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Re: Saturable Reactors as Ballast
Original poster: "Mike" <induction@xxxxxxxxxxx>
Hi Carl,
About the PRS (Primary Saturable Reactor) control
systems, we use that often at work with induction heaters.
Three usual modes of power level control are used, SCR, Thyratron and PSR.
On the PSR's three ways to effect control, one is a mag amp that
takes pot input or instrument input (pyrometers, etc),
the other is a simple SCR zero to ~ 90 volts, up to 25 amps available
to control windings. Both of the prior are 115 volt input, 0 tom 90 VDC out.
The third and less costly is simple variac and rectifier as you said.
For three phase, there is one saturable reactor per leg going to the
plate transformer primary.
While the three PSR's are isolated phase to phase, the DC control
loop is in series with each DC winding of the 3 PSR's.
Also, each usually has a 100 Ohm resistor across both ends of each DC
or control winding to equalize.
Typical current for a 10 through 50 kW machine, through all 3 control
windings, is in the area of 4 to 5 amps.
The greater current, the more "pass through". Most PSR control
systems limit voltage to 90 volts figuring a group of 3 reactors will
be in the loop. Expect a ~ 10 volt drop so set the XMFR taps one step
below your line voltage.
It is important that you understand a few things about PSR control,
first, the things are very load dependant. The plate transformer may
park at minimum of ~5 to 10 percent power, even when the control is
saying minimum power, with the oscillator tube as the minimum load,
so PSR.s rarely go all the way off.
Also, they are a tad slow, it may take 2 or 3 seconds to flatten out
where you set the power control pot.
Fine for long term jobs but if somebody is looking for on, heat, off
cycles in a .5 or 1 second window, this is not the control system to
use but rather SCR or thyratrons.
If you remove the minimum load, where the plate kV meter may say 1000
volts, the plate transformer will go right up to full voltage. If I
have a Tech Support call saying HV overloads, I have them remove the
Osc tube filament connection so the tube won't conduct. Any current
reading past that is Cap leakage, or other items like blocker caps,
etc. As soon as the Fil;ament is off the tube, the voltage goes
pretty much all the way up so it's a self contained hipot.
The inductance, the transformer impedance, the load current range
minimum to maximum, need to be known to get the right swing of
minimum to maximum control and stay within the control range of the
pot for linearity.
About the windings, most of the ones we use in rebuilding machines
have a Burndey connector jumper setup so we can set 240 or 480 volt
range per leg.
This leaves two wires, one per leg in, one to transformer, out. The
two small wires go to the DC control loop and the equalizing 100 ohm
resistor across each DC winding.
The input is of course on a contactor, the output connects to each
plate transformer input.
So, you need to know expected load current swings at least to get a
PSR in the right area, else you will not have good control on it.
You can often get these in the KVA range you are working at by
finding an old tube induction heater with PSR in it. Some even had an
outside, roll around PSR mini frame you could connect to the machine.
Auctions, Ebay, you name it.
Decent PSR's are about 1800 Bux each, not cheap.
Slow but very smooth control, also helps wash out line spikes from
getting your diode stacks.
Hope this helps.
Mike
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, December 21, 2005 5:05 PM
Subject: Saturable Reactors as Ballast
Original poster: "Carl Litton" <Carl_Litton@xxxxxxxxxx>
Does anyone on this list have any experience building saturable
reactors for ballasting purposes? I found some older notes in the
archive suggesting that a second winding on an iron core inductor
can be used to introduce a variable DC voltage that will give full
range control of the inductance as the core approaches saturation.
I have been able to demonstrate the effect on a small scale with a
simple step down transformer by putting the primary in series with a
120 VAC ciruit and connecting the the secondary to the rectified out
put of a Variac. A 2.7 Henry inductor was reduced to a little less
than 1 Henry with 140 VDC in the control winding, allowing a small
light bulb just enough current to give off some visible light
(measured current 0.188 Amp with no DC control and no light to 0.42
Amp with 140VDC control and visible soft glow from bulb).
However, all attempts to do this on any large (20 to 250 lbs.)
inductors controlling a 240 volt circuit in the 30 to 150 Amp range
have been not only fruitless but have almost instantly slagged the
25 Amp bridge rectifier connected to the control winding.
I need to understand what I am missing here. Any theory or
especially winding diagrams of working reactors would be greatly
appreciated. I did find one article that suggested 2 AC power
windings in series and in phase have to be used with 2 DC windings
in series and "out of phase" with each other in order to cancel the
effect of induced AC in the control winding. But here again, no
practical application, turns ratios, winding configurations, etc.
Any thoughts?
Thank you,
Carl Litton
Memphis HV Group