ECN Forum Forums Technical Discussion Electrical Theory and Applications Core Losses

Guys, I appreciate the responses.........

However, I need a little more help.

A customer has (5) 75kva-112.5Kva general purpose dry transformers, feeding (5) machines in one area of production. they are 480X208.
(4) come from one 400a 480v panelboard, and the other one is it's own feed. Both feeds are about 200' from the main service MDP.


How would someone calculate the savings on this?
How much better would one transformer save vs 5 or 6 smaller "general purpose".

What would you do? Any suggestions?


Dnk....

One more very important piece of infomation is needed, the load profile. Published loss data is usually only provided at 100% loading so the actual loading is important.

Core loss is present 100% of the time a transformer is energized. Conductor loss is only present when the transformer is loaded. This loss varies with the square of the loading, at 90% load there will only be 81% of the full load conductor losses.

So if your transformer is fully loaded 24hx7d (like a production line) you would want a design that minimizes conductor losses. But if your transformer was only loaded 80% for 16hx5d and 30% for 8hx5d + 24hx2d (like an office building) you might want a design that minimizes core losses.

For a not-completely-unrelated question, what would the in-rush be if a typical 7200/240VAC pole transformer were to be back energized? Say, by a clueless homeowner supplying the grid? I tried to calculate this last night but couldn't find the right xfmr specs to work it out

Dnkldorf: Personally? I'd keep the 5 transformers, but I'd bus them on both ends with ACBs, so that if I lost any of them, I could isolate it and use the others to assume the load. If I was designing new, I'd run the numbers first, but I'd probably just use 2, each capable of carrying the full load in case of failure. But then, efficiency savings from the transformers is very low on my priority list

[This message has been edited by SteveFehr (edited 12-04-2005).]

Just to add further complexity to winnie's excellent description, the losses are also temperature dependant, increasing by about 0.4% per degree C over ambient. And you have to factor in the quality of the iron sheets used in the core laminations, because 'impurities', [carbon, phosphorus, sulphur, silicon, manganese, copper, etc.], affect permeability. Historically, many of these impurities could not be commercially totally removed from the iron in smelting, so pure ores were used; the best being pure red haematites from Sweden smelted with charcoal- because coal and derived cokes contaminated the metal. Hence the term Swedish Iron. Absolutely no idea what iron is used today though.

Alan

math error

[This message has been edited by Alan Belson (edited 12-05-2005).]

I put in a 75kva zig zag today. 480X208Y.

Anyway, I remembered to take readings before and after....

75kva that I took out, Dry transformer..general purpose, yada,yada...

With no load, primary current was 7.5-8 amps/phase...

New transformer...1 amp/phase, no load....

The old transformer had 6.5kw of core losses and magentizing currents?


Dnk......

Dnk,
That sounds rather worrying for a transformer of that size.

This may sound extremely odd - and even down right incorrect, but the idling current on the original ("Old") Transformer may have been drawing mostly Reactive Power (AKA "VARs"), and very little True Power (AKA "Watts"), within the complete Apparent Power (AKA "Volt-Amps") package.

Simply put, the measured 7.5 Amps on the Primary side would result in 6.2 KVA - the complete Apparent Power Package.
Since the Transformer is at idle - and the only True Power carried within the input KVA would be the resultant of internal losses of the Transformer (see "Losses" below for more info.), the "remainder" of the KVA input would be Reactive Power.

Example figures (just for fun!):

<OL TYPE=A>

[*] Input Apparent Power "package" (KVA): 6200 VA (+/- 7,5 Amps @ 480V 3Ø),


[*] Input True Power (Wattage): 416 Watts (0.5 Amps @ 480V 3Ø),


[*] Input Reactive Power (VARs): 6186 VARs (7.435 Amps @ 480V 3Ø)
</OL>

The True Power would be creating _Most_ of the heat that could be felt at the Transformer [makes sense... True Power = Heat energy...in a simple way ...], directly from "Resistive-Like" losses,

... BUT:

The Reactive Power would do nothing more than flow between the Poco's Transformer and this Transformer - however, due to it's Magnetising abilities, it will result in exibiting "Resistance-Like" losses; and thus will create a scenario where heat energy is released, and therefore, an additional True Power load on the Primary side.

