Application of VFD (Variable-frequency Drive) in Cylindrical Grinding Machine

The grinding wheel motors of the cylindrical grinders used in standard machining are all operated according to the traditional starting circuit. After the motor is started, it runs at the rated speed. Due to the certain fluctuations in the grid voltage, the friction load between the grinding wheel and the workpiece is constantly changing, which will affect the speed error of the motor. The starting circuit of the standard grinding wheel motor generally has only one processing speed, which is difficult to adapt to the different relative processing speeds required for different workpiece sizes, so that the processing precision of the processed workpiece is difficult to guarantee. Therefore, from the perspective of improving processing quality and processing efficiency, saving energy, etc., applying variable frequency speed regulation technology to cylindrical grinders can achieve satisfactory results.

The cylindrical grinders in the machining industry process a wide variety of products, with different sizes of workpieces and different processing precision requirements. The relative requirements for the grinding wheel speed are different from the spindle speed. It is difficult to adjust the spindle speed to the ideal state to meet the processing speed of the workpiece. Due to the stress bending of the shaft during the processing, the grinding wheel feed torque will be different during the grinding process, which will bring about different changes in the output speed/torque of the grinding wheel, and correspondingly produce vibration marks/burning marks, etc., and the grinding accuracy is difficult to guarantee, resulting in low production efficiency and low quality rate.

With the development of power electronics technology, variable frequency speed regulation technology is becoming more and more popular, and the application of VFD (Variable-frequency Drive) in the mechanical processing industry has received good results. The stepless speed regulation, soft start, and constant torque output of VFD (Variable-frequency Drive) greatly meet the requirements of mechanical processing equipment for constant speed/constant torque.

Load characteristics of cylindrical grinding machine processing

According to the size of the different shaft diameters, the speed of the main grinding wheel motor is equal to the speed of the grinding wheel output. The size of the 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 grinding wheel remains unchanged, the diameter of the workpiece being processed is small, and the grinding feed is small; when processing large-diameter, long-axis workpieces, the motor that rotates the workpiece is in the low-speed section, the grinding wheel feed is large, and the grinding force of the grinding wheel is also increasing, and it is difficult for the motor to operate at constant torque/constant speed. The change in speed produces changes such as vibration marks/burning marks/processing accuracy. The stepless speed regulation and constant torque output of the variable frequency speed regulating motor can make up for the above defects.

VFD (Variable-frequency Drive) selection

According to the characteristics of system operation, considering the use requirements of mechanical processing cylindrical grinders and equipment investment costs, a general-purpose V/FVFD (Variable-frequency Drive) with automatic torque boost function is selected. Taking the Sunwoele HFV3000 series as an example, 18.5KW/15KW VFD (Variable-frequency Drive) is selected according to the power of the grinding wheel motor. Taking 15KW as an example, the rated voltage is 380V three-phase, 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. It has an automatic torque boost function and flexible PWM control, which can achieve lower noise operation and more than 10 speed control levels.

System startup and speed regulation

The main grinding wheel motor starting circuit of the cylindrical grinder mostly adopts a star-delta conversion starting circuit. Replace the original circuit with VFD (Variable-frequency Drive), and use the normally open contacts of the original contactor to control the operation of VFD (Variable-frequency Drive). Considering the inertia of the grinding wheel startup, the VFD (Variable-frequency Drive) soft start time is set to 10S, and the stop brake time is set to 15S. The starting torque of different types of grinding wheels can be adjusted between 50%--80%. The system speed is adjusted by an external adjustable potentiometer, and the speed is adjusted according to the different workpiece diameters. Due to the limitations of the mechanical strength/noise/vibration of the motor at high speed, the maximum frequency of the motor is set to 60HZ; due to the poor heat dissipation of the motor at low speed, considering the speed change factor of the workpiece rotation and the actual maximum processing workpiece size, the minimum frequency is set to 35HZ and the base frequency is set to 50HZ. From the previous load characteristic analysis, it can be seen that the high-speed section is a constant power nature, the low-speed section is a constant torque nature, and the load requires a large torque and strong overload capacity at low speed. For motors above the base frequency, in order to ensure that the motor is not over-voltage, constant voltage variable frequency speed regulation is adopted. From the motor theory, when the voltage remains unchanged and the frequency increases, the magnetic flux under each pole of the motor decreases with the increase of the frequency; when the motor current is the rated current, the maximum allowable output torque decreases, and the allowable output power remains unchanged. It is a constant power speed regulation and is suitable for constant power loads. Below the base frequency, in order to ensure that the magnetic flux under each pole of the motor remains unchanged, a variable voltage and 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, the internal resistance of the motor cannot be ignored. If V/F is still kept constant, the motor torque will decrease, which cannot meet the high torque and strong overload capacity required by the load at low speed. Therefore, the torque automatic compensation function of VFD (Variable-frequency Drive) is adopted to select the compensated V/F curve for correction.

Precautions for system design

The accuracy of grinding wheel balance should be improved: the motor is 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: Because the motor is cooled by a self-cooling fan, the heat dissipation conditions deteriorate at low speed, and the load torque is large, the motor temperature will increase, so the minimum speed is set based on the surface temperature difference of the motor not exceeding the allowable value.