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RE: How are 240 VAC variacs made?



Original poster: "John H. Couture" <couturejh-at-mgte-dot-com> 


Finn -

Thank you for the new wiring diagrams. They explain the wiring very clearly.
When using 120 V. single phase equipment with 240 V. service the coiler
should be aware of the grounding hazards. For example the single phase 120
V. variac normally has the low voltage end of the coil connected to the
metal enclosure. This should be disconnected for 240 V. use. The typical
single phase service is 120/240 volts with a mid tap that is grounded. If
the service is three phase the grounding varies. The new three phase
services are generally 120/208 V. wye connected with the center of the Wye
grounded. This means that the 120 V. legs are phasors at 120 degrees. Check
with the electric utility or an electrical engineer for your local setup.

Large Tesla coil system grounding is inherently complicated and when the
service is three phase the coiler should be very careful of how he connects
the grounding to avoid the hazards.

John Couture

-------------------------------------


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Tuesday, March 02, 2004 8:21 PM
To: tesla-at-pupman-dot-com
Subject: Re: How are 240 VAC variacs made?


Original poster: Finn Hammer <f-h-at-c.dk>

John,

Yes, I have it in my power controller.
Perhaps it helps to show the wiring like this, it is just rearranging the
schematic by flipping one variac 180 degrees:

              9                   9                      9 <---
              9                   9                      9
       -------9            -------9 <---          -------9
        ^     9             ^     9                ^     9
        |     9             |     9                |     9
        |     9             |     9                |     9
        |     9             |     9                |     9
        |     9             |     9                |     9
        |     9<---         |     9                |     9
        |     |             |     |                |     |
        240V  |    0V       240V  |  240V          240V  |  280V
        |     9<---         |     9                |     9
        |     9             |     9                |     9
        |     9             |     9                |     9
        |     9             |     9                |     9
        |     9             |     9                |     9
        v     9             v     9                v     9
       -------9            -------9 <---          -------9
              9                   9                      9
              9                   9                      9 <---

In this case, the wipers are moved out from the center, till beyond the
240V tap point (or 120 tap points, if you like)

Tesla list wrote:
 >Original poster: "John H. Couture" <couturejh-at-mgte-dot-com>
 >Finn -
 >Have you actually tested this wiring setup? I believe you will not get the
 >280 volts with the setup you have shown which is a short circuit across the
 >output.

Only in the shown position, which is also the 0 Volts position.
But ordinary variac`s short the output too:

              9  <---possibility of 280V
              9
240-------- 9
              9
              9
              9
              9
              9
              9
              9<-----------0
   0-----------------------0


If you move the two wipers to the other ends of the windings you
 >will get a lower voltage.

No, this is where I get the 280 V

   If the 240 volt input is grounded there will also
 >be differences in the voltage vectors to ground.

It may not be a smart thing to ground the intermediate feed from variac,
thru` current limiting inductor, to stepup transformer.....

However, I do not have two
 >variacs to make any tests.

You don`t have to, I`m sure you can see it with the new drawing.

Cheers, Finn Hammer


 >             +----------9 <----------------
 >             |          9            ^
 >             |          9            |
 >             |          9            |
 >             |          9            |
 >             |          9          0-280V
 >      -------)----------9            |
 >     ^       |          9            |
 >     |       |          9            |
 >     |       |                       v
 >     |       +----------9 <-----------------
 >    240V                9
 >     |                  9
 >     |                  9
 >     |                  9
 >     v                  9
 >     -------------------9
 >                        9
 >                        9
 >The "9`s" are the windings.
 >If morale is high, he connects them on the same shaft, but this is not
 >necessary.
 >
 >Cheers, Finn Hammer
 >