Research Advances in the Design and Control Technologies of Electric Spindle Motors for CNC Machine Tools

Abstract

The electric spindle serves as a critical component in enabling a highly dynamic response, stable torque output, and precise motion control for the main cutting operations of CNC machine tools. The design precision and control performance of its drive motor directly influence the geometric accuracy, surface quality, and overall machining efficiency of the workpiece, thereby determining the comprehensive performance of advanced CNC systems. This paper begins with a systematic review of the global industrial layout of CNC machine tool and electric spindle manufacturers, highlighting regional clustering patterns and technological development trends across key manufacturing regions. Subsequently, it classifies and elaborates on the differentiated technical requirements for the electric spindle motor in terms of wide-speed-range servo capability, high-efficiency operation, adaptability to high-speed and high-power cutting loads, and precision maintenance under high-speed conditions, based on the process characteristics of different types of CNC machine tools. A comprehensive overview of the current state of research is provided with respect to electric spindle motor design and control technologies. Finally, forward-looking perspectives are presented on future development directions, particularly in the areas of multi-physics coupling co-design and the integration of intelligent control algorithms, aiming to offer a solid theoretical foundation and strategic guidance for the advancement and engineering application of high-performance electric spindles.

1. Introduction

With the rapid advancement of emerging industries—including new energy vehicles, lithium-ion batteries, photovoltaic power generation, wind energy, and 5G communications—the demand for manufacturing complex and high-precision components in next-generation rockets, satellites, and aerospace systems has been steadily increasing. Against the backdrop of concurrent trends toward integration, large-scale design, lightweight construction, and ultra-high precision, the market demand for high-end CNC machine tools and precision electric spindles continues to grow significantly. The primary function of an electric spindle is to deliver stable power output and high-precision motion control for the main cutting operation in CNC machine tools. Its performance critically determines the geometric accuracy of machined components, production efficiency, and the dynamic stability of the entire machine system. As the principal power source and key actuating component in CNC machine tools, the electric spindle motor enables the precise execution of complex numerical control commands through highly accurate and responsive rotational motion, facilitating micron-level and sub-micron-level precision machining.

The sustained enhancement of electric spindle motor performance fundamentally relies on the synergistic integration of advanced materials, state-of-the-art manufacturing techniques, and innovative motor topologies, necessitating the systematic incorporation of multidisciplinary frontier technologies into the design framework. Recent advancements in the design and control of electric spindle motors have not only significantly improved their comprehensive performance—particularly in power density, rotational speed range, and thermal stability—but have also driven the evolution of electric spindles from single-function mechanical components into intelligent, integrated high-end functional units. This transformation has profoundly reshaped both the developmental trajectory of electric spindle technology and the competitive dynamics within the modern CNC machine tool industry.

2. Global Competitive Landscape

2.1. Overview of the Global Distribution of CNC Machine Tool Manufacturers

Based on industry data publicly released by national and international trade associations worldwide, this paper provides a systematic overview of the global distribution of CNC machine tool manufacturers and presents a selection of representative companies, as detailed in

Table 1. Overview of the global distribution of CNC machine tool manufacturers.

Country	CNC Machine Tool Manufacturers
United States	MAG, HAAS, HARDINGE, DRAKE, Gleason
Germany	CHIRON, DMG MORI, HERMLE, TRUMPF, GROB, EMAG, SIEMENS, HEIDENHAIN
Switzerland	GF Machining Solutions (MIKRON, LIECHTI), STARRAG (BUMOTEC, Heckert, SIP), UNITED GRINDING (STUDER, Walter), Willemin-Macodel, TORNOS
Italy	PAMA, INNSE, LAZAATI, PROMAC, Breton
United Kingdom	RENISHAW, Holroyd, Colchester, Churchill, MATRIX
Japan	MAZAK, FANUC, TSUNEZO MAKINO, AMADA, OKUMA, TSUGAMI, MITSUI SEIKI, Okamoto, YASDA
South Korea	DN Solutions, Samsung, Hyundai WIA
China	Topo CNC, Guangzhou CNC, Beijing Machine Tool Research Institute, General Technology Group, KND, Beijing Jingdiao, Jinan No. 2 Machine Tool, Shanghai Machine Tool Works, Haomai Technology, Shenyang Machine Tool, Kedex CNC, Huazhong Numerical Control, Wuhan Heavy-duty Machine Tool, Qinchuan Machine Tool

.

In North America, the U.S. machine tool industry focuses on high-end CNC systems and intelligent manufacturing technologies for the aerospace and defense sectors, demonstrating strong capabilities in system integration and technological innovation.

In Europe, the German machine tool industry maintains a leading global position due to its high precision-to-lifetime ratio and advanced nano-composite coating technologies. Swiss manufacturers hold an irreplaceable role in micro- and nano-scale ultra-precision machining, particularly in the production of high-end timepieces and medical devices. The Italian machine tool industry demonstrates distinct strengths in manufacturing heavy machinery, defense equipment, and large aerospace components. Meanwhile, the UK has established a specialized development model centered on precision metrology and proprietary machining technologies.

In Asia, Japanese machine tools have long dominated the automotive and mold manufacturing sectors, driven by high cost-effectiveness, consistent batch production precision, and a fully integrated industrial chain. South Korean manufacturers demonstrate strong international competitiveness in high-tech domains, including semiconductor equipment, display panels, shipbuilding, and energy infrastructure. China accounts for one-third of the global market share in both production and sales of CNC machine tools. While its output remains predominantly mid-to-low-end, the industry is progressively advancing into high-end segments. A regional pattern of coordinated development and industrial clustering has emerged, led by Taiwan, the Yangtze River Delta, and the Pearl River Delta, with accelerated growth observed in the Bohai Rim and central and western regions.

2.2. Overview of the Global Distribution of Electric Spindle Manufacturers

Based on industry data publicly released by national authorities and international industry associations, this paper presents a systematic overview of the global distribution of electric spindle manufacturers and identifies a selection of representative companies. Detailed information is provided in

Table 2. Overview of the global distribution of electric spindle manufacturers.

Country	Leading Electric Spindle Manufacturers
United States	Celera Motion, SETCO, INGERSOLL, PRECISE, Ex-Cell-O
Germany	GMN, SIEMENS, SYCOTEC, IMT, CYTEC, KLUFER, Kessler, HEIDENHAIN, FAG, HULLER
Switzerland	FISCHER PRECISE, IBAG, MCT, ABB, STEP-TEC, GEPY
Italy	FAEMAT, Reckerth, CAMFIOR, OMLAT, FAMUP
United Kingdom	WESTWIND, ABL
Japan	Mitsubishi, NAKANISHI, NSK, Panasonic, MSIY, Yaskawa
China	ROYAL, Parfaite, POSA, WLT, WOLONG, Haozhi Electromechanical, Aibike Precision, Kede Numerical Control, Beijing Jingdiao, Beijing Chaotongbu, Luoyang Bearing Industry Research Institute, Huazhong Numerical Control

.

The United Kingdom-based WESTWIND and Switzerland-based IBAG are global leaders in precision spindles for semiconductor cutting applications. Switzerland manufacturer FISCHER and Germany’s GMN hold significant technological advantages in the electric spindle market for precision mold machining. Kessler of Germany has established a robust technical foundation in the research, development, and manufacturing of electric spindles for PCB drilling. The United States has emerged as a key technology hub for the development of high-end electric spindles in aerospace-specific applications. Japan possesses a mature manufacturing system and highly efficient large-scale production capabilities in the mass production of medium- and high-end machining center spindles. In China, Haozhi Electromechanical, Kede Numerical Control, the Bearing Research Institute, and Beijing Jingdiao have experienced rapid advancement in recent years. Furthermore, leading international bearing manufacturers have increasingly expanded into the electric spindle sector, while most major foreign CNC machine tool companies maintain in-house capabilities for electric spindle development.

