The variac would spit, and spark under load, and the sparking
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will cut the sparking wire in half, ruining the variac.
The brush *MUST* contact more than one wire always. That is
crucial to the performance of the variac.
The left hand and right hand stop is fully adjustable to fix
the very problem the OP is complaining of.
Roy Morgan wrote:
I have seen Variac brushes that were not point shaped at the contact but rather had s linear shape. The contact end was as long as the brush diameter but narrowed along the sides so as to contact the full length of the flattened portion of one or two windings.
Without examining any of the ones here, I suspect they do contact more than one winding if positioned between two wires in order to create smooth transition from one winding to the next.
I do have one very old GR Variac whose brush is simply round, and is flat on the contact end. It may date from the 40’s or even 30’s.
On Dec 9, 2019, at 10:57 PM, DW <wilson2115@...> wrote:
What you describe makes sense Chuck. I observe no issues with smoke, hum, and the variac works throughout its full operating range other than not going to zero. Therefore I been convinced the variac as far as the winding is fine.
Having ruled out the variac, Bob made a good point about the carbon brush, I believe he is on to something about the contact point being worn down widening the contact area and potentially causing what I am experiencing. I will try to rework the brush to a point with some sand paper and see if I notice any improvements.
I would like to thank everyone here for their valuable help, I see this being useful for someone with collector supply issues to check the wiper brush!