The heating problem of stepper motor and its treatment

    (1) Why does the stepper motor heat up?

    For various stepping motors, the interior is composed of iron core and winding coils. The winding has resistance, and power will produce loss. This loss is called copper loss. The size of copper loss is proportional to the square of the resistance and the current. If the current is not a standard DC or sine wave, harmonic losses will also occur. The iron core has a hysteresis eddy current effect, and loss will also occur in an alternating magnetic field. Its size is related to the material, current, frequency, and voltage. This is called iron loss. Copper loss and iron loss will show up in the form of heat, which affects the efficiency of the motor. Stepper motors generally pursue positioning accuracy and torque output, with relatively low efficiency, relatively large currents, and high harmonic components. The frequency of current alternation also changes with the speed. Therefore, stepping motors generally have heat generation, and the situation is more than normal. The AC motor is serious.

    (2) Reasonable range of stepping motor heating

    The allowable degree of motor heating mainly depends on the internal insulation level of the motor. The internal insulation performance will be destroyed at high temperature (above 130°C). Therefore, as long as the internal temperature does not exceed 130°C, the motor will not be damaged, and the surface temperature will be below 90°C.

    (3) The effect of speed change on the heating of stepper motor

    When using constant current drive technology, the current of the stepper motor will remain relatively constant at static and low speed to maintain constant torque output. When the speed is high to a certain level, the internal back EMF of the motor will increase, the current will gradually decrease, and the torque will also decrease. Therefore, the heat generated by copper loss is related to speed. The heat is generally high at static and low speeds, and low at high speeds. However, the iron loss (although it accounts for a small proportion) changes in a different situation, and the entire heat generation of the motor is the sum of the two, so the above is just a general situation.

    (4) The effect of fever

    Although the heating of the motor generally does not affect the life of the motor, it is unnecessary for most customers to bother. But severe fever will bring some negative effects. For example, the different thermal expansion coefficients of the internal parts of the motor 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 will easily lose step at high speed. Another example is that in some occasions, excessive heating of the motor is not allowed, such as medical equipment and high-precision test equipment. Therefore, necessary control should be performed on the heating of the motor.

    (5) How to reduce the heating of the motor

    To reduce heat is to reduce copper loss and iron loss. There are two ways to reduce copper loss, reducing resistance and current. This requires the selection of motors with small resistance and low rated current when selecting models. For two-phase motors, motors that can be connected in series do not need to be connected in parallel. But this often contradicts the requirements of torque and high speed. For the selected motor, you should make full use of the drive's automatic half-current control function and offline function. The former automatically reduces the current when the motor is in a static state, and the latter simply cuts off the current. In addition, since the current waveform of the subdivision drive is close to sinusoidal, there are fewer harmonics, and the motor will generate less heat. There are not many ways to reduce iron loss. The voltage level is related to it. Although the high-voltage drive motor will bring about the improvement of high-speed characteristics, it also brings an increase in heat generation. Therefore, the appropriate drive voltage level should be selected, taking into account the high speed, stability, heat generation, noise and other indicators.

(Huiputech)