Development of an FPGA-based sub-module as three-phase spindle motor speed controller for CNC PCB milling machine

Spindle motor (three-phase asynchronous AC motor) is an important part of Computer Numerical Control (CNC) Printed Circuit Board (PCB) milling machine, which plays role to engrave the copper layer based on the PCB pattern design. To perform the engraving processes, it is necessary to adjust the spindle motor speed in accordance with the PCB material used. This paper presents the results of implementing a three-phase spindle-motor speed controller based on Field Programmable Gate Array (FPGA). The control method used is fixed Voltage/Frequency (V/F) with Sinusoidal Pulse Width Modulation (SPWM) technique, so that the maximum torque of the motor can be maintained and the harmonic signals can be minimized. This sub-module consists of two main components, namely FPGA and three-phase inverter, which consists of opto-isolation circuit, logic inverter, MOSFET driver, and three-phase full-bridge inverter circuit. The FPGA unit generates digital SPWM signal as input to the inverter to convert DC voltage into three-phase AC voltage according to the given input frequency. Results show that the sub-module can control the speed in the range of 14000-28000 RPM with the corresponding frequency range of 250-500 Hz. The Sub-module also has constant V/F and Speed/Frequency (ω/F) characteristics. In addition, a ramp speed profile used during acceleration and deceleration has also been successfully implemented to address the problem of low starting torque due to the increase of supply frequency.