yoke. In high-speed motors, non-magnetic metal sheaths are required, or non-woven glass ribbons are wrapped and then cured with epoxy glue. The rotor structure of the permanent magnet motors in our basic series products has been gradually changed to an internal permanent magnet (IPM) structure. From the perspective of mechanical strength, it fully meets the requirements of high-speed motors and has higher reliability. The internal permanent magnet structure has obvious benefits for improving the cogging torque [5], is beneficial to the symmetrical distribution of rotor mass, and is helpful to reduce the torque fluctuation of the motor and improve the noise and vibration of the motor, which is especially important for high-speed motors. The characteristics of the electromagnetic design of the basic series servo motors are: high-performance permanent magnet materials are usually used, generally sintered ndfeb; high effective material utilization rate, that is, high electric and magnetic loads are used; most of them are three-pole or four-pole mechanisms to obtain their high power (torque) density and high response characteristics. The author's design is no exception, and it is basically a four-pole structure. At 9500 r/min, the electromagnetic frequency of a four-pole permanent magnet motor is 633 Hz, which is already in the medium frequency (≥400 Hz) range. According to commonly accepted calculations, the core loss [img=53,22]http://www.ca800.com/uploadfile/maga/servo2007-1/___wmf93.jpg[/img] would be 27 times that at the power frequency (50 Hz). This shows that standard electromagnetic design would result in excessively high core losses and unsustainable temperature rise. Therefore, reducing the core loss of the motor will become a key design feature and critical technology for high-speed servo motors. The attached table shows the no-load test data for a motor with only the number of turns in the prototype winding. [img=436,616]http://www.ca800.com/uploadfile/maga/servo2007-1/dwgb1.jpg[/img][font=黑体] [color=black]Modification Design for Increasing Speed [/color][/font] The main characteristic of the modification design is to make full use of the basic series and change the design of the basic series components as little as possible, especially the parts that require changes to the mold, such as the geometry of the core laminations. Some changes are only made when there is no simple way to achieve the modification goal. Because of these practical considerations, the obtained modification design is unlikely to be an ideal design, but only a usable, but generally economical, solution. The design for changing the rated speed is quite simple; under the condition that everything else remains unchanged, only the number of turns in the winding needs to be changed. If the power supply voltage remains constant, the phase potential at the rated speed can be kept constant. That is, the potential coefficient ke1 is inversely proportional to the rated speed nn, which means that the effective number of series turns n of the phase winding is inversely proportional to nn. Let the rated speed required for the modification be: [img=59,22]http://www.ca800.com/uploadfile/maga/servo2007-1/___wmf94.jpg[/img]（1） The number of turns of the modified motor can be taken as: [img=59,16]http://www.ca800.com/uploadfile/maga/servo2007-1/___wmf95.jpg[/img]（2） If the slot fill factor is kept constant, the cross-sectional area of ​​the phase winding conductor ([img=21,24]http://www.ca800.com/uploadfile/maga/servo2007-1/___wmf96.jpg[/img]) should be k times that of the original. [align=left][img=58,22]http://www.ca800.com/uploadfile/maga/servo2007-1/___wmf97.jpg[/img]（3） If the rated current density of the winding is kept constant, the rated current will be k times that of the original, and the power will also increase to k times. In fact, since the electromagnetic load (magnetic flux density and line load) of the motor remains unchanged, the electromagnetic torque tn is the same, and the output power pn increases by a factor of k with the increase of the speed. [img=44,19]http://www.ca800.com/uploadfile/maga/servo2007-1/___wmf98.jpg[/img]（4） [img=54,20]http://www.ca800.com/uploadfile/maga/servo2007-1/___wmf99.jpg[/img]（5） The above analysis actually reveals a basic law of motor design, namely, the size of the motor determines the torque of the motor, and the power is directly related to the speed. However, this law only applies when the speed is within a certain range. When the speed exceeds a certain range, due to the increase in the power loss of the motor, especially the increase in the iron core loss, the temperature rise of the motor exceeds the allowable value and is not easy to achieve. In fact, under the condition of keeping the current density and the amount of copper constant, the copper loss of the winding is constant. The situation with iron loss is different. When the magnetic flux density remains constant, the frequency of magnetic field alternation increases with the increase of rotational speed, and the iron loss pfe increases. Among them, hysteresis loss increases proportionally to the frequency f, and eddy current loss increases proportionally to the square of the frequency. It is generally believed that the total iron loss is proportional to the (1.3)th power of the frequency, so: [img=73,23]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm100.jpg[/img] (6) It can be seen that although from the perspective of electromagnetic design, a motor with a certain geometric size can obtain a certain torque, so that the output power of the motor increases proportionally with the rotational speed, it will be limited by heat generation. When the rotational speed reaches a certain value, the total loss of the motor exceeds the allowable value, so the current density j must be reduced, so that the power cannot increase proportionally, or even decreases instead of increasing. In extreme cases, i.e., when the speed is quite high, the iron loss (plus mechanical loss) of the motor exceeds the allowable value of heat generation, and the motor will not be able to carry a load. The modification design by simply changing the number of winding turns cannot be realized. At this time, it is necessary to consider taking measures to reduce iron loss. The main methods should be: reducing the magnetic flux density in the iron core, reducing the alternation frequency of the magnetic field; and using silicon steel sheets with lower loss or thinner silicon steel sheets. Based on this modification design example, the prototype selected is the 92bl(3)d100-30h(st) type motor, which is a motor with pn=1000w, nn=3000r/min and 8 poles. Since the basic series of motors already uses cold-rolled non-oriented low-loss silicon steel sheets with a thickness of 0.35mm, it is not very practical to change the core material to reduce losses. Reducing the number of poles, for example, changing the original 8-pole