Practical testing the specs of a power transformer?

[Mark]

I recently had a power transformer rewound rather than buy an aftermarket one. The motivation was rewinding was $160 cheaper and the original part was reused. This was a bad move as the transformer wasn’t putting out the power that was required.

https://ampgarage.com/forum/viewtopic.p ... 05#p467805

I would think in order to measure the output of a power transformer I would have to know the mains input voltage, and measure the output voltages under load. The filament voltage would be easy enough, but the secondary winding supplying the amp is another matter.

The rail voltage with no signal would be higher than the rail voltage at full output, and current flow would have to be measured. My thoughts are that I would need a meter measuring current flow as signal is applied to the amp and a meter measuring the rail voltage sagging. So at least two meters are required in order to measure power. I would think the power should be relatively constant once signal is applied.

290FEX.pdf

I did contact Hammond about their 290FEX transformer and explained what I was encountering, they very kindly replied to my concerns. Regarding rewinding transformers, this is what they had to say.

“The issue with rewound transformers is that they are much less efficient than they were originally because the lams could never be re-stacked back together nearly as effectively as a new product, but it is what it is -working again.”

It’s part of my nature to be sceptical about nearly everything, my scepticism is it is in their interest to sell new transformers, but what stated could well be true, I don’t know enough about transformers to agree or disagree on this.

What are your thoughts on testing power transformers and that rewound transformers are less efficient than new transformers?

Thanks for your time and patience.

[R.G.]

I've been debating whether or not to post a series of transformer articles specifically on how to think about power transformers.

You can model a power transformer as an ideal transformer hidden inside some real-world imperfections. How closely this resembles the real world stuff depends on how much work you want to go through.
An ideal transformer is only a ratio - the Vprimary to Vsecondary ratio. everything else falls out of that, plus imperfections. We know heuristically that Npri/Nsec = Vpri/Vsec; and that Isec = Vpri/Vsec * Ipri.

The first order of approximation to make is to measure:
1. The DC resistance of the primary wires and secondary wires.
2. The no-load current into the primary; this "magnetizing current" is the price for energizing the core so transforming can take place. To a first approximation, this is Vpri(rms)/Xlpri so you can calculate the magnetizing inductance. A poor stack on re-used laminations will increase this magnetizing current by lowering the primary inductance. This is what I think the guys at Hammond were talking about.
3. The no-load voltage on the secondary, measured accurately at the same time the voltage on the primary is measured. From the preceding measurements, you can calculate the turns ratio/voltage ratio reasonably accurately. The voltage on the ideal transformer primary inside the real transformer is lowered by the magnetizing current from 2. times the primary wire resistance. What remains is the ideal primary voltage, and since there is no load on the secondary, it does not lose voltage to current losses.
These measurements are all you need to do a reasonably accurate model and calculations of a real world power transformer.
In practice, re-winding a transformer and re-using the laminations requires a lot of manual skill. It is artistically difficult to not bend or damage laminations getting them out, clean them adequately, and insulate them from one another at restacking, while at the same time getting them closely and tightly stacked. Shorts between laminations run up eddy current losses in the iron, and loose stacking drops the primary inductance, both of which run up the no-load magnetizing current.
There are additional tests for the leakage inductance and capacitances, but for most power applications, these are not needed.

[bepone]

About stacking laminations.. old ones have some parts of ex varnish on them , so E and I when stacked will be with "varnish air gap"..
they still will be on place, E will go on I and i dont see any problem with efficiency..B - flux will be in some minor percent more linearised. L primary inductance in some small percent will be lower.

If on the rewound transformer is used thinner wire, then PT will have under load more voltage drop, so also the resulting output power will be sligtly less.. and some light compression added.

[R.G.]

About stacking laminations.. old ones have some parts of ex varnish on them , so E and I when stacked will be with "varnish air gap"..

I have just re-used laminations without worrying about the effects, as you say (I think). I don't hand-wind any transformer where there are mains safety requirements, nor where there is not a reasonably large margin for error. I don't have the equipment to do it accurately enough.

It may be different in other countries, but all of the new laminations I had to work with were covered in the blue-black oxide of iron, not varnish, to start with. The lamination makers stated that this was for lamination-to-lamination insulation for eddy current reduction.

When a transformer is varnished, the varnish can seep between lamination sheets. Un-stacking them pries apart this interlayer varnish and leaves bits of it on both surfaces. When re-stacking, these bits don't align again, so each lamination is a little further apart than it was originally. This makes the core stack be a little bigger than it was originally, and all of the original laminations might not fit. Whether this is a problem or not depends on the use of the transformer - if it was originally designed with more laminations (or - not as high a Bmax fux density) then loss of a lamination or two may not matter. If it was running at the edge of saturation, core losses would go up.

This gets to be an issue when a 60Hz transformer is used on 50Hz, and when a transformer is used on a higher AC mains voltage than it was designed for. In the USA, the AC voltage has crept up, from 110 to 112 to 115 to 120 and beyond. My house measures 124-125Vac as a rule. So a transformer designed for 60Hz and 112Vac would be pushed further towards laturation losses plugged into my 125Vac. Looked at another way, ideally a re-wind of a 112Vac transformer needs more laminations than it originally had, not less.

But yeah, usually a rebuilder gets away with it with only a little more heat in the transformer.

[bepone]

When you dissasemble old one, varnish is everywhere also between laminations, also there is internal and external I side (external is with the varnish). During second assembly it is possible to insert I part, varnished side , to "inside", and then it is creating gapped lamination with E side.

But i didnt see any major issue with that, if they were reffering to it..

[R.G.]

But i didnt see any major issue with that, if they were reffering to it..

As I said, usually a rebuilder gets away with it with only a little more heat in the transformer. Usually.

I wonder if commercial re-winder businesses soak the laminations in some kind of varnish remover. Probably not, but since iron oxide on the laminations will not be removed by most solvents, it could be possible.

[Mark]

I would appreciate an article on transformers, I find that I need the various terms explained to me so I know what to measure.

[bepone]

just winding 1kV 30W - nominal (42Wmax) Single ended output transformer for HiFi use..
and strong transmitter triodes.

IMG_2024.jpg

[R.G.]

What's the core Bmax and interleaving like on that?