I wonder if someone could advise on the wiring of this (pictured) variac wiring. I want to wire it for a 240V supply with output upto 270V (which the plate says it is capable of). The terminals, which are simply numbered, do not match with any wiring diagrams I have found and this variac does not seem to be manufactured any longer.

 

If capable of 270 out, input 240v at 1 & 4 , slider output at 3.
Max.

 

The output would connect between 1 ("neutral") and 3.
The input would connect between 1 (neutral) and 4 ("hot").

That might make for the wrong direction of rotation, which would require essentially "mirroring" the connections. In any event, the "hot" wire should be one tap in from the end. This assumes the number of turns between 2 and 5 is the same as between 4 and 1, which is very likely true but should be confirmed. You can confirm by applying the input voltage between 1 and 5, then measuring the voltages between 1 & 2 and 4 & 5.

I recommend performing initial testing with a time-delay fuse of no more than 1 ampere rating in the line input. That should prevent damage if something is not as it should be.

The movable contact of a variac must be fused for a current not greater than the nominal current rating of the transformer. A variac is still a transformer and if you have the moveable contact at the middle position, transformer action would allow twice as much current through the contact as would flow from the line. This can cause damage.

 

I would also suspect that if used on 120v the input would be 1 & 6 and output would be 1 and 3 (slider) in order to output 270v.
Max.

 

Thank you @ebp & @MaxHeadRoom. Those directions seem to work perfectly!