As a digital actuator, stepping motor is widely used in motion control system. When using the stepping motor, many user friends feel that the motor has a large fever when working. They have doubts about whether this phenomenon is normal. In fact, heating is a common phenomenon of stepping motor, but what kind of heating degree is normal and how to reduce the heating of stepping motor as much as possible?
First of all, we should understand why the stepping motor is hot
For all kinds of stepping motors, the interior is composed of iron core and winding coil. If the winding has resistance, power on will produce loss, and the loss is directly proportional to the square of resistance and current, which is what we often call copper loss. If the current is not standard DC or sine wave, it will also produce harmonic loss; The iron core has hysteresis eddy current effect and will also produce loss in alternating magnetic field. Its size is related to material, current, frequency and voltage, which is called iron loss. Copper loss and iron loss will be expressed in the form of heating, which will affect the efficiency of the motor. Stepper motors generally pursue positioning accuracy and torque output, with low efficiency, large current, high harmonic component, and the frequency of current alternating also changes with speed. Therefore, stepper motors generally have heating, which is more serious than ordinary AC motors.
Moreover, the heating of stepping motor is controlled within a reasonable range, and the allowable degree of motor heating mainly depends on the internal insulation level of the motor. The internal insulation performance will be damaged at high temperature (above 130 degrees). Therefore, as long as the internal temperature does not exceed 130 degrees, the motor will not be damaged, and the surface temperature will be below 90 degrees. Therefore, the surface temperature of stepping motor is normal at 70-80 degrees. The simple temperature measurement method can be used with a point thermometer or a rough judgment: you can touch it with your hand for more than 1-2 seconds, no more than 60 degrees; It can only be touched by hand, about 70-80 degrees; If a few drops of water vaporize rapidly, it will be more than 90 degrees; Of course, it can also be detected with a temperature measuring gun.
Third, when the heating of the stepping motor changes with the speed, when the constant current driving technology is adopted, the current of the stepping motor will remain relatively constant at static and low speed to maintain the constant torque output. When the speed is high to a certain extent, the back EMF inside the motor increases, the current will gradually decrease, and the torque will also decrease. Therefore, the heating caused by copper loss is related to speed. Generally, the heating is high at static and low speed, and low at high speed. However, the change of iron loss (although it accounts for a small proportion) is not always the case, and the whole heating of the motor is the sum of the two, so the above is only a general case.
Fourth, the impact of heating. Although the motor heating generally will not affect the service life of the motor, it is unnecessary for most customers to pay attention to it. However, severe fever can have some negative effects. If the thermal expansion coefficient of each part of the motor is different, it will lead to the change of structural stress and small change of internal air gap, which will affect the dynamic response of the motor, and it will be easy to out of step at high speed. For example, excessive heating of the motor is not allowed in some occasions, such as medical devices and high-precision testing equipment. Therefore, the heating of the motor should be controlled as necessary.
Finally, reducing the heating of the motor is to reduce copper loss and iron loss. There are two directions to reduce copper loss and reduce resistance and current, which requires that motors with low resistance and low rated current should be selected as far as possible. For two-phase motors, motors that can be connected in series do not need parallel motors. However, this often conflicts with the requirements of torque and high speed. For the selected motor, the automatic half current control function and offline function of the driver shall be fully utilized. The former automatically reduces the current when the motor is static, and the latter simply cuts off the current.
In addition, because the current waveform of subdivision driver is close to sinusoidal, there are less harmonics and less motor heating. There are few ways to reduce iron loss. The voltage level is related to it. Although the motor driven by high voltage will improve the high-speed characteristics, it will also increase the heating. Therefore, appropriate driving voltage level should be selected, taking into account high-speed, stability, heating, noise and other indicators.