As a digital actuator, stepper motors are widely adopted in motion control systems. Many users notice considerable heat generation during stepper motor operation and wonder whether such phenomenon is normal. In fact, heating is common for stepper motors. However, what temperature range is regarded as acceptable, and how can heat generation be minimized?

First, let us analyze the causes of heat generation in stepper motors.All types of stepper motors consist of iron cores and winding coils. Windings possess resistance and produce power loss when energized. The loss is proportional to resistance and the square of current, commonly known as copper loss. If the current waveform is not standard DC or sine wave, harmonic loss will also occur. The iron core generates loss due to hysteresis and eddy current effects under alternating magnetic fields. Such loss, termed iron loss, is related to material properties, current, frequency and voltage. Both copper loss and iron loss manifest in the form of heat and reduce motor efficiency. Stepper motors are generally designed for positioning accuracy and torque output, resulting in relatively low efficiency, high operating current and abundant harmonic components. In addition, the alternating frequency of current varies with rotational speed. Consequently, stepper motors routinely generate significant heat, more severely than ordinary AC motors.

Second, controlling stepper motor temperature within a reasonable range.The permissible operating temperature mainly depends on the motor’s internal insulation class. Internal insulation will degrade only at temperatures above 130°C. Therefore, the motor will not suffer damage as long as its internal temperature stays below 130°C, corresponding to a surface temperature lower than 90°C. In practice, a surface temperature of 70–80°C is normal for stepper motors.Temperature can be measured with a point thermometer, or roughly judged by touch:

• If the surface can be touched by hand for 1–2 seconds or longer, the temperature is below 60°C;

• If only momentary contact is tolerable, the temperature is approximately 70–80°C;

• If water droplets vaporize instantly upon contact, the temperature exceeds 90°C.A thermal gun can also be used for accurate measurement.

Third, variation of heat generation with rotational speed.When constant-current driving technology is applied, the current remains relatively constant at standstill and low speeds to maintain constant torque output. Once the speed rises above a certain threshold, the back EMF inside the motor increases, causing gradual reduction of current and torque. Hence, heat generated from copper loss varies with speed: heat is normally higher at standstill and low speeds, and lower at high speeds. Nevertheless, the variation of iron loss (which accounts for a smaller proportion) follows a different trend. Since total heat generation equals the sum of copper loss and iron loss, the above rule only serves as a general reference.

Fourth, impacts of motor heating.Moderate heating generally does not shorten motor service life and can be ignored for most applications. Nevertheless, excessive heating may trigger adverse effects. Different thermal expansion coefficients of internal components lead to changes in structural stress and tiny variations of internal air gap, which degrade dynamic response and easily cause step loss at high speeds. Furthermore, some applications prohibit excessive motor heating, such as medical equipment and high-precision testing instruments. Thus, necessary measures should be taken to control motor temperature.

Finally, methods to reduce motor heat generation.Reducing heat generation means lowering copper loss and iron loss.There are two approaches to cut copper loss: reducing resistance and operating current. During motor selection, preference should be given to motors with low resistance and low rated current. For two-phase stepper motors, series connection is preferred over parallel connection where applicable. However, this often conflicts with torque and high-speed performance requirements.For already selected motors, users should make full use of the driver’s automatic half-current control function and de-energization function. The former automatically reduces current when the motor is stationary, while the latter completely cuts off winding current. Moreover, microstep drivers produce less motor heat because their current waveform approximates a sine wave with fewer harmonics.

Few solutions are available to reduce iron loss, which is related to drive voltage level. Although high-voltage driving improves high-speed performance, it also increases heat generation. Accordingly, an appropriate drive voltage grade shall be selected to balance high-speed performance, running smoothness, heat generation, noise and other indicators.