Causes of motor heating.
Causes of motor heating.

As a digital actuator, stepping motor is widely used in motion control system. Many users feel that the shell of the stepping motor is hot when they use it. They want to know why the stepping motor is hot. In fact, heating is a common phenomenon of stepper motors, but what is the normal degree of heating and how to minimize the heating of stepper motors?

 

First of all, we need to understand what causes the stepping motor to be hot and hot. For all kinds of stepping motors, the interior is composed of iron core and winding coil. The winding has resistance, and power on will cause loss. The loss is proportional to the square of resistance and current. This 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. This 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 and high harmonic components. The frequency of current alternating also changes with speed. Therefore, stepper motors generally have heating, which is more serious than general AC motors. Under static and low speed conditions, the current of the stepping motor will remain relatively constant to maintain the constant torque output. When the speed reaches 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 heating caused by copper loss is related to the 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 overall heating of the motor is the sum of the two. To sum up, the causes of stepping motor heating are as follows:

 

1. The current set by the driver is greater than the rated current of the motor

 

2. Motor speed too high

 

3. The inertia and positioning torque of the motor itself are large, so even the medium speed operation will generate heat, but it will not affect the service life of the motor. The demagnetization point of the motor is 130-200 ℃, so the motor is normal at 70-90 ℃. As long as it is less than 130 ℃, there is generally no problem. If you really feel overheating, set the current of the driver to about 70% of the rated current of the motor or reduce the speed of the motor.

 

So, how to control the heating of stepping motor within a reasonable range? The allowable degree of motor heating mainly depends on the internal insulation grade of the motor. The internal insulation performance will be damaged at high temperature (above 130 ℃). Therefore, as long as the internal temperature does not exceed 130 ℃, the motor will not be damaged, and the surface temperature will be below 90 ℃. Therefore, the surface temperature of the stepping motor is normal at 70-80 ℃. The simple temperature measurement method can be roughly judged by using a point thermometer: it can be touched by hand for more than 1-2 seconds, not more than 60 degrees; It can only be touched by hand, about 70-80 degrees; A few drops of water vaporize rapidly, and the temperature is above 90 degrees; Of course, it can also be detected with a temperature measuring gun.

 

In this case, how to reduce the motor heating, that is, how to reduce copper loss and iron loss. There are two ways to reduce copper loss and reduce resistance and current, which requires that the motor with low resistance and low rated current should be selected as far as possible during model selection. For two-phase motors, the motor that can be connected in series does not need to be connected in parallel. But 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 the subdivision driver is close to sinusoidal, there are less harmonics, and the motor will generate less heat. There are few ways to reduce iron loss. The voltage level is related to it. Although the high-voltage driven motor 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.

 

Will you consider whether the heating phenomenon has any impact on the motor? Although the motor heating generally does 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, the structural stress and the internal air gap will change slightly, which will affect the dynamic response of the motor, and the high-speed will be easy to out of step. 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.

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