Although electric spindles have become the core component and prevailing trend in the future development of the machine tool industry, mechanical spindles continue to hold a significant share of the low-end CNC machine tool market. In the domain of ultra-high-speed (>50,000 r/min) and ultra-precision (<0.1 μm) electric spindles, leading international brands from Europe, the United States, and Japan remain dominant. In contrast, Chinese manufacturers, leveraging their competitive cost-performance ratio, have achieved the technical capability to fully substitute comparable foreign products under conventional operating conditions (rotational speed ≤ 20,000 r/min, positioning accuracy ≥ 1 μm). It is noteworthy that in specialized application domains—including new energy vehicles, industrial robotics, wind power generation, and energy storage systems—the market share of electric spindles manufactured in China has surpassed 50%.

3. Performance Requirements

3.1. Performance Requirements for Electric Spindles of CNC Machine Tools

A detailed analysis of the performance requirements for electric spindles used in different types of CNC machine tools is presented in

Table 3. Performance requirements for electric spindles across different types of CNC machine tools.

Electric Spindle Type	Speed Range (r/min)	Acceleration or Deceleration Time (s)	Speed Fluctuation Rate (%)	Main Features
Grinding spindle motor	2000~ 150,000	1~30	≤0.02	Speed fluctuation is minimal across the entire speed range
Machining center spindle motor	0~ 40,000	0.5~10	≤0.05	Enabling compatibility with both low-speed heavy-duty cutting and high-speed precision machining
Engraving and milling spindle motor	5000~ 60,000	0.1~5	≤0.05	Precise control of micro-overshoot is emphasized
CNC milling spindle motor	250~ 42,000	1~15	≤0.1	Wide speed range with high torque output at low speeds
Turning spindle motor	0~ 12,000	0.5~10	≤0.05	Supports C-axis indexing, thread turning, and bidirectional precise motion control
Drilling spindle motor	0~ 300,000	0.3~15	≤0.1	Ensures axial stability along with ultra-high-speed operation and fast dynamic response
Multi-process machining center spindle motor	0~ 80,000	0.5~10	≤0.02	Wide-range, fast-response, high-precision servo, turning and milling switching

.

The grinding process imposes stringent requirements on workpiece surface quality and dimensional accuracy. The core performance of the electric spindle is characterized by operational stability and high-precision control under high-speed conditions, ensuring smooth grinding wheel operation and effective suppression of vibration-induced surface defects, thereby enhancing surface integrity and geometric accuracy consistency. Dynamic response during acceleration and deceleration is less critical; instead, the speed regulation process prioritizes exceptional smoothness in speed transitions and rigorous control of torque fluctuations to prevent minor vibrations from adversely affecting the precision of the ground surface.

The electric spindle used in machining centers must exhibit broad processing adaptability. At low speeds, it is required to deliver sufficient torque to support high-force cutting operations on hard materials and large-diameter tools. At high speeds, the spindle enables precision machining with small cutting tools, ensuring complex components such as molds achieve high-precision contour accuracy and superior surface integrity.

The electric spindle used in milling and engraving applications is characterized by high rotational speed, high precision, and high dynamic response performance. It places particular emphasis on high-precision trajectory control for complex contours, thereby imposing stringent requirements on the servo system’s resolution, dynamic tracking accuracy, and positioning accuracy.

The electric spindle employed in CNC milling must exhibit excellent low-speed cutting performance and a broad speed regulation range, enabling stable output torque and consistent dynamic behavior under complex and variable machining conditions.

The electric spindle used in turning applications must provide flexible bidirectional rotation, a wide speed regulation range, and robust radial load-carrying capability. It should also integrate essential functions such as analog speed control, thread cutting, and constant surface speed control to meet the diverse requirements of modern turning operations.

Electric spindles for drilling applications demand high dynamic response performance, requiring rapid acceleration and deceleration capabilities to ensure efficient transient response under varying operating conditions, as well as high rotational accuracy and thermal stability during continuous operation. Ultra-high-speed electric spindles based on air hydrostatic bearing technology, owing to their superior low-friction and high-stiffness characteristics, have emerged as the primary trend in the technological development of this domain.

The electric spindle used in compound machining centers exhibits the highest level of integration, capable of incorporating multiple machining functions such as turning, milling, drilling, and tapping. It imposes the most stringent technical demands on the dynamic response speed, positioning accuracy, and overall control performance.

3.2. Technical Requirements for Key Parameters of Electric Spindle Motors

The electric spindle motor is a shell-less motor integrated within the electric spindle assembly. Its direct-drive configuration substantially shortens the primary transmission chain of the machine tool. This compact design not only improves the motion system’s operational smoothness and dynamic response characteristics but also enhances spatial utilization efficiency. It is widely employed in medium- and high-end CNC machine tools. The current research status and developmental trends are summarized in

Table 4. Technical Requirements for Key Parameters of Electric Spindle Motors.

Technical Direction	Performance Indicator	Current Status	Development Trend
Wide-range speed servo control	Speed range (r/min)	1:50~1:10,000	1:100~1:30,000
	Acceleration time (s)	0.5~30	0.1~15
	Overshoot (%)	≤5	≤1
	Speed fluctuation (%)	±0.05	±0.01
High-efficiency servo control	Motor efficiency (%)	90~95	>98
	Temperature rise (°C)	≤40	≤25
	Thermal error (mm/°C)	≤0.005	≤0.003
High-speed and high-power cutting	Constant Power Ratio	1:2~1:12	1:2~1:15
	Power Density (kW/kg)	≥3	≥5
	Maximum Power (kW)	≥200	≥300
High-speed and high-precision capability retention	Vibration velocity (mm/s)	≤0.6	≤0.2
	Radial runout (μm)	≤5	≤2
	Surface roughness (Ra)	≤0.05	≤0.03

.

3.2.1. Wide-Range Speed Servo Control

The wide-speed servo performance of the electric spindle motor primarily comprises three aspects: wide-range speed regulation, rapid-response speed regulation, and stable speed regulation.

Regarding wide-range speed regulation, significant variations exist in the speed regulation range across different types of CNC machine tool spindles. General-purpose milling and drilling spindles typically exhibit a speed regulation range of 1:100 to 1:500. In high-speed precision and semi-precision machining applications—such as mold manufacturing and aerospace component processing—the speed regulation range can reach 1:1000 to 1:2000. For ultra-high-speed machining spindles equipped with ceramic bearings, magnetic levitation bearings, or air hydrostatic bearings, the speed regulation range can extend up to 1:3000 to 1:10,000.

With regard to rapid-response speed regulation, the system’s dynamic response capability is typically characterized by speed loop bandwidth or torque response time, reflecting its accuracy in tracking speed command changes and its effectiveness in suppressing external load disturbances. A higher speed loop bandwidth signifies faster and more precise responses to speed variations; whereas a shorter servo sampling period and control cycle duration constitute fundamental prerequisites for achieving high-bandwidth control. Electric spindles generally must accelerate to the designated speed and attain a stable operating condition before initiating cutting operations; necessitating minimization of acceleration and deceleration times. Currently, electric spindles employed in high-end high-speed machining centers have achieved start–stop response times on the order of seconds or even sub-seconds, demonstrating advanced acceleration performance.

With respect to stable speed regulation, maintaining a constant cutting linear velocity is critical for ensuring consistent workpiece surface roughness and surface finish quality. Overshoot as an indicator of the system’s transient response characteristics can induce dynamic errors that directly compromise dimensional contour accuracy during the initial phase of machining; which may adversely affect tool life. Frequent rotational speed fluctuations lead to variations in cutting force which can result in surface vibration marks or non-uniform machining traces. Each tool type has a designated optimal cutting speed range defined by its design parameters; sustaining a constant spindle speeds enables the tool to operate under ideal conditions thereby effectively preventing abnormal wear or edge chipping caused by stability issues.

