Servo motors are generally categorized into stepper motors, brushed DC servo motors, brushless DC servo motors, and AC servo motors. With continuous technological advances, many special servo motors have emerged, such as piezoelectric ceramic motors, linear motors and voice coil motors. This article mainly discusses the selection of servo motors in the conventional sense.

The choice of servo motor largely depends on the physical characteristics of the load, load operating characteristics, system requirements and working environment. Once the system requirements are confirmed, regardless of the type of servo motor selected, the first consideration is to determine the appropriate motor size, which mainly relies on the physical properties of the load including load torque and inertia. For servo motors, size is usually measured by torque or force. Therefore, motor selection starts with calculating the load torque or force reflected to the motor shaft. A safety margin must be reserved after torque calculation; generally, the continuous torque of the selected motor shall be no less than 1.3 times the load torque to ensure reliable motor operation. In addition, the load inertia reflected to the shaft must be calculated. As a general guideline, the ratio of load inertia to motor rotor inertia should be less than 5:1 to guarantee fast response of the servo system. If inertia and torque mismatch exists between the motor and the load, the only solution is to sacrifice speed and install a reducer between the motor and load, which requires comprehensive trade-offs.

After determining the required motor torque, it is necessary to analyze the load operating characteristics and working environment. Load operating characteristics cover operating speed (high or low), required acceleration, demand for frequent start-stop cycles, cycling frequency, system motion accuracy and other parameters. There are no fixed rules for servo motor selection at this stage. The core principle is that the selected motor must satisfy the motion requirements of the load.

For instance, stepper motors serve as an ideal option for systems with low precision requirements, operating speeds ranging from several revolutions up to 500 rpm (higher than 1 rpm), and no demand for frequent start-stop operations. This is because stepper motors adopt open-loop control with limited positioning accuracy. Their torque drops drastically at excessively high speeds, resulting in insufficient driving capacity for loads, while low speeds may trigger discontinuous rotational crawling. Furthermore, stepper motors feature slow response and are unsuitable for applications requiring frequent start-ups.

DC or AC servo motors are preferred for applications with speeds from several revolutions to under 3000 rpm and relatively high control accuracy. In most cases, AC servo motors exhibit inferior low-speed performance compared with DC servo motors. For loads running at low speeds, DC servo motors are recommended. Nevertheless, brushed DC motors generate commutating sparks via brush commutation, making them inapplicable in vacuum, explosion-proof, underwater and similar environments. Moreover, such sparks cause thermal expansion of the motor shaft and connected components, disqualifying brushed DC motors from high-precision systems.

Permanent magnet synchronous AC servo motors are widely adopted in industrial applications. Thanks to their constant-torque characteristic below rated speed, they are extensively used in scenarios with constant or slightly fluctuating load torque, such as machine tool feed systems. Motor selection is relative: either AC or DC servo motors can be viable for the same application, and the final decision sometimes hinges on environmental conditions. For certain robotics projects where AC power supply is difficult to access, DC servo motors become the only feasible solution.

Conventional servo motors struggle to meet the demands of many special working conditions. For example, ultra-low-speed stable operation (in some cases as low as several revolutions per year) cannot be achieved by standard servo motors, and torque motors are required instead. For applications demanding frequent start-stops, rapid response and high acceleration, ordinary servo motors are inadequate. The frequent start-stop frequency of loaded standard AC servo motors generally does not exceed 5 Hz, whereas linear motors deliver superior performance, capable of achieving acceleration up to 30G and start-stop frequencies reaching 20 Hz.

The fundamental rule of motor selection is to fully understand load characteristics, working environment and motor performance attributes — only in this way can a suitable servo motor be chosen.