Y connected ? and transformer efficiency ?

[bob g]

haven't had time to devote to windpower but have been doing

a bunch of research related to alternators and their design.

i came across a boost scheme that is used in some applications

and wondered if anyone tried it with their homebuilt alternators

and what the result was.

not being able to post a schematic i will try to put it into words

if one has an alternator stator connected in a Y or star configuration

therefore there is a common connection of the phases (center of the Y)

the usual connection is to connect the outer legs of the Y to the bridge rectifier

such as 6 diodes (3 upper and 3 lower) of course.

in the scheme i am refering to the center connection is also brought out

and another rectifier (1 upper and 1 lower) is also used.

it is reported that some alternators gain approx 10% in output by using this method.

either by higher output or an increase in efficiency.

has anyone done this? and what was the result?

any comments from the Electronic guru's are also welcome.

the second question relates to transformer efficiency

from what i have been able to determine from my research

transformer efficiency can be quite high most especially in very large distribution

transformer where 98% and above efficiencies are common.

smaller transformers from my earlier understanding are much less in efficiency

commonly between 85-90% on a good day (at least the ones under a few kva capacity)

i am now beginning to think perhaps some of the smaller units might also be over 90% efficient because of evidence found in some of the high power sine wave inverters which use transformers in the final stage. many of these units report over 90% efficiency for the total inverter, so it would stand to reason that the transformer used internally would have to be well over 90% because of other losses in the inverter to start with.

so my question becomes

are these transformers that are used in these inverters more efficient than typical off the shelf power transformers?

and what would you expect the effiency of inverter transformers to be?

i am thinking they too must be ~95% or better effiency?

comments?

bob g

Moved to Controls.

[joestue]

I have nothing to say about the alternator connection, i'll try to find the time to get a simulation going, i suspect if the third harmonic is high enough, some of that voltage could be dumped into the dc buss, but that would have to be one crappy alternator.

Transformers, well, in theory the iron loss is proportional to weight, and a function of flux density and frequency.

What makes small transformers inefficient is a combination of the fact that only a part of the iron is aligned to the magnetic field, because the core is stamped from a sheet. Distribution transformers are built up from separate strips of grain-aligned electric steel. They operate at 1.7 T and higher, where the amorphous sheet E-I cores typically are no higher than 1.2 T.

Automatically this means more turns of a smaller wire. it's also a squared function, double the flux density, halve the turns, 1/4th the resistance, 4 times the VA's.

Perhaps the real reason small transformers are inefficient is because they can be, proportionally they have 1000 times more cooling surface area, compared to a 100MVA Tx.

[bob g]

Joestue:

thanks for the comments

my question is,

if small transformers are less efficient, how do inverters that use them

in the final stage (60 hz) reach overall efficiencies of ~90%?

it would seem to me that perhaps the transformers used in inverters must be

more efficient than their off the shelf counterparts.

if they were 80% efficient, then the rest of the circuit, controls, cooling fans, etc would have to be at the very least 100% efficient for an average of 90% overall.

(and we all know nothing in the real world is 100% efficient)

the reality would seem that the electronics would be closer to maybe 95% efficient

and the transformer to be 85% for it to average out at 90%?

that is if the losses are about evenly shared between the electronics and the final stage transformer?

it would seem to me that the losses would be weighted toward the transformer instead of the electronics in reality, so would it not follow that the transformer would have to be much closer to 90% for the total of the inverter to achieve 90% efficiency?

i have two sets of transformers out of inverters, they are both EI cores

but are built up that way, out of seperate pieces interweved to form the E section

and the I section rather than the more common stamped complete E section with its complimentary I stampings.

perhaps the inverter transformer cores are made to a higher standard and therefore achieve higher efficiency?

just trying to get an understanding of these efficiencies,, there is not much written about transformers in this kva rating that i have found.

if it is true that inverter transformers are built to a higher standard and achieve higher efficiencies it would appear to me that old inverters/ups transformers might be useful for power conversion/transmission of diy'er windgenerators.

thoughts?

bob g

[ghurd]

I tried boosting the 3-ph voltage, but starting with crap usually means ending with crap.

Ran out of time before I had the transformer figured out for a decent match. The thing has maybe 60 wires.

I'm not clear on the scheme you are talking about.  It almost sounds like a Delta-Y switch?

I think most mentioned increases in efficiency are when transmitting high voltage 3 phase a long distance, then stepping it down near the batteries.  Less line losses.

G-

[bob g]

i am not in reference to a Wye/delta switch

those i am familiar with, but thanks.

also my math is a bit off

if the electronics are 95% efficient

and the transformer is also 95%

the net is ~90 % for the inverter.

therefore an inverter transformer would have to exceed 90% in efficiency

in a inverter that uses one in the final stage?

maybe typing this i have answered one of my questions?

bob g

[joestue]

Are these true sine wave inverters you speak of?

If not, then the manufacturer is forced to use better iron, because harmonics quickly dominate iron loss in square wave driven circuitry.

I have used a 450va rated transformers from an APC ups as a 600 watt oil cooled battery charger, it didn't burn up, also the no load current draw was 1 amp from a 12 volt lead acid, when operating as an inverter. now that I think of it, that is really good, considering the iron core weighed a few pounds. I would not be surprised if the iron is of a much better grade than it was 10 years ago.

[commanda]

The iron used in transformers does come in several "grades".

The good stuff is considerably better than the low-grade stuff.

Looking in my Jaycar catalog, I see several transformers with the following annotation:

Made from special high grade silicon steel for compact dimensions.

This same high-grade stuff is often used for valve amplifier audio output transformers as well.

Amanda

[Jerry]

Carefull Bob G. Your gona start up that dredfull Jerry Rigged VS Star debate again (LOL).  More to follow.

                          JK TAS Jerry

[Flux]

Not sure about the alternator connection without seeing a diagram. It sounds as though it is a double half wave rectifier where a positive line is half wave rectified with respect to neutral and a similar negative line is done. Useful for things that need a split positive and negative supply.

For a single supply it is the same as a bridge rectifier when the single load is taken from plus to minus. May be an advantage in cases where dual supplies are needed but otherwise it's just a 3 phase bridge and the neutral is irrelevant.

Now the transformer bit.

I feel certain that the transformers in the inverters you are referring to are high frequency ferrite devices. In general the inverters using power frequency transformers have lower efficiency.

I suppose within reason a transformer can have as high an efficiency as you want if you make it big enough and costly enough. You can reduce copper loss to negligible proportions by using thick enough wire. You can't entirely loose the iron loss, but by keeping flux density low ( lots of turns/volt) and using the highest grades of "iron" possible you may be able to get a low power transformer up to low 90s%

Small electrical machines are inherently larger in proportion and less efficient than their monster power station brothers. I think Joestue hit it exactly with the cooling bit. To be practical and cost effective for most applications the thing needs to be efficient enough to survive its temperature rise. A 1kVA transformer may be able to get rid of 15% loss easily enough. Trying to dissipate 15% of 50mVA is a very different problem. The cooling is a major issue for units running at over 98%.

Typical small transformers are probably considerably worse than many imagine with many cheap units coming in at below 80%. The average customer will not pay the price or accept something weighing 5 times as much to gain a few %.

The other factor is frequency, the lower the frequency the larger it becomes for a decent efficiency, but above about 250Hz the choice of iron becomes very critical. Many of the aircraft transformers are a fraction of the weight of the 60Hz things but the efficiency may be little better as once again they are optimised to be most cost effective for the job.

Typically you need to be up in the 10kVA region to see efficiencies in the 90% region with standard production devices.

Flux

[Flux]

Bob having looked again at your description I think the type of construction you are referring to is a "C" core design. Something that was common a while ago but seems to be less common these days.

As Joestue said the ideal core is a strip of grain oriented steel wound through the coil bobbin so the flux is along the grain axis. This is costly to manufacture and the usual compromise is a wound strip core cut in half and the faces polished. When tightly clamped together this comes close to the ideal but with some effects from the cut.

These are usually assembled in pairs side by side giving an effective "E" type construction.

To justify this more costly method it is invariably used with the best grade core materials. Mainly used for higher frequency ( 400Hz aircraft) equipment or for compact high performance power frequency devices. If made the same size as a standard unit the efficiency will be higher but it comes with a fair cost penalty. If the efficiency needs to be high and cost is not a factor then you may occasionally find them but most commercial units now seem to get by with a new type of conventional core material that has replaced the old silicon iron punchings ( likely a nickel alloy as it seems soft more like mumetal).

Flux

[joestue]

Just to bring it to your attention, i disassembled a 10KV oil furnace ingniter transformer, it was a stamped elongated 'C' core construction, with windings on both sides, (I presume it ran continuously during operation).

non-aligned transformer iron has about 30% less loss in the rolling direction, (i'm no engineer so i don't remember the proper term here) By lengthening the core in this direction you can add a disproportional amount of copper. but its a 2-4% increase.

Another point to bring up is large transformers have a core built up from 15 or more sizes of steel, the core has about a 90% or better fill factor in a circular area, so the copper loss is 3.1415/4 or 78% that of a square form.