motor to a 4-pole motor, should be a more promising method. However, it should be considered that the yoke height of multi-pole motors is small. If the air gap magnetic flux density or pole arc coefficient is not specifically reduced after changing to 4 poles, the magnetic flux density of the yoke will be too high due to the increase in magnetic flux per pole, which will lead to a sharp increase in iron loss, so that the effect of reducing frequency cannot be seen. This modified design example is to artificially reduce the pole arc coefficient so that the magnetic flux density of the yoke does not increase. With a suitable number of winding turns, the purpose can be achieved. [/align][b][color=black][font=黑体]Iron Loss Experiment Example[/font][/color][/b] The motor under test is a prototype motor with only the number of winding turns changed. The prototype is the aforementioned 92bl(3)d100-30h(st) type motor. Since the magnetic system has not changed, the iron loss at the same speed remains unchanged, and the no-load loss is also basically unchanged. When the motor is unloaded and the speed is open-loop, the data of no-load operation at different speeds can be measured by adjusting the applied DC power supply voltage, as shown in the attached table. In the table, vs is the applied DC power supply voltage, is is the DC power supply output current, and the total input power of the motor at this speed is: p0=vsis (7) ia0 is the effective value of the phase winding current, r1 is the phase winding resistance, and the basic copper loss of the phase winding is: pcu0=3 i2a0 r1 (8) Considering that the current is very small, the power tube loss and additional copper loss are ignored, and the electromagnetic power during no-load operation is: pe0=p0－pcu0 (9) The electromagnetic torque and the no-load loss torque are balanced as follows: [img=49,37]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm101.jpg[/img] (10) Rotor angular velocity: [img=52,32]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm102.jpg[/img]（11） The measured relationship between no-load loss torque and angular velocity is shown in curve (1) in Figure 2. When the part with a relatively low speed is not considered, a straight line (2) can be used to fit it, and the expression is: [img=86,21]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm103.jpg[/img]（12） [img=137,35]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm104.jpg[/img]（13） Wherein: [img=17,18]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm105.jpg[/img] represents the constant no-load loss torque, mainly including mechanical dry friction torque and core hysteresis loss torque; [img=14,19]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm106.jpg[/img] represents the viscous friction coefficient; [img=25,19]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm107.jpg[/img] represents the viscous friction loss torque, which actually mainly reflects the core eddy current loss torque. The viscous friction torque lubricated by air and bearing oil accounts for only a small part, and can be measured by a motor with an unmagnetized rotor if necessary. The attached table shows that the total no-load loss of the 8-pole prototype motor at the rated speed nn=3000r/min is 31.1w, which is only 3.11% of the rated power, and is an appropriate value for a high-performance servo motor. When the motor was tested on a commonly used bracket in the factory laboratory, the surface heat dissipation coefficient (natural cooling) of the motor was about 1.68w/℃, so the surface temperature rise of the motor at 3000r/min should be [align=left][img=103,34]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm109.jpg[/img]℃ (14) There is a large margin to carry the load. When the speed increases to twice the rated speed (6000r/min), the total no-load loss increases to 95.12w, which is about three times that at nn. When the speed increases to 3nn (9000r/min), p0 increases to 175.35w, which is about 5.6 times that at nn. The surface temperature of the casing will reach: [/align][align=left][img=109,30]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm110.jpg[/img]℃ (15). In addition to the ambient temperature, it has exceeded the allowable temperature of the B-class insulation winding, and there is no margin for load. For the motor, improving its insulation class and using high-temperature resistant bearing grease and permanent magnet materials can allow for a higher temperature rise. However, if the motor temperature is too high, it will be transmitted to the milling tool and seriously affect the surface processing quality of the workpiece. Therefore, the modified design scheme that only changes the number of winding turns is not advisable. The modified motor with reduced pole number is model 92bl(2)d100-100h(st-1), where (2) indicates that the position sensor uses a resolver with analog output, rated power pn=1000w, rated speed [img=72,16]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm111.jpg[/img], the rotor is a 4-pole structure, and the ndfeb permanent magnet poles have a small pole arc coefficient. Using the same method to conduct no-load experiments, as expected, the no-load loss, which is mainly iron loss, is significantly reduced compared with the 8-pole motor. The measured curve of the relationship between the no-load loss torque and the speed is shown in Figure 3 (1). In the range above a certain speed, it can be fitted by the straight line (2), and we get: [/align][align=left][img=91,25]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm114.jpg[/img] (16) [img=151,44]http://www.ca800.com/uploadfile/maga/servo2007-1/___wm115.jpg[/img] (17) Compared with Figure 2 and equations (12) and (13), it can be seen that the iron loss of this modified motor is significantly reduced. Taking the speed of 9000 r/min as an example, the total no-load loss is reduced by about 57.16%, and the surface temperature rise is reduced to about 44.3°, which leaves a certain margin for the motor to carry the load. When the modified motor was applied in the field, its actual operating speed was 9500 r/min. Since the motor operates intermittently in the actual system, and the user added auxiliary air cooling, the temperature rise of the motor during operation was not high, and it did not affect the workpiece. Its reliability was also very good, meeting the requirements of the actual application. [ Conclusion ] This article introduces an example of the application of a high-speed servo motor system, taking the requirements of the milling cutter motion system of a shoe last machine as an example. It discusses the design of high-speed servo motors, especially the key technologies and practical problems of modified design. Test data examples of the relationship between no-load loss and speed, mainly iron loss, are given, and an example of a modified motor that meets the requirements of the shoe last machine cutter motion system is presented.