3.2.2. High-Efficiency Servo Control

In terms of high-efficiency servo performance, operation of electric spindle motors under flux-weakening control induces a series of adverse effects, including reduced efficiency, degraded power factor, and elevated temperature rise. Flux-weakening control necessitates an increase in the d-axis demagnetizing current, resulting in higher stator current amplitude and a consequent significant rise in copper losses. Although the main magnetic flux is weakened, the associated increase in high-frequency harmonic content leads to substantially increased eddy current losses. Under high-speed flux-weakening conditions, harmonic magnetic fields within the permanent magnets induce eddy currents, giving rise to additional parasitic losses. These loss components grow exponentially with rotational speed, constituting one of the primary causes of the sharp efficiency drop observed in the ultra-high-speed operating range. Furthermore, during flux-weakening operation, multiple loss mechanisms increase simultaneously and are confined to a limited physical space, leading to a pronounced loss concentration effect—particularly evident in the end windings and permanent magnet regions where thermal buildup is most severe. As the degree of flux weakening increases, the proportion of reactive current rises, contributing to a decline in power factor. Concurrently, increased PWM depth results in higher harmonic content from switching devices causing current waveform distortion and an enlarged phase difference between current and voltage; thereby further deteriorating the system’s power factor. Effective thermal management directly influences precision retention and operational lifespan of the electric spindle. For every 10 °C rise in bearing temperature service life is approximately halved; additionally thermal expansion may result in axial spindle elongation ranging from 0.01 to 0.05 mm. Presently, temperature rise in mainstream electric spindles can be maintained within 40 °C while certain models incorporating advanced cooling technologies exhibit temperature increases of no more than 25 °C even under continuous-duty operation.

3.2.3. High-Speed and High-Power Cutting

In the domain of high-speed and high-power cutting, driven by continuous advancements in cutting tool technology and high-speed feed systems, along with the proliferation of various composite machining platforms, there is a growing demand for CNC machine tools capable of delivering both low-speed, high-torque heavy cutting and high-speed, precision machining performance. Composite machining centers not only streamline manufacturing processes and enhance production efficiency but also significantly reduce setup errors and minimize work-in-process handling time. Multi-functional composite machines incorporating process-integrated design principles have emerged as a key trend in the future development of advanced machine tool technology. The ability to deliver constant power output enables electric spindles to fully utilize motor power across a broad speed range, which is critical for achieving efficient material removal in heavy-duty operations and maintaining high accuracy in finishing processes. Key enabling technologies include motor magnetic field regulation strategies, variable-parameter control schemes that adaptively adjust current limits and modulation modes based on rotational speed, the adoption of three-level inverters or matrix converters to improve voltage utilization, and the integration of silicon carbide (SiC) power devices to reduce switching losses and thereby extend the constant power operating range. From a structural design perspective, motors with high short-term overload capacity must be incorporated to meet low-speed, high-torque requirements, supported by advanced cooling systems to ensure effective thermal dissipation under sustained high-torque loads. Simultaneously, high-performance spindle bearings must be employed to maintain mechanical rigidity and operational stability under the combined effects of high rotational speeds and large torque transmission.

The constant power ratio is closely tied to specific machining requirements. Heavy-duty spindles used in applications such as large gantry milling machines and boring-milling centers generally have a constant power ratio of 1:2 to 1:3, primarily intended for rough machining with large material removal allowances and typically paired with large-diameter face milling cutters. General-purpose machining center spindles usually feature a constant power ratio of 1:4 to 1:6, enabling both low-speed, high-torque heavy cutting and high-speed semi-finishing or finishing operations under constant power conditions. High-speed spindles can achieve a constant power ratio of 1:8 to 1:12, often fitted with small-diameter ball-end milling cutters, requiring sufficient output power at high rotational speeds to maintain adequate material removal rates. Ultra-high-speed spindles—such as those used in precision grinding and PCB drilling—typically have a constant power ratio exceeding 1:12, with maximum speeds surpassing 100,000 r/min. However, these spindles generate relatively low cutting forces. Their power design primarily addresses air friction losses and ensures rotational stability, resulting in relatively modest demands for high-power output.

3.2.4. High-Speed and High-Precision Capability Retention

With regard to the maintenance of high-speed and high-precision performance, maintaining a low vibration level is essential for electric spindles to achieve high-accuracy machining. Excessive vibration not only accelerates tool wear but also degrades workpiece surface quality and compromises dimensional and geometric accuracy. Key technologies encompass high-performance bearing systems, real-time vibration suppression through piezoelectric or electromagnetic actuators, as well as online dynamic balancing and active unbalance compensation techniques capable of autonomously correcting mass imbalances. From a structural design perspective, finite element modal analysis is employed to identify and avoid natural frequencies within the operational speed range, thereby preventing resonant conditions. Furthermore, the integration of smart materials such as shape memory alloys and magnetorheological fluids enables real-time modulation of damping and stiffness characteristics, effectively mitigating vibration responses during high-speed machining operations.

3.3. Selection Strategy for Electric Spindle Motors

A comparative analysis of the performance characteristics of different electric spindle motors is provided in

Table 5. Comparative analysis of the performance characteristics of different electric spindle motors.

Type	Permanent Magnet Synchronous Spindle Motor	Variable Flux Permanent Magnet Spindle Motor	Asynchronous Induction Spindle Motor	Switched Reluctance Spindle Motor
Applicable operating conditions	CNC grinding machines, high-speed turning, and composite machining centers	Ultra-high-speed electric spindles and composite material drilling applications	Conventional CNC lathes, general-purpose milling machines, and drilling machines	Mining machinery, and operations under high-temperature and high-humidity conditions
Principle of flux-weakening speed extension	Flux-weakening control	Online adjustment of magnetization state	Slip frequency control and flux-weakening control	Angular position control
Speed regulation performance	Wide speed regulation range, high power density, and fast dynamic response	Lossless magnetic flux adjustment, and high efficiency in the high-speed region	Relatively wide speed regulation range, but efficiency varies with rotational speed	Extremely wide speed regulation range, yet substantial output fluctuation
Methods for expanding constant power	Optimize flux-weakening control and increase DC bus voltage	Active flux regulation, optimize the magnetization control strategy	Field-oriented vector control, enhance the power supply voltage	Angle control, single-pulse voltage control
Speed range	10,000~100,000 r/min	10,000~150,000 r/min	≤20,000 r/min	≥100,000 r/min
Control accuracy	±0.1%	±0.2%	±0.5%	±1%
Power factor	95%~98%	92%~96%	85%~93%	88%~92%
Motor efficiency	0.9~1.0	0.8~0.95	0.7~0.9	0.6~0.8
Manufacturing cost	Higher	Medium	Lowest	Lower
Current market share	50%~60%	5%~10%	20%~30%	<5%
Application status	Mainstream and mature technology, widely adopted in machining centers	Not yet widely implemented in the field of machine tool spindles	Commonly employed in cost-effective electric spindle applications	Continued application under specific operating conditions
Future growth trend	5%~10%	4%~8%	−8%~−15%	Remains unchanged

.

In the selection of electric spindle motors for CNC machine tools, switched reluctance electric spindle motors exhibit outstanding environmental adaptability and operational reliability under harsh operating conditions—such as high temperature, high humidity, and dusty environments—owing to their robust mechanical structure and absence of permanent magnet losses.

Asynchronous induction electric spindle motors are widely employed in general-purpose machining applications with relatively low requirements for machining accuracy and dynamic response, including conventional milling and turning operations. However, due to the ongoing decline in market demand for cost-sensitive machine tools, their market share has been steadily decreasing.

Variable-flux permanent magnet electric spindle motors, as representative emerging technologies, have transitioned progressively from laboratory research to industrial deployment and are particularly suitable for ultra-high-speed operation and high-temperature electric spindle systems. However, the manufacturing process is highly complex, and the equipment investment cost exceeds three times that of conventional radial motors. The industrial supply chain currently lacks sufficient capacity for high-coercivity permanent magnets and high-performance soft magnetic composite (SMC) materials. Additionally, variable-flux motors require specialized control strategies and advanced algorithms, resulting in heightened control complexity and significant development challenges. To enable large-scale commercial deployment of variable-flux permanent magnet electric spindle motors, future efforts must focus on breakthroughs in material innovation, structural design optimization, thermal management enhancement, control algorithm development, and coordinated advancement across the industrial supply chain, and the industrial maturity should be gradually enhanced. Moreover, a balanced trade-off between performance improvement and the complexities of manufacturing processes and control systems must be carefully considered in strategic decision-making.

Permanent magnet synchronous electric spindle motors achieve full-speed-range efficiency and wide-range speed regulation through integrated optimization of high-precision sensors, silicon carbide-based power converters, and advanced intelligent control algorithms. These motors are now extensively adopted in mid-to-high-end CNC machining applications. Owing to their comprehensive advantages in machining precision, dynamic response performance, and energy efficiency, they have become the dominant technical solution and preferred choice for high-end CNC electric spindle systems. Their market share in the CNC electric spindle sector is expected to continue growing in the future.

