When selecting a feed drive motor for a machine tool, you need to consider factors such as the matching of the transmission structure of the mechanical part and the motor, the running speed of the motor, the acceleration and deceleration time of the machine tool, and the stopping distance of the motor.
Mainly include the following factors:
1. Load inertia ratio;
2. Acceleration and deceleration characteristics (short-term processing factors);
3. Continuous load torque;
4. Motor speed;
5. Root mean square value of torque;
6. Dynamic braking distance.
It should be noted that the motor needs to be selected through correct calculation methods.
1. Load inertia ratio
The load inertia ratio refers to the ratio of the load inertia of the feed axis to the inertia of the feed axis motor. This value reflects the motor's ability to control the load. The smaller the value, the stronger the motor control. To ensure that the servo motor can work effectively, it is necessary to select a motor with the appropriate inertia for the machine tool. The technical index selected is called load inertia ratio (load inertia/motor inertia).
In particular, when the surface quality of workpiece processing is the priority, the ratio of the inertia of the machine tool to the inertia of the selected feed axis servo motor should be within the recommended load inertia ratio range, and the load inertia ratio should be kept as small as possible.
If the load inertia ratio is too large, the control of the motor will be unstable and it will be very difficult to debug the motor. At the same time, the accuracy and roughness of the machined surface will be reduced, and the positioning time will be longer.
Recommended selection range: load inertia/motor inertia = 3~5.
Note: In special processing situations, such as woodworking machining. Curves and grooves need to be processed during high-speed movement. In this case, it is recommended to choose a motor with an inertia greater than or equal to the load inertia to meet the needs of high-speed processing. In addition, when performing high-speed, high-precision processing and mold processing, it is recommended to select the inertia value of the motor so that the load-inertia ratio is less than 3.
2. Continuous load torque
Continuous load torque includes mechanical friction and the torque of gravity in the gravity axis. Generally, the torque of continuous load should not exceed 70% of the locked-rotor torque of the servo motor.
If the continuous load torque is the same as the locked-rotor torque, when calculating the root mean square value, it will cause the average torque value during the entire processing process (including acceleration/deceleration torque) to exceed the locked-rotor torque of the motor.
In the use of gravity axis motors, external mechanical factors will cause the torque to increase during the rising and stopping processes. In this case, it is recommended to use a torque value not exceeding 60% of the locked-rotor torque as a standard.
Note: The standard of stall torque should be measured based on the actual characteristics of the machine tool and the actual mechanical structure.
Based on actual debugging experience, it is recommended that the continuous load torque should not exceed 45% of the motor's locked-rotor torque. (In the case of the gravity axis, it is also recommended to make corresponding adjustments according to the actual situation. It is recommended that 30% of the stalled torque should be used as the standard.)
3. Acceleration and deceleration characteristics (short-time processing factors)
In mechanical processing, in addition to ensuring the continuous thrust to push the load for processing, short-term processing factors must also be considered, namely: the output characteristics of the motor during acceleration and deceleration. During the acceleration and deceleration process, the maximum thrust required by the machine will be reached. Therefore, when selecting a motor, you need to consider whether the maximum torque of the motor matches the maximum torque required during mechanical acceleration and deceleration.
The maximum torque of the motor directly affects the setting of the acceleration and deceleration time constant.
4. Motor speed
In actual mechanical operation, the rotation speed of the motor cannot exceed the maximum speed of the motor rotation.
