Application of VFD (Variable-frequency Drive) on grinders in the mechanical processing industry

1 Introduction:

The grinding machines and grinding wheel motors used in standard machining all operate according to the traditional starting circuit. After the motor is started, it runs at the rated speed. Due to certain fluctuations in the grid voltage and the constant changes in the friction load of the grinding wheel and workpiece grinder, the speed error of the motor grinder will be affected. The standard grinding wheel motor starting circuit generally only has one processing speed, which is difficult to adapt to different workpiece size grinders. Different grinding machines are required to process relative linear speeds, so it is difficult to guarantee the processing precision of the workpiece grinder. Therefore, from the aspects of improving processing quality, processing efficiency, and saving energy, applying frequency conversion speed regulation technology to grinders can achieve satisfactory grinding results.

There are many types of grinding machine products processed by grinders in the mechanical processing industry, with different sizes of workpieces and different processing accuracy requirements. The grinding wheel speed required by a relative grinder is different from the spindle grinder line speed. It is difficult to adjust the spindle grinder speed to the ideal state by simply adjusting the spindle grinder speed to meet the workpiece grinder processing line speed. In addition, due to the stress bending of the grinder caused by the machining process of the shaft, the grinding process will produce different grinding wheel feed torques, which will lead to changes in the grinding wheel output speed/torque, and the corresponding grinding machine will produce chatter marks/burning marks, etc. , it is difficult to guarantee the grinding accuracy, resulting in low production efficiency and low quality rate.

With the development of power electronic technology grinders, variable frequency speed control technology grinders are becoming more and more popular, and the application of VFD (Variable-frequency Drive) grinders in the mechanical processing industry has received good grinding results. The VFD (Variable-frequency Drive) grinder has stepless speed regulation, soft start, and constant torque output. The grinder meets the requirements of mechanical processing equipment for constant speed/constant torque grinders.

2.Grinder processing grinder load characteristics:

According to different grinder shaft diameters and grinder sizes, the grinder speed of the main grinding wheel motor is equal to the grinding wheel output grinder speed. The size of the grinding force depends on the output torque of the grinding wheel motor. In the high-speed section of the grinder's spindle motor that drives the workpiece rotation grinder, the output speed of the grinding wheel remains unchanged. The diameter of the workpiece processed by the grinder is small and the grinding feed is small. When processing large-diameter and long-axis workpieces, the workpiece rotates the grinder motor in the low-speed section, and the grinding wheel advances. When the feed amount is large, the grinding force of the grinding wheel also increases, and it is difficult for the motor to operate at constant torque/constant speed. Changes in the speed of the grinder will produce changes in the grinder such as vibration patterns/burning lines/machining accuracy. The variable frequency speed motor grinder has stepless speed regulation and constant torque output, which can make up for the above defects.

3.VFD (Variable-frequency Drive) grinder selection:

Based on the characteristics of the grinding machine operating in the system, considering the requirements for use of the mechanical processing grinder and the equipment investment cost, a vector-type VFD (Variable-frequency Drive) for the grinder with automatic torque boost function was selected. Taking the SANCH-S2000 series as an example, an 18.5KW/15KW grinder VFD (Variable-frequency Drive) is selected according to the power of the grinding wheel motor grinder. 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 0.1%, overload capacity 150%/60S. It has automatic torque boost function and flexible PWM control, which can achieve lower noise operation and more than 10-stage grinder speed control.

4. System grinder startup and speed adjustment:

The starting circuit of the main grinding wheel motor of a grinder mostly uses a star-delta conversion starting circuit. Use VFD (Variable-frequency Drive) to replace the original circuit, use the original contactor grinder normally open contact to control the operation of the VFD (Variable-frequency Drive) grinder, consider the inertia of the grinder when starting the grinding wheel, and the VFD (Variable-frequency Drive) soft start time Set to 10S, the stop braking time is set to 15S, and the starting torque of the grinding wheel can be adjusted between 50% and 80% according to different types of grinders. The system speed adjusts the frequency through an external adjustable potentiometer. The grinder adapts to the speed adjustment according to the different diameters of the workpiece. Due to the grinding machine limitations due to factors such as the mechanical strength/noise/vibration of the grinder when the motor is at high speed, the maximum frequency of the motor is set to 60HZ; due to the heat dissipation of the motor at low speed The effect is poor. Considering the actual maximum workpiece size to be processed by the workpiece rotation grinder speed change factor, the minimum frequency is set to 35HZ and the base frequency is set to 50HZ.

From the previous analysis of the load characteristics of the grinder, it can be seen that the high-speed section has constant power characteristics and the low-speed section has constant torque characteristics. At low speeds, the load requires large torque and strong overload capability. For motors above the base frequency, in order to ensure that the motor does not overvoltage, constant voltage frequency conversion speed regulation is used. It can be seen from the motor theory that when the voltage remains constant and the frequency increases, the magnetic flux under each pole of the motor decreases as the frequency increases; when the motor current is the rated current, the maximum allowable output torque decreases and the allowable output The power remains unchanged and it is a constant power speed regulation, 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, V/F = constant grinder voltage variable frequency speed regulation is adopted. When the motor grinder current is the rated current, the allowable output torque of the motor remains unchanged, which is a constant torque regulation. speed, suitable for constant torque loads. 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 and cannot meet the load requirements at low speed. The grinder has large torque and strong overload capability, so VFD is used. (Variable-frequency Drive) Automatic torque compensation function of the grinder. After selecting the compensation, the V/F curve of the grinder is corrected. 5. System design grinder precautions:

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

The heat dissipation situation of the grinder at low speed: Since the motor uses a self-cooling fan to dissipate heat, the heat dissipation conditions become worse at low speed, and the load torque is larger, the motor temperature will increase, so the minimum speed grinder is set so that the surface temperature difference of the motor does not exceed the allowable value. in accordance with.

During variable frequency speed regulation, the VFD (Variable-frequency Drive) output contains high-order harmonics, which will cause electromagnetic noise to the motor. Therefore, a zero-phase noise filter must be installed at the output end to reduce auxiliary interference and induced noise.

The VFD (Variable-frequency Drive) grinder parameter setting item mainly locks functions such as reverse/maximum frequency/stall!

This article analyzes in detail the application of VFD (Variable-frequency Drive) grinding machines in the mechanical processing industry. As the main motor of the power grinder grinding wheel, variable frequency speed regulation technology is introduced to adapt to various types of grinding grinding machines.