COMPARATIVE ANALYSIS AND CLASSIFICATION OF PULSE-CONTROLLED FERROMAGNETIC VOLTAGE AND CURRENT REGULATORS

V. P. OBRUSNIK

(Presented by the Scientific and Technical Seminar of the Research Institute of AEM)

Magnetic amplifiers, saturation chokes, and premagnetized transformers are widely known ferromagnetic voltage and current regulators. The simplicity of their design and manufacture, their low cost, and their high operational reliability at one time ensured their rapid introduction into technical devices of the most varied purposes. In a short period of time a theory of the processes of ferromagnetic regulators (FMR) was created, methods for their design were developed, and serial production was organized both of the regulators themselves and of systems based on them.

However, the appearance of thyristor and transistor switches, possessing regulating properties identical to those of magnetic switches while having much greater speed and smaller weight-and-size characteristics, sharply narrowed the practical application of FMR in the field of power engineering. Many specialists consider these regulators unpromising and not competitive with valve devices.

Without sharing extreme judgments about FMR, it must nevertheless be admitted that a significant part of the criticisms directed at them is justified, if one has in mind ferromagnetic regulators with continuous control.

In a comparative assessment with valve regulators, the greatest shortcomings are found in self-saturating magnetic amplifiers, which in their time were recognized as the best among FMR of other types. Here they have no additional advantages, while possessing great weight and inertia. The noncompetitiveness of self-saturating FMR in relation to valve regulators is evident even at a high supply-voltage frequency, when the weight, dimensions, and inertia of both become comparable. Self-saturating FMR have uncontrolled valves in the power circuit, and it is sufficient to make these controllable; this will not worsen the weight-and-size characteristics of the system, will preserve the same regulating properties, and will not require any substantial complication of the control circuit, while the need for the ferromagnetic part will become obvious. The conclusion here suggests itself: FMR operating in the self-saturation mode are always worse in specific economic indicators than valve regulators (thyristor or transistor) [9].

However, the conclusion drawn will be incorrect for FMR that do not have positive feedbacks (internal and external) with respect to load current. These regulators are 30–40% heavier than self-saturating FMR and have a small power gain coefficient (not

more than 100), but, while being excellent current regulators, they acquire advantages not characteristic of valve devices. The central ones among these advantages are: self-protection against overloads and short-circuit currents, natural stabilization of the load current, and the absence of valves in the power circuit under alternating-current loads.

The properties listed are decisive for a number of electrical systems (especially in very high-power power-engineering installations); therefore, choke-type ferromagnetic regulators are used and will continue to be used in many electrical installations, despite their inferior indices with respect to inertia, weight, dimensions, and sensitivity to the control signal. With a substantial improvement of these indices, the possibilities for practical application of FMRs will expand still further; moreover, it is possible to achieve, and even to provide ferromagnetic regulators with, fundamentally new properties if their magnetic elements are controlled discretely.

In discrete control of an FMR, the control action that leads to a change in the magnetic state of the cores and, correspondingly, to regulation of the electrical parameters of the output circuit must be not continuous but pulsed, metered at definite time intervals.

All known methods of pulse control of magnetic elements are based on the fact that particular circuits of these elements are “closed and opened” with some switching frequency and duty factor. Discretely controlled FMRs have been developed and are being investigated in which the following is carried out:

a) switching of the premagnetization circuit from a dc voltage source [12];

b) switching of current-feedback circuits (most often by current harmonics) [5];

c) simultaneous switching of the circuits of the premagnetization source and the feedback circuits;

d) switching of alternating-current windings [4, 6, 13, 21].

Ferromagnetic devices controlled according to item “a” are current regulators that, in open-loop systems, have external characteristics analogous to the curves in Fig. 3a. Devices controlled according to items “b” or “c” are voltage regulators with external characteristics possessing current limitation (Fig. 3b), while creation of a constant component of induction by the pure-switching method according to item “d” gives voltage regulators with external characteristics without current limitation in Fig. 3c.

Naturally, in closed-loop automatic-control systems all groups of discretely controlled ferromagnetic devices can perform any functions, including the functions of voltage, current, or power regulator-stabilizers. In classification it is convenient to establish their properties for open-loop systems and to subdivide them into two classes:

a) pulse-premagnetized ferromagnetic regulators — PP FMRs, in which regulation of the output quantities is effected by switching the dc premagnetization circuits;

b) pulse-switched ferromagnetic regulators — PS FMRs, in which the output quantities are regulated only by switching alternating-current windings.

