For the cylindrical grinder used in standard machining, the grinding wheel motor operates according to the traditional starting circuit. After the motor is started, it operates at the rated speed. Due to the fluctuation of the grid voltage and the constant change of the friction load between the grinding wheel and the workpiece, the speed error of the motor will be affected. The starting circuit of the standard grinding wheel motor generally has only one processing speed, which is difficult to adapt to the requirements of different workpiece sizes and different processing relative linear speeds, resulting in the precision of the workpiece processed Degree is hard to guarantee. Therefore, from the aspects of improving processing quality and efficiency, saving energy and so on, the application of frequency conversion speed regulation technology in cylindrical grinder can achieve satisfactory results.
There are many kinds of products processed by cylindrical grinder in mechanical processing industry. The sizes of workpieces are different, and the machining accuracy is required to be different. The relative requirements of the grinding wheel speed is different from the linear speed of the spindle. It is difficult to adjust the spindle speed to meet the processing linear speed of the workpiece. In addition, due to the stress bending produced in the process of grinding, the feed torque of grinding wheel will be different in the grinding process, which will lead to different changes in the output speed / torque of the grinding wheel, resulting in vibration blade pattern / burning paste grain, etc., which makes the grinding accuracy difficult to guarantee, resulting in low production efficiency and low quality rate.
With the development of power electronic technology, frequency control technology is becoming more and more popular. The application of frequency converter in mechanical processing industry has received good results. The frequency converter's stepless speed regulation, soft start and constant torque output greatly meet the requirements of mechanical processing equipment for constant speed / constant torque.
According to the different diameter of the shaft, the speed of the main grinding wheel motor is equal to the output speed of the grinding wheel. The grinding force depends on the output torque of the grinding wheel motor. In the high-speed section of the spindle motor which drives the workpiece to rotate, relative to the output speed of the grinding wheel, the diameter of the workpiece processed is small and the grinding feed is small; when processing large diameter and long axis workpieces, the motor of the workpiece rotating is in the low-speed section, the feed rate of the grinding wheel is large, and the grinding force of the grinding wheel is also increasing, so the motor is difficult to operate under constant torque / constant speed. The change of speed results in the change of vibration pattern / burning pattern / machining accuracy. The stepless speed regulation and constant torque output of variable frequency adjustable speed motor can make up for the above defects.
According to the characteristics of the system operation, considering the requirements of machining cylindrical grinder and equipment investment cost, the general-purpose V / F converter with automatic torque lifting function is selected. Taking Dongyuan speech? 7200ma series as an example, 18.5kw/15kw frequency converter is selected according to the different power of grinding wheel motor. Taking 15kw as an example, the rated voltage is 380V, the output current is 32a, the frequency control is 0.5? 400Hz, the speed control is 0.1%, and the overload capacity is 150% / 60s. With automatic torque lifting function, flexible PWM control, it can achieve lower noise operation and speed control of more than 10 segments.
The starting circuit of the main grinding wheel motor of the cylindrical grinder mostly adopts the star delta conversion starting circuit. The frequency converter is used to replace the original circuit, and the normal open contact of the original contactor is used to control the operation of the frequency converter. Considering the inertia of the grinding wheel starting, the soft start time of the frequency converter is set to 10s, and the stop braking time is set to 15s. According to different types of grinding wheel starting torque, the setting can be adjusted between 50% and 80%. The system speed is adjusted by an external adjustable potentiometer to adjust the speed according to the different diameter of the workpiece. Due to the limitation of mechanical strength / noise / vibration and other factors when the motor is at high speed, the high frequency of the motor * is set to 60Hz; Due to the poor heat dissipation effect of the motor at low speed, considering the variable speed factor of workpiece rotation and the actual size of large workpiece, the * low frequency is set to 35Hz, and the reference frequency is set to 50Hz. From the previous load characteristics analysis, it can be seen that the high-speed section is of constant power, and the low-speed section is of constant torque, and the load requires large torque and strong overload capacity at low speed. For the motor above the reference frequency, constant voltage variable frequency speed regulation is adopted to ensure that the motor is not over-voltage. According to the motor theory, when the voltage is constant and the frequency is increased, the magnetic flux under each pole of the motor decreases with the increase of frequency; when the motor current is the rated current, * large allowable output torque decreases and the allowable output power remains unchanged, which belongs to constant power speed regulation and is suitable for constant power load. Below the fundamental frequency, in order to ensure that the magnetic flux under each pole of the motor is constant, the variable voltage and frequency conversion speed regulation with V / F = constant is adopted. When the current of the motor is rated current, the allowable output torque of the motor remains unchanged, which belongs to constant torque speed regulation and is suitable for constant torque load. However, when the speed is low, the internal resistance of the motor can not be ignored. If V / F is still kept as a constant, the motor torque will be reduced, which can not meet the performance of high torque and strong overload capacity required by the load at low speed. Therefore, the automatic torque compensation function of frequency converter is adopted and the V / F curve after compensation is selected for correction.
On 2018-08-08 11:45:46
