Re: Saturable Reactors as Ballast

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FIFTYGUY@xxxxxxx 

In a message dated 12/21/05 5:06:32 PM Eastern Standard Time, 
tesla@xxxxxxxxxx writes:

>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.

    I have no experience with Saturable Reactors or Magnetic 
Amplifiers. But I have glanced at the theory before, and after your 
question I figured I'd take a closer look.
    It appears that the "out-of-phase" two-winding connection for 
the DC control is mandatory to prevent AC from being induced in the 
control side, as you have found. Moreover, doing so only suppresses 
the fundamental frequency and odd harmonics thereof. Therefore, the 
flow of even-harmonic currents in the control winding is a design issue.
    In fact, there's a distinction between "free-flow even-harmonic 
current" and "suppressed even-harmonic current" designs. To suppress 
the even harmonics, the DC control circuits impedance should be high 
relative to the induced currents. I've seen mention of filtering 
chokes in the DC control circuit. I guess the aim would be to achieve 
DC control with the lowest DC current, most control wire turns possible?
    The correct operation point on the magnetization curve is also 
important, as has already been mentioned.
    As to two AC ("gate") windings required, I don't see why this 
would be necessary. One explanation is that the texts consider a 
Saturable Reactor as a single DC winding with a single AC winding. 
Magnetic Amplifiers are built using one or more SR's, so a MA built 
with two out-of-phase DC windings requires two SR's. This gives the 
necessary configuration on the DC side, but you end up with two AC windings.
    There are also some distinctions between whether the AC windings 
are parallel or series connected. In addition, by putting a half-wave 
rectifier in series with each AC winding, but both connected to the 
load (the two rectifiers in opposite direction), AC is still run 
through the load, but the evil even-harmonic currents are suppressed. 
Each winding conducts through half a cycle.

    Now for the part where I go off on wild, inexperienced speculation:

    It seems to me that the current through the load from an SR/MA 
is much like that from an SCR controller, since it's a phase-angle 
controller. At a controlled point on the AC wave, the SR goes into 
saturation, and current through the load increases. I suppose the 
advantages of the SR would be that it could be very simple and very 
robust, requiring no tubes or semiconductors to fry (could run off a 
car battery). You've also already got the big chunk of iron because 
of the ballast. The turn on ramp from the SR might be more gentle 
than the sudden commutation of an SCR, so EMI, noise, and di/dt 
stresses would be less.
    OTOH, SCR controllers are very mature technology, and darn 
tough. they can also be very simple to install, particularly a 
single-phase, relatively low-voltage/low current application such as 
the input to a pig.

-Phil LaBudde