transformer controlled generator

[mobile_bob]

when it comes to excitation of a generator, we have the typical st harmonic, the avr, the pm type and variants of these, plus
another type known as the transformer controlled generator.

the problems with the harmonic excitation st type system is not so much problems with decent voltage stability, but problems
with increased harmonic distortion of the waveform caused by coupling between the stator windings and the harmonic winding.

the problem with avr excitation is complexity and relative ease of damage to the regulator

the problem with pm excitation is you have to really have excellent design, engineering, consistent materials, magnet strength
cooling, and careful assembly, done right the result is quite good.

the transformer controlled generator is an interesting animal in that it is much more robust than an avr, does not couple harmonics
from a harmonic winding, and can deliver tighter voltage regulation than a pm head from no load to full load.

the theory of operation is as follows

a transformer wound with a standard primary and secondary takes power from the generator output to step down to the required excitation
current and sends that product to the rectifier and on  to the field of the generator, this provides excitation for the generator and when spun up
to the required rpm will allow the alternator to provide the correct voltage at no load.  the up side is there is no coupling in the stator so the
output waveform is not distorted by a harmonic winding would do. the downside is as the load to the alternator is increased the voltage starts to decline because there is no provision for additional excitation current,, however...

the transformer has a second heavy conductor primary winding that is in series with the output to the load, the current flowing through this winding to the load will increase with the load and in doing so increase the transformer current induced into the secondary which increases the output of the secondary, which in turn increases the excitation to the field via the rectifier.

the transformer is known as a "current compound transformer" or a "current boost transformer"

when the transformer is designed and sized properly for the alternator it is to be used with, then the excitation follows closely with additional loading in a linear fashion.

the transformer is a standard EI core unit, that has the I core stacked separate so that the airgap can be adjusted.

the airgap is adjusted to get the no load excitation set for proper output voltage, and there are primary taps to set the full load voltage.

once set the core is locked in place and there is no need to revisit the setup again for the life of the generator.

from what i have found out so far, manufactures such as stamford newage, onan, winpower, kohler and many others have used
this method of control/regulation of generators from small units to quite large, encompassing all the sizes we typically would be interested in using.

there is no reason that transformer control of an st head could not be accomplished, the result would be a cleaner waveform, good regulation
(5% is typical) and the ability to start heavy inductive loads (which the st was not designed to do)

stamford used this type of control on generators that provided 450% increased output for 7 seconds to provide for very heavy motor starting loads,  and it is written that the generators had motor starting ability of 1hp per kva of generator capacity.

personally i am thinking of doing some research in this type of generator control, to see what the outcome might be, what the pros and cons are, and what the cost of implementation might be.  if i can come up with a prototype transformer to test and prove the concept, i can then send it out for replication by a transformer manufacture. 

i am not sure what an acceptable cost for such a transformer might be?  i would think that if the result was a generator with better than 5% thd
better surge capability, better regulation, perhaps 100 bucks might be within reason for one to fit an st5 or st7.5?

any comments are welcome.

bob g

[mike90045]

All I can see, is adding a cap across the rectifier output, or is that supposed to be a pulsing DC ?

Would it work with the existing ST windings, or would the generator need to be re-wound ?

[mobile_bob]

the cap is omitted for simplicity in the illustration

yes it should work with a standard st head without any alterations, except for the disconnection of
the harmonic winding, which would not be used with the transformer control/excitation system.

bob g

[wiebe]

Ive got one here ,try to find a honda em 2500/1500.
The old type gens with v belt drive and side valve motors.
They come with a nice red box on it with volmeter and a pometer vor some variation .
In the red box is the scematic and the transvormer .

mmmm seen some newer scematichs with what seems a elektronic avr ,mine has a transvormer so seek the old ones.

[Ronmar]

The only issue I see, which is the same old issue, is feeding and regulating for a US domestic panel with the center tapped 240V configuration...  which side gets sensed, and how do you balance for that?  In theory, you could use one on each hot leg of the split 240, and combine the outputs after they are rectified.  Each unit would provide field current based on their individual load...  Or perhaps a second current winding in a single transformer, but my transformer theory is a little rusty, and I am not sure how things would interact...  In a 120V only setup, It should work.  I have read about these a long time ago, but never actually seen one in practice...   

[mobile_bob]

thanks wiebe

it stands to reason that honda would also have used the system too.

what i think is interesting as it relates to this systems use with an st head, is the use with the head as driven by
a heavy flywheel engine.

an engine such as a 6/1 has a lot of surge capacity owing to the massive flywheels, couple that with a transformer controlled
st head and well starting or other heavy inductive starting loads such as heat pumps, A/C, welders,  etc. would be greatly improved.

i suspect it would put some of the more marginal listeroid generators over the top and make them much more useful.

from those that have experience with the system, it is thought of as the most bulletproof and effective excitation system, even if it is not
quite as good a voltage regulation as an AVR system. from what i have been able to determine voltage regulation is within 5% which seems adequate to me. probably as good as the harmonic excitation when it comes to voltage regulation under steady state conditions, and clearly provides for less voltage sag under heavy startup loads.

one reference i found for a stamford reports that the voltage climbs approx 5 volts from no load to full load under motor startups, now that is
quite different when most genheads droop under this sort of load condition.  any system that not only maintains the voltage but actually picks
it up a bit on heavy inductive load start up condition ought to really perform well starting those difficult loads that folks seem to be most concerned with.

also of note, most generators i found that use the transformer control are rated with a power factor of 0.8 lagging, while the harmonic
controlled heads are almost always 1.0 power factor. this tells me that the alternator using the transformer control is much better suited to
powering inductive loads than those that use the harmonic excitation.

anyway this is something that is very interesting to me.

bob g

[mobile_bob]

Ronmar:

i should have expected to be caught on the need for 240 split phase systems

the transformers for such duty have dual compound windings on the same core, that way both legs are sensed
and able to provide boost to the secondary winding.

i didn't illustrate the second winding for reasons of clarity, figuring it might be easier to understand the concept if only
one of the compound windings was illustrated.

you are correct though, if there is only one compound winding, then the system either has to be 120 only or 240 only

if it is the latter, then one could use a heavy step down transformer to get the two legs of 120 each and get by with a single
compound winding on the control transformer, however unless one just happened to have such a step down transformer laying
around it would make more sense to have the compound transformer with dual compound windings.

at this point i would really like to get my hands on some design text for a compound current transformer, but like many things in life
it would seem that if they tell you they would then have to kill you sort of thing.

while the system was widely used, widely understood, it would appear that the engineering of such a system was probably a cut and try
determined by emperical evidence sort of thing,  especially given it had to match a specific generator and could not be used effectively on
any other generator.

bob g

[SteveU.]

Hi Guys
My Italian made in 1999 Mecc Alte four pole 8 kW is transformer controlled.
It is a model CT3LA/4 brushless 120/240 60 cycle head.
All of thier later "ECO" models are AVR controlled.
Wiring diagram for mine can be downloaded at
www.meccalte.it/
click open the centered dark blue "mecc alte"
click open "Downloads"
click open "Instruction manuals of obsolete generators"
go to the page 2 of the listings
then down load the "CT3" manual
Info you want is on pages 15-18 & 31-32.
Took you the scenic route there in case some other of their info might strike your fancy.
Parts for this including the transformer assembly is suppose to be available from two different US/Canada sources.
Regards
Washington State Steve Unruh

[vdubnut62]

VERY interesting thread! Keep us posted on your findings, Bob.
Is it possible to to fine tune the transformer with a rheostat? I guess I mean to fine tune the output voltage
Ron

[SteveU.]

Hey Ron
No. As M.Bob said you voltage tune these with squeeze screws going through the transformer laminated mag core stack.
The Mecc Alte manual describes this well.

Regards
Steve Unruh

[Chris]

Some of the later Lister startomatic CS generators used transformer regulation. I was familar with two generator versions. One used some sort of shunt control that I never understood. (I called them the fat boy generators) and the other had a transformer in series with the output (skinney boy)similar to what Bob is describing.
Both generators had DC windings and brushes that were part of the starter motor system, battery charger and to provide no load excitation. There was a variable resistor to adjust the no load voltage. The Skinney boy with the transformer in series had a variable resistor to adjust full load voltage. The secondaries of the transformer went to a retifyer (I could never spell) and back to the field coils. I can't remember if the variable resistor was before or after the current was retified.

There is someone on this site or the other that purchased a virtually new, very low hours, VA generator that had the wiring diagram for the skinney boy generator.
If I remeber correctly the A/C load current was pulled off the slip ring (Rotor) and not the stator.

Chris

[mobile_bob]

there are two main types of alternators

one which we are familiar with is the rotating field type where the power is taken off the fixed stator, and

the other is known as the revolving armature type where the fields are fixed and the power is taken off the
armature which rotates, necessitating large heavy brushes (relative to the much smaller and lighter rigged revolving field type)
via the slip rings.

apparently there were lots of manufactures that did both types,  concurrently.

why i am not sure, the advantage of one design over the other is unclear to me at this time, it would seem to me that a revolving field
would be the best design, however for some reason just about every manufacture also made the revolving armature type so there
must be some advantage to the design.

bob g

[mobile_bob]

Steve

thanks for the mec alte link, it provides a bit more detail to the setup of the transformer

a little piece of the puzzle here, and  little there, somemore yet another place, before long i should have a complete
picture.

bob g

[wiebe]

Here are some pics of the internals of the control box.schematic is lost but wil find it!!









Now hope i can find the backplate is is here somewhare ,thats ware the scematic is .

Greetings Wiebe.

[Ronmar]

Cool pics, thanks.

Note the 6 diode bridge rectifyer.  It looks as if they combine the output of different windings to ultimately derive the field current.  I see one variable resistor, and two adjustable band resistors.  With the number of leads on that transformer, and all appear to be connected, I would hazard a guess that tome are used in conjunction with the adjustable resistors to drive the output.  Hope you can find a schematic, I am curious as to how it works.

Thinking about this last night, I wonder if perhaps this could be accomplished using 2 or 3 smaller off-the-shelf transformers.  You need one fixed, voltage transformer to provide the no-load drive, say 1.5A on an ST-5.  You then need a current transformer that provided the remaining full load field, say an additional 2A at 25A of 120V load.  Current transformers usually provide a ratio of current.  IE: a 12:1 transformer should provide 1A output for 12A thru the primary.  So should provide 2A @24A of load.  These outputs would be combined after rectification...  A combination of different ratios could be used to fine-tune the field to load relationship, or create a secondary path for a split 240 setup....