I have done this for a power source to be used in telecom overvoltage testing. I needed to take 208 or 240 in and produce an AC power supply adjustable from 50 to 1000 VAC. The output is then connected through current limiting resistors (.17 to 20 amps, momentary) to the tip and ring leads of various telecom equipment to determine their immunity to "power cross" conditions.
In my application the 240 comes in, is fused, goes to a powerstat 246U 0-280V 4.2 KVA autotransformer. 0-280 VAC output of autotransformer goes to two sola/hevi-duty HS5F3AS 3KVA 240 volt-wired secondaries in parallel. The Sola/hevi-duty transformers' primaries are configured for 480V each and put in series to get the 1000VAC output I need. Definitely for lab use only! This was built mostly with parts that were on hand (had some nice parts lying about). I will be rebuilding this one with bigger and better parts over the next two months. Unfortunately, still using 240 on the input - don't want to get the facilities department involved with a 480 v installation. Also want some portability - "portability" LOL, the new transformers I'm looking at are going to total about 500 pounds, plus about 100 pounds of 225 watt resistors. Most of the stuff in the (well grounded metal) box I've got is rated to 600 VAC but being used at 1000 VAC. As I said, lab use only.
Don't think this sort of thing is advisable for a permanent installation...