What does all this mean?

Well, not very much!

Seriously though, when taking an Amp Reading on an idling Transformer (and even some Induction Motors running unloaded), the measured readings may not directly reflect the entire KW drawn.

One big clue to the amount of True Power (KW) drawn is the amount of heat "Pouring Out" of the Transformer's Enclosure.

Say you took an Amp reading on the Primary side, and found the idling Transformer to have a "sort-of equal" reading of 3.5 Amps on all 3 conductors, you could "Guesstimate" the consumed KW by:

If there is just a small amount pouring out - like the amount of heat that would be felt from a 100 Watt Incandescent Lamp, the input KW would be small (possibly 100 Watts!)

However, if the amount of heat pouring out is large - like what comes out of a 1500 Watt Portable Heater, the input KW would be much higher (possibly 1500 Watts!).

*** Losses ***
(and some other stuff)

Losses at the Transformer may be from the Windings' Resistance (most responsible), effects in the core, and other stuff.
But what causes a large VAR draw, and why would a Transformer be designed to draw such a large VAR?

The VAR input is Magnetising power, and it's a resultant of the core design.
The VARs are stored in the core/windings assembly, and having them "readily available" makes the Transformer react better when there is a large draw on the Secondary side (such as a loaded Motor starting).

Another item is the Power Factor of the idling Transformer's Primary Windings.
At idle, the Power Factor is very low - only True Power coming in is heating the surrounding air, and maybe passed on to the charging of the circuitry + windings on the Secondary side. The rest of the power is Reactive.

When the Transformer is in operation, the Power Factor changes.
Off hand, I think the "changing points" are at 35% and 65% of the Full-Load ratings.

With heavy draws across the core (from winding to winding), the losses may become higher - especially with cores made from more reluctant materials - or using "El-Cheapo" brand of Silicon Steel (more impurities than the "Non El-Cheapo" brand of Silicon Steel).

Some Transformers are designed to idle "high", others aren't. Some are designed with larger air gaps on the core (which, even with low permeable materials, increases the core's reluctance drammatically), some designed with EMI / RFI sheilding, some designed with "Impedance Protection", yadda-yadda-yadda.
All these things effect the "lossy-ness" of the Transformer.

Here in California, Transformers need to conform to the State's Energy Commission guidelines (AKA: "TP-1").
Losses, which result in excessive input KW draws, are frowned upon bigtime!

I wish there was enough time to include as much Transformer information as should be, but the Dinner Bell is ringing - and I am STARVING!!!

So see ya later everyone!

Let me know if there are way too many inconsistancies (sp???) in this message.

Scott35

edited to fix spelleeng airrerereerrrszzz

[This message has been edited by Scott35 (edited 12-10-2005).]

Thanks everyone for your input..........

Now then, let's say the transformer in question here has a 6.5kw loss at idle.

Does this loss increase with load by a percentage?

At no load, we have a 6.5kw loss, what could we expect if the transformer was loaded at 75%. Would the losses increase porportionally to the load, or not?

Would more magnetizing currents and eddy currents be present, therfore increasing the total amount of losses, or is it the other way around. Whereby loading the transformer more, decreases the idle losses?


Dnk.....

[This message has been edited by Dnkldorf (edited 12-11-2005).]

In almost all cases _total_ losses will increase as the load increases.

However the various internal losses will change in relative proportion.

My gut feeling is that the 'core' losses (magnetizing and eddy current) would _decrease_, since the input voltage for magnetizing would drop slightly. But this is from experience with induction motor design; the various loss aspects mentioned above might have the opposite effect on core loss.

At the same time the load conduction losses would increase in proportion to the square of the current. I expect lower losses in the steel and higher losses in the copper as the load increases.

-Jon

I should of checked PF before.....

Didn't think of that one for some reason..

And I own the meter, DUH.....


Thanks guys...


Dnk......