sizing servos for a cnc lathe

I have decided to abandon trying to reuse the existing dc servos on my Cincinnati lathe, they don't appear to work like any drive I have any understanding of, there is little to no documentation and they are inefficient DC drives with questionable operating characteristics. 

currently the DC drives on the machine are sized at 

5hp X 

7.5hp Z 

30hp on the spindle. with a max rpm of around 1500 rpm iirc. 

amd I right in thinking I can half size these with AC servos running 2x the speed ?

2.5 hp x 

3.75hp Z 

15hp spindle 

 

Well, you _could_ downsize but one area that DC servos win is low speed "grunt"

I would suggest doing some speculative operations on a feeds and speeds calculator and getting a feel for what 30 spindle HP means.

Your lathe is a monster outside my personal experience.

But: to answer your specific question:

am I right in thinking I can half size these with AC servos running 2x the speed ?

No.

You can halve the _torque_ running 2x the speed with appropriate gear changes, but the power remains the same (that's why we use power in the calcs)

But, what are you making? Do you need to fill a skip with swarf every hour? 30hp will remove about 500kg of metal per day (wild guess, but it is more then 50 and less then 5000)

I don't need 30hp, and i am ok with reducing power. Option b is replacement dc brushed drives, and obviously they'd need to be correctly sized, so meh. 

I'm open to replacing the drives and keeping the motors, but is the financially sensible ??

I see no reason to change the spindle motor, though. It will only take the power needed. Or if it's truly weird and hard to drive, fit a big conventional indiction motor.

But can i use a big induction motor like a dc servo ?? I need to be able to orient the spindle, hold it, cut splines etc.  It's got a very difficult to control reliance v*s drive in it now.

Are there any big dc brysged servi controllers out there ?? Outside of this old junk

smc.collins wrote:

But can i use a big induction motor like a dc servo ?? I need to be able to orient the spindle, hold it, cut splines etc. 

No. 

You can turn, thread, etc with an induction motor but it won't hold position like a DC servo. In fact a DC servo might be better than a modern AC servo in that role. 

So, why don't we look at a different layer: What about the existing drives is hard to control? You probably don't want to afford equivalent drives.