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DC and AC Spindle Drive Systems for VMC Machining Center

1. ANTISHI VMC machining center DC spindle drive system

The DC spindle drive mostly adopts the thyristor speed regulation method. The DC feed servo system is a double-loop control system composed of a speed loop and a current loop to control the armature voltage of the DC spindle motor. The pivot windings are independent of each other. From the minimum value to the rated value, the motor speed keeps the excitation current unchanged, and realizes the voltage regulation and speed regulation, which belongs to constant torque control; from the rated value to the maximum value, the excitation current decreases, and the magnetic speed regulation is realized, which belongs to constant power control.

2. ANTISHI VMC Machining Center AC Spindle Drive System

With the development of AC speed regulation technology, the spindle drive of CNC machine tools mostly adopts the control method of spindle motor and frequency converter. The grid-side inverter is composed of a three-phase bridge-type fully-controlled rectifier circuit composed of six thyristors. The circuit can work in the rectification state, directly supply power to the intermediate circuit, or work in the inverter state to complete the task of energy feedback to the grid. The load-side inverter consists of six power transistors with anti-parallel freewheeling diodes. Through the control of the magnetic field computer, the three-phase sinusoidal pulse width modulation (PWM) voltage output by the inverter at the load end enables the motor to obtain the required torque current and excitation current. The output three-phase PWM voltage amplitude range is 0~430V, and the frequency adjustment range is 0~300Hz. During feedback braking, the motor energy is charged to the capacitor through the six freewheeling diodes of the AC. When the voltage on the capacitor exceeds 600V, the energy on the capacitor is returned to the grid through the control regulator and the grid-side inverter.

Fault phenomenon: In an ANTISHI VMC machining center, it was found that the Y-axis had vibration during the feeding process.
Analysis and processing process: The vibration and crawling of the coordinate axis during the processing of the ANTISHI VMC machining center are related to various reasons. The fault may be the cause of the mechanical transmission system, or the adjustment and setting of the servo feed system may be improper, etc.

In order to determine the cause of the failure, the operation mode of the ANTISHI VMC machining center was set to the manual mode, and the Y-axis feed was controlled by the hand-cranked pulse generator, and it was found that the Y-axis still vibrated. In this mode, the OVC alarm light on the Y-axis speed unit is on after a long period of movement. It is proved that an overcurrent alarm has occurred in the Y-axis servo drive. According to the above phenomenon, the possible reasons are analyzed as follows:

1) The motor is overloaded.

2) Defective mechanical transmission system

3) The position loop gain is too high.

4) Defective servo motor, etc.

Through the interchange method during maintenance, it is confirmed that the cause of the fault lies in the DC servo motor. Remove the Y-axis motor, after inspection, it is found that the springs of 2 of the 6 brushes have been blown, causing the armature current to be unbalanced and the motor output torque to be unbalanced. In addition, it was found that the bearing of the motor was also damaged, which caused vibration and overcurrent of the Y-axis.

After replacing the motor bearings and brushes, the ANTISHI VMC machining center returned to normal.

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