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Re: Saturable Reactors as Ballast



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