User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0
Thomas,
I would connect the Variacs as pictured below. View ASCII art as a
monospaced font to have it look right. Unfortunately, your diagram got
mangled in transmission.
240 Line -----| CW end of Variac 1
)
)
Variac 1 )
)
) <- Wiper 1 --------> \
) \
| \
Neutral ------+ CCW End both Variacs > 0 - 240 VAC output
| /
) /
) <- Wiper 2 --------> /
Variac 2 )
)
)
)
240 Line -----| CW End of Variac 2
It won't matter to your bridge if the two legs are not precisely
balanced across the 240 line -- the bridge will see only the two
variable tap voltages, and will have no connection to the neutral. It
won't matter if it floats a few volts above or below neutral potential.
Be aware that both the - and + ends of the bridge output will be "Hot"
relative to earth, and you can not ground either of them.
PFC caps provide no protection against voltage spikes. They are only
present to reduce reactive power imbalances resulting from heavily
inductive loads. You can use a beefy line filter across the wiper
outputs to keep RF hash out of your house wiring, and to protect your
Variacs (though they Variacs are much more tolerant of transients than
the rectifier bridge is.)
Dave
On 9/24/2015 1:25 PM, Thomas Shurtz via Tesla wrote:
I have two 1156 powerstat autotransformers that I'm planning to use for control of a 10-15KVA DRSSTC. I am trying to determine the appropriate way (if it exists) to use these in series. If I want to use my split phase 240v service as input then how do I want to wire these up? I assume I would connect the common terminals of the two powerstats to each other and then either wire from the mains to the other ends of each primary. But I have a few questions.
1) Should I connect the common terminals to: a. neutral? b. ground? c. nothing - let it float? d. use it as a centertap for full wave voltage doubler? (My gut tells me to go with a. and d.)
hot1 powerstat 1_______ ) )<-----------------------------neutral ) _______)_________________ to full wave doubler ) powerstat 2 )hot2 )<------------------------------
_______)
2) I plan to use a single shaft tied to both wipers, but certainly the output voltages won't be exactly opposite and equal. Does that matter somewhere down the line for my full bridge? Do I need to do anything special to ensure proper current sharing through the powerstats?
3) If I choose to forgo PFC how are these units likely to hold up under the current spikes I expect from the rectifier/doubler? I seem to remember seeing a datasheet for the 1156d series that indicated they could withstand up to twice the rated current for several minutes, but what does experience say?
I'm hoping that someone has used a similar configuration at some point and could share some pointers/lessons learned.
Thomas