One of the most important components in a high speed spindle package is the motor. The spindle shaft (and cutter) must rotate, often at high speed, and provide adequate power over the speed range. Without sufficient power, we cannot cut metal and produce parts. To further increase productivity and efficiency, we now must boost the RPM even higher to produce parts faster.
How Did We Start
The first metal cutting spindles were driven by gang belts. A single belt was connected to a large, master motor in the shop. Each machine received rotation from this system. The belts fed both the spindles and feed screws.
AC and DC Motors
Following that, manual milling machines had individual, single speed AC motors. With the introduction of NC control, most main spindles were belt driven by DC motors. At first, the spindle speed was set using stepped pulleys. As electronic technology evolved, variable voltage power supplies were used to control the speed of the DC motor. How does a DC motor work?
A DC motor works by converting direct current (DC) electrical energy into mechanical energy through the interaction of magnetic fields. When DC current flows through the motor's coils, it generates a magnetic field in the stator, which interacts with the magnetic field of the rotor, causing it to rotate. The commutator and brushes ensure that the current direction in the rotor coils is periodically reversed, allowing for continuous rotation.
The main benefit of using a DC motor is high torque at low speed. The amount of torque is determined by both the diameter of the rotor and the length. However, a commutating DC motor is limited in rotation speed and reliability. That makes it less attractive for a high speed spindle.
Integral Spindle + Asynchronous AC Motor
DC motors proved to be efficient, but lacked the higher speed capability required to be used for a high speed spindle. To achieve much higher speeds, it was necessary to make one critical change. A belt driven setup could not support over 15K RPM, so, an integral spindle design was necessary. And, an induction AC system was the motor technology of choice.
AC motors use the back-and-forth flow of AC electricity to generate a rotating motion. They consist of two main parts: a stationary stator and a rotating rotor. When AC power is supplied, it creates a magnetic field in the stator that interacts with the magnetic field of the rotor, causing it to turn. AC motors can provide high power and high speed. They have no brushes, are low cost and last a long time. However, precise speed control is difficult.
The speed of an AC induction motor is equal to the frequency based speed less the slip. As the motor is loaded the speed will decrease. The slip, and current, will increase. Since the actual speed will be less than calculated, it is called the Asynchronous speed. It is usually about 95% of full speed at base speed.
However, since the spindle was required to have variable speed, a Variable Speed Drive (VFD) was necessary. How do they work?
Variable Speed Drive (VFD)
To make an AC motor run at a specific speed, the motor is sent a voltage at a specific frequency. The speed of an AC motor is based upon the frequency. So, to make a spindle run at high RPM requires a drive that provides high current and over 60 Hz frequency. This is generally accomplished using a Variable Frequency Drive, or VFD. The VFD takes supply voltage at 60 Hz and, through electronics, generates a variable output frequency. To control motor speed, the drive is commanded by the CNC to output a specific frequency that matches the programmed spindle speed. To do this, the input voltage is rectified to a DC Buss and then “Stepped Out” in a quasi waveform at a specific frequency. A basic layout of a VFD is provided below.
Courtesy https://electraschematics.com/
Basic open-loop VFD’s have a maximum frequency of 400 Hz. For a 2 pole asynchronous motor, a frequency of 400 Hz would result in a motor speed of 24,000 RPM! For 60,000 RPM, an output frequency of 1000 Hz is required. This technology provided a very good solution to controlling AC spindle motor speed.
It should be noted that all VFD’s were not built for the same applications. Many basic VFD’s are designed for simple tasks other than machine tool spindles, like pumps and fans. For a high speed spindle, the VFD must be capable of very high output frequency (up to 2000Hz) and have a very clean output waveform. Filtering is always required. Without induction filtering, the AC waveform can result in excessive heating and damage to the motor windings. The last thing a spindle needs is more heat to remove!
