Common problems and solutions of stepper motors -Part 2

In the previous article, we analyzed some common problems of stepper motors and their solutions, but we will also encounter other problems during use. Next, we will introduce these problems one by one.

1. Q: Why does the torque of a stepper motor decrease as the speed increases?

Answer: When the stepper motor rotates, the inductance of each phase winding of the motor will form a reverse electromotive force; the higher the frequency, the greater the reverse electromotive force. Under its action, the phase current of the motor decreases as the frequency (or speed) increases, resulting in a decrease in torque.

2. Q: What is the accuracy of stepper motors? Is it cumulative?

Answer: Generally, the accuracy of stepper motors is 3-5% of the step angle, and it is not cumulative.

3. Q: What is the allowable surface temperature of a stepper motor?

Answer: If the temperature of the stepper motor is too high, the magnetic material of the motor will first demagnetize, resulting in a decrease in torque and even loss of step. Therefore, the maximum temperature allowed on the surface of the motor should depend on the demagnetization point of the magnetic material of different motors; generally speaking, the magnetic material The demagnetization point of the stepper motor is above 120 degrees Celsius, and some are even as high as above 200 degrees Celsius, so the surface temperature of the stepper motor is completely normal at 70-80 degrees Celsius.

4. Q: Why can the stepper motor run normally at low speeds, but cannot start and make a whistling sound if it is higher than a certain speed?

Answer: The stepper motor has a technical parameter: no-load starting frequency, which is the pulse frequency at which the stepper motor can start normally under no-load conditions. If the pulse frequency is higher than this value, the motor cannot start normally and may lose steps or block. change. Under load, the starting frequency should be lower. If you want the motor to rotate at high speed, the pulse frequency should have an acceleration process, that is, the starting frequency is low, and then rises to the desired high frequency at a certain acceleration (the motor speed increases from low speed to high speed).

5. Q: How to overcome the vibration and noise of the two-phase hybrid stepper motor when running at low speed?

Answer: The inherent shortcomings of stepper motors include loud vibration and noise when rotating at low speeds. Generally, the following solutions can be adopted:

• If the stepper motor happens to work in the resonance zone, the resonance zone can be avoided by changing the reduction ratio and other mechanical transmission;
• Use a driver with subdivision function, which is the most common and simplest method;
• Change to a stepper motor with a smaller step angle, such as a three-phase or five-phase stepper motor;
• Switching to AC servo motors can almost completely overcome vibration and noise, but the cost is higher;
• Add a magnetic damper to the motor shaft. This product is already on the market, but the mechanical structure has changed significantly.

6. Q: What is the difference between the series connection method and the parallel connection method of the four-phase hybrid stepper motor and driver?

Answer: Four-phase hybrid stepper motors are generally driven by two-phase drivers. Therefore, the four-phase motor can be connected into two phases using series connection or parallel connection. The series connection method is generally used in situations where the motor speed is low. The required driver output current is 0.7 times the motor phase current, so the motor generates less heat. The parallel connection method is generally used in situations where the motor speed is high (also known as high-speed connection). method), the required driver output current is 1.4 times the motor phase current, so the motor generates greater heat.

7. Q: What are the differences in the application scenarios of two-phase and five-phase hybrid stepper motors?

Answer: Generally speaking, two-phase motors have large step angles and good high-speed characteristics, but there is a low-speed vibration zone. The five-phase motor has a small step angle and operates smoothly at low speed. Therefore, five-phase motors should be selected when the motor's operating accuracy is required to be high, and mainly in the medium and low speed range (generally less than 600 rpm); conversely, if the high-speed performanceof the motor is pursued, there is no need for precision and stability. In situations with too many requirements, a lower-cost two-phase motor should be used. In addition, the torque of five-phase motors is usually above 2NM. For applications with small torques, two-phase motors are generally used. The problem of low-speed smoothness can be solved by using subdivided drives.