I'm rebuilding a Bridgeport Series II for the last 2 years. Cleaned out a garbage can of shavings, re-scarped the saddle piece, cleaned ball screws, painted it and now reto an Ajax/Centroid control. The old Heidenhain 3 kw auto transformer will work on 240 single ph. and deliver 85-90 volts ac @ 30 amps for the DC servo supply. This works out well by rearrainging the taps.

My question is: This is an "auto transformer" (no secondary winding). Some have said this is spooky if you lose the gnd conn. Blue smoke and all. I'm torn about winding a new pwr transformer with a secondary if I don't have to. 50 years ago I designed and built transformers but I'm old and tired now and square wire is nasty.

This is my first post. I hope I'm doing this right.

Thanks for any info.

Billy Bird

I have never had a autotransformer smoke on me and I have seen a pile of them, usually on Interact machines from Europe. Even had one at a plant where one leg was tied to ground. The spindle drive did not like this, but everything else was fine.

George

I think I was not clear in my post when I mentioned smoke/ autotransformer. What I meant was that one end of the transformer winding is grounded and the singular coil has many taps as the turns go up the hot side to a 480 volt tap. I have applied 240 volts across the gnd. and 480 tap. This results in the availability of 90 V ac @ 30 amps for the DC servo supply as well as 110 V for machine lamps etc. An engineer friend told me that this is all well and good but make sure that the ground lead is of good size and that it is mechanically well secured to the neutral connection. He said if for some reason that lead failed (opened up) the 240 V would cook (smoke) my servo supply. He said a better design would not employ an auto transformer but rather a transformer with a secondary winding. Is this true? Has anyone faced this problem? Should I continue this way or not? That is my delemma.
Thanks,
Billy Bird

Billy I have a brand new 240v trans/Cap power supply from ajax setting here. I starting to go that way but changed my mind so it is unused. If you are interested let me know. (:~)= Terry