The Next Step – Close That Loop
As spindle speeds increase, spindle producers are always looking for the incremental advances provided by advancing technologies. For high speed spindles, this means increasing not only spindle speed, but also adding cutting power and process monitoring controls.
Open-loop VFD’s have limitations. In an open-loop system, the VFD outputs the requested voltage and frequency to the spindle motor. The VFD, however, has no feedback on rotor speed or position, so the actual spindle speed, considering slip, will be a maximum of 95% accurate. A better way to control the speed more accurately would be to close the loop!
A closed-loop spindle system is achieved by adding a rotary encoder to the spindle. The encoder feeds back position location to the VFD. This data can be used to better control the speed accuracy up to 99.5%. And, the spindle can now be positioned for tool change orientation. More like a servo.
Diagram of Closed-Loop Motor Control Courtesy Manufacturingtomorrow.com
What Is the Future?
Most people have heard of servo drives. CNC machine tools have used DC servo drives for many years. Over the years, servo drives have evolved to be faster, more accurate and more efficient, primarily through advanced electronics development. How can this technology contribute to the development of high speed spindles? What would the benefits be?
Synchronous is the Word
Modern servo systems rely upon Synchronous Brushless DC motors. A brushless DC motor, or, Permanent Magnet Motor is quite different when compared to an AC induction motor. AC induction motors rely upon an induced current in the rotor to create a magnetic field. This also creates heat in the rotor that will adversely affect the bearings. In a brushless motor, the magnets are integrated into the rotor and provide a magnetic field without creating any excess heat. This helps to boost bearing life and allows higher speeds. Removing rotor heat is very difficult.
The systems are also Synchronous. There is encoder feedback used to provide closed-loop control. This means that the spindle motor is running at exactly the same speed as the commanded rotating field. There is no slip in a synchronous motor. Speed control is very accurate and the spindle can be positioned quite accurately, like a servo motor.
How Did We Start
The first metal cutting spindles were driven by gang belts. A single belt was connected to a large, master motor in the shop. Each machine received rotation from this system. The belts fed both the spindles and feed screws.
AC and DC Motors
Following that, manual milling machines had individual, single speed AC motors. With the introduction of NC control, most main spindles were belt driven by DC motors. At first, the spindle speed was set using stepped pulleys. As electronic technology evolved, variable voltage power supplies were used to control the speed of the DC motor. How does a DC motor work?
A DC motor works by converting direct current (DC) electrical energy into mechanical energy through the interaction of magnetic fields. When DC current flows through the motor's coils, it generates a magnetic field in the stator, which interacts with the magnetic field of the rotor, causing it to rotate. The commutator and brushes ensure that the current direction in the rotor coils is periodically reversed, allowing for continuous rotation.
The main benefit of using a DC motor is high torque at low speed. The amount of torque is determined by both the diameter of the rotor and the length. However, a commutating DC motor is limited in rotation speed and reliability. That makes it less attractive for a high speed spindle.
Integral Spindle + Asynchronous AC Motor
DC motors proved to be efficient, but lacked the higher speed capability required to be used for a high speed spindle. To achieve much higher speeds, it was necessary to make one critical change. A belt driven setup could not support over 15K RPM, so, an integral spindle design was necessary. And, an induction AC system was the motor technology of choice.
AC motors use the back-and-forth flow of AC electricity to generate a rotating motion. They consist of two main parts: a stationary stator and a rotating rotor. When AC power is supplied, it creates a magnetic field in the stator that interacts with the magnetic field of the rotor, causing it to turn. AC motors can provide high power and high speed. They have no brushes, are low cost and last a long time. However, precise speed control is difficult.
The speed of an AC induction motor is equal to the frequency based speed less the slip. As the motor is loaded the speed will decrease. The slip, and current, will increase. Since the actual speed will be less than calculated, it is called the Asynchronous speed. It is usually about 95% of full speed at base speed.
However, since the spindle was required to have variable speed, a Variable Speed Drive (VFD) was necessary. How do they work?
