Stepper motor heating principle and acceleration and deceleration process control technology

Heat generation principle of stepper motor.

 

 

1, usually see all kinds of motors, the internal are iron core and winding coil. The winding has resistance, energized will produce loss, the size of the loss is proportional to the square of the resistance and current, which is often referred to as copper loss, if the current is not the standard DC or sine wave, will also produce harmonic loss; core has hysteresis eddy current effect, in the alternating magnetic field will also produce loss, its size and material, current, frequency, voltage, which is called iron loss. Copper loss and iron loss will be manifested in the form of heat, thus affecting the efficiency of the motor. Stepper motors generally pursue positioning accuracy and torque output, efficiency is relatively low, the current is generally relatively large, and high harmonic components, the frequency of current alternation also varies with the speed, and thus stepper motors generally have heat, and the situation is more serious than the general AC motor.

2, the reasonable range of stepper motor heat.

Motor heat allowed to what extent, mainly depends on the motor internal insulation level. Internal insulation performance at high temperatures (130 degrees or more) before being destroyed. So as long as the internal does not exceed 130 degrees, the motor will not lose the ring, and the surface temperature will be below 90 degrees at this time.

Therefore, the stepper motor surface temperature in 70-80 degrees are normal. Simple temperature measurement method useful point thermometer, you can also roughly determine: with the hand can touch more than 1-2 seconds, not more than 60 degrees; with the hand can only touch, about 70-80 degrees; a few drops of water quickly vaporized, it is more than 90 degrees.

3, stepper motor heating with speed changes.

When using constant current drive technology, stepper motors at static and low speed, the current will remain constant to maintain a constant torque output. When the speed is high to a certain level, the motor's internal counter potential rises, the current will gradually drop, and the torque will also drop.

Therefore, the heating condition due to copper loss will be speed dependent. Static and low speed generally generate high heat, while high speed generates low heat. But the iron loss (although a smaller proportion) changes are not the same, and the motor as a whole heat is the sum of the two, so the above is only the general situation.

4, the impact of heat.

Although the motor heat generally does not affect the life of the motor, the majority of customers do not need to pay attention to. But seriously will bring some negative impact. Such as the different coefficients of thermal expansion of the internal parts of the motor lead to changes in structural stress and small changes in the internal air gap, will affect the dynamic response of the motor, high-speed will be easy to lose step. Another example is that some occasions do not allow the excessive heat of the motor, such as medical equipment and high-precision test equipment, etc.. Therefore, the heat of the motor should be necessary to control.

5, how to reduce the heat of the motor.

Reduce heat generation, is to reduce copper loss and iron loss. Reduce copper loss in two directions, reduce the resistance and current, which requires the selection of small resistance and rated current of the motor as much as possible, the two-phase motor, the motor can be used in series without parallel motor. But this often contradicts the requirements of torque and high speed. For the selected motor, the drive's automatic half-current control function and offline function should be fully utilized, the former automatically reduces the current when the motor is at rest, and the latter simply cuts off the current.

In addition, the subdivision drive, because the current waveform is close to sinusoidal, less harmonics, motor heating will also be less. There are few ways to reduce iron loss, and the voltage level is related to it. Although a motor driven by high voltage will bring an increase in high-speed characteristics, it also brings an increase in heat generation. So we should choose the right drive voltage level, taking into account the high speed, smoothness and heat, noise and other indicators.

Control techniques for acceleration and deceleration processes of stepper motors.

With the widespread use of stepper motors, the study of stepper motor control is also increasing, in the start or acceleration if the stepper pulse changes too quickly, the rotor due to inertia and not follow the electrical signal changes, resulting in blocking or lost step in the stop or deceleration for the same reason may produce overstepping. In order to prevent blocking, loss of step and overshoot, improve the working frequency, the stepper motor to lift the speed control.

The speed of a stepper motor depends on the pulse frequency, the number of rotor teeth and the number of beats. Its angular speed is proportional to the pulse frequency and is synchronized in time with the pulse. Thus, if the number of rotor teeth and the number of running beats are certain, the desired speed can be obtained by controlling the pulse frequency. Since the stepper motor is started with the help of its synchronous torque, the starting frequency is not high in order not to lose step. Especially as the power increases, the rotor diameter increases, the inertia increases, and the starting frequency and the maximum running frequency may differ by as much as ten times.

The starting frequency characteristics of the stepper motor so that the stepper motor start can not directly reach the operating frequency, but to have a start-up process, that is, from a low speed gradually ramp up to the operating speed. Stop when the operating frequency can not be immediately reduced to zero, but to have a high-speed gradual speed reduction to zero process.

 

The output torque of the stepper motor decreases with the rise of the pulse frequency, the higher the starting frequency, the smaller the starting torque, the poorer the ability to drive the load, the start will cause a loss of step, and in the stop will occur when the overshoot. To make the stepper motor quickly reach the required speed and not lose step or overshoot, the key is to make the acceleration process, the acceleration torque required to make full use of the torque provided by the stepper motor at each operating frequency, and not to exceed this torque. Therefore, the operation of the stepper motor generally has to go through the acceleration, uniform speed, deceleration three stages, the acceleration and deceleration process time as short as possible, the constant speed time as long as possible. Especially in the work requiring a rapid response, from the starting point to the end of the running time required to be the shortest, which must require acceleration, deceleration process is the shortest, while the highest speed at constant speed.

 

Scientists and technicians at home and abroad have conducted a lot of research on the speed control technology of stepper motors, and established a variety of acceleration and deceleration control mathematical models, such as exponential model, linear model, etc., and on the basis of this design and development of a variety of control circuits to improve the motion characteristics of stepper motors, to promote the application range of stepper motors exponential acceleration and deceleration takes into account the inherent moment-frequency characteristics of stepper motors, both to ensure that the stepper motor in movement without losing step, but also give full play to the inherent characteristics of the motor, shorten the lift speed time, but due to changes in motor load, it is difficult to achieve while linear acceleration and deceleration only consider the motor in the load capacity range of angular velocity and pulse proportional to this relationship, not due to fluctuations in the supply voltage, load environment and the characteristics of the change, this speed-up method of acceleration is constant, the disadvantage is that it does not fully consider the stepper motor output torque With the characteristics of speed change, stepper motor at high speed will occur out of step.

 

This is an introduction to the heating principle and acceleration/deceleration process control technology of stepper motors.

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