CNC machine tools

Application of KERUN Inverter in CNC Machine Tools
-- Example of ACD320 Series

1. Requirements of CNC Machine Tools for Application Technology

1.1. Motor Requirements
Typically, the use of inverter-duty motors or standard motors with added cooling fans is required to meet motor heat dissipation needs at low frequencies. A wide motor speed regulation range is also demanded.

1.2. Technical Requirements for the Inverter

High Low-Frequency Torque
Select a vector inverter capable of outputting 150% rated torque at low frequencies (1–10 Hz).

Fast Torque Dynamic Response, High Speed Stability Precision
Choose a vector inverter to achieve excellent dynamic response. It quickly responds to load changes through adjustments in output torque, thereby stabilizing the spindle speed.

Fast Deceleration and Stopping
Acceleration and deceleration times for CNC machine tools are typically short. Acceleration time relies on the inverter's performance, while deceleration time depends on externally added braking resistors or braking units.

Perform Motor Parameter Auto-tuning
After selecting a vector inverter, achieving optimal control performance usually requires auto-tuning the motor parameters. The purpose is to obtain accurate internal motor parameters for vector control calculations. The motor nameplate parameters needed for auto-tuning are: Motor Rated Power, Motor Rated Frequency, Motor Rated Speed, Motor Rated Voltage, Motor Rated Current. Some inverter-duty motor nameplates may not list the rated speed; it can be estimated based on experience. Ensure auto-tuning is performed under no-load conditions (motor shaft disconnected from the load) to guarantee the accuracy of the learned parameters.
If on-site conditions prevent no-load operation, consider trial operation using the inverter's factory-default motor parameters.

Frequency Command and Run Command
For inverters used on CNC machine tools, both the frequency command and run command originate from the CNC controller. Generally, there are two command channels: analog input and multi-step speed setting, or both simultaneously with multi-step speed taking priority. Analog input is primarily voltage-type, though current-type also exists. The inverter can accept both types.

1.3. Anti-Interference Issues
Inverters undergo rigorous anti-interference testing at the factory, possessing strong anti-interference capabilities. However, inverters themselves are also sources of interference, and it is difficult to avoid interfering with other equipment during use. On CNC machine tools, the CNC controller is most susceptible. Once the CNC controller is interfered with, the system may malfunction. Particularly, the inverter's frequency command and run command might be affected, potentially causing unstable frequency commands or erroneous inverter operation in severe cases. A solution is to add ferrite cores on the inverter's output cables to reduce high-frequency radiation. Generally, imported CNC systems have stronger anti-interference capabilities.

2. Debugging of ACD320 Series on Machine Tools:

1) Wiring

Figure 1-1 Inverter Location in the Cabinet and Simplified Wiring Diagram

2) Parameters

3. Several Fault Phenomena Caused by Non-Inverter Quality Factors:

Discrepancy between actual spindle speed and set speed.
Since the inverter's frequency command comes from the system's 0-10V or 4-20mA analog signal, if there is a deviation between the set speed and the displayed speed, it can be corrected by adjusting the correspondence between the maximum frequency and the analog input value. Generally, calibrate the high speed first, then adjust the low speed.

Insufficient spindle speed accuracy.
Usually caused by large errors in motor parameters. Dynamic auto-tuning of the motor is required to obtain accurate parameters.

Insufficient cutting power during machining.
Caused by large errors in motor parameters. Dynamic auto-tuning of the motor is required to obtain accurate parameters.

Overcurrent fault during acceleration or operation.
Accelerating overcurrent is often due to too short acceleration time. It can also be caused by significant motor parameter deviations, requiring dynamic auto-tuning for accurate parameters.

Overvoltage fault during deceleration/stopping.
One reason is too short deceleration time. Another possibility is improper braking resistor configuration or an open circuit in the braking resistor loop. Adjust the deceleration time or configure a suitable braking resistor.

Abnormal noise from the spindle within a specific frequency range during operation?
This is usually caused by mechanical resonance. It can be eliminated by setting a frequency jump band or modifying the mechanical structure.

CNC system cannot control the inverter normally?
First, determine if the cause lies with the inverter itself or the system. This can be judged by attempting control via the keypad or by directly applying signals to the control terminals.

4. Conclusion
For the spindle motor of CNC lathes, using sensorless vector control with a variable frequency drive offers the following significant advantages: Substantially reduces maintenance costs, even enabling maintenance-free operation; allows for highly efficient cutting and high machining accuracy; delivers robust torque output at both low and high speeds. The ACD320 series inverter, with its unique features (low starting current, smooth speed regulation, wide speed range, energy saving & environmental protection, stable operation, high precision, high low-frequency torque, comprehensive protection functions, high reliability, easy operation and maintenance, etc.) and superior cost-performance ratio, is rapidly gaining prominence in CNC machine tool applications.

After implementing the inverter on CNC machine tools, mechanical components like clutches and gears for speed changing are eliminated, making maintenance more convenient. The use of an inverter enables multi-step speed control and constant surface speed machining. The machine tool can feed according to command signals, shortening machining cycles and improving production efficiency. The machine tool exhibits good speed reproducibility, ensuring stable product quality. It also easily achieves high-speed operation, enabling efficient machining of soft materials like aluminum.

Precautions:

Due to the wide adjustable speed range of the inverter, consider matching with mechanical components to prevent resonance. Measures may include changing mechanical properties or using frequency jump to avoid resonance points.

Braking units and braking resistors must be installed to shorten the deceleration time during high-to-low speed switching and prevent deceleration overcurrent faults (unrelated to inverter quality).

The power rating of the braking resistor is determined by the deceleration frequency and should be selected based on the most demanding operating mode.