High Voltage Forum

General Chat => Laboratories, Equipment and Tools => Topic started by: klugesmith on October 11, 2020, 06:18:10 PM

Title: Recording a variac
Post by: klugesmith on October 11, 2020, 06:18:10 PM
Rikkitikkitavi might like this. 
A nice 1.4-kVA variac had a power cord so old, its rubber jacket was cracked and splitting along most of its length.  Was becoming scary to use.

It was time to replace the cord, which had 2 conductors and a polarized plug complementing the variac's output receptacle.
This variac is for the lab, not for a museum, so it received the cord from a dead MWO.  16 AWG with three conductors and a ring lug on the grounding conductor.  How to connect the ground?

Die-cast cover had a blind hole molded into one inside corner, as if for 3-wire corded model.
Not threaded, but matching the tap drill size for an 8-32 screw, like the long screws for cover attachment.
My only 8-32 tap has a long taper, so I needed to drill the hole a little deeper to finish the job.

[edit] The major diameter of 8-32 thread is 4.2 mm.  In my experience it's the "standard" thread for securing wires in household electrical devices in USA.   In the original Powerstat, all connections were soldered except for spade lugs under 8-32 screws at the internal variac terminal plate.
Title: Re: Recording a variac
Post by: johnf on October 11, 2020, 08:44:54 PM
I would have drilled a hole through the side offset from the cord grip to allow use of a nut and bolt and spring washer.
AS/NZS3100 states a minimum of a 4mm bolt that is solely used for earthing purposes
Title: Re: Recording a variac
Post by: rikkitikkitavi on October 11, 2020, 09:47:31 PM
You sexy little thing :)
(the variac)

I presume you test GND from wall plug to the main body chassis, so that the screws holding the cover to the main body makes a good connection?
It looks like aluminium and hence some oxide might give a higher GND resistance. Anyhow nice work.

I have had a few Powerstats passing thru my hands, even all the way over to Sweden and feel their ruggedness and the impressive "Powerkote" and black goo on firmly anchoring the winding.

The old catalouge in a nice 60ies layout/type from Superior Electric.

I have modified one old variac in just the manner johnf describes, but it was a steel chassis, perhaps for magnetic shielding but I mostly use it with my insulating transformer OR a earth fault circuit breaker.  Anyhow  I cant choose not to because I had an electrican remake the entire fuseboard in my house just after moving in  and hence EFCB where mandatory but atleast I have split all 40some fuses on three groups so not everything goes black when I f-k up... and that I do sometimes.

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