Guess the Fault.

[Fred_Fanackapan]

In the 70s I was employed as the engineer at an advertising agency's television studio.

One of my first jobs was in the preview theatre. The house lights were dimmed by a big Variac. The problem was that the Variac's brushes kept on burning out.

What was the fault?

 

[Taylortwocities]

If it was a wound true Variac then i expect the output load was way too high for the Variac's brushes.

Why do you ask these questions, grasshopper?

 

[Fred_Fanackapan]

No, the rating of the Variac was correct for the load.

 

[JohnD]

was it wired across the supply in parallel to the load?

 

[Fred_Fanackapan]

No, not in parallel.

 

[racket]

Go on then.......the suspence is killing me

 

[sparkyspike]

Since Variacs are American, I guess it was rated at 110v and wired into a 240v supply?

 

[Fred_Fanackapan]

Saying that is a bit like saying Ford is American so Ford cars are LHD.

Anyway the term Variac can mean any variable auto transformer.

From Wikipedia

From 1934 to 2002, Variac was a U.S. trademark of General Radio for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. In 2004, Instrument Service Equipment applied for the Variac trademark for the same type of product. The term has acquired the status of a genericized trademark and is often used to describe similar variable autotransformers made by other makers.

The transformer in question was rated for 240v.

 

[sparkyspike]

Ok, running out of ideas now.

All I can think of is it was wired in reverse. But it wouldn't dim fully then, unless the primary winding was set quite low?

 

[jj4091]

Was this problem there from instalation or appeared after being ok for years?

 

[conny]

Incorrect grade of brush fitted or not bedded in correctly or combination of both.

 

[Fred_Fanackapan]

Was this problem there from instalation or appeared after being ok for years?

The fault was there from installation.

 

[Fred_Fanackapan]

Incorrect grade of brush fitted or not bedded in correctly or combination of both.

No.

 

[sparkyspike]

What was the fault?

 

[Fred_Fanackapan]

When I looked at the wiring diagram then the wiring the fault was apparent.