Core imbalance in RCDs

[Franc Zabkar]

On Tue, 10 May 2005 09:25:05 +0100, Tony Williams
<tonyw@ledelec.demon.co.uk> put finger to keyboard and composed:

In article <m7mv711o7d1optojtlsk2del9d5qsnih9a@4ax.com>,
Franc Zabkar <fzabkar@optussnet.com.au> wrote:

Unfortunately I don't have access to a meter, so I am unable to
test my idea.

I still have some CT test kit still around,
and it is a trivial experiment to do, as below.

The CT has a 1000 turn secondary and I still have
an old 100-way plug+socket arrangement, with 100
wires, arranged as 50+50 turns. So those 50+50
turns can be the primary, wired either as series-
-adding or series-opposing. A primary current
of 1A is equivalent to 100A in a bar-primary.

Here are the results.

2. Series-opposing 50-50 turn pri, 1000 turn sec.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2a. Straight through, return bundle as far away as poss.
Ipri= 1.020A, Isec= 7.6uA.... yes, microamps.

Hmmm, that means there must be a net flux equivalent to
7.6mAmpere.Turns. This in turn suggests that a perfectly balanced load
current of 100A is creating a core imbalance equivalent to that
expected for a leakage current of 7.6mA. I wonder if you now pass a
single wire through the core whether an ammeter will measure 7.6mA.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

[Terry Given]

Mark wrote:

There is no such thing as a fractional turn in a transformer.

There are only integer turns coupling and leakage inductance.


Mark

Not quite true. Its possible to wind half-turns on E cores, but of
course this unbalances the flux in the outer legs, which may cause
problems. for example see Billings, Switch Mode Power Supply Handbook
section 4.23 - transformer half-turn techniques.

A toroid cannot have half-turns, for the reason John Perry pointed out -
current flows in loops therefore the circuit *always* completes a loop
around the toroid.

Cheers
Terry

 

[Tony Williams]

In article <xn0e23d116cg796007@news.sunsite.dk>,
Roger Johansson <no-email@no.invalid> wrote:

[snip]

We could also put the wire through the core once, to see how much
stronger the influence gets, to compare the result with the
fractional turns experiments.

Sorry, I've been careless with my typing again.

That is not one wire carrying 1A, it is the
100-wire bundle, each wire carrying 1A. It would
probably try to produce kilovolts if the bundle passed
through the core.

As already remarked, the induction from wires outside
a toroidal core is insignificant compared with a wire
passing through the centre.

--
Tony Williams.

 

[Tony Williams]

In article <efs4819nqgpsgsnerrntqr2q09er9du99s@4ax.com>,
Franc Zabkar <fzabkar@optussnet.com.au> wrote:

2. Series-opposing 50-50 turn pri, 1000 turn sec.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2a. Straight through, return bundle as far away as poss.
Ipri= 1.020A, Isec= 7.6uA.... yes, microamps.

Hmmm, that means there must be a net flux equivalent to
7.6mAmpere.Turns. This in turn suggests that a perfectly balanced
load current of 100A is creating a core imbalance equivalent to
that expected for a leakage current of 7.6mA. I wonder if you now
pass a single wire through the core whether an ammeter will
measure 7.6mA.

I'm ok with the series-adding measurements, but
would be a little cautious about using the numbers
off the series-opposing expt.... there was too
much variation across the three conditions. I
suspect some other effects are going on.

--
Tony Williams.