Accurate electric motor sizing is one of the most critical phases in industrial machine design. Underestimating power or torque can compromise the entire production line, while oversizing leads to energy waste and higher costs. Choosing the right motor—stepper, brushless, or DC—depends on mechanical, electrical, and functional parameters that determine automation system efficiency and reliability.
INDEX
1. Importance of correct motor sizing
2. Key parameters for sizing calculations
3. Torque and application load analysis
4. Selecting the motor type: stepper, brushless, or DC
5. Influence of speed and acceleration
6. Power and efficiency evaluation
7. Effects of torque ripple and dynamic response
8. Thermal sizing and heat dissipation
9. Motor driver and control: a key factor not to overlook
10. Practical sizing example
11. Ever Motion Solutions motion control solutions
12. Toward smarter and more automated sizing
1. Importance of Correct Motor Sizing
Sizing an electric motor is not just about rated power—it is about balancing efficiency, service life, and performance. An undersized motor can overheat, generate vibrations, and reduce the useful life of the mechanical system; conversely, an oversized motor increases energy consumption and makes thermal management more complex. In industrial applications where precision and repeatability are essential, the right balance between torque, inertia, and electronic control is decisive.
2. Key Parameters for Sizing Calculations
Sizing starts with defining the main mechanical parameters:
- Required torque (Nm): determined by the load and the motion profile.
- Angular speed (rpm): linked to the machine cycle time.
- Load inertia (kg·cm²): affects accelerations and decelerations.
- Gear ratio and its efficiency: define the relationship between the motor and the final motion.
Environmental parameters also matter, such as operating temperature, vibration levels, and the motor IP protection rating.
3. Torque and Application Load Analysis
The motor must be able to provide sufficient continuous torque to move the load and peak torque to handle acceleration phases. The torque–speed curve helps verify that the operating point remains within a safe operating area.
Load types:
- Constant loads: such as conveyors or pumps, with stable torque demand.
- Variable loads: such as robots and pick & place systems.
- Inertial loads: with fast accelerations and decelerations.
An incorrect torque analysis can cause motion instability, vibration, and excessive current draw.
4. Selecting the Motor Type: Stepper, Brushless, or DC
Motor type selection is strictly connected to the required performance:
- Stepper motor: ideal for precise motion at low speed. It provides direct position control without an encoder, but it may suffer from step loss if overloaded.
- Brushless motor (BLDC): suitable for high speed and dynamic profiles. It offers constant torque, low maintenance, and high efficiency.
- DC motor: simple and versatile, suitable for linear motion or constant-torque regulation.
Ever Motion Solutions offers all these motor types, allowing designers to select the most efficient configuration based on the duty cycle.
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5. Influence of Speed and Acceleration
Rated speed and required accelerations directly affect the choice of both the motor and the driver. A motor with high peak torque enables fast starts, but it can generate oscillations if not properly controlled. In servo systems, high-resolution encoders enable fine motion-profile control and precise dynamic response even in the shortest cycles.
6. Power and Efficiency Evaluation
Required power is calculated as:
P (W) = (T × ω) / 9.55, where T is torque in Nm and ω is speed in rpm.
Motor efficiency (η) impacts power supply and driver sizing. Brushless motors reach efficiencies above 90%, while stepper motors typically have lower efficiency but higher low-speed torque. Proper sizing also accounts for losses due to friction, transmissions, and mechanical tolerances.
7. Effects of Torque Ripple and Dynamic Response
Torque ripple—i.e., periodic torque variation—can reduce motion accuracy in high-resolution systems. It is important to select motors and drivers that minimize this effect through:
- Vector control (FOC)
- Sinusoidal commutation / sinusoidal drive
- High-resolution encoder feedback
Ever Motion Solutions integrates these solutions into its brushless motors and programmable drivers, ensuring constant and smooth torque even under variable load conditions.
8. Thermal Sizing and Heat Dissipation
During operation, part of the electrical energy is converted into heat. If not properly dissipated, heat can damage windings and bearings. Therefore, it is necessary:
- to verify the ambient temperature of the working environment
- to ensure adequate ventilation or passive cooling
- to use high-IP motors only when needed, to avoid overheating due to excessive sealing
9. Motor Driver and Control: A Key Factor Not to Overlook
The driver is an integral part of the sizing process. The system’s effective torque depends on the driver’s ability to provide steady current and accurately regulate the motion profile. Ever Motion Solutions manufactures drivers with fieldbus options (CANopen, Modbus RTU, EtherCAT, Profinet, Ethernet/IP, Powerlink, Modbus TCP/IP) as well as programmable versions, compatible with PLC and embedded systems, ensuring seamless integration in industrial processes.
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10. Practical Sizing Example
Assume we need to size a motor for a rotary table with:
- Load: 5 kg
- Radius: 0.1 m
- Acceleration: 500 rad/s²
Required torque is:
T = J × α = (m × r²) × α = (5 × 0.1²) × 500 = 25 Nm
If maximum speed is 1000 rpm, required power is:
P = (25 × 1000) / 9.55 ≈ 2,600 W
A 20–30% safety margin should be added to account for friction and losses. The selected motor should therefore have a rated torque of about 30 Nm and a power rating of 3,200 W.
11. Ever Motion Solutions Motion Control Solutions
- 2- and 3-phase hybrid stepper motors, also available with brake, encoder, or gearbox.
- AC and DC brushless motors with high torque density and low torque ripple, also available with brake, encoder, or gearbox.
- Programmable and fieldbus drives for complex automated systems.
- Motors with integrated electronics, ideal for compact and modular solutions.
Each product is supported by a technical team that assists customers in defining sizing parameters and validating the project.
12. Toward Smarter and More Automated Sizing
Correct electric motor sizing is a key factor in ensuring machine efficiency, reliability, and long-term durability. The combination of mechanical analysis, electronic simulation, and real-world testing makes it possible today to design motors that are increasingly precise and high-performing.
With the evolution of digital control technologies and Industry 4.0 systems, sizing is moving toward greater automation, with software capable of adapting motor parameters to the motion profile in real time.