The Heating Problem of the Stepper Motor and It's Solution

1. The Heating Reason for The Stepper Motor 

As for various stepper motors, it’s internal structure consists of an iron core and winding coils. There is resistance in the windings, the current will cause a loss, which is proportional to the resistance the square of the current. If the current is not a standard current or sine wave, it will produce a harmonic loss. There is a hysteresis eddy effect in the iron core, which will produce a loss in the alternating magnetic field and it is related to the material, current, frequency, voltage, which is an iron loss. The copper loss and iron loss are in the form of heat, which will affect the efficiency of the motor. The stepper motor pursues positioning accuracy and torque output, the efficiency is relatively low, the current is relatively large, and the harmonic components are high. The frequency of alternating current will change with speed, so the stepper motor has a heating condition, which is more serious than the AC motor.

2. The Reason Scope of Heating of The Stepper Motor

The heating scope of the motor is allowed to what extent, which mainly depends on the internal insulation level. The performance of internal insulation will be damaged at high temperatures(above 130°)。 So, as long as the internal temperature is not above 130°, the stepper motor will not be damaged, the surface temperature will be under 90°. So, the surface temperature ranges from 70 to 80°, it is normal. The temperature measurement method can be thermometer or hand: 

(1) you can touch it with your hand for more than 1-2 seconds, the temperature is no more than 60°. 

(2) You can touch it with your hand, the temperature is about 70-80°.

(3) A few of water vaporizes quickly, the temperature is more than 90°.

3. The Changing of The Heating Problem of The Stepper Motor with Speed

By constant current drive technology, when the stepper motor is under the condition of static and low speed, the current will remain relatively constant, which will remain constant torque output. When the speed reaches some extent, the Back EMF will rise, the current will drop, and torque also. So, the heating caused by copper loss is related to speed. When the motor is in the static and low speed, the heating is high, when the motor is at high speed, the heating is low. But the iron loss is different. 

4. The Effect Caused by Heating

The heating of the motor will not affect life. However, the server condition will have a bad impact. If the coefficient of thermal expansion of each part in the motor is different, which will contribute to change in the structural stress and internal gap, and affect the dynamic response and lose a step in high speed. If some application such as medical apparatus and high precision equipment does not allow excessive heating, the heating should be controlled. 

5. How to Reduce Heating of The Stepper Motor

To reduce heating is to reduce copper loss and iron loss. The decrease of copper loss has two ways: reducing resistance and current, which requires that the resistance and rated current of the motor should be small. For a two-phase phase motor, choosing a series motor instead of the parallel motor which is contrary to the torque and speed. For the selected motor, you should take full advantage of the driver’s automatic half current control function and off-line function. The former automatically reduces current when the motor in the static, the latter will cut off the current.

Besides, the current waveform of the subdivision driver is close to the sine wave, the harmonic is less and the motor heating is relatively small. Although the high voltage can improve high-speed performance, the heating will increase.

So, in the selection process of drive, the criteria such as heating, noise, stability should consider.