Core imbalance in RCDs

[Tony Williams]

[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.

 

[John Perry]

Tony Williams wrote:


Nice pictures.

....but very wrong (electrically speaking).

The first picture shows a 1/3 turn, as the wire goes straight
tangentially on the outside, result 35mV.

There's no such thing as a partial turn. Think about it. If you have
current flow, you have a circuit, which means you have a complete loop
(i. e., turn). It doesn't matter much whether the return path is 1/16"
away, or 12" away, or 100ft away. It's still a complete loop.

The larger the loop is, the more opportunity there is for parasitics to
siphon off some of the field; but the core is generally very high mu,
and offsets the parasitics almost completely.

That's why standard current transformers (remember the discussion of a
few days ago?) work quite well with just a single cable threaded through
the core.

Picture 1 gave a tiny voltage because a tiny fraction of the field
passed through the core hole, inducing voltages into the near wires
without being cancelled by corresponding induction into the opposite
wires on the core.

Picture 2 had roughly equal and cancelling fields inside the hole.

Picture 3 had a slightly higher voltage than 1, probably because the
wires were not bent exactly at the same angles, so got a trifle more of
the uncancelled field in the hole.

Picture 4 is picture 1 plus 2 copies of picture 2. Same voltage out as 1.

I don't know how many turns you had on the secondary, butput the primary
wire inside the hole and the output voltage will be very high if you've
wound it at all like a CT (which is what a GFCI or RCD is, after all).

Put the primary inside the hole, loop it back inside again without going
around the core, and you'll have zero out. That's a GFCI/RCD.

jp

 

[Roger Johansson]

There's no such thing as a partial turn. Think about it.

Think about coils without a core which goes around the wires. Air cores
and cores which are like a straight stick.

In all such cases partial turns work exactly as expected. You can use
it to fine-adjust the transformed voltage relation to the input voltage.

Toroids are just a special case, which cannot change the basic laws of
physics, and cannot change the fact that partial turns give different
voltages,

but the partial turns have much less effect in a toroid transformer,
because the effect is masked by the strong effect of going through the
core or not.

 

[Franc Zabkar]

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

Here are the results.

<snip>

Thanks very much for that.

A wire through the middle of the toroidal CT constitutes
one full turn, what happens around the outside is second
order, and there is no such thing as fractional turns.

I guess another way of looking at it is to say that a "turn" refers to
the number of times a conductor passes *through* the core rather than
*around* the core.


- Franc Zabkar

 

[John Perry]

John Perry wrote:




Think about coils without a core which goes around the wires. Air cores
and cores which are like a straight stick.

Of course. Here there's no closed magnetic circuit to confine the
field. There's plenty more leeway for field from one segment of
conductor to avoid all other conductors, or cancel from conductor to
conductor. There is no such leeway in a toroidal or other closed
magnetic circuit.

In all such cases partial turns work exactly as expected. You can use
it to fine-adjust the transformed voltage relation to the input voltage.

Toroids are just a special case, which cannot change the basic laws of
physics, and cannot change the fact that partial turns give different
voltages, but the partial turns have much less effect in a toroid transformer,
because the effect is masked by the strong effect of going through the
core or not.

Actually, no. Toroids enforce the basic laws of physics by not allowing
parts of fields to go astray.

Depends upon your point of view .

jp

 

[Franc Zabkar]

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

 

[Terry Given]

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]

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.

[/QUOTE]

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.