Lower Flux density and Lower current density in transformer

[ravindranathan]

How does lower flux density and lower current density help to improve life of Power Tranformers.

 

[waross]

Lower flux density suggests that the transformer is running at less than optimum voltage.
Lower current density suggests that the transformer may be running at less than the optimum load.
There are studies correlating extra heat with loss of transformer life.
I have not seen any studies suggesting that lower temperatures will extend transformer life.
Many may consider lower flux density and lower current density to be wasteful.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Transformers typically have very high efficiency over 98% and a long life of over 40 to 50 years with proper maintenance.

They operate at the knee point of magnetization for stability. Lowering the magnetic flux density will affect that stability.

Lower current density will reduce the copper loss and will result in a slight increase in life but you will pay more upfront.

Muthu

http://www.edison.co.in

 

[bacon4life]

The life of paper insulation has a log relationship with temperature. At or above rated operating temperature, the length of life of a transformer life can be dramatically shortened by the operating temperature. If the transformer operates more than 20 degrees C below rated temperature, the end of life will more likely be related to non-thermal failure modes like rust or leaky gaskets.

Rather than specifying a specific flux/current density, the typical practice is to specify the lifetime costs of load losses and excitation losses. Then the manufacturer can optimize the transformer to achieve the most economical design.

Also, different kinds of core steel have different hysteresis curves. Transformer manufacturers might choose to use a smaller amount of higher quality core steel if there are size/weight constraints.

 

[ravindranathan]

In our specification its mentined that Flux density should not exceed 1.6Tesla and Current density should not exceed 2.5Amps/mm2.None of the vendors came back to us stating that this is not ok or they should be allowed to choose flux/current density.What could be the reason?

 

[waross]

They can easily meet the spec by supplying a re-rated, higher voltage transformer.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

You cannot dictate the flux density and current density to a designer. The usual way is to ask them for the guaranteed copper and iron loss, capitalize those losses and look at the payback period. Obviously, lower these losses, quicker is the payback period and longer life is implied.

Muthu

http://www.edison.co.in

 

[waross]

You cannot dictate the flux density and current density to a designer.

Probably not.
However if you have a target for flux density, you may consider an existing line of transformers with flux density and current density given.
Selecting a transformer with a higher than needed voltage rating will result in a lower flux density at the target voltage.
Likewise, selecting a transformer with a higher KVA rating than needed will result in a lower current density at the target KVA.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

In our specification its mentined that Flux density should not exceed 1.6Tesla and Current density should not exceed 2.5Amps/mm2.None of the vendors came back to us stating that this is not ok or they should be allowed to choose flux/current density.What could be the reason?

I would say most likely the values in your spec are not restrictive compared to what the manufacture would ordinarily provide. If it was, they would probably engage you to discuss it further during the bid process.

We use a motor spec with similar design requirements related to volts per mil of solid insulation, requirement for dedicated turn insulation, etc. Those type of requirement arose from an industry group with experts chiming in on what they thought were bounding requirements that would help ensure a reliable design based on some assumptions about the types materials and processes that would be used during manufacture. Are these useful requirements. or are they just people trying to pretend they're contributing something useful to a spec? I don't know, but it does create an opportunity to bring to your attention if a vendor intends to provide a design that will deviate significantly from what the spec-writers envisioned (because the bidder should take exception, which gives you a chance to talk it through and understand it better)

In other words, my assumption is for the most part the vendor is usually going to quote what they think is right based on the performance requirements and will take exception (or at least contact you for discussion) if these type of detailed design requirements in the spec don't match what they would otherwise recommend. At that point you as a customer have a decision to make based on how you weigh your own specification / understanding vs the vendor recommendation.

 

[prc]

The reason for manufacturers not coming to you with requests for change is simple. They know you want those limits!
In India, users generally put a limit to flux density (B) and current density ( usually 1.65 T and 2.5 A/mm2.) Most of the users avoid loss capitalization but specify a maximum limit for copper loss, core loss and I2R loss. No benefit is given for any lower losses. Users consider they are getting better transformers by this route instead of asking for optimum losses with capitalization rates for losses.

 

[bacon4life]

The slides at

https://www.brown.edu/Departments/Engineering/Courses/ENGN1931F/mag_cores_dataAKSteel-very good.pdf

have some good comparisons of using different grades/thicknesses of core steel to achieve specific loss values. Core steel has improved over the decades, so any specific flux density requirement should be routinely reviewed to see if it is still appropriate.

As to why vendors did not push back, there can be several reasons:
1) Your restriction is not an unusual request, and it only slightly constrains their design. Since all bidders have to follow the same rules, the vendor does not have a direct financial incentive to suggest changes to the specification.
2) Vendors try to give customers what the customer asks for, sometimes even when the vendor's staff thinks the specification is wacky. There was one case where I had inadvertently included an outdated legacy requirement from reusing old specifications, and none of the vendors officially pushed back. However during Factory Acceptance Testing, one of the factory technicians expressed curiosity about why we included the outdated provision. Even if a vendor had pushed back during the official bidding window, it is unlikely I would have delayed the project by rebidding the equipment.
3) Some purchasing departments automatically reject any bid that includes exceptions. Based on this, vendors try very hard to take exception to a bid document.
4) The person preparing the Vendor's bid might not fully understand all aspects of the specification. Or they might just miss certain aspects of specification during bid preparation.

In my region it is common to routinely operate the grid almost 5% higher than the nominal voltage. For a recent transformer I purchased, the flux density at nominal voltage was 1.697 T and the flex density at 105% of nominal voltage was 1.782 T. If you actually want to operate at a certain flux density, it would be important to consider your how the actual grid voltage aligns with the transformer nominal voltage, as well as considering any DETC or LTC settings.