Curiosity: Iron Core transformers; which is more efficient?

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,091
Of types A, B & C, which iron core transformer is most efficient and which is least? And would one have an advantage over another in certain circumstances?

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BobTPH

Joined Jun 5, 2013
2,543
Your drawings are ambiguous. Does A have an iron core through the middle? I assume it does since that is the most common configuration. C could be just another view of B, but I assume it has no iron wrapping around.

With those assumptions, the order of efficiency would be A, B, C from high to low.

Bob
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,091
A is an EI iron core. B has a core going through the two bobbins; and so does C. Yes, B & C are much alike. I've seen these sorts of configurations for transformers and was wondering why someone would use one of these config's. A seems to me to be the most efficient, but I wonder if there's a time when B or C would have an advantage. Maybe it has to do with space. I don't know. That's why I ask. (out of curiosity only) (no plans)
 

MrChips

Joined Oct 2, 2009
21,890
My guess is that in theory there is no difference, assuming all the magnetic flux is trapped in the core.
Since we have to compare the surface area of the part of the core at the center of the coils and we assume they are the same then in theory they all reach saturation at the same flux level.
 

BobTPH

Joined Jun 5, 2013
2,543
Wouldn't A have a higher saturation level than B or C assuming similar core cross section? That is why I said it would be more efficient. If not, why don't manufacturers save money by making B or C instead of A?

Perhaps efficiency is the wrong word, I think A is more cost effective.

Bob
 

MrAl

Joined Jun 17, 2014
7,814
Of types A, B & C, which iron core transformer is most efficient and which is least? And would one have an advantage over another in certain circumstances?

View attachment 213011
Hello,

The A core looks like an E I configuration but in that case it is not drawn correctly as the top and side legs of an E I core have only 1/2 the area of the center leg. Taken as is though, that means that a somewhat large part of the core only has about 1/2 the flux density as the center leg and that would mean less core loss than B or C.

Now A and B both have their windings close together along the magnetic path while C has windings far apart. That makes the coupling in C less than in A or B.

Since A has coils close together and lower flux density than B or C, A is best, and since C has a longer magnetic path between coils, that makes C the worst. B is in the middle.

If A was a true E I configuration, then it is probably close to B in efficiency but has better shielding.

So A is probably best while C is clearly the loser, unless of course you need built in leakage inductance and then C is the clear winner.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,091
Thanks for all the input. Just a curiosity question, that's all. Wondering when one type would be favored over another. And the EI core - - - I've seen two types: One where all the E's are stuck together and the I's are welded over the ends. The other was an EI laminated with a reversed IE (EI backwards)to build up the core. The first - I've seen lots of those in microwave oven transformers. The second, I've seen them in smaller transformers. I would imagine the second method is more labor intensive (or machine labor intensive).
 

MaxHeadRoom

Joined Jul 18, 2013
21,443
Much of the manufacturing techniques were to reduce audible hum, the welding, reverse EI, etc.
Early on I dismantled quite a few EI types in order to customize, very laborious process!:(
Max.
 

MrAl

Joined Jun 17, 2014
7,814
Thanks for all the input. Just a curiosity question, that's all. Wondering when one type would be favored over another. And the EI core - - - I've seen two types: One where all the E's are stuck together and the I's are welded over the ends. The other was an EI laminated with a reversed IE (EI backwards)to build up the core. The first - I've seen lots of those in microwave oven transformers. The second, I've seen them in smaller transformers. I would imagine the second method is more labor intensive (or machine labor intensive).
Since the E and the I are two different parts they can vibrate when 50 or 60Hz or higher frequencies are used with them. If they are bolted together with side brackets they can still vibrate significantly so often the entire construction is vacuum varnished. Welding is another way to stop the two from vibrating without the need for vacuum varnishing. A third method is to run a stainless steel band around the entire outside of the construction and use a crimp buckle to keep it banded tightly although varnishing after that is still a good idea for the audible frequencies.
Even with vacuum varnishing high power converters can make a lot of noise. You dont want to have to sit in the same room with those type. Sound deadening material is often used within the enclosure to try to decrease the audio level.

The E and the I are usually arranged as E with I as EI with the I either tightly pressed to the open ends of the E or some insulating material placed across the tips of the three legs of the E so that when the I is placed there is a gap between the E and the I. That creates more leakage inductance when filtering has to be part of the function of the transformer. With no gap the tightest coupling is achieved.

If the construction really is made with the I first and E second as IE then they must have been after a large mount of leakage inductance and very low effective permeability which would be beneficial in DC filtering applications. I've never come across one constructed like that though perhaps it is a tuning mechanism also.

There are many, many other configurations we havent looked at yet. For example, the pot core. Also, with EI laminations (maybe with other types too) sometimes small "shunts" are added to tune the resonant frequency of the circuit it is being used in. This is common in resonant converters. Microwave ovens use this idea too.
 

KeithWalker

Joined Jul 10, 2017
1,224
Large laminated iron cored chokes were common in the days of mains driven tube radios. They were built with the I laminations all stacked together and similarly the E laminations. There was an insulated gap between the I and E sections to minimize saturation because they were connected in series with the anode supply and carried the DC supply current. I remember seeing one radio that used the smoothing choke as the magnet for the loudspeaker.
Regards,
Keith
 

sparky 1

Joined Nov 3, 2018
339
While some power transformers have less droop other transformers the choice can be trade offs.
Adjusting a set of cheap current probes is an example showing various degrees of phase shift.
The ANSWER 3 explains the burden resistor.
It is a common misunderstanding. The voltage and current being perpendicular why calibrate.
(The analog of digital ZCD phase sync also shown on oscilloscope)
https://electronics.stackexchange.com/questions/378306/why-im-seeing-a-90-degree-phase-shift-with-a-current-transformer
 

KeithWalker

Joined Jul 10, 2017
1,224
So am I thinking of a choke? A coil of wire turned around an iron core?
Probably. That type is just a big inductor so ignoring the resistive component, AC voltage and current will be 90 degrees out of phase.. There is another type of choke that has two parallel windings. The supply is connected to the pair on one end of the windings and the load on the other. The DC current flows in opposite directions in the pair so the magnetic fields from the conductors cancel each other out and the core does not saturate. Any AC interference also cancels out.
Keith
 
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