A while back, I got a nice deal on an early 90’s Enco 100-1525 milling machine. It’s a Chinese 2/3 size Bridgeport copy with an 8″x32″ table. This machine has a 3-phase motor, which I consider a plus because it means I can have knob-controlled speed by adding a VFD (Variable Frequency Drive).
Adding a VFD is usually not too complicated with typical two-speed three-phase motors, but this machine turned out to have a 4.3-to-1 speed ratio between the two ranges (unlike the typical 2-to-1 ratio). With a 2-to-1 ratio, it’s usually fine to run the motor only on the high speed range and then use the VFD for speed reduction. There is a small torque reduction at low speeds when doing this, but for home shop use, it’s fine.
But with the Enco mill motor’s 4.3-to-1 ratio, if I only drive the high speed motor coils and then try to dial down the speed to cover the slow range, the torque loss is too much. This meant I had to keep BOTH motor speed ranges and devise a way to switch between them.