4. Research on the Optimal Design of Electric Spindle Motors

Electric spindle design does not entail the isolated pursuit of peak performance in the individual domains of electromagnetism, thermal management, and dynamics. Rather, it involves identifying a globally optimal, integrated solution under the constraints imposed by the interdependent coupling among these three physical fields, tailored to specific application requirements such as maximum rotational speed, rated power, and machining accuracy. This approach necessitates the effective resolution of multi-physics coupling conflicts among electromagnetic–thermal, electromagnetic–structural, and thermal–structural interactions.

4.1. Integrated Optimization of Electromagnetic Performance and Topological Configuration

4.1.1. Pursuing High Power Density and High Efficiency

The performance comparison of permanent magnets for electric spindle motors is presented in

Table 6. Performance Comparison of Permanent Magnets for Electric Spindle Motors.

Parameters	Neodymium Iron Boron (NdFeB)	Ferrite	Aluminum Nickel Cobalt (AlNiCo)	Samarium Cobalt (SmCo)
Energy product (MGOe)	>50	4~5	5~10	20~30
Coercivity (kOe)	>12	3~4	<2	>25
Maximum operating temperature (°C)	80~200	250	550	350

.

NdFeB exhibits the highest energy product among permanent magnet materials and offers an optimal balance between superior electromagnetic performance and cost-effectiveness, making it the preferred choice for mass-produced electric spindle motors. In contrast, SmCo permanent magnets demonstrate outstanding thermal stability, rendering them particularly suitable for high-temperature operation of electric spindles. From the standpoint of electromagnetic load design, the use of NdFeB magnets with high remanence and high coercivity, in conjunction with built-in rotor magnetic circuit configurations such as V-type and U-type topologies, can significantly enhance the motor’s electromagnetic loading capability and achieve higher power density [1,2].

Enhancing the motor slot fill factor is widely recognized as one of the key technological pathways for achieving high power density, minimizing material consumption, and enabling motor miniaturization. From the standpoint of winding structure optimization, the implementation of advanced flat-wire winding technologies—such as hair-pin windings and Litz wire—has emerged as a critical strategy to improve slot fill factor and maximize spatial utilization. A higher slot fill factor facilitates a more compact structural design, increased power density, and enhanced output capability.

4.1.2. Wide-Speed-Range Motor Structure Design

By applying short-duration current pulses to the stator, the magnetization state of permanent magnets—such as AlNiCo, ferrite, and SmCo—can be effectively regulated, enabling dynamic control of the air gap flux. Motors employing this principle are referred to as variable flux memory motors (VFMMs). From the standpoint of magnetic field regulation mechanisms, variable-flux permanent magnet motors [3,4] can be categorized into four primary types: main flux regulation, leakage flux regulation, magnetic circuit reconfiguration, and winding connection switching. A comparative performance analysis of these configurations is provided in

Table 7. Performance Comparison of Variable-Flux Permanent Magnet Synchronous Motors.

Type	Representative Motor	Magnetic Flux Modulation Mechanism	Topological Structure
Variable main flux	Hybrid permanent magnet memory motor	Pulse current	Combination of multiple low- coercivity and high-coercivity permanent magnets
Variable leakage flux	Variable-leakage-flux motor with anti-salient-pole structure	Leakage flux path or magnetic reluctance distribution	Anti-salient-pole configuration with rotor-embedded magnetic barriers
	Switched-flux memory motor	On-off control of switched-flux windings	Permanent magnets embedded in stator teeth with salient-pole rotor structure
	Flux-switching memory motor	Flux-switching principle	Stator-mounted permanent magnets, rotor with salient-pole structure without permanent magnet material
Variable magnetic circuit	Mechanical variable-flux motor	Mechanical actuation mechanism	Alters the leakage flux cross-sectional area
Variable winding configuration	Variable-pole permanent magnet motor	Winding switching mechanism	Reconfigurable stator windings with hybrid permanent magnet rotor

.

Although commercial applications of variable-flux electric spindle motors in the field of CNC machine tools remain limited, with most systems still at the experimental prototype stage, ongoing advancements in novel variable-flux motor topologies, flux linkage estimation techniques, and integrated control strategies for current-torque regulation and flux-speed coordination are expected to enable broader adoption of adjustable-flux motor systems in wide-speed-range permanent magnet motor drives for CNC machine tools.

The rotor of the hybrid permanent magnet memory motor incorporates both high-coercivity and low-coercivity permanent magnets. By applying controlled pulse currents, the magnetic field can be regulated in real time, enabling efficient operation across a wide speed range with minimal armature losses. This makes the motor well suited for electric spindles that frequently transition between high- and low-speed operating conditions. However, the low-coercivity permanent magnets exhibit limited resistance to demagnetization, posing significant challenges to the stability of their magnetic state. In contrast, the salient-pole variable leakage flux motor employs a carefully designed rotor magnetic barrier structure to achieve higher quadrature-axis inductance than direct-axis inductance. It enables dynamic adjustment of leakage flux in response to speed variations through magnetic bridges or barriers, offering advantages such as reduced torque ripple and low risk of permanent magnet demagnetization under diverse operating conditions—features that better meet the demands of high-precision machining for smooth and stable operation. The primary challenge lies in the complexity of electromagnetic design, requiring precise parameter identification and support from advanced control strategies. These two types of variable-flux electric spindle motors strike a favorable balance between speed regulation capability and controllability, demonstrating strong potential for engineering applications. They are expected to be deployed in electric spindle systems of high-end CNC machine tools and turning–milling compound machining centers, with emphasis on overcoming critical engineering challenges such as online algorithm optimization and thermal rise mitigation.

4.1.3. Enhance Power Output in the High-Speed Range

The electric spindle motor must possess efficient flux-weakening control capability to maintain constant power output and deliver sufficient cutting force during high-speed operation. Optimal design can be approached through the following five aspects, as detailed in

Table 8. Method for Enhancing Power Output in the High-Speed Region.

Objectives	Optimization Strategies
permanent magnet flux regulation	Prioritize high-coercivity grades of NdFeB permanent magnets, such as the SH, UH, EH, and TH series, or samarium cobalt magnets characterized by high coercivity and relatively low remanent flux density
Minimize direct-axis inductance	Optimize the magnetic barrier structure by employing wider and thinner barriers or incorporating flat permanent magnets to increase the reluctance of the direct-axis magnetic path
Reduce the design of direct-axis inductance	Reasonable design of the width, position and number of layers of the rotor magnetic barrier can effectively adjust the ratio of Ld to Lq, and the attempt of reverse salient pole motor is made
Double-winding structure design	Utilizing electronic switches to switch windings to achieve low-speed series connection and high-speed parallel connection
Increase the base speed of the motor	Raising the rotational speed at which the back EMF reaches the peak bus voltage

.

4.1.4. Optimize Dynamic Response and Control Performance

Reducing the electrical time constant is critical for enhancing current response speed. By shortening the air gap length, employing a semi-closed slot structure, utilizing magnetic slot wedges, and incorporating a magnetic barrier design to constrain the quadrature-axis (q-axis) flux path, the quadrature-axis inductance Lq can be effectively reduced, thereby significantly improving the motor’s current response performance.

A lightweight design can be effectively achieved by incorporating weight-reducing holes in the rotor structure or adopting a hollow shaft configuration, which enables the passage of broaches and coolant, and by replacing conventional alloy sheaths with low-density, high-strength materials such as carbon fiber. Furthermore, the use of high-energy-product permanent magnets and optimization of the pole-slot combination can significantly enhance torque density, thereby increasing the motor’s output torque capability and improving its dynamic response [5] performance. Additionally, optimizing the mass and structural design of chucks and cutting tools, along with employing carbon fiber-based lightweight tool holders, reduces load inertia, thereby minimizing the inertial resistance during acceleration and enhancing torque output per unit current.

4.1.5. Achieve Extremely High Precision and Smooth Operation

Electric spindle motors can be optimized in the following three aspects, as detailed in

Table 9. Optimization measures for motor motion accuracy and operational smoothness.