Each of the two classes of pulse-controlled FMRs possesses different properties, gives different control and external characteristics, and therefore has an independent purpose. For pulse control of magnetic elements, high speed is required—

ing. It is better when the switching processes occur without inertia; therefore, effective implementation of these methods became possible with the appearance of semiconductor switches (transistor and thyristor).

Fig. 1.

In Fig. 1, a, b, c, d are shown typical variants of the basic use of a pulsed FVR. The time variation of the principal quantities of these regulators in steady-state modes with an active-inductive load is illustrated in Fig. 1, e. The circuits are built on the simplest magnetic elements—a saturable reactor (SR), Fig. 1, a, and, in the best version of its implementation, a saturable reactor with combined AC and DC windings (SRW), Fig. 1, b, c, d. The connection here of matching transformers (autotransformers), or their structural combination with reactor elements, which gives devices of the type TRPSH, ATRPSH, TRPN, etc. [1], as well as the three-phase implementation of the pulsed FVR circuits, does not affect the essence of the arguments set out below.

In the circuits in Fig. 1, a, b, pulsed control of the FVR is carried out by switching the circuit of the DC voltage source. Characteristic here is the presence of a valve \(V\), shunting the bias circuit, and of a fast-acting switch \(K\) (transistor or thyristor), connected in series with the source of unregulated DC voltage \(U_0\). The valve \(V\) performs two functions: it protects the switch \(K\) from overvoltages during “opening” and accom-

... provides positive feedback with respect to the current of the harmonics. Pulse biasing of a ferromagnetic regulator (FMR) was first tried on self-saturating amplifiers using power semiconductor PWMs [2, 20]. Here the only effect achieved was that the semiconductor regulator controlling the current \(i_y\) was given its most advantageous mode—switching. However, the use of PWMs for controlling ferromagnetic elements without positive feedback with respect to the load current led to contradictory phenomena. Switches operated in one and the same system either very well, providing the required FMR characteristics and large amplification factors, or else gave no additional effect at all in comparison with continuous control.

Studies have shown [9, 14] that the cause of everything is the harmonic currents of the biasing circuits, whose amplitude and phase depend substantially on many factors. If the pulse of the PWM output voltage is incorrectly oriented with respect to the instantaneous value of the current of the alternating component of the magnetic element biasing circuit (compare positions A and B in Fig. 1d), then a very large current \(i_y\) will flow through the switch, which will lead to a sharp decrease in the amplification factor of the semiconductor regulator-switch and to its thermal overload. The dependences of the FMR amplification factor on the load current (in relative units) and on the phase of the arrangement

Fig. 2. Dependence of the rms value of the amplification factor of a pulse FMR:
a) on the rms load current at constant settings of the amplitude of the controlling voltage pulse (the phase and width of the pulse do not change); the quantities \(I_{\mathrm{dc},r}\) and \(I_{\mathrm{n}}^{d}\) are expressed as fractions of the nominal values; b) on the phase-shift angle of the biasing pulse relative to the beginning of the half-period of the supply voltage.

of the control pulses relative to the beginnings of the half-periods of the supply-line voltage are shown, respectively, in Figs. 2a and 2b (here the amplitude and relative pulse duration \(\gamma\) are taken as constant). As can be seen, for the same parameters of the control pulse the value of the FMR amplification factor is strongly dependent on the phase of the pulse arrangement and on the load, which is explained by the influence of the indicated factors on the magnitude of the current \(i_y\) flowing through the switch from the source \(U_0\) (see positions A and B in Fig. 1d).

In order to stabilize the amplification factor of switch \(K\), a method of pulse control of choke-type magnetic elements has been proposed [12], in which the controlling voltage pulse is applied to the biasing circuit at the minimum of the current flowing through it (see position A in Fig. 1d). The optimum repetition frequency of the control pulses is a multiple of the frequency of the fundamental harmonic of the biasing current ...

...requirements: for single-phase and grouped three-phase FMRs—\(f_u=2f\) of the network; for three-phase ones—\(f_u=3f\) of the network.

