Application of the hottest frequency converter in

2022-10-24
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Application of frequency converter in CNC grinding machine

1 Preface:

the cylindrical grinder and grinding wheel motor used in standard machining operate according to the traditional starting power and will not damage the painting performance of the material surface. After the motor is started, it operates according to the rated speed. Due to the certain fluctuation of the electric voltage and the continuous change of the friction load between the grinding wheel and the workpiece, it will affect the speed error of the motor. The standard grinding wheel motor starting circuit generally has only one processing speed, which is difficult to meet the requirements of different workpiece sizes and different processing relative linear speeds, so that the processing precision of the workpiece is difficult to be guaranteed. Therefore, from the aspects of improving processing quality and efficiency, saving energy and so on, the application of variable frequency speed regulation technology to cylindrical grinder can achieve satisfactory results

cylindrical grinding machines in the machining industry process a wide range of products, with different workpiece sizes and sizes, requiring different machining accuracy. Relatively, 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 to the ideal state. In addition, due to the stress bending produced by the shaft and rod in the processing process, different feed moments of the grinding wheel will be produced in the grinding process, which will lead to different changes in the output speed/torque of the grinding wheel, and corresponding vibration marks/burn marks will be produced. The grinding accuracy is difficult to ensure, resulting in low production efficiency and low quality

with the development of power electronics technology, frequency conversion and speed regulation technology is becoming more and more popular, and the application of frequency converter in mechanical processing industry has received good results. The stepless speed regulation of frequency converter is restricted by a large number of risks and uncertainties like our forward-looking statement. The soft start and constant torque output greatly meet the requirements of machining equipment for constant speed/constant torque

2. Load characteristics of cylindrical grinder processing:

according to the size of different shaft and rod diameters, the speed of the main grinding wheel motor is equal to the speed of the grinding wheel output. The grinding force depends on the output torque of the grinding wheel motor. In the spindle motor that drives the workpiece to rotate, the high engineers can reduce the number of fixed bolts required. Compared with the constant output speed of the grinding wheel, the diameter of the workpiece processed is small and the grinding feed is small; When machining large diameter and long axis workpiece, the motor rotating the workpiece is in the low-speed segment, the feed rate of the grinding wheel is large, and the grinding force of the grinding wheel is also increasing, so it is difficult for the motor to operate at constant torque/speed. The change of speed leads to the change of vibration pattern/burn pattern/machining accuracy, etc. The stepless speed regulation and constant torque output of variable frequency adjustable speed motor can make up for the above defects

3. Selection of frequency converter:

according to the characteristics of system operation, considering the use requirements of machining cylindrical grinder and equipment investment cost, select vector type frequency converter with automatic torque lifting function. Taking sanch-s2000 series as an example, 18.5kw/15kw frequency converter is selected according to the power of grinding wheel motor. Taking 15kw as an example, the rated voltage is three-phase 380V, the output current is 32a, the frequency is controlled from 0.5 to 400Hz, the speed is controlled by 0.1%, and the overload capacity is 150%/60s. With automatic torque lifting function and flexible PWM control, it can realize lower noise operation and speed control of more than 10 sections

4. Starting and speed regulation of the system:

the starting circuit of the main grinding wheel motor of the cylindrical grinder mostly adopts the star triangle conversion starting circuit. Replace the original circuit with the frequency converter, and use the normally open contact of the original contactor to control the operation of the frequency converter. Considering the inertia of the starting of the grinding wheel, the soft start time of the frequency converter is set to 10s, and the stop braking time is set to 15s. The setting can be adjusted between 50% and 80% according to the starting torque of different types of grinding wheels. The system speed is adjusted by an external adjustable potentiometer to adjust the frequency according to the adaptive speed of different workpiece diameters. Due to the limitation of mechanical strength/noise/vibration and other factors of the motor at high speed, the maximum 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 maximum workpiece size, the minimum frequency is set to 35Hz and the reference frequency is set to 50Hz. From the previous analysis of load characteristics, it can be seen that the high-speed section is of constant power nature, and the low-speed section is of constant torque nature, and the load requires large torque and strong overload capacity at low speed. For the motor above the reference frequency, in order to ensure that the motor does not have overvoltage, constant voltage variable frequency speed regulation is adopted. According to the motor theory, when the voltage is constant and the frequency increases, the magnetic flux at each pole of the motor decreases with the increase of frequency; When the motor current is the rated current, the maximum allowable output torque decreases and the allowable output power remains unchanged. It belongs to constant power speed regulation and is suitable for constant power loads. Below the fundamental frequency, in order to ensure that the magnetic flux under each pole of the motor remains unchanged, the variable voltage variable frequency speed regulation with v/f= constant is adopted. When the current of the motor is the 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, because the internal resistance of the motor cannot be ignored, if v/f is still kept constant, the motor torque will be reduced, which cannot meet the performance of high torque and strong overload capacity required by the load at low speed. Therefore, the torque automatic compensation function of the frequency converter is adopted, and the compensated v/f curve is selected for correction

5. Precautions for system design:

the accuracy of grinding wheel balance should be improved: run the motor at the rated speed. In order to reduce the vibration at high speed, it should be mentioned not to touch any button on the equipment to increase the dynamic balance of the motor rotor and the grinding wheel and correct it

heat dissipation at low speed: because the motor is cooled by self cooling fan, the heat dissipation condition becomes worse at low speed, and the load torque is large, and the motor temperature will increase. Therefore, the setting of the minimum speed is based on the fact that the temperature difference on the motor surface does not exceed the allowable value

during variable frequency speed regulation, electromagnetic noise will be generated in the motor because the output of the frequency converter contains high-order harmonics. Therefore, zero phase noise filter should be installed at the output end to reduce auxiliary radiation interference and induced noise

the parameter setting items of the frequency converter are important to lock the reverse/maximum frequency/stall and other functions

this paper analyzes in detail the application of frequency converter in cylindrical grinder in mechanical processing industry. As the power of cylindrical grinder, the main motor of grinding wheel introduces frequency conversion speed regulation technology to meet the processing needs of cylindrical grinding of various shafts, improve work efficiency, improve the processing quality of cylindrical grinding of shafts, and save energy. Combined with the actual field application, the operator's response is better

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