Objectives	Optimization Strategies
Reduce electromagnetic torque ripple	Reasonably adjust the shape of the magnetic poles and the pole arc coefficient to make the counter electromotive force waveform closer to the ideal sine wave
	Suppress harmonic components by optimizing the magnetic pole structure and winding distribution
Suppress cogging torque	Adopt fractional-slot winding structure and rationally configure the pole-slot matching relationship
	Adopt rotor skewed pole structure or optimize the shape of magnetic poles to reduce the amplitude of cogging torque
	Adopt closed-slot or semi-closed-slot structure and optimize the slot opening width
Weaken the radial electromagnetic force wave	Optimize the shape of the magnetic poles and the pole arc coefficient to reduce the harmonic content in the air gap magnetic flux density
	Appropriately increase the air gap length to reduce the amplitude of the magnetic flux density and the amplitude of the electromagnetic force wave

.

4.2. Analysis of Loss Characteristics and Thermal Management System Design

The thermal design [6,7,8] of the electric spindle motor represents a synergistic integration of loss minimization in electromagnetic design and heat dissipation capacity maximization in structural design. Optimization can be achieved through five key aspects, as outlined in

Table 10. Optimization methods for heat dissipation of electric spindle motors.

Direction	Optimization Strategies
Loss reduction and heat source analysis	Ultra-thin design of non-oriented silicon steel sheets combined with advanced insulation coating technology effectively reduces stator iron losses
	The adoption of flat wire windings, graphene composite superconducting copper windings, and novel hybrid windings effectively reduces winding copper losses
Thermal path optimization	High-thermal-conductivity impregnating varnish and slot insulation materials are employed to fill winding gaps
	Interference fits are implemented between the stator core and the spindle motor housing, as well as between the rotor core and the spindle shaft, to minimize interfacial thermal contact resistance
Innovative cooling structure	Advanced cooling channels, including spiral, axial Z-shaped flow passages, and pin fin configurations
	a novel design incorporating heat pipes embedded within the rotating shaft or end cover
Application of cooling medium	forced air cooling via casing air ducts, water cooling circulation along the stator periphery, spray oil cooling, internal oil cooling within stator slots, phase change cooling, and multi-path coordinated cooling

.

4.3. Analysis of Rotor Strength and Dynamic Characteristics

It is essential to conduct multi-physics coupling simulations involving electromagnetic fields, stress fields, thermal effects, and rotor dynamics, with optimization achievable through the four aspects outlined in

Table 11. Methods for enhancing rotor strength and optimizing dynamic characteristics.

Direction	Optimization Strategies
Improve mechanical strength of rotor and permanent magnet materials	Rotor cores are advancing toward high-strength and ultra-high-strength materials, while the development of permanent magnets with enhanced tensile strength is actively underway
Strength guarantee and process collaborative innovation	An ultra-high pre-tightening force winding process, coupled with an optimal interference fit design between the sheath and the permanent magnet
Advanced rotor structure optimization	Adopt carbon fiber composite material sheath, high-strength non-magnetic alloy sheath and segmented sheath structure
Dynamic characteristics and vibration analysis	Accurately calculate the critical speed of the rotor and conduct modal analysis and harmonic response analysis

.

Due to factors such as material inhomogeneity, manufacturing errors, and thermal deformation, the rotor exhibits an imbalance in mass distribution, leading to synchronous vibrations at frequencies matching the rotational speed during operation. These vibrations not only degrade machining accuracy and reduce equipment service life but may also induce severe mechanical failures, thereby compromising system reliability. The primary objective of active vibration control and online imbalance compensation technologies is to enable real-time, proactive suppression of vibrations during rotor operation, significantly improving dynamic stability and operational precision—particularly critical in high-speed and high-precision rotating machinery. To achieve stable, accurate, and self-adaptive rotor performance, systematic optimization and technological advancements are required in real-time computational capabilities, sensor configuration architectures, and coordinated actuator control strategies.

The conventional passive balancing method, which relies on counterweight adjustment, typically necessitates machine shutdown, resulting in low operational efficiency and delayed response. In contrast, active vibration control and online imbalance compensation technologies allow for continuous monitoring of vibration signals during rotor operation. Through the control system, these technologies generate control forces or corrective actions that are equal in magnitude and opposite in phase to the unbalance forces, enabling dynamic vibration cancelation and demonstrating clear advantages of active control. Currently, two principal control strategies are employed: The first is automatic balancing control, designed to align rotor rotation with its principal inertia axis to minimize the transmission of synchronous vibratory forces to the bearings. Typical implementations include LMS algorithm-based adaptive filters and generalized notch filters. The second is imbalance compensation control, which aims to maintain rotor rotation as closely as possible around the geometric center axis by suppressing vibration displacement amplitudes, making it suitable for applications with stringent rotational accuracy requirements. Representative approaches include coordinate transformation-based feedforward compensation and variable-step-size LMS algorithm-based adaptive compensation mechanisms.

In the real-time control of high-speed rotors, several critical technical challenges must be addressed: (1) The challenge of real-time computation and delay compensation arises due to the rapid dynamics of rotor vibration signals, which demand high sampling rates and broad control bandwidths. However, time delays are inherently introduced across multiple stages. When the cumulative system delay approaches the order of the vibration period, control performance degrades significantly and may even result in system instability. While advanced control algorithms such as model predictive control (MPC) and adaptive filtering can enhance control accuracy and dynamic responsiveness, their high computational burden poses a significant obstacle to achieving microsecond-level real-time execution under constrained embedded hardware resources. (2) Sensor architecture faces a performance–cost trade-off. High-precision encoders and laser displacement sensors offer superior feedback quality but entail high costs, limiting their scalability in industrial applications. Conversely, low-cost sensors often lack sufficient resolution and fidelity for capturing high-frequency vibration components. To enable comprehensive state monitoring of the rotor, multi-sensor data fusion is required—integrating heterogeneous measurements such as displacement, velocity, acceleration, and acoustic emission signals. (3) Controlling piezoelectric and electromagnetic actuators presents significant control-theoretic challenges. Piezoelectric actuators offer fast response and high positioning precision; however, their inherent nonlinearities—such as hysteresis and creep—complicate accurate modeling and degrade both open-loop accuracy and closed-loop control performance. Electromagnetic actuators, including magnetic bearings and active balancing units, are capable of generating large control forces, but exhibit strong nonlinear force characteristics and significant inter-axis coupling in multi-degree-of-freedom systems.

To address the aforementioned challenges, future technological development will focus on three key directions: intelligence, integration, and high performance: (1) With regard to smart materials and novel actuators, there is a pressing need to develop advanced smart materials, and improved linearity, thereby enhancing actuator performance and energy efficiency. Concurrently, research should prioritize the design of integrated actuator-sensor architectures operating under multi-physical-field coupling, enabling self-sensing and self-diagnostic capabilities in actuators and thus improving system reliability and maintainability. (2) Embedded artificial intelligence and edge computing represent a critical pathway toward intelligent control. Lightweight AI models—based on deep learning, reinforcement learning, and other advanced algorithms—should be embedded into low-level controllers to enable real-time vibration mode identification, automatic parameter tuning, and feedforward compensation, significantly enhancing the system’s autonomous decision-making capability. The adoption of a heterogeneous computing architecture combining multi-core processors and FPGAs is recommended. (3) Digital twin [9] and virtual sensing technologies offer transformative potential for system monitoring and control. High-fidelity digital twin models of rotor systems should be developed to enable dynamic, bidirectional mapping between physical and virtual domains, supporting predictive maintenance planning and online optimization of control parameters. Furthermore, virtual sensing techniques should be advanced to estimate critical but difficult-to-measure state variables.

5. Research on Control Strategies for Electric Spindle Motors

5.1. Wide-Speed Servo Control of Electric Spindle Motors

Field-weakening control is typically implemented by increasing the negative direct-axis current to enable flux weakening. Furthermore, by employing an enhanced space vector modulation technique to improve DC link voltage utilization, the speed regulation range can be effectively extended while simultaneously enhancing the motor’s output power capability.

