Application of VFD in CNC Machine Tool

In CNC machine tools, the frequency converter is mainly used to control the AC motor. It not only plays the role of energy saving and speed regulation, but also its soft start can protect the auxiliary electrical equipment, avoiding the impact of direct start on the mechanical equipment, which may cause mechanical failure. . Therefore, the frequency converter is an ideal speed regulation and control device. This article elaborates on the application of frequency converter on CNC machine tools and its common problems in use and maintenance.

1. Application of frequency converter on CNC machine tools

CNC machine tools require a wide range of spindle speed regulation, which can realize stepless speed regulation, automatic acceleration and deceleration control when the spindle rotates forward and reverse, and the acceleration and deceleration time should be short, requiring a wide range of constant power. The inverter can adjust the speed of the AC motor by changing the frequency of the output AC. The machine tool is controlled by a frequency converter. When starting, the frequency and voltage are correspondingly increased with the acceleration of the motor. The starting current is generally limited to below 150% of the rated current. However, when the power frequency power supply is used to start directly, the starting current is 6 to 7 times the rated current, which will cause a great impact on the power grid and load, affect the work of peripheral electrical appliances, increase the wear of mechanical transmission parts, and reduce the equipment’s operating efficiency. life. In addition, the torque of the motor will decrease as the speed decreases. After using the frequency converter to control the motor, the situation of insufficient torque at low speed of the motor will be improved, and the frequency converter can perform constant torque speed regulation at the rated frequency. The inverter’s stepless speed regulation, soft start, and constant torque output greatly meet the requirements of CNC machine tools for constant speed, constant torque, acceleration and deceleration control, etc., with higher production efficiency and more convenient use and maintenance. Therefore, CNC machine tools Frequency converters are widely used for main drive control.

2. Common faults in the use of inverters for CNC machine tools

From the perspective of the cause of the fault, the main reasons and solutions for the inverter not working properly are:

3.1 Overload, including inverter overload and motor overload
The overload of the inverter is often caused by short acceleration time, excessive DC braking or low grid voltage. Generally, the fault can be solved by prolonging the acceleration time, prolonging the braking time, and checking the grid voltage. The overload of the motor is caused by the power of the frequency converter is too small and the load of the motor is too heavy, so the system should choose a suitable frequency converter to adapt to the motor.

3.2 Undervoltage and overvoltage
When the voltage is too low or too high, the detection element of the frequency converter will automatically protect the frequency converter, and the frequency converter will stop working. Undervoltage and overvoltage faults are mainly caused by the fault of the external power supply, and a few faults are caused by the damage of the detection circuit. Such as overvoltage may be caused by too short deceleration time or damage to the braking resistor. When encountering undervoltage or overvoltage faults, first check whether the detection circuit is intact, and then check the external power supply.

3.3 Overcurrent
When the current is too large, the inverter starts over-current protection, and the inverter stops working. Generally, the overcurrent fault can be analyzed from the external electrical appliances and the frequency converter itself.

Overcurrent faults caused by external electrical appliances:
(1) The load of the motor suddenly increases, causing an impact and causing overcurrent.
(2) The insulation of the motor and the motor cable phase or phase to ground is damaged, causing occasional or phase-to-ground short circuits, resulting in overcurrent.
(3) When the speed measuring encoder is installed, if the speed feedback signal is lost or abnormal, it will also cause overcurrent. At this time, it is necessary to check the encoder and its cable.

3.4 Circuit fault interference
The control coils of all relays and contactors around the inverter need to be equipped with absorbing devices to prevent shock voltage. The wiring distance of the control loop should be shortened as much as possible, and it should be separated from the main line, and the shielded wire loop should be used.
When the frequency converter of CNC machine tools is installed, due to its high integration, compact overall structure, and large heat dissipation, it has high requirements for the temperature, humidity and dust content of the installation environment, as well as the quality of the power grid. Running under temperature is the basic requirement to prolong the life of the inverter. The external conditions of the site should be checked before the inverter is used. If the requirements cannot be met, effective measures should be taken to solve it, so as to prevent the external environment from affecting the inverter.