Motor type selection is very simple, but also very complicated. This is a problem that involves a lot of convenience. If you want to quickly select the type and get the result, experience is the fastest.
In the mechanical design automation industry, the selection of motors is a very common problem. Many of them have problems in the selection, either too large to waste, or too small to move. It’s okay to choose a big one, at least it can be used and the machine can run, but it’s very troublesome to choose a small one. Sometimes, in order to save space, the machine leaves a small installation space for the small machine. Finally, it is found that the motor is selected to be small, and the design is replaced, but the size cannot be installed.
In the mechanical automation industry, there are three types of motors used most: three-phase asynchronous, stepper, and servo. DC motors are out of scope.
Three-phase asynchronous electricity, low precision, turn on when powered on.
If you need to control the speed, you need to add a frequency converter, or you can add a speed control box.
If it is controlled by a frequency converter, a special frequency conversion motor is required. Although ordinary motors can be used in conjunction with frequency converters, heat generation is a problem, and other problems will occur. For specific shortcomings, you can search online. The control motor of the governor box will lose power, especially when it is adjusted to a small gear, but the frequency converter will not.
Stepper motors are open-loop motors with relatively high precision, especially five-phase steppers. There are very few domestic five-phase steppers, which is a technical threshold. In general, the stepper is not equipped with a reducer and is used directly, that is, the output shaft of the motor is directly connected to the load. The working speed of the stepper is generally low, only about 300 revolutions, of course, there are also cases of one or two thousand revolutions, but it is also limited to no-load and has no practical value. This is why there is no accelerator or decelerator in general.
The servo is a closed motor with the highest precision. There are a lot of domestic servos. Compared with foreign brands, there is still a big difference, especially the inertia ratio. The imported ones can reach more than 30, but the domestic ones can only reach about 10 or 20.
As long as the motor has inertia, many people ignore this point when selecting the model, and this is often the key criterion to determine whether the motor is suitable. In many cases, adjusting the servo is to adjust the inertia. If the mechanical selection is not good, it will increase the motor. Debugging burden.
Early domestic servos did not have low inertia, medium inertia, and high inertia. When I first came into contact with this term, I didn’t understand why the motor with the same power would have three standards of low, medium, and high inertia.
Low inertia means that the motor is made relatively flat and long, and the inertia of the main shaft is small. When the motor performs high-frequency repetitive motion, the inertia is small and the heat generation is small. Therefore, motors with low inertia are suitable for high-frequency reciprocating motion. But the general torque is relatively small.
The coil of the servo motor with high inertia is relatively thick, the inertia of the main shaft is large, and the torque is large. It is suitable for occasions with high torque but not fast reciprocating motion. Because of the high-speed movement to stop, the driver has to generate a large reverse drive voltage to stop this large inertia, and the heat is very large.
Generally speaking, the motor with small inertia has good braking performance, quick start, fast response to acceleration and stop, good high-speed reciprocation, and is suitable for some occasions with light load and high-speed positioning. Such as some linear high-speed positioning mechanisms. Motors with medium and large inertia are suitable for occasions with large loads and high stability requirements, such as some machine tool industries with circular motion mechanisms.
If the load is relatively large or the acceleration characteristic is relatively large, and a small inertia motor is selected, the shaft may be damaged too much. The selection should be based on factors such as the size of the load, the size of the acceleration, etc.
Motor inertia is also an important indicator of servo motors. It refers to the inertia of the servo motor itself, which is very important for the acceleration and deceleration of the motor. If the inertia is not well matched, the motor’s action will be very unstable.
In fact, there are also inertia options for other motors, but everyone has weakened this point in the design, such as ordinary belt conveyor lines. When the motor is selected, it is found that it cannot be started, but it can move with a push of the hand. In this case, if you increase the reduction ratio or power, it can run normally. The fundamental principle is that there is no inertia matching in the early stage selection.
For the response control of the servo motor driver to the servo motor, the optimal value is that the ratio of the load inertia to the motor rotor inertia is one, and the maximum cannot exceed five times. Through the design of the mechanical transmission device, the load can be made.
The ratio of inertia to motor rotor inertia is close to one or smaller. When the load inertia is really large, and the mechanical design cannot make the ratio of the load inertia to the motor rotor inertia less than five times, a motor with a large motor rotor inertia can be used, that is, the so-called large inertia motor. To achieve a certain response when using a motor with a large inertia, the capacity of the driver should be larger.
Below we explain the phenomenon in the actual application process of our motor.
The motor vibrates when starting, which is obviously insufficient inertia.
No problem was found when the motor was running at low speed, but when the speed was high, it would slide when it stopped, and the output shaft would swing left and right. This means that the inertia matching is just at the limit position of the motor. At this time, it is enough to increase the reduction ratio slightly.
The 400W motor loads hundreds of kilograms or even one or two tons. This is obviously only calculated for power, not for torque. Although the AGV car uses 400W to drag a load of several hundred kilograms, the speed of the AGV car is very slow, which is rarely the case in automation applications.
The servo motor is equipped with a worm gear motor. If it must be used in this way, it should be noted that the speed of the motor should not be higher than 1500 rpm. The reason is that there is sliding friction in the worm gear deceleration, the speed is too high, the heat is serious, the wear is fast, and the service life is relatively reduced. At this time, users will complain about how such rubbish is. Imported worm gears will be better, but they cannot withstand such devastation. The advantage of servo with worm gear is self-locking, but the disadvantage is loss of precision.
Inertia = radius of rotation x mass
As long as there is mass, acceleration and deceleration, there is inertia. Objects that rotate and objects that move in translation have inertia.
When ordinary AC asynchronous motors are generally used, there is no need to calculate the inertia. The characteristic of AC motors is that when the output inertia is not enough, that is, the drive is too heavy. Although the steady-state torque is enough, but the transient inertia is too large, then When the motor reaches the unrated speed at the beginning, the motor slows down and then becomes fast, then slowly increases the speed, and finally reaches the rated speed, so the drive will not vibrate, which has little effect on the control. But when choosing a servo motor, since the servo motor relies on the encoder feedback control, its startup is very rigid, and the speed target and position target must be achieved. At this time, if the amount of inertia that the motor can withstand is exceeded, the motor will tremble. Therefore, when calculating the servo motor as a power source, the inertia factor must be fully considered. It is necessary to calculate the inertia of the moving part that is finally converted to the motor shaft, and use this inertia to calculate the torque within the startup time.
Post time: Mar-06-2023