5.1.1. Flux-Weakening Control Speed Regulation of Permanent Magnet Synchronous Motor for Spindle Motor

The flux-weakening control [10,11] strategies for permanent magnet synchronous electric spindle drives can be categorized into five distinct types, as presented in

Table 12. Classification of flux-weakening control strategies for permanent magnet synchronous electric spindle motors.

Classification	Core Concept	Main Approach
Closed-loop feedback method	Regulate the amplitude and phase angle of the voltage or current vector to ensure operation within the voltage limit circle boundary	Negative d-axis current compensation method, leading-angle field-weakening method, single-current regulator method
Model-based formula calculation method	The required d-axis current reference value for field weakening is directly computed using the mathematical model of the motor	Table lookup method, analytical formula method
Online search optimization method	The optimal operating point at the intersection of the voltage limit circle and the current limit circle is determined through online iterative computation	Gradient descent method, Newton–Raphson method
Model formula calculation method	Calculate the required d-axis current reference value for field weakening directly using the motor’s mathematical model	Table lookup method, formula method
Hybrid flux-weakening strategy	Improve and combine the existing methods to make up for the inherent limitations and deficiencies of a single method	Based on the improvement of the closed-loop feedback method, the combination of online search and intelligent control method

.

The closed-loop feedback method is the most widely adopted, classical, and technically mature control approach in the motor servo systems of CNC machine tool electric spindles. It strikes an effective balance between control performance and system complexity, and has been extensively integrated into mainstream commercial electric spindle servo drives, establishing itself as the standard configuration in current industrial applications. For a detailed comparison of its control performance, refer to

Table 13. Comparison of closed-loop feedback flux-weakening control performance.

Classification	Core Principle	Control Essence
Negative direct-axis current compensation method	An additional negative d-axis current reference is incorporated into the voltage loop output to compensate for the back electromotive force	Voltage sag is compensated through feedforward of the negative d-axis current
Leading angle flux-weakening control method	A negative d-axis current is indirectly generated by directly applying a leading voltage phase angle	The weak magnetic effect is achieved through voltage phase advancement
Single-current regulator control	Remove the q-axis current regulator and retain only the single d-axis current regulator	Control voltage vector angle η

.

In the study of negative direct axis current compensation flux-weakening control methods, the most fundamental implementation involves calculating the required negative d-axis current compensation by monitoring the DC bus voltage or output voltage to maintain voltage stability. However, when applied independently to electric spindle drive systems, this method is prone to cause current loop oscillations or even saturation and instability under deep field-weakening conditions. To address these limitations, researchers worldwide have proposed various improvements and optimizations based on the negative d-axis current compensation approach. Liang Jinhua and Xu Haiping [12] from the Institute of Electrical Engineering, Chinese Academy of Sciences developed an enhanced optimal field-weakening trajectory control algorithm incorporating modified negative d-axis current compensation. This method significantly improves the speed stability of electric spindle motors during high-speed field-weakening operation by adjusting the maximum torque per ampere (MTPA) curve and applying a first-order low-pass filter for preprocessing the U, V, and W phase current signals.

In the field of leading angle flux-weakening control research, the accurate determination of the advanced angle is crucial for effective control implementation. By adjusting the magnitude of the advanced angle, extended speed operation under weak magnetic conditions can be achieved. However, the current regulator is susceptible to saturation and instability under high-load conditions, and the optimal switching point during the transition from the constant-power weak magnetic region to the deep weak magnetic region remains challenging to identify with precision. Consequently, recent studies have increasingly focused on integrating advanced angle control with complementary control strategies. Reference [13] proposes a composite weak magnetic control scheme that combines the advanced angle with maximum torque per volt (MTPV) control, enabling coordinated regulation of d-axis and q-axis currents.

In the research of single-current regulator control methods, the evolution from the traditional dual current regulator to the single-current regulator aims to solve problems such as severe coupling of the quadrature and direct axis currents and the decline in system stability during high-speed operation. Among the three types of single-current regulator methods, the variable voltage vector angle method has the widest speed regulation range and good high-speed operation stability, which can effectively meet the working conditions of high-speed cutting of the electric spindle. The current research focus is mainly on the optimization of the load-carrying capacity and the realization of smooth and disturbance-free switching between the constant torque region, the constant power region, and the deep field-weakening region to avoid torque fluctuations or even loss of control at the moment of switching. Reference [14] proposes a hybrid single-current regulator control algorithm for permanent magnet synchronous motors in high-speed operation.

In the study of model-based formula calculation methods, the table lookup approach fundamentally relies on constructing a data mapping table derived from the relationships among flux linkage, torque, and current. In contrast, the formula method directly employs the motor’s mathematical model to compute the reference current for the next time step in real time based on its current operating state, offering faster transient response performance. However, this method is highly dependent on an accurate and well-characterized motor model, and is therefore limited to applications where motor parameters are precisely known and remain relatively constant during operation.

The online search optimization method is well suited for applications characterized by significant parameter variations and stringent requirements on speed regulation range and operational efficiency. Among the available techniques, the gradient descent method focuses on accurate identification of the field-weakening region and an effective correction strategy for the field-weakening current. While it exhibits relatively slow convergence, it offers excellent stability. In contrast, the Newton-Raphson method achieves faster convergence but entails a higher computational burden, necessitating support from high-performance hardware platforms such as digital signal processors (DSP) or field-programmable gate arrays (FPGA).

In intelligent control methods [15], fuzzy control-based weak magnetic strategies are particularly suitable for systems characterized by model uncertainty and strong nonlinearities. The results demonstrate that the adaptive FLSC effectively enhances DC bus voltage utilization and exhibits superior demagnetization performance. Furthermore, intelligent optimization techniques such as particle swarm optimization (PSO) and evolutionary algorithms can effectively accommodate the nonlinear behavior of motor parameters and variations across different motor units, enabling online optimization of response speed and operational efficiency under complex operating conditions. However, these methods rely on cost function evaluation and iterative computation, thereby imposing higher computational demands on the motor control processor.

The hybrid weak magnetic control algorithm [16] represents one of the most promising approaches in weak magnetic control, exhibiting significant potential for future advancement, and has emerged as a key research focus in the application of electric spindles for high-end CNC machine tools.

5.1.2. Expand the High-Speed Operation Range Through Modulation Strategies

The over modulation strategy [17,18] is designed to further improve the torque and power output capabilities of motors in the high-speed operating region by mitigating the limiting effects of back electromotive force (EMF) on drive performance, while maintaining effective voltage utilization under weak magnetic control. Theoretically, the six-step voltage mode can enhance DC bus voltage utilization by approximately 15%. To ensure a smooth transition from the linear modulation region to the over modulation region and ultimately into the six-step voltage mode, a robust switching mechanism must be implemented to minimize torque pulsations and speed transients.

Current research challenges in over modulation strategies primarily involve three key aspects: first, maximizing DC bus voltage utilization without compromising motor torque ripple characteristics, current harmonic content, and operational smoothness; second, effectively balancing the trade-off between dynamic performance and harmonic distortion; third, mitigating the impact of sampling blind spots during current measurement on the accuracy of phase current reconstruction. To address these challenges, Zhang Huixuan and Fan Tao from the University of Chinese Academy of Sciences [19] proposed a dynamic over modulation strategy that integrates system stability and dynamic responsiveness, enabling significant improvement in motor dynamic performance within the weak magnetic region while maintaining stable operation. Wang Wenjie and Yan Hao from Harbin Institute of Technology [20] enhanced the conventional vector pulse insertion method by leveraging hybrid pulse width modulation (PWM) technology, achieving accurate phase current reconstruction in the over modulation region and thereby extending the motor’s operational range.

5.1.3. Selection of Flux-Weakening Control Strategies for Permanent Magnet Synchronous Electric Spindle Motor

For the drive control of most electric spindles in CNC machine tools, closed-loop feedback control or its integration with model-based feedforward compensation—referred to as hybrid control strategies—is recommended as the preferred approach. These strategies achieve a favorable balance among technical maturity, dynamic performance, and system robustness. The negative d-axis current compensation method and the single-current regulator method have been extensively validated through industrial practice and demonstrate high engineering applicability. For high-end CNC applications demanding wide speed regulation ranges and high operational efficiency, online search optimization methods or intelligent control techniques may be considered. In contrast, for cost-sensitive economic drive systems, the single-current regulator method or advanced angle-based field weakening remains practically viable. With continuous advancements in chip computing capabilities and the evolution of advanced control technologies, deep learning-based online adaptive weak magnetic control is emerging as a promising future direction, capable of real-time learning of motor parameter variations and load dynamics to enable globally optimal efficiency operation.

