[TCML] NST polarity

[David Speck]

Ahhh, yes.

If John wants to parallel one or more NSTs for increased current, he 
could use the technique I used.  There are several ways to go about it.

If you have identical NSTs, there's a pretty good chance that they have 
the same relative polarities.  Regardless or whether they are identical 
or not, I started by arbitrarily marking one of the 120 VAC input 
terminals as "+" on both NSTs, and one of the HV terminals on ONE of the 
NSTs also as "+".  To determine which HV terminal on the second NST saw 
also "+", connect 120 VAC to both NST primaries, with the hot (black) 
lead to BOTH "+" input terminals, and the neutral (white) wire to the 
other terminals.  Ground both cases together  to the green wire.

I then connected a length of neon sign tube (a 4 foot conventional 
fluorescent tube would also work) between the arbitrary "+" HV terminal 
on the first NST, and one of the HV terminals on the unknown NST.  It is 
critical that both cases are grounded, as they will complete the 
circuit.  (NSTs usually have the center tap of their secondaries 
connected to the case).

Apply power to the pair of NSTs, by slowly the applied voltage through a 
Variac.  If the tube lights at a low applied primary voltage, you have 
the "Wrong" polarity, and the opposite terminal on the unknown 
transformer is the "+" terminal.

If you get to full, or nearly full primary voltage without the tube 
lighting, then you have identified the connected terminal on the unknown 
NST as also being a "+".

(Fine print:  if the NSTs have slightly different output voltages, the 
tube may light when nearly full voltage is applied even if the 
polarities match.  You can verify the correct arrangement by reversing 
the HV terminal selection on the unknown transformer.  The tube will 
light at a much lower applied voltage if you have connected a "+" to a 
"-" terminal.)

Select one transformer as a reference unit, and you can proceed to phase 
all the rest of your NSTs if you want to have a bank of three or more in 
parallel.

You can now safely parallel the two NSTs by parallel connecting all the 
"+" power input terminals, and the corresponding "+" HV terminals together.

I've also heard of a similar process that involves drawing arcs between 
terminals but I think my way may be a bit safer and more controlled.

Dave

On 3/26/2011 10:59 AM, dave pierson wrote:
>>> I was wondering if the output on NSTs have specific polarities.
>>> (positive/negative) If so, how can you tell which on is positive
>>> and negative?
>> Conventional iron core NSTs produce alternating current output.  Each
>> terminal is positive on one half of the AC line cycle, and negative on
>> the other half.
>
>> When one HV terminal is positive, the opposite terminal is negative,
>> and vice versa.
>
>      Concur.  However, IF the question relates to paralleling,
>      the instantaneous polarity is critical.  The archives
>      will have much, keyword:
>
>         phasing
>
>      .
>
>       best
>        dwp
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