Many circuit board drilling machines still use obsolete frequency converters to drive their very high speed spindles. These typically have a maximum speed of 80,000RPM and pre-date the modern general purpose types that now go that fast.
It can be difficult to retro fit these old machines with new drives for a number of reasons. Some of the spindles are 2-phase types, and these types of converters are no longer manufactured. Others use low voltage 3-phase motors that are not capable of withstanding the high voltage PWM pulse produced by modern drives.
The frequency converter pictured also has an unusual interface which uses a square wave to determine the speed of the drive as well as being a low voltage drive.
There is no documentation for this drive so it was reverse engineered to obtain a schematic. This wouldn't normally be necessary, but the drive in question had a fault that was difficult to trace without a deeper understanding of how it worked.
The are still a good number of machines using Sieb & Meyer, Kavo and Precise frequency converters driving a variety of spindle types. All of these can be supported even though they are obsolete.
Frequency converter repairs
Wessel frequency converter for Jaeger low voltage 60,000 RPM spindles
Contact me on - email@example.com