The integration of over modulation strategy and field-weakening control represents a key enabling technology for enhancing the high-speed performance of electric spindles in CNC machine tools. This combined approach provides enhanced available voltage support for field-weakening operation, thereby preserving stronger air gap magnetic field regulation capability under identical DC bus voltage conditions. The synergistic interaction between these two techniques ensures stable delivery of required torque and power across the high-speed field-weakening operating range. However, when the electric spindle motor operates in deep field-weakening conditions, significant challenges arise, including increased current harmonics, notable reduction in motor efficiency, decreased power factor, elevated reactive power, and intensified thermal rise. Consequently, while achieving wide-range speed regulation remains a primary objective, it is equally critical to address the stringent demands of high-precision machining with respect to operational stability, low vibration characteristics, and effective thermal management. Therefore, the selection of a weak magnetic control strategy with superior comprehensive performance is essential.

5.2. High-Speed Response Control of the Electric Spindle Motor

5.2.1. Methods for Improving the Speed Response of Electric Spindle Motors

The control optimization techniques aimed at improving the speed response characteristics of the electric spindle motor are detailed in

Table 14. Control optimization methods for improving the speed response characteristics.

Classification	Core Principle	Applicable Scenarios
Current loop optimization	Enhance the bandwidth and dynamic response of the current loop to achieve accurate and rapid tracking of the torque command	fundamental requirement for ensuring precise torque output and high-speed dynamic performance
High-gain PID regulation	Increasing proportional gain of the speed loop enhances the system’s capability to correct speed errors	Supplementary strategy in conjunction with more advanced control algorithms
Feedforward control	The required control input is calculated in advance based on known reference commands and superimposed onto the system output	This method compensates for known system inertia, significantly reducing tracking errors
Model predictive control	Predicts future behavior over a finite time horizon and performs online rolling optimization to achieve optimal control performance	Applicable to high-end electric spindles requiring extremely high dynamic performance
Automatic Parameter Tuning	Key parameters are automatically identified by injecting test signals into the system or by analyzing operational data	Enables rapid adaptation of control parameters following tool changes or workpiece load variations

.

Current loop optimization serves as the foundation and a fundamental prerequisite for all advanced control functionalities. Feedforward control offers the highest cost-effectiveness in performance enhancement and has become a standard feature in modern high-end electric spindle drives. Model predictive control represents a forward-looking development direction, providing unmatched capabilities in managing multi-variable systems and nonlinear constraints, and is recognized as one of the core algorithms for next-generation ultra-high-performance drives. Automatic parameter tuning embodies system intelligence by significantly reducing the complexity of controller commissioning, thereby enabling electric spindles to adapt more effectively to varying operating conditions.

5.2.2. Reducing Acceleration and Deceleration Time of Electric Spindle Motors

Reducing the acceleration and deceleration time of an electric spindle motor primarily involves minimizing the moment of inertia, increasing acceleration torque, and enhancing deceleration torque. Priority should be given to reducing the motor’s own inertia, followed by minimizing the inertial load from tools and other accessories. High acceleration torque is achieved through a drive system capable of 2–3 times continuous overload, combined with Maximum Torque Per Ampere (MTPA) control and advanced deep flux-weakening control algorithms. Deceleration torque is predominantly determined by the braking method employed. A comparative classification of braking technologies is presented in

Table 15. Comparison of braking methods for electric spindle motors.

Method	Advantages and Disadvantages	Applicable Scenarios
Regenerative feedback braking	Returns energy to the power grid, providing smooth, high-torque braking with minimal heat dissipation; however, it entails higher initial costs	Recommended as the preferred solution for electric spindles subject to frequent start–stop cycles
Energy consumption braking	Offers low implementation cost, mature technology, and effective braking performance; however, it generates significant heat through the braking resistor	Widely adopted as the most economical and practical solution for mid-range electric spindle applications
DC Braking	Provides high braking torque and remains effective at zero speed; however, it induces significant motor heating and poses a risk of magnetic demagnetization	Auxiliary braking for asynchronous induction electric spindles or for precise standstill positioning at extremely low speeds
Reverse braking	The braking torque is huge, but it is extremely destructive and has a strong impact	Incompatible with the high-precision requirements of the electric spindle

.

Regenerative feedback braking is the preferred solution for high-performance applications, representing the advancement toward energy efficiency, operational efficiency, and superior performance. Despite its higher initial investment, it offers significant long-term savings in electricity costs and contributes to improved workshop environmental conditions. For cost-effective performance, dynamic resistance braking—utilizing a braking resistor—is the mainstream approach and the most widely adopted configuration in the current market, providing an optimal balance between technical requirements and economic considerations. Proper selection of the braking resistor’s power rating and allowable duty cycle is critical to system reliability and performance. In contrast, DC injection braking and plugging may induce irreversible damage to precision electric spindles, including excessive heat generation, magnetic demagnetization, and mechanical shock. These methods are rarely employed as standalone solutions in modern CNC machine tool spindle drive systems.

5.3. Speed Stability Control of the Motor for the Electric Spindle

Speed overshoot is a critical issue during dynamic operations. The primary control strategies are summarized in

Table 16. Comparison of methods for suppressing speed overshoot in electric spindle motors.

Method	Core Principle	Applicable Scenarios
Adjust the PID parameters	Increase system damping by reducing the proportional gain (P) or increasing the integral time (I), at the expense of response speed to reduce overshoot	It can only be used for final fine-tuning, as it sacrifices the dynamic performance of the electric spindle
Output differential negative feedback	Introducing the differential of the actual rotational speed as negative feedback can predict the overshoot trend in advance and suppress it, thereby increasing the damping of the actual output	Tiny noise fed back by the encoder of the electric spindle may cause significant interference after differentiation
IP speed controller	Eliminate differential term (D) of speed loop and retain only proportional (P) and integral (I) terms. Generate torque command by integrating error	large load disturbances, and an absolute zero tolerance for overshoot in precision grinding spindles
Proportional gain compensation	A high gain is used for large errors to achieve a rapid response, while a low gain is employed for small errors to prevent overshoot	It is applied in mid-range electric spindles, which can balance the response speed and stability
Variable gain scheduling	It automatically switches or smoothly transitions to the preset multiple sets of PID parameters based on the operating status of rotational speed and load current	Common functions of high-end electric spindles can ensure good stability during low-speed engraving and high-speed milling
State observer	Real-time estimation of load torque disturbance feedforward compensation using Romberg observer or Kalman filter algorithm	Suitable for suppressing overshoot and speed drop when heavy cutting or feed rate changes

.

The electric spindle domain imposes stringent requirements on overshoot suppression, as it directly influences machining accuracy, surface finish quality, and tool longevity. The selection of control methods varies significantly across different application scenarios. A high-performance electric spindle drive system typically employs a composite control strategy integrating feedforward control, state observers, and gain-scheduling techniques to achieve optimal dynamic performance with high speed, high precision, and minimal or no overshoot.

Speed fluctuation [21] refers to periodic or random variations in rotational speed during steady-state operation, primarily induced by external disturbances and mechanical resonances. To address periodic disturbances, a repetitive control strategy is employed, which learns the error from the previous cycle to correct the current cycle’s output. For load disturbances, a state observer is utilized to effectively mitigate speed fluctuations and dropouts caused by varying cutting forces, thereby ensuring stability during heavy cutting operations. In the case of resonances, notch filters and low-pass filters are implemented to identify and attenuate energy at specific mechanical resonance frequencies, preventing the excitation of vibrations. This approach is particularly effective in suppressing chatter arising from long tools or tool holders and in eliminating surface waviness on the machined workpiece.

Suppressing speed overshoot and rotational fluctuations in electric spindles constitutes a systematic engineering challenge that necessitates integrated design and optimization across four key domains: mechanical structure, feedback sensing, power electronics, and control algorithms. No single approach is sufficient to address all performance limitations. The success of modern high-end electric spindles relies on the seamless integration of these multidisciplinary technologies, enabling a transformative advancement from merely high-speed operation to comprehensive high-performance capability.

5.4. Thermal Error Prediction and Control of Electric Spindle

The traditional thermal error compensation [22] (TPEC) typically relies on static thermal models constructed offline, with its primary limitation being the inability to adapt to dynamic variations in machine tool operating conditions—such as fluctuations in ambient temperature, changes in coolant flow rate, and variable cutting parameters. The future direction of development lies in establishing a predictive thermal control system endowed with self-learning and self-adaptive capabilities. The core technical approaches encompass the following three aspects:

Development of an adaptive dynamic thermal model. Critical model parameters—including thermal resistance, thermal capacitance, and thermal deformation coefficient—should no longer be treated as constant but instead be estimated and continuously updated in real time using recursive least squares (RLS) or extended Kalman filter (EKF) techniques based on online sensor data. A lightweight EKF algorithm can be implemented within a PLC or embedded industrial PC to enable periodic parameter updates. A gray-box modeling approach is recommended: employing a physics-based heat conduction model as the foundational framework to ensure extrapolation reliability under varying operational conditions, while simultaneously incorporating neural networks to model residual errors dynamically. These residual states should be integrated into the Kalman filter’s state vector, thereby achieving synergistic integration of physical constraints and data-driven corrections.

Implementation of advanced control strategies with embedded predictive functionality. The adaptive thermal model should serve as the prediction engine within a model predictive control (MPC) framework. This enables the controller not only to correct current thermal errors but also to generate a sequence of preemptive compensation commands through rolling optimization, leveraging future-time-horizon information such as planned machining trajectories and spindle speed profiles, thus actively mitigating anticipated thermal drift. In this context, long short-term memory networks (LSTM) or temporal convolutional networks (TCN) can be employed to extract features and forecast trends from multivariate time-series data—including temperature and power inputs—thereby enhancing the system’s foresight into future thermal behavior.

System-level integration for closed-loop control. Most existing TPEC systems remain confined to feedforward open-loop compensation schemes. True closed-loop control necessitates direct measurement of the relative displacement between the tool and workpiece at the contact point. To this end, a non-contact in-machine measurement system based on laser displacement sensors should be developed to rapidly assess critical dimensional features during machining intervals, with the resulting data fed back to the thermal model for online recalibration. This facilitates coordinated execution of thermal compensation commands in conjunction with PLC logic and cooling unit control, ultimately forming a fully integrated intelligent thermal management system that unifies perception, modeling, prediction, decision-making, and actuation.

6. Conclusions

This comprehensive review and analysis of electric spindle motor design and control technologies yields the following conclusions:

The integrated optimization of electromagnetic performance and topological structure is aimed at achieving ultra-high precision and operational smoothness by prioritizing high power density and efficiency. This approach extends the processing application range through wide-speed-range motor designs, enhances cutting torque via improved high-speed power output, and leverages optimized dynamic response and control performance as critical enablers for improving production efficiency. The high-speed operation of electric spindles represents a key development trend in the future. Electric spindles developed by British WESTWIND and German GMN have already achieved maximum rotational speeds exceeding 250,000 r/min. Driven by continuous advancements in cutting tool technology and high-speed feed systems, the output power of electric spindles under high-speed operating conditions continues to increase—exemplified by the TSE series from GMN, Germany, which delivers a maximum power output of over 350 kW. Concurrently, there is a growing trend toward high-torque designs, where electric spindles are required not only to achieve high rotational speeds but also to provide maximized output torque at low-speed ranges. A representative case is the electric spindle integrated into the MEGA-8800 heavy-duty horizontal machining center manufactured by MAZAK Corporation of Japan, which achieves a peak output torque of up to 1249 N·m.

Thermal design in electric spindle motors represents a synergistic integration of minimized electromagnetic losses—achieved through electromagnetic design—and maximized heat dissipation capacity derived from structural design. Key research directions encompass loss and heat source characterization, thermal path optimization, innovative cooling architectures, and the effective application of cooling media. Advancements in cooling technology have led to the development of innovative approaches—including phase change cooling, multi-channel cooperative cooling, bionic cooling channels, 3D-printed customized flow paths, and composite thermal conductive materials—that are progressively extending the limits of heat dissipation performance. Concurrently, advanced oil-based lubrication and oil-air lubrication techniques are significantly improving operational reliability and durability under high-speed conditions. Thermal management systems are increasingly shifting toward thermal compensation and thermal suppression strategies, with focused research efforts on developing artificial intelligence-driven models for spindle thermal deformation prediction, enabling accurate forecasting and active control of temperature rise behaviors.

Rotor strength and dynamic behavior are investigated through multi-physics coupling simulations involving electromagnetic fields, stress distributions, thermal effects, and rotor dynamics. Research efforts primarily focus on enhancing the mechanical strength of rotor components and permanent magnet materials, ensuring structural integrity through process-structure co-design, advancing rotor topology optimization, and conducting comprehensive dynamic and vibration analyses. From a technological advancement perspective, the field is progressing toward online dynamic balancing by integrating adaptive filtering algorithms and artificial intelligence-based diagnostic models, enabling systems to autonomously learn and accurately identify imbalance characteristics under varying operating conditions. Concurrently, research is advancing in adaptive vibration suppression, with emphasis on wideband and multi-source vibration sensing and separation techniques, integrated with intelligent actuators and energy recovery mechanisms to achieve a transformative shift from passive compensation to active control. Furthermore, the trend is moving toward intelligent and multifunctional integration, where multidimensional sensor data—including vibration, temperature, displacement, current, and acoustic signals—are fused to construct comprehensive fault databases. These databases support the training of deep neural networks and transfer learning models, thereby enhancing the generalization capability of diagnostic systems across diverse operating conditions and electric spindle configurations.

In wide-speed-range servo control, the implementation of advanced deep field-weakening algorithms, combined with efficient DC bus voltage regulation techniques, enables an extended speed regulation range while simultaneously increasing the motor’s output power capability. An online parameter identification algorithm has been developed to enable real-time compensation for flux linkage variations induced by inductance saturation and temperature fluctuations. For the coordinated optimization of voltage and current limit circles, an online optimal current trajectory planning algorithm based on model predictive control is proposed, together with a voltage reconstruction technique that accounts for inverter nonlinearities, thereby enhancing the system’s dynamic response and control precision. In the context of collaborative optimization between DC bus voltage regulation and field-weakening control, a Lyapunov stability theory-based cooperative controller is designed to achieve dynamic temporal coordination between the two control loops, ensuring stable system operation across the full speed range.

High-speed response control in advanced electric spindle drive systems typically integrates high-bandwidth current loops, feedforward control, automatic parameter adaptation, model predictive control, and regenerative braking. These control elements are hierarchically coordinated to ensure superior performance in high-speed operation, high-precision positioning, and rapid dynamic response. The adoption of high-switching-frequency power semiconductor devices such as GaN and SiC provides the necessary physical foundation for high-bandwidth control. Delay compensation technology is introduced to effectively mitigate the inherent control delay of 1.5 to 2 switching cycles in digital control systems. Furthermore, energy efficiency management strategies are optimized through the implementation of “peak shaving and valley filling” energy scheduling, enabling one axis in the braking mode to simultaneously supply energy to another axis during acceleration, thereby facilitating internal energy circulation and efficient utilization across multiple axes.

Rotational speed stability in high-performance electric spindle drive systems is achieved through composite control strategies that integrate feedforward control, state observers, and gain scheduling techniques. These strategies enable optimal dynamic performance characterized by high speed, high accuracy, and minimal or negligible overshoot. Effective suppression of speed overshoot and rotational speed fluctuations necessitates holistic design and optimization across four critical domains: mechanical structure, feedback sensing, power electronics, and control algorithms. To suppress speed fluctuations during low-speed operation, a more accurate nonlinear inverter model is established, and a corresponding online dead-time compensation algorithm is developed to eliminate torque ripple induced by switching delays and device voltage drops. Based on the outputs of a state observer or online parameter identification, a parameter-adaptive gain scheduling strategy is implemented to enable real-time dynamic adjustment of controller gains, thereby ensuring that the control system consistently operates at its optimal performance point.