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Nice three phase variac conversion



Hello list members

I have just converted a 3-phase variac to single phase for my TC control
box and thought this information might be useful.

The variac (actually a "Varitrans" - made by TWNZ) consists of three cores
linked by a common shaft. Each core is rated to supply 15 amps at 0-280
volts with an input voltage of 230 volts (i.e. the nominal line voltage in
NZ).

A power bus supplying a neutral and 230 volt line is connected to each
core. The output from each core is wired in series with a 48 mH inductor
and two of the inductors are each connected to seperate 20 amp switches. In
ASCII (fixed-width courier etc):
        
        [Variacs] [Inductors] [Switches]

L-in
----+-----0--3
    |        3                   
    |        3<---O---mmm---O------------+
N   |        3                           |
----)--+--O--3------------------------+  | 
    |  |                              |  |
    +--)--0--3                        |  |
    |  |     3                   /    |  |  L-out
    |  |     3<---O---mmm---O---/  .--)--+---
    |  |     3                        |  |
    |  +--O--3------------------------+  |
    |  |                              |  |
    +--)--0--3                        |  |
       |     3                   /    |  |
       |     3<---O---mmm---O---/  .--)--+
       |     3                        |     N
       +--O--3------------------------+------


The 48 mH inductor limits the maximum current through each core to 15 amps.
The inductors also prevent the formation of current loops. The maximum
measured difference in output voltage is 2 volts - so the maximum loop
current is (with a 50 Hz line):

V = I*X
2 = I*(2*Pi*50*48*10^-3)
I = 133 mA

I consider this an acceptable loss (less than 1% per core).

The purpose of the switches is to allow the output current to be controlled
in steps of 15 amps. This corresponds to power output levels of 3450VA,
6900VA, and 10,350VA - plenty for my coiling needs. 

Safe coiling,

Gavin Hubbard