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Re: Variacs in parallel ?
At 06:55 AM 4/18/00 -0600, you wrote:
>Original Poster: "Daniele Bortoluzzi" <dbortoluzzi-at-sogeda.it>
>
>can I put smaller variacs in parallel to withstand more power ?
Hello Daniele
You can do this, although you usually need to add some hardware to stop a
current loop forming. I have attached two posts explaining how to do this.
The first is an excellent explanation by Bert Hickman, and the second is an
explanation by myself describing a specific conversion I made only last
week. Let me know if you need more information.
Safe coiling,
Gavin Hubbard
Bert Hickman wrote:
<<
Commercial paralleling chokes are actually small, high current, 1:1
transformers. They are simply a single center-tapped high-current
winding wound on a silicon-steel core. The winding ends go to two variac
wipers, and the center tap is the combined output. Any slight voltage
mismatch between the pair of variacs is cleverly "cancelled" by the
transformer action, and there's virtually NO circulating current.
If the voltage difference between outputs of a pair of variacs is
defined as Vx = V1-V2, the balanced 1:1 transformer circuit (below) will
share this difference. The trick is to make the windings robust enough
to handle the current, and the core large enough to handle the maximum
expected voltage difference between pairs of wipers on linked variacs
(typically only a few volts).
Vout = [V1-(V1-V2)/2]
|
|
To Wiper of + Vx/2 - | + Vx/2 - To Wiper of
Variac 1 <---------OOOOOOO----o----0000000---------> Variac 2
V1 ----------------------- V2
-----------------------
-----------------------
Common Core
For a 3-phase variac you'd need chokes between pairs in a tree
configuration:
W1 ------------
O
o-----------
0 |
W2 -------o---- |
| |
| o----------- Vout
| |
| |
----- |
O |
o-----------
0
W3 ------------
A bit of heavy gauge wire or strapping and some transformer laminations
should be sufficient to do the job, and one heck of a lot less expensive
new paralleling chokes.
A less "elegant" way to handle this would be to use a small high current
single-winding reactors in the output leg of each variac to be
paralleled (except possibly the first). Two chokes would be necessary to
connect three variacs together. While this method reduces the magnitude
of circulating currents, it does not eliminate them as in the first
circuit. However, each choke only needs to carry approximately the
output current from its variac plus a small circulating current.
>>
Gavin Hubbard wrote:
<<
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.
>>