It has been established [14, 15] that, if the rules indicated above are observed, then, when the average value of the pulse voltage is equal to the average value of the control voltage of continuous character, all static and dynamic characteristics will be identical both for the discrete and for the continuous principles of control. In this case:

— choke-type FMRs retain their characteristics as current regulators, possessing the natural properties of stabilizing the load current and protection against overloads and short circuits (see in Fig. 3, a the external characteristics of a pulse-magnetized DCS on magnetic cores made of E320 steel, obtained experimentally in relative units);

— the power gain increases by a factor of 20–25 and more;

— the rated power of the PWM decreases by a factor of 10–20;

— the overall efficiency of the system rises by (2–3)%, which is very important for powerful voltage and current regulators;

— pulse magnetization according to [12] makes it possible to create, on the basis of FMRs with combined windings, voltage and current regulators of a new type, which in their technical and economic indices surpass all known ferromagnetic devices, and in a number of cases prove to be better than thyristor and transistor regulators.

The last advantage is central. Ferromagnetic devices with combined windings, being the simplest and most economical in construction and not requiring additional control windings, could not find wide application under continuous control. Having low gain factors, they additionally required the inclusion in the dc circuit of choke filters or low-voltage, high-current voltage sources, which in weight and dimensions exceeded the regulator itself. Pulse magnetization according to [12] removes the indicated shortcomings.

On the basis of pulse-magnetized ferromagnetic devices, a number of automatic installations have been developed and are being successfully operated in practice. Among them may be mentioned: single-phase ac voltage stabilizers with a power of (200–1000) VA [17]; power supplies for electrolytic-heating baths with a power of 150 kW [18] and 75 kW [13], automated charging units [19], etc.

A large volume of theoretical studies has been carried out on the operating modes, characteristics, and parameters of pulse-magnetized FMRs. Methods have been developed for the analysis and synthesis of processes and characteristics [14, 15]; a method has been proposed for designing PM FMR systems that are optimal in terms of technical and economic indices [8, 9]; simple circuits of thyristor and transistor switches have been developed for them [7, 16]. It is most effective to use PM FMRs as voltage and current regulators with a power of 1–150 kW.

A substantial shortcoming of pulse-magnetized FMRs is their great inertia for transient processes under disturbances from the control-circuit side. Their duration is the same as for ordinary saturable reactors. This shortcoming can be eliminated if the feedback valve of the FMR for the harmonic current is made controllable. Variants of such discrete control by feedback are shown in Fig. 1 c, d.

The circuit in Fig. 1 c, at \(i_\partial=\mathrm{const}\), has a response time of one half-period, but requires an additional control winding and, most importantly, contains a linear choke in the control circuit, whose dimensions and weight are relatively large.

Fig. 3. External characteristics of pulse-controlled FMRs made on laminated magnetic cores of E-320 steel, 0.35 mm:
a) for the IP FMR in Fig. 1a, b;
b) for the IP FMR in Fig. 1c;
c) for the IK FMR in Fig. 4b.
Everywhere \(U_n^d = U_n : U_{n \mathrm{nom}}\), \(I_n^d = I_n : I_{n \mathrm{nom}}\).

The best variant is the circuit of two-switch pulse control of an FMR in Fig. 1, c. Here switch \(K_2\) sets at the DNC input magnetizing-voltage pulses that are constant in amplitude and duration, providing the system with properties determined by the control method according to [12]. Switch \(K_1\) performs the functions of a regulator in closed-loop automatic control systems, as described in [5]. The system in Fig. 1, d has, in terms of quality, the same external (Fig. 3, b) and regulating characteristics as the system in Fig. 1b, but is inertia-free and smaller in weight and dimensions, which in a number of cases justifies its somewhat greater complexity due to the introduction of an additional control unit for switch \(K_1\).

A variety of pulse-controlled ferromagnetic regulators of the IK FMR class is based on magnetic-valve devices (MVDs), in which the alternating-current winding of the magnetic element (or part of it) is periodically, for a certain interval of time shorter than a half-period, short-circuited by a semiconductor switch (once per period of the supply voltage) [7, 13, 21]. The typical structural and circuit implementation of the simplest MVD is shown in Fig. 4, a, b. There also exist MVD circuits with matching transformers, which are sometimes combined with the choke element into a single structure, yielding devices of the MPTSh type [13]. The essence of the processes occurring is not changed by this.

Two principal variants of commutation of MVD switches are possible—natural and forced. In magnetic-valve devices with natural—

Fig. 4.

With natural commutation [6, 7, 21], the switching element “closes” the