It's still a DC "drive" system though, it just didn't use semiconductors as we know them today. It was a resistance ribon and a 99 position switch. In other words it was not the MOTOR that was inherently variable speed, it was HOW you applied power to it that made it so. That is the same as it is with AC motors now. The only difference really is that the technology for allowing this to happen with AC motors took longer to come about.
Thanks, Jraef.
A little digression on speed control of AC motors.
I'm pretty old. I started in employment as a development/commissioning engineer. Well, I got roped into doing a fair bit of troubleshooting on old systems, some of it manufactured before I was born to designs that pre-dated that by quite some years. I suppose, in part, this was because I was young, had no family, and was prepared to travel with little or no notice.
Much of it was on paper machines where a number of DC drives have to run in precise speed control and and defined close speed ratios. Speed holding to better than 0.1% is a process requirement for good product.
One early system was the faceplate regulator. The armature voltage to all the DC motors came from a DC generator driven by an AC motor. The DC generator output voltage was controlled by adjusting its field either directly or by the field of a common shaft driven exciter. A Ward-Leonard system, so conventional for that era.
The precise speed control of the individual motors was by a face-plate rheostat in the shunt field. The position of the rheostat arm was determined by the output of a differential gearbox. One input was from the drive motor and the other from a synchronous motor. Any difference between the two resulted in the arm moving until there was no difference. The resultant steady state was zero speed error.
The speed of the synchronous motor was determined by the frequency of the Master Alternator. That, in turn, was driven by the Master Motor, the speed of which determined overall machine speed. Variable frequency.
The last one I worked on was originally in the UK but, as with many others, the mill closed. The entire paper machine, including the drives, was shipped to the far east. The drives had worked well but, being of 1930s vintage, replacement parts were getting difficult to source. We were in the process of replacing some bits of the system with electronics. In particular, the faceplate rheostats were in poor condition so we converted them to electronic regulators. We replaced the main generator with a DC drive. Quite a few other major and minor changes were made.
Along with the whole kit and caboodle being shipped to the far east, so was I and a mechanical engineer. Baptism by fire in some ways. I was in my mid twenties and not many years qualified but I had done the design work. I was 10,000 miles from home with an eight hour time difference. And, forty years ago, there was no internet. Essentially, I was on my own. For eight months from laying the sole plates to getting paper off the end of the machine.
You learn from such experiences.
