Application of frequency converter on machined cylindrical grinding machine

The cylindrical grinding machine used in standard machining, the grinding wheel motor is operated according to the traditional starting circuit. After the motor starts, it runs at the rated speed. Because the grid voltage has a certain fluctuation, the friction load of the grinding wheel and the workpiece will change continuously, which will affect the motor speed error. The standard grinding wheel motor starting circuit generally has only one processing speed, which is difficult to adapt to different workpiece sizes. The requirement for different machining relative line speeds is such that the machining precision of the machined workpiece is difficult to guarantee. Therefore, from the aspects of improving processing quality, processing efficiency, energy conservation, etc., the frequency conversion speed regulation technology is applied to the cylindrical grinding machine, and satisfactory results can be obtained.

The cylindrical grinding machines in the machining industry process a wide range of products, with different workpiece sizes and different machining precisions. Relatively speaking, the rotational speed of the grinding wheel is different from the linear speed of the main shaft. It is difficult to adjust the rotational speed of the main shaft to meet the machining line speed of the workpiece. Due to the stress bending caused by the shaft during the machining process, the torque of the grinding wheel is different during the grinding process, which causes different changes in the output speed/torque of the grinding wheel, and correspondingly generates the vibration pattern/ Burning paste, etc., grinding accuracy is difficult to guarantee, resulting in low production efficiency, low quality.

With the development of power electronics technology, the frequency conversion speed regulation technology has become more and more popular, and the application of the frequency converter in the machining industry has received very good results. The stepless speed regulation, soft start and constant torque output of the frequency converter greatly satisfy the requirements of constant speed/constant torque of machining equipment.

2. Load characteristics of cylindrical grinding machine processing:

Depending on the diameter of the shaft type, the speed of the main grinding wheel motor is equal to the speed of the grinding wheel output. The amount of grinding force depends on the output torque of the grinding wheel motor. In the high-speed section of the spindle motor that drives the workpiece to rotate, the output speed of the workpiece is small with respect to the output speed of the grinding wheel, and the machining of the workpiece is small, and the grinding feed is small; when machining a large-diameter, long-axis workpiece, the motor rotating the workpiece is at a low speed section. The grinding wheel has a large feed rate, the grinding wheel grinding force is also increasing, and the motor is difficult to operate at constant torque/constant speed. The change in speed produces changes such as vibrating knives/burning lines/machining accuracy. The stepless speed regulation of the variable frequency speed regulating motor and the constant torque output can make up for the above defects.

3. Selection of the inverter:

According to the characteristics of the system operation, considering the requirements of the machining of the cylindrical grinding machine and the investment cost of the equipment, the vector inverter with automatic torque boosting function is selected. Taking the SANCH-S2000 series as an example, the inverter of 18.5KW/15KW is selected according to the power of the grinding wheel motor. Taking 15KW as an example, the rated voltage is three-phase 380V, the output current is 32A, the frequency control is 0.5-400HZ, and the speed control is 0.1%. Load capacity 150% / 60S. With automatic torque boosting, flexible PWM control for lower noise operation, more than 10 segments of speed control.

4. System startup and speed regulation:

The main grinding wheel motor starting circuit of the cylindrical grinding machine mostly adopts a star-delta conversion starting circuit. Replace the original circuit with the inverter, use the normally open contact of the original contactor to control the operation of the inverter, consider the inertia of the grinding wheel start, the soft start time of the inverter is set to 10S, and the stop brake time is set to 15S, according to different The type of grinding wheel starting torque can be adjusted between 50% and 80%. The system speed is adjusted by the external adjustable potentiometer, and the speed is adjusted according to the different diameters of the workpiece. Due to the mechanical strength/noise/vibration of the motor at high speed, the maximum frequency of the motor is set to 60HZ; the motor is dissipated at low speed. The effect is poor. Considering the shifting factor of the workpiece rotation and the actual maximum machining workpiece size, the lowest frequency is set to 35HZ and the reference frequency is set to 50HZ. It can be seen from the analysis of the previous load characteristics that the high speed section is of constant power nature, the low speed section is constant torque property, and the load requirement torque is large at low speed and the overload capability is strong. Above the motor reference frequency, in order to ensure that the motor is not under pressure, constant voltage frequency control is adopted. It can be known from the motor theory that when the voltage is constant and the frequency is increased, the magnetic flux at each pole of the motor decreases with increasing frequency; when the motor current is rated current, the maximum allowable output torque is reduced, and the allowable output is obtained. Constant power, constant power speed regulation, suitable for constant power loads. Below the fundamental frequency, in order to ensure that the magnetic flux at each pole of the motor is constant, the V/F=constant variable frequency variable frequency speed regulation is adopted. When the current of the motor is the rated current, the allowable output torque of the motor is constant, which belongs to the constant torque adjustment. Speed, suitable for constant torque loads. However, when the speed is low, the internal resistance of the motor can not be neglected. If the V/F is kept constant, the motor torque will be reduced, and the torque required for the load at low speed cannot be satisfied, and the overload capability is strong. Therefore, the frequency conversion is adopted. The torque automatic compensation function is selected and the compensated V/F curve is selected for correction.

5. System design considerations:

The accuracy of the grinding wheel balance should be improved: the motor should be operated at the rated speed. In order to reduce the vibration at high speed, the dynamic balance of the motor rotor and the grinding wheel should be improved and corrected.

Heat dissipation at low speed: Since the motor is cooled by the self-cooling fan method, the heat dissipation condition is deteriorated at low speed, and the load torque is large, and the motor temperature is increased. Therefore, the minimum speed is set so that the motor surface temperature difference does not exceed the allowable value. in accordance with.

In the frequency conversion speed regulation, the inverter output contains higher harmonics, which will generate electromagnetic noise in the motor. Therefore, a zero-phase noise filter should be installed at the output end to reduce the auxiliary radiation interference and the induction noise.

The parameter setting item of the inverter is important to lock the functions of reverse/highest frequency/stall!

In this paper, the application of the frequency converter in the external grinding machine of the machining industry is analyzed in detail. The main motor of the cylindrical grinding wheel as the power introduces the frequency conversion speed regulation technology to meet the needs of various shaft cylindrical grinding. Improve work efficiency, improve the processing quality of shaft cylindrical grinding and save energy. Combined with the actual field application, the operator's reaction is more convenient, the surface is smooth and the imaging degree is high, and the yield is greatly improved.