Questions about transformer

**The Electrician** · Nov18-09, 08:17 PM

I have a 30 VA Radio Shack transformer that's very handy for making measurements of the effects of variations in core configuration because they didn't varnish the laminations. I can unstack it and restack it however I wish.

I restacked it with all the E laminations together and all the I laminations together. I put the stacks on a flat surface and with a soft mallet, I lined up all the edges and then put a nylon screw through a hole in the laminations to keep everything together.

With all the E's together and placed in the bobbin, I could then take the stack of I's and put them across the ends of the E's so that there was one combined butt joint.

I then clamped the E's and I's together with a few pounds of pressure and measured the power loss with the same setup I described in post #14, applying 120 VAC to the primary with the secondary open circuited..

The DC resistance of the primary is 33.2 ohms.

The first image shows the measurement with the two stacks clamped together. The exciting current is .0787 amps, which gives a copper loss of .0787*.0787*33.2 = .2056 watts. The core loss was 1.65 - .2056 = 1.44 watts.

The second image shows the measurement with the two stacks separated by a .002" piece of nomex, then clamped. The exciting current is .180 amps, which gives a copper loss of 1.076 watts. The core loss was 2.54 - 1.076 = 1.464 watts.

Adding gap to the core substantially increased the exciting current, but hardly changed the core loss at all. This is what I would expect, since the reluctance which the gap adds to the flux path is in air, and air is not a lossy core material.

When the laminations are stacked in interleaved fashion, increasing the gap between an E lam and its mating I lam causes flux which would normally cross the gap to move to the side, and pass through a neighboring lam, increasing the flux density locally there. That would increase loss in the iron locally.

I'll make a pair of measurements on an interleaved stack and post the results.

**The Electrician** · Nov18-09, 08:48 PM

This time the laminations were stacked in standard interleaved fashion. The secondary was open circuited, and 120 VAC applied to the primary.

The first image shows the measurements on the interleaved stack; a soft mallet was used to reduce the interleaved butt joints to a minimum. The exciting current was .0542 amps for a copper loss of .0542*.0542*33.2 = .0975 watts. The core loss was 1.77 - .0975 = 1.6725 watts.

The second image shows the measurements after I took a needle and thrust it into each gap on one side of the stack, moving each I lam on that side about .01" away from its mating E lam. The exciting current increased to .174 amps, giving a copper loss of 1.005 watts. The core loss was 2.74 - 1.005 = 1.735 watts.

So, adding all those gaps increased the exciting current from .0542 to .174 amps, and the core loss from 1.6725 to 1.735 watts.

The point of all this is that exciting current is only a very rough indicator of core quality; you really have to measure the true core loss to determine core quality.

**kudjung** · T, 04:45 AM

Hi There,

Seem like I need to study a lot more about transformer. This transformer was there for a while and I happen to assign to look at the issue. I think we first get this transformer, we only provide them electrical spec. and ask the transformer supplier to design the transformer for us. Look like we might have relying on them too much.

BTW, for the question with welding. I tracked the history. It seem that this transformer was creating humming noise and the supplier put this welding to eliminate the noise.

I'll take a look at all the suggestion.

Thanks once again, very useful.
Kudjung

**uart** · T, 08:49 AM

Originally Posted by Bob S

Both your current waveforms are ugly. You are driving the iron into saturation.

Yes but that is par for the course with very small transformers like this Bob. Linearity of the magnetizing circuit is not really the same issue as it may be with inductor design.

Originally Posted by kudjung

BTW, for the question with welding. I tracked the history. It seem that this transformer was creating humming noise and the supplier put this welding to eliminate the noise.

The weld could increase the eddy current losses but if it's shallow enough (as looks to be the case) it mightn't be any problem.

Assuming that there are no major differences in the physical construction (as seems to be the case) then my diagnosis is simply that they've used a different type of transformer steel on the new transformers.

The newer steel ("bad" transformer) has the following characteristics relative to the original one :

1. Lower magnetic permeability (slope of the B verus H characteristics). This is why it requires more magnetizing ampere turns.

2. Very similar saturation flux levels as the original (knee of the B/H curve happens at approx the same value of B).

3. The newer steel possibly has a smaller hysteresis loop (lower iron loss) than the original. This might sound counter intuitive given the higher magnetizing current, but notice how the magnetizing current in the newer version has slightly better symmetry between the left and right "shoulders" of the waveform. This actually corresponds to lower hysteresis loss.

Overall I would suggest that the new design is not necessarily "Bad". Yes it has higher magnetizing current and you have to assess whether or not that extra copper loss is indeed an issue for you. You should compare the overall losses of the two, as the new core just might have a lower hysteresis loss (offsetting or partially offsetting the higher copper loss).

Can you make some temperature rise tests of the new and old transformers. Just leave them both running at 130V and no load for about 30 minutes and make some measurement with a contact thermometer on the core. It wouldn't hurt to repeat the test at full load secondary current and again compare temperature rise of the two.