Here's a motor story since we're on the subject.
Back in the 80's when I was wet behind the ears and didn't know shit about like I do now, I bought a new USA brand of CNC vertical mill. I added 3 more later of the same model, for commonality of parts, maintenance, operation, --as used ones came onto the market at good prices, mostly from auctions. There were lot's of great equipment acquistion opportunities, as our economic collapse was just beginning at the time, and few recognized it.
The mills used what would now be considered a crude...
[yet complex, you should have seen the 6 phasing drive boards light up and their TO-3 transistor tops blow holes in them with sillycone fire spurting out, after I replaced all the Motorola Darlington power transistors on them, about 18 transistors, looked like a Christmas tree and I wasn't a happy camper.
I remember staring at them after I threw the main power switch and if "WTF" has a real meaning, that was it.
The transistors needed to be matched and I didn't know that until I went to buy a new set, about $400. So's I made a high current matching test setup and matched up about 5 or so sets of 18 transistors each, for future use.
The matching worked good and when I installed the new set the machine's 3 phase drive ran great for at least 15 more years.
]
...continuing .......3 phase motor drive that had control inputs similar to modern compact drives. The little modern drives are easily retrofitted into these mills. I sold one recently [with the original drive] that I had hung onto, to a machine shop and they immediately put it into production. Really good machines that may have had 50,000 hours on them when we finished with them in the mid 2000's. Talk abt getting your money's worth!
These machines did rigid tapping which means the motor slows down and reverses as the Z axis moves the tap into the workpiece and back out in synchronization. Real slow speeds. We tapped literally must be millions of small holes.
Anyhow, they used a standard GE 5 HP 3 phase motor for the spindle drive and I had one motor crack a motor housing. The machine was no longer supported but parts were generally supported by aftermarket companies. I learned the motor needed to have some sort of 'quality' to work well with the motor drive but I forget what it was.
Something about the massiveness of the casting, wimpy 5 HP motors weren'r suppose to work well.
I remember connecting a modest quality 4 HP motor to the drive and it made noise. The GE motors were plain old high quality ones, nothing designed for variable speed and they worked real well.
After seeing how much a used replacement motor cost, I took the broken motor apart and figgered a way to make a temporary repair. The repair was two steel bands that clamped the cracked housing.
This expedient repair...
...the same sort of a repair that would be made in a SHTF when a replacement might not be available at any cost...
...worked from about 1988 to well into the 2000's when the machine was given to another company and ran there.
The motor ran perfectly and milled and tapped for maybe 20,000 more hours.