Variable Speed Drive (VFD)
To make an AC motor run at a specific speed, the motor is sent a voltage at a specific frequency. The speed of an AC motor is based upon the frequency. So, to make a spindle run at high RPM requires a drive that provides high current and over 60 Hz frequency. This is generally accomplished using a Variable Frequency Drive, or VFD. The VFD takes supply voltage at 60 Hz and, through electronics, generates a variable output frequency. To control motor speed, the drive is commanded by the CNC to output a specific frequency that matches the programmed spindle speed. To do this, the input voltage is rectified to a DC Buss and then “Stepped Out” in a quasi waveform at a specific frequency. A basic layout of a VFD is provided below.
Courtesy https://electraschematics.com/
Basic open-loop VFD’s have a maximum frequency of 400 Hz. For a 2 pole asynchronous motor, a frequency of 400 Hz would result in a motor speed of 24,000 RPM! For 60,000 RPM, an output frequency of 1000 Hz is required. This technology provided a very good solution to controlling AC spindle motor speed.
It should be noted that all VFD’s were not built for the same applications. Many basic VFD’s are designed for simple tasks other than machine tool spindles, like pumps and fans. For a high speed spindle, the VFD must be capable of very high output frequency (up to 2000Hz) and have a very clean output waveform. Filtering is always required. Without induction filtering, the AC waveform can result in excessive heating and damage to the motor windings. The last thing a spindle needs is more heat to remove!
The Next Step – Close That Loop
As spindle speeds increase, spindle producers are always looking for the incremental advances provided by advancing technologies. For high speed spindles, this means increasing not only spindle speed, but also adding cutting power and process monitoring controls.
Open-loop VFD’s have limitations. In an open-loop system, the VFD outputs the requested voltage and frequency to the spindle motor. The VFD, however, has no feedback on rotor speed or position, so the actual spindle speed, considering slip, will be a maximum of 95% accurate. A better way to control the speed more accurately would be to close the loop!
A closed-loop spindle system is achieved by adding a rotary encoder to the spindle. The encoder feeds back position location to the VFD. This data can be used to better control the speed accuracy up to 99.5%. And, the spindle can now be positioned for tool change orientation. More like a servo.
Diagram of Closed-Loop Motor Control Courtesy Manufacturingtomorrow.comWhat Is the Future?
Most people have heard of servo drives. CNC machine tools have used DC servo drives for many years. Over the years, servo drives have evolved to be faster, more accurate and more efficient, primarily through advanced electronics development. How can this technology contribute to the development of high speed spindles? What would the benefits be?
Synchronous is the Word
Modern servo systems rely upon Synchronous Brushless DC motors. A brushless DC motor, or, Permanent Magnet Motor is quite different when compared to an AC induction motor. AC induction motors rely upon an induced current in the rotor to create a magnetic field. This also creates heat in the rotor that will adversely affect the bearings. In a brushless motor, the magnets are integrated into the rotor and provide a magnetic field without creating any excess heat. This helps to boost bearing life and allows higher speeds. Removing rotor heat is very difficult.
The systems are also Synchronous. There is encoder feedback used to provide closed-loop control. This means that the spindle motor is running at exactly the same speed as the commanded rotating field. There is no slip in a synchronous motor. Speed control is very accurate and the spindle can be positioned quite accurately, like a servo motor.
Courtesy webmnotor.org
Conclusion
New motor technology, including faster processing VFD’s and Synchronous Brushless Permanent Magnet motors are being offered by some high speed spindle suppliers today. This new technology allows higher spindle speeds, boosts torque and has cooler operating conditions. This will result in spindle designs that achieve more capability and last longer. At this time, the higher technology comes with a high price tag. Large, permanent magnet motors and synchronous capable drives are more expensive. This will reduce over time, in the same way CNC replaced NC and those old shop belts!
Next Technology Update Chapters to Follow:
- Tool Holding Technology
- Sensor & Monitoring Technology
