Here is a description of a switched reluctance (DVR) vs a stepper motor. A servo motor is the same as a stepper but has positional feedback.
Switched reluctance motors operate by switching currents in the stator windings when the reluctance in the magnetic circuit changes.
www.motioncontroltips.com
My thoughts are: (comment to turning relevance)
> Servos excel at positioning (n/a)
> Servos have better speed control/consistency at slow rpm, below ~200 rpm or so (relevant for XL turning)
> Unsure of relevant torque capabilities at typical turning speeds, say 500 rpm and up
I tend to think the servo might bring some advantage at low rpm, relevant if you turn a lot of big stuff. At more typical turning speeds, I doubt there is any advanteage vs a VFD or switched reluctance power supply. As noted in the article, an advantage of switched reluctance is power/size density. My experience with vfd lathes suggests to me that the DVR does "out-torque" a similarly rated VFD/AC unit, but I have not been able to find any data to substantiate my impression, and have not found relevant data to compare servo type motors. I think it provides a good marketing strategy for Harvey, though!
Life experience tells me if there was a clear technical advantage to any of the 3, the data would be more accessible. All 3 work, with some strong points here or there, but certainly nothing conclusive pro or con. I have to say the relatively easy replacement of a vfd or motor vs the other 2 technologies are worth strong consideration. If my Galaxi DVR drive system dies there is a chance I'm the market for another lathe and not replacement parts, depending on the cause. To the positive side I haven't heard of many can't repair stories with the system, and I really like the how the lathe works.