Sub arc welding polarities range from AC, DCEP,DCEN, DCEP with AC trailing dandem, all dependent on welding application coupled with flux choices and wire numbers and wire diameters along with travel speed.
Some with much better results than others. DCEP is probably the most popular in single wire and "semi auto" controlled applications.
Visit their website for a glimpse of their welding fixtures and applications.
http://www.ogdenwelding.com/products.html