Stepper motor maintenance

Stepper motors are widely used as actuators in motion control. When many users use a stepper motor, they worry that the motor will generate too much heat when it is working, and do not know whether it is normal. In stepping motors, heating during operation is a very common situation, but what kind of heating is considered normal, and how to reduce the heating of the stepping motor.

Why a stepper motor generates heat: The inside of a stepper motor is composed of an iron core and a winding coil. Winding coils have resistance and heat loss occurs when energized. Loss is proportional to resistance and current, and is commonly referred to as copper loss. If the current is not standard direct current, harmonic losses will also occur. The iron core has a hysteresis eddy current effect, and loss also occurs in an alternating magnetic field. Loss is related to material, current, frequency and resistance, and is usually called iron loss. Copper loss and iron loss are manifested in the form of heat, which will affect the efficiency of the motor. Stepping motors pursue positioning accuracy and torque output. The efficiency is relatively low, the current is usually relatively large, and the harmonics are relatively high, and the frequency of current alternation also changes with speed. Therefore, stepping motors generally generate heat, and the situation is more serious than that of ordinary AC motors. How to control the heating of the stepper motor within a reasonable range, the heating degree of the motor mainly depends on the internal insulation level of the motor, high temperature (above 130 degrees) will destroy the internal insulation performance. Therefore, as long as the interior does not exceed 130 degrees, the motor will not be damaged, and the surface temperature of the motor is less than 90 degrees. Therefore, it is normal for the maximum temperature of the stepper motor surface to reach 70-80 degrees. The rough judgment method of the surface temperature of the motor: touch it by hand for more than 2 seconds, less than 65 degrees; after touching about 70-80 degrees by hand, it will immediately retract; after dripping, it will quickly evaporate, greater than 90. The temperature gun can be used for precise measurement.

When the heat of the stepping motor changes with the speed, when using constant current drive, the current of the stepping motor will remain relatively constant at rest and at low speed to maintain a constant torque output. When the speed increases, the internal back electromotive force of the motor will also increase, the current will gradually decrease, and the torque will also decrease. Heat is generally higher at static and low speeds, and lower at high speeds. However, the iron loss (although it accounts for a small proportion) will change under different circumstances, and the total heat generation of the motor is the sum of the two, so the above is just a general situation.

The heating of the motor usually does not affect the life of the motor, and most customers do not need to worry. But severe fever will bring some negative effects. For example, the different thermal expansion coefficients of the internal parts of the motor will lead to changes in the structural stress and small changes in the internal air gap, which will affect the dynamic response of the motor, and it is easy to lose step at high speeds. Another example is in certain occasions where the motor is not allowed to generate too much heat, such as medical equipment and high-precision test equipment. Therefore, the heating of the motor should be controlled reasonably.

How to reduce the heating of the motor, reducing the heating is to reduce the copper loss and iron loss. There are two main points to reduce copper loss and reduce resistance and current. When choosing a model, you need to choose a motor with low resistance and low rated current. For two-phase motors, a series connection can be used instead of a parallel connection, but this usually contradicts the requirements of torque and high speed. For the selected motor, make full use of the static half current function and enable the control function of the drive. Static half current means that when the motor is in a static state, the current is automatically halved. The enable function can stop the current output when the motor stops. . Using a subdivision driver can make the current waveform closer to a sine wave, with fewer harmonics and less heat. The way to reduce iron loss is to use as low voltage as possible. Although high-voltage drive motors will bring improvements in high-speed characteristics, they will also increase iron loss. When selecting a motor, please consider various parameters in combination with torque usage, high-speed performance and calorific value.

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