Stepper Motor vs Servo Motor: Torque-Speed ​​Characteristics and Selection for Industrial Applications

1. Introduction

In modern engineering, drive motor selection is a critical factor that directly influences the accuracy, speed and reliability of automated systems. For demanding sectors such as aerospace and energy, where the slightest failure can have significant operational and economic consequences, the choice between a stepper motor and a servo motor is fundamental. This technical article examines in depth the torque-speed characteristics of these two technologies, offering a rigorous guide for informed selection, essential to the robustness and sustainability of industrial installations. Understanding their intrinsic principles allows maintenance engineers and plant managers to optimize equipment performance and minimize unplanned downtime, thereby contributing to compliance with industry standards like NF EN 60204-1.

2. Fundamental Principles

2.1. Stepper Motor

The stepper motor is a brushless synchronous motor that converts electrical pulses into discrete angular movements. Its operation is based on the magnetic attraction between a toothed rotor and a stator composed of windings. Each electrical pulse sequentially activates the stator windings, moving the rotor a fixed angle called a "pitch." This open loop mode of operation does not require a position sensor for applications with low torque requirements, as long as the motor is not overloaded.

• Working principle: The stator generally contains 4 to 8 electromagnetic poles. The rotor is a permanent magnet or variable reluctance structure. The sequential powering of the stator coils by a driver creates a rotating magnetic field which attracts the rotor.
• Control: The control is mainly open loop. The driver sends a sequence of pulses to the motor, and each pulse corresponds to a mechanical step. The position is inferred by the number of steps taken. Micro-stepping modes allow the angular resolution to be increased, typically up to 256 micro-steps per integer step.
• Torque-Speed ​​Characteristic: The stepper motor has a high maximum torque at standstill (holding torque) and at very low speed. This torque decreases rapidly as speed increases. At high speeds, the motor loses the ability to maintain the necessary torque, potentially leading to loss of steps and therefore positioning errors. For example, a NEMA 23 stepper motor may offer 2 Nm of holding torque, but only 0.2 Nm at 500 rpm.

2.2. Servo Motor

The servo motor is a closed-loop drive system designed for precise control of position, speed and torque. It consists of a motor (often brushless, permanent magnet synchronous type), a feedback sensor (encoder or resolver), and a controller (servodrive). The controller compares the desired position/speed to the value measured by the sensor and adjusts the motor power accordingly.

• Operating principle: The motor generates torque proportional to the input current. The encoder (e.g., 20 bits for a resolution of 1,048,576 pulses/rev) provides instantaneous feedback of rotor position and speed to the servo drive.
• Control: Closed-loop control generally uses PID (Proportional-Integral-Derivative) algorithms to minimize the error between the setpoint and the measurement. This continuous feedback ensures high precision and rapid dynamic response, capable of correcting disturbances in real time.
• Torque-Speed ​​Characteristic: The servo motor maintains a constant rated torque over a much wider speed range than the stepper motor, before the torque begins to decrease at very high speeds. It is also capable of providing peak torque several times higher than its nominal torque for acceleration or transient loads. A 750 W servomotor can thus deliver a nominal torque of 2.4 Nm up to 3000 rpm, with a peak torque of 7.2 Nm.

3. Technical Specifications and Standards

The selection and integration of motors into industrial systems are governed by strict standards to ensure safety, performance and interoperability. Key technical specifications include resolution, torque (rated and peak), rated speed, inertia and response time.

3.1. Key Parameters

• Resolution: For stepper motors, it is defined by the number of steps per revolution (eg: 200 steps/revolution, or 1.8°/step). In micro-stepping mode, this resolution can be increased electronically. For servomotors, the resolution is determined by the encoder (eg: 17 bits equals 131,072 positions per revolution).
• Rated Torque: The torque that the motor can produce continuously without overheating. It is expressed in Newton meters (Nm).
• Peak Torque: The maximum torque that the motor can provide temporarily (a few seconds) for acceleration. Typically 2 to 3 times the rated torque for servo motors.
• Rated Speed: The maximum speed at which the motor can operate while delivering its rated torque.
• Inertia: The resistance of the rotor to changes in motion. Low inertia allows rapid acceleration and deceleration.
• Response Time: The time it takes for the motor to reach a target position or speed. Servo motors have response times of the order of milliseconds, compared to tens of milliseconds for steppers.

3.2. Applicable Standards

Compliance with standards is an imperative in French and European industry, particularly in regulated sectors. UNITEC-D ensures that distributed components meet these requirements.

• NF EN 60034-1:2018 (Rotating electrical machines - Part 1: Rated characteristics and operating characteristics): This standard specifies the rated characteristics (voltage, current, power, speed, torque) and performances of rotating electrical machines, including motors. It is essential for evaluating the ability of a motor to operate under defined conditions.
• NF EN 60204-1:2018 (Safety of machines - Electrical equipment of machines - Part 1: General requirements): This AFNOR standard is critical for the integration of motors into machines. It covers functional safety and electrical aspects, including emergency stop devices and overcurrent protection.
• Machines Directive 2006/42/EC (CE Marking): All motors intended for the European market must comply with this directive, evidenced by the CE marking, ensuring compliance with essential health and safety requirements.
• NF EN ISO 13849-1:2016 (Safety of machines - Safety-related parts of control systems - Part 1: General design principles): For applications where safety is paramount (e.g. emergency stops, safe speed limits), this standard is fundamental for the design of motor control circuits.
• NF EN 60529:2000 (Degrees of protection provided by the enclosures - IP code): The protection of motors against the intrusion of solid and liquid bodies is specified by the IP code, essential for harsh industrial environments. For example, an IP65 motor is protected against dust and water jets.

4. Selection and Sizing Guide

Optimal selection of a motor requires careful analysis of application requirements. This section presents the engineering criteria and a decision matrix.

4.1. Selection Criteria

• Precision and Repeatability:Applications requiring extremely precise (micron) and reproducible movements are directed towards servomotors (e.g.: CNC machine tools). For precisions of the order of a tenth of a millimeter, stepper motors may be sufficient.
• Speed ​​and Acceleration: Dynamic applications with fast cycles and high acceleration/deceleration require servo motors. Slow, controlled movements are suitable for stepper motors.
• Required Torque: Applications requiring constant torque over a wide speed range or high peak torque for frequent starts/stops favor servo motors. Steppers are effective for moderate torques at low speeds.
• Cost and Complexity: Stepper motors are generally cheaper and simpler to implement (no PID adjustment). Servo motors, with their controller and encoder, represent a higher initial investment and require finer adjustment.
• Power Efficiency: Servomotors are generally more efficient, consuming only the power needed to maintain position or speed, which is an advantage in continuous operation. Stepper motors draw current even when stopped to maintain their holding torque.
• Environment: The ability of a motor to operate in hostile environments (temperature, humidity, vibrations) is crucial. Industrial servomotors are often designed to meet specific certifications such as ATEX for explosive atmospheres or Nadcap for aerospace.

4.2. Sizing Formulas

Sizing begins by calculating the dynamic requirements of the application.

Acceleration torque (T_acc):

Tacc = (Jtotal * Δω) / Δt

• J_total: Total inertia (load + motor), in kg·m².
• Δω: Change in angular velocity, in rad/s.
• Δt: Acceleration time, in seconds.

Load torque (T_load): Includes friction torque, gravitational torque, etc.

Maximum torque required (T_max):

Tmax = Tacc + Tcharge

The chosen motor must have a peak torque greater than T_max and a nominal torque sufficient for continuous operation.

4.3. Application Decision Matrix

The following table provides a comparative view to guide selection based on typical industry requirements.

5. Good Installation and Commissioning Practices

Installation and commissioning in accordance with the manufacturer's specifications and current standards are imperative for the performance and lifespan of motorized systems.

5.1. Mechanical Installation

• Precise Alignment: Couplings must be aligned to an accuracy of 0.05mm to avoid excessive radial and angular stresses on the motor bearings. The use of flexible bellows or reed type couplings can compensate for slight misalignments while maintaining rigid torque transmission.
• Rigid Mounting: Fix the motor on a flat, rigid surface to minimize unwanted vibrations and resonances, in accordance with the NF EN ISO 10816-3 standard relating to the evaluation of machine vibrations.
• Heat Dissipation: Ensure adequate ventilation. Industrial motors are often designed for a maximum ambient temperature of 40°C. Operation beyond this limit can significantly reduce the life of the winding insulation (halving for every 10°C increase).

5.2. Electrical Wiring

• Shielded Cables: Use EMC (ElectroMagnetic Compatibility) type shielded cables for the power and feedback connections of the servomotors in order to prevent electromagnetic interference, in accordance with the requirements of the NF EN 61000-6-4. The shielding must be earthed correctly and over 360°.
• Grounding: Proper, low impedance grounding of the entire system (motor, driver, machine) is essential for safety and suppression of electrical noise.
• Cable Separation: Maintain a minimum distance (e.g. 200mm) between power cables and signal (encoder) cables to avoid inductive coupling.

5.3. Commissioning and Adjustments

• Adjusting the Stepper Driver: For stepper motors, adjust the driver current according to the required torque and the temperature allowed by the motor. Use micro-stepping for smoother and more precise movements, but be careful of the associated torque reduction.
• Servo Motor PID Tuning: Proportional, Integral and Derivative gain tuning is critical to servo motor performance. Improper adjustment can cause oscillation, overshoot, or slow response. Auto-tuning functions integrated into modern servo drives make this step easier. Gain values ​​should be adjusted to minimize tracking error and ensure system stability, with a typical settling time of 20 ms for rapid positioning applications.
• Functional Tests: Perform motion tests under load and empty to validate the behavior of the system under different conditions, verifying positioning accuracy and the absence of resonances.

6. Failure Modes and Root Cause Analysis

Identifying and understanding failure modes is crucial for preventative maintenance and reducing downtime. UNITEC-D offers solutions to minimize these risks.

6.1. Stepper Motor

• Pitch Loss: Occurs when the load exceeds the available torque of the motor, usually at high speeds or when accelerating too aggressively. Visualization: The motor is not positioned correctly, progressive position shift. Cause: Mechanical overload, motor undersizing, insufficient driver supply voltage.
• Overheating: Excessive current or insufficient heat dissipation may damage the coil insulation. Visualization: Motor housing hot to the touch, burning smell. Cause: Driver current poorly adjusted, duty cycle too high, ventilation obstructed.
• Resonance: At certain speeds, the stepper motor may resonate, resulting in excessive vibration and noise. Visualization: Abnormally loud noise, significant mechanical vibrations. Cause: Insufficient mechanical rigidity, poorly adapted load inertia, absence of electronic damping (micro-steps).

6.2. Servo motor

• Encoder Failure: The encoder provides position feedback. Its failure results in loss of control of the servodrive. Visualization: Servo drive alarm (e.g. “Encoder Fault”), erratic movements or motor shutdown. Cause: Damaged wiring, contamination (dust, oil), mechanical wear (encoder bearings), EMC interference.
• Motor/Drive Overheating: May result from prolonged overload or poor heat dissipation. Visualization: Drive overtemperature alarm, hot motor box. Cause: Insufficient sizing, excessive duty cycle (especially for peak torque), blocked ventilation, fan failure.
• Oscillations / Overshoots: Improper adjustment of the servodrive PID parameters can cause system instability. Visualization: The motor oscillates around the target position, jerky movements. Cause: PID gains too high (excessive Proportional gain), load inertia underestimated.
• Bearing Failure: Motor bearing wear is a common cause. Visualization: Abnormal noise (squeaking, whistling), increased vibration, increased temperature near the bearings. Cause: Contamination, excessive radial or axial load, lack of lubrication, service life exceeded.

7. Predictive Maintenance and Condition Monitoring

The implementation of predictive maintenance strategies makes it possible to detect anomalies before failure, thus optimizing equipment availability and operational safety, in accordance with certification requirements such as Nadcap for aeronautics.

7.1. Monitoring Techniques

• Vibration Analysis (NF EN ISO 10816): Fundamental technique for detecting bearing wear, imbalances, misalignments or coupling problems. Accelerometer sensors mounted on the motor body or load track frequency spectra and identify fault signatures. For example, an increase in vibration amplitude at specific frequencies may indicate bearing degradation (cage, ball or raceway failure).
• Infrared Thermography: Allows detection of abnormal hot spots on the motor, driver or electrical connections, a sign of overheating due to overload, excessive friction or poor connection. A rise of 10°C above normal may indicate an impending problem.
• Motor Current Monitoring (MCSA - Motor Current Signature Analysis): Analysis of motor current harmonics can reveal mechanical faults (broken rotor bars, bearing problems) or electrical faults (phase imbalances, winding faults) before they are audible or visible.
• Encoder Signal Quality Monitoring: For servomotors, analysis of the encoder output signal (amplitude, cleanliness, phase shift between channels) can anticipate sensor failure due to contamination or wear.
• Servo Drive Data Analysis: Modern servo drives record event logs and performance data (tracking errors, average torque, current peaks, internal temperature). Analyzing this data makes it possible to identify degradation trends and optimize settings.

7.2. Outils et Systèmes

The integration of sensors and SCADA (Supervisory Control and Data Acquisition) systems or IIoT (Industrial Internet of Things) platforms enables continuous monitoring and real-time data analysis, facilitating condition-based maintenance.

8. Comparison Matrix: Stepper Motor vs Servo Motor

This matrix provides a direct comparison of key performance to help make the final decision.

9. Conclusion

The distinction between stepper motor and servo motor does not come down to an intrinsic superiority of one over the other, but to a rigorous adequacy with the specific requirements of the application. Servo motors excel in tasks requiring high dynamics, sub-millimeter precision and sustained torque over a wide speed range, criteria common in aerospace or critical power generation equipment. Stepper motors, on the other hand, offer a cost-effective and simple solution for low-speed positioning and moderate loads, where occasional step loss is tolerable or managed by a simple feedback system. A thorough analysis of torque-speed curves, load inertia, work cycles and safety standards (NF EN 60204-1, NF EN ISO 13849) is essential. UNITEC-D, as a specialist supplier of MRO components, has the expertise and a complete range of motors and their control systems to meet the most demanding specifications of French industry. For reliable and compliant integration of your motorized systems, it is essential to choose the partner who guarantees quality, performance and compliance with the highest standards.

Explore our e-catalog to discover our drive solutions and engineering services tailored to your specific needs: https://www.unitecd.com/e-catalog/

10. References

• NF EN 60034-1:2018. Rotating electrical machines - Part 1: Rated characteristics and operating characteristics. AFNOR.
• NF EN 60204-1:2018. Safety of machinery - Electrical equipment of machinery - Part 1: General requirements. AFNOR.
• NF EN ISO 13849-1:2016. Safety of machinery - Safety-related parts of control systems - Part 1: General design principles. AFNOR.
• KENJO, Tak. Stepping Motors and their Microprocessor Controls. 3rd edition, Oxford University Press, 2000.
• BOLDEA, Ion; NASAR, Syed A. Electric Drives. CRC Press, 2005.