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RE: ballasting, alternators, Saturable, controllable cores

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Hi Jim,

Sounds like a good big generator :-) I see you have a main and an exciter, so it is really two machines in one. I think the exciter is a permanent magnet alternator that is rectified by a SCR bridge to supply DC to the main field. The voltage regulator should be a transistor circuit that looks at the main output voltage and adjusts the SCR firing angles to change the main field strength in such a way as to keep the output voltage constant.

With any luck, you will be able to just swap any SCRs in there for diodes, and replace the voltage regulator circuit with a small variac. (or small stack of three if the exciter is 3-phase.) This will control the voltage going from the exciter into the main field rectifier, and thus the main field strength and ultimately the output voltage. I imagine a 5 amp variac would be enough, and it gets rid of those pesky transistors ;^)

But please do post the schematic and w e can comment further. Maybe the existing voltage regulator already allows you to adjust the voltage from 0 to 100% in which case your life would be easy.

You are running a DC system so you should not have any trouble with back EMFs smoking your generator- the HV diodes get in the way of any nasties. So you'll smoke them instead ;-) Sounds like a great project, I wish I was there to see it.

Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: "Jim Mora"

Thanks for the reply!

Presently, I am not using the genset as a machinist is setting up the
registration between the diesel flywheel and the flex plate from the
armature. I am limiting my dedicated single phase 70amp 240v Edison circuit.
However, the idea of limiting the field current in the alternator is
intriguing and since T esla invented the rotating magnet field to begin with,
highly appropriate.

Clearly, I am going to have to crack the books on this. The alternator
schematic clearly shows the exciter field bridge diode assembly's connection
to the regulator transformer along with the suppressors rv1 and rv2 across
the main and exciter fields and the six pulse diode assembly.

The alternator is presently wired for 277/480 volt wye output operation. I
planned to scale it back to 139/240 which is where the variac consideration
also came into play when the "big one" hits here. I mention the suppressors
on the fields as it is nagging to me that the generators smoked in Colorado
Springs due to some reflected harmonics back from Tesla's coil, did they

So much to learn, so little time... I should mention that this is a heavy
duty alternator, very conservatively rated at 18.7 KVAR with very heavy
windings. The armature shaft is 2".

Jim Mora

Harvey, if Terry would be so kind, I could scan the schematic inside the
termination box for your clarification remarks, and to augment my limited
education in this area. My neighbor down the street rewinds motors for the
oil patch here; but, I would rather not have to humbly bring it to him :-^)

-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Sunday, February 05, 2006 1:39 PM
To: tesla@xxxxxxxxxx
Subject: Re: ballasting not revisited, Saturable, controllable cores

Original poster: Harvey Norris

--- Tesla list wrote:

> Original poster: Steve Conner
> >Still working on my 3 phase monster... 3 2950
> Superiors in a stack
> >for voltage control. A 20KW 3 phase generator
> driven by an Isuzu
> >diesel (not a factory unit!)
> Why are you bothering to lug a big variac stack and
> saturable
> reactors, when you can easily control voltage by
> adjusting the field
> excitation on the generator?
> Steve Conner
> http://www.scopeboy.com
Yes I was also going to comment on this matter as
there may be more here then mere control of output
voltage by the generator, if designed as the rotating
electromagnet field of an alternator by amount of
causitive field excitation. In this situation the
stator output can be tested for its current limitation
by shorting the output windings, (which also
determines R(int) by division of respective V and I
readings) and then increasing the field current until
the designated current output is reached. It is here
in making this assumptive field measurement that we
discover that the field itself being a DC pole face
assembly that mimics an AC output by design of
triangula r flux leakage in mechanical rotation
interacting with the circular stator rim: this field
itself is a non-linear resistance, easily appearing
higher in resistance in rotation then when not in
motion. In fact it is not until the stator output
voltage is doubled from its prexistant rotational one
that the amount of field current begins to
significantly reflect the availability of stator
current. In any case the fact that the primary of the
transformer can be ballasted by the amount of field
current in the AC alternator as a current limited
supply; this has further ramifications on the
transformer output end which is operating a primary
arc gap. In this case the effect of shorting the
alternator stator itself is to cause its output
voltage to drop to its lowest possible operational
point, thus also withdrawing voltage for the operation
of the primary arc gap, making for the possibility of
a better quenched arc, all of this depe ndent on the
time lag between currents on the secondary and the
primary, and how the secondary draw influences the
primary one.