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Spindle Motor Control of CNC Precision Machining

1. Motors selected for CNC precision machining

Most of the spindle motors of CNC presion machining use cage-type AC three-phase asynchronous motors, because they does not have the problem of commutation of the carbon brushes of DC motors. The vector frequency conversion technology also makes the performance of cage-type asynchronous motors fully reach the performance of DC motors. In addition, they have strong overload capacity, small size, and fully enclosed maintenance-free.

2. CNC precision machining spindle motor control

The motor characteristic curve of CNC precision machining is divided into two parts, the transverse torque at low speed and the constant power at high speed. There is a node at the intersection of these two curves, called the basic rotating speed. According to this basic rotating speed, mechanical parts processing plants usually design the gearbox into two gears, which is convenient for the control of the controller. Because of the two different situations, the adjustment situation is essentially different.

(1) The transverse torque at low speed is adjusted by adjusting the armature voltage.

(2)  Constant power at high speed, speed is regulated by adjusting the excitation current.

No matter which adjustment method is adopted for the spindle speed of CNC precision machining, it is done through the frequency converter. If a permanent magnet synchronous AC servo motor is used, it cannot work well in the weak magnetic state. At this time, there must be an auxiliary system to complete the action of the main shaft. For such an auxiliary system, in the structural design, a gear will be individually so as to control, which is why some large precision mechanical parts processing machine tools adopt three gears.

At present, the constant torque speed regulation range ratio of CNC precision machining spindle drive device can reach 1:100, the constant power speed regulation range ratio can reach 1:30, and the constant torque and constant power speed regulation range ratio can reach 1:2. It can still work continuously for 30 minutes when overload is 1.5 times. For example, if the rated speed of the main shaft is 1500/min, the gears should be separated at 500r/min, and the low gear should be at least 5r/min. A better gear pair in the mechanical parts processing plant can reduce the speed to 0.5r/min.

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