I. Stepper Motor Selection Principles
1. First, determine the torque required for the stepper motor to drive the load. The simplest method is to add a lever to the load shaft, pull the lever with a spring scale, and multiply the pulling force by the lever arm length to get the load torque. Alternatively, it can be calculated theoretically based on the load characteristics. Since stepper motors are control motors, the maximum torque of commonly used stepper motors currently does not exceed 45 Nm. The higher the torque, the higher the cost. If the torque of the motor you choose is large or exceeds this range, you can consider adding a reduction gear.
2. Determine the maximum operating speed of the stepper motor. The speed is crucial when selecting a stepper motor. A characteristic of stepper motors is that torque decreases as the motor speed increases. The rate of torque decrease depends on many parameters, such as the driver's drive voltage, the motor's phase current, the motor's phase inductance, and the motor size. Generally, the higher the drive voltage, the slower the torque decreases; the larger the motor's phase current, the slower the torque decreases. In the design, the motor speed should be controlled at 1500 rpm or 1000 rpm. However, this is not a very precise description; please refer to the section on "Torque-Frequency Characteristics."
3. Based on the two important indicators of maximum load torque and maximum speed, and referring to the <torque-frequency characteristics>, you can select a suitable stepper motor. If you feel that the motor you have chosen is too large, you can consider adding a reduction gear, which can save costs and make your design more flexible. To select a suitable reduction ratio, you need to comprehensively consider the relationship between torque and speed to choose the optimal solution.
4. Finally, it is also necessary to consider leaving a certain amount of torque margin (such as 30%) and speed margin.
5. Try to choose a hybrid stepper motor, as its performance is higher than that of a reactive stepper motor.
6. Try to select a microstepping driver and make the driver work in microstepping mode.
7. When selecting a motor, avoid the misconception of only looking at the motor torque. In other words, it is not always better to have a larger motor torque. The speed should also be considered.
8. When a higher speed is required, a driver with a higher drive voltage can be selected.
9. When purchasing, there are no specific requirements for whether to use two-phase or three-phase power supplies, as long as the step angle meets the usage requirements.
II. Parameters to Consider When Selecting a Stepper Motor
Interpretation of Stepper Motor Parameters and Characteristics
1. Step error
It refers to the difference between the measured step angle and the theoretical step angle under no-load conditions. It reflects the accuracy of the stepper motor's angular displacement.
The step error of domestically produced stepper motors is generally within the range of ±10′ to ±30′, while the step error of high-precision stepper motors can reach ±2′ to ±5′.
2. Maximum static torque
This refers to the maximum applied torque that a stepper motor can withstand when one phase is always energized and the motor is stationary; that is, the maximum electromagnetic torque it can output. It reflects the stepper motor's braking capability and load capacity during low-speed stepping operation.
3. Start-up torque-frequency characteristics
This refers to the relationship between the maximum step input pulse frequency (also known as the starting frequency) that a stepper motor can accept to start normally without losing steps when there is an external load torque, and the load torque.
4. Activate inertial frequency characteristics
This refers to the relationship between the starting frequency and the moment of inertia when a stepper motor drives a purely inertial load to start.
5. Operating torque-frequency characteristics
This refers to the relationship between the output torque and the input pulse frequency when a stepper motor is running. When selecting a stepper motor, the operating frequency and the operating point corresponding to the load torque should be below the operating torque-frequency characteristic to ensure that the stepper motor operates normally without losing steps.
6. Stepping motion and low-frequency oscillation
When the input pulse frequency is very low, if the pulse period is greater than the transient process time of the stepper motor, the stepper motor will operate in a step-and-stop state, which is called stepping operation. Stepper motors have a relatively low natural frequency. When the stepping frequency or low-speed operating frequency is equal to or close to this natural frequency, resonance will occur, causing the stepper motor to oscillate and stop moving. This phenomenon is called low-frequency oscillation.
Methods to avoid low-frequency oscillations:
One approach is to make the operating frequency avoid the natural frequency. Another approach, if the former is not feasible, is to change the natural frequency by adjusting the damper on the stepper motor.
7. Maximum phase voltage and maximum phase current
These refer to the maximum power supply voltage and the maximum current that can be applied to each phase winding of the stepper motor, respectively.
