Due to the inherent mechanical limitations of DC motors, their applications are subject to numerous restrictions. The surface linear velocity, commutation current, and voltage of the commutator are all limited, increasing the difficulty and cost of motor manufacturing, as well as the complexity of the control system. This restricts the improvement of their speed and power, and their constant speed range is also relatively small. The commutator requires periodic shutdown for inspection and maintenance, making its use and upkeep quite cumbersome.
Since the 1980s, significant advancements have been made in electronic technology, speed regulation theory, and modern control theory. Simultaneously, the maturation of new high-power semiconductor devices, high-power transistors (GTRs), and insulated-gate bipolar transistors (IGBTs) has created the necessary conditions for AC drives to enter the practical application stage. Currently, most CNC machine tools utilize induction AC motors, which suffer from the weaknesses of onboard commutation and limitations in adjustment and high power. Variable frequency AC drives with converter control have achieved performance levels comparable to DC drives. Furthermore, AC motors are small, lightweight, and feature fully enclosed enclosures, providing better protection against dust and oil contamination. Therefore, the complete replacement of DC motors by AC motors is an inevitable trend.
A Brief Analysis of AC Spindle Drives in CNC Machine Tools
There are two types of servo motors: squirrel-cage induction motors and permanent magnet synchronous motors. Currently, the vast majority use AC permanent magnet synchronous motors. Spindle motors mostly adopt the structure of squirrel-cage induction induction motors, and rarely use permanent magnet synchronous motors. This is because the capacity of current permanent magnet synchronous motors is not large enough, and the cost is also relatively high. However, with the development of motors, permanent magnet motors may be used in the future. Of course, through the introduction of DC spindles, we can see that the control of DC spindles does not have the same performance requirements as servo control, meaning that squirrel-cage induction motors can also meet the requirements of machine tools.
A squirrel-cage induction asynchronous motor consists of a three-phase winding distributed on the stator, with these three phases spatially separated by a potential angle of 120 degrees. Its rotor is simply a shorting bar and end ring cast from aluminum alloy, a very simple structure. Clearly, a common induction motor can be used instead of a CNC spindle motor. An AC spindle motor is structurally similar to a common motor, but its electrical and mechanical strengths are significantly enhanced to meet higher requirements. Furthermore, it has many structural features, including improvements in ventilation. Ventilation holes have been added to the stator, and instead of a traditional casing, a polygonal structure made of steel sheets is used as the outer shell, which improves heat dissipation. To improve starting and running characteristics, the rotor is made of skewed cast aluminum. A pulse generator or pulse encoder for detection is coaxially mounted at the tail of the AC spindle motor.
Electric motors are available in two installation methods: flange type and foot type, which can be selected according to different needs.
AC spindle motors also use power-speed curves to reflect their performance. Their characteristic curves show a similar principle to DC spindle motors: constant torque speed regulation below the base speed and constant power speed regulation above the base speed. However, some motors experience a decline in speed beyond a certain value, failing to maintain constant power. For typical spindle motors, this constant power speed regulation ratio is 1:3, with a speed range of 2000~6000 r/min when the motor is outputting constant power. Furthermore, AC spindle motors also possess strong overload capacity, typically 1.2~1.5 times the rated value, with overload time ranging from several minutes to half an hour.
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