An In-Depth Analysis of the ST Series Permanent Magnet Synchronous Servo Motors

On the automated production lines of 2026, if variable-frequency motors are the “muscles” of industry, then the ST series of permanent magnet synchronous servo motors are its “fingers.” No longer content with merely “turning,” they are designed for ultra-fast start-up and stopping, absolute positioning accuracy, and extremely high dynamic response.

As the flagship products of Xi'an Xima Motors and Lu'an Jianghai Motors in the field of precise drive, the ST series motors represent the cutting-edge level of domestic servo motors in the field of high-performance motion control.

一、Core Definition: What is the ST Series Servo?

The ST series is a permanent magnet synchronous motor (PMSM) specifically designed for high-performance, high-response applications. The key differences between this series and standard industrial motors lie in the optimization of its “moment of inertia” and the precision of its “feedback system.”

S (Servo): Represents servo control, emphasizing closed-loop feedback, and capable of precisely executing position, speed, and torque instructions. 

T (Torque/Tough): Represents high torque density and strong dynamic overload capacity.

二、Technical Core: The Leading Edge of the ST Series

1. Exceptional dynamic response (low-inertia design)

The rotors of the ST series motors have been specially lightweighted and electromagnetically optimized to achieve extremely low rotational inertia.

Performance: The motor accelerates from a standstill to its rated speed (typically 1500 rpm or 3000 rpm) in just a few milliseconds.

Application Benefits: In CNC machine tool feed systems with frequent forward and reverse cycling, or in high-speed reciprocating packaging machinery, ultra-fast response translates to higher production rates.

2. Ultra-high torque density (compact design)

By utilizing high-performance neodymium-iron-boron (NdFeB) permanent magnet materials, the ST series delivers remarkable torque within an extremely compact footprint.

Comparison: For the same output torque, the ST Series occupies only one-third to one-quarter of the volume of a traditional asynchronous motor. This is critical for collaborative robot joints or all-in-one automation systems where space is limited.

3. Precise feedback pulses

The ST series typically comes standard with high-resolution encoders (such as 23-bit multi-turn absolute encoders).

Accuracy: Each revolution can be subdivided into millions of pulses, enabling positioning accuracy at the micrometer ($mu m$) or even nanometer ($nm$) level.

No Cumulative Error: Absolute encoders ensure that even after a power loss and restart, the motor immediately knows its exact position without needing to seek a mechanical home position.

三、Performance Parameter Matrix (for reference)

四、Shift to Application: The "Hardcore" Battlefield of the ST Series

Although servo motors are ubiquitous, the ST series is more commonly found in applications with extremely demanding requirements for precision and cycle time:

1. CNC Machine Tools and Metalworking

In multi-axis machining centers, the ST series drives lead screws for high-speed feed. It works seamlessly with the host control system to ensure precise cutting paths, which directly determines the surface finish of the workpiece.

2. Collaborative Robots and Industrial Arms

Leveraging the ST series’ high torque density and low cogging torque, robots can achieve exceptionally smooth motion. Particularly in precision assembly (such as mobile phone component assembly), ST motors endow robotic arms with “embroidery-like” precision.

3. Laser Cutting and Printing/Packaging

Laser cutting heads require extremely frequent fine adjustments to maintain focus. The ST series’ ultra-short response time ensures clean, precise cuts, even during complex free-form cutting operations.

五、The technological support from Simo and Jianghuai

As leading manufacturers in Northwest and East China, Simo and JiangHuai have incorporated unique manufacturing processes into the production of the ST series:Vacuum Pressure Impregnation (VPI): Even our small servo motors undergo high-standard impregnation to ensure that the winding insulation does not age under the electromagnetic stress caused by long-term high-frequency acceleration and deceleration.

Precision Dynamic Balancing: The motor achieves an extremely high level of dynamic balance, reducing mechanical vibration during high-speed rotation and protecting the optical accuracy of the encoder.

Electromagnetic Compatibility (EMC) Design: To address high-frequency interference generated by servo drives, Simo has implemented special shielding and grounding measures within the motor to ensure stable control signals.

六、Selection Tips: How to Accurately Select the Model?

When selecting the ST series motors, not only the power should be considered, but also the relationship in the following formula must be taken into account:

Determine the rated torque ($T$): particularly the static friction force that needs to be overcome and the instantaneous peak torque during acceleration.

Matching load inertia ratio: The ratio of load inertia to motor inertia is recommended to be controlled within 5:1 (lower ratios are preferable in high-performance scenarios) to achieve the best dynamic stability.

Feedback interface: Based on the requirements of the control system, choose incremental, absolute value or line-sensing encoders.

The ST series permanent magnet synchronous servo motors not only serve as power sources but also form the cornerstone of precise motion control. In the technical blueprint of SIMO and Jianghuai Motors, the ST series is evolving towards smaller size, higher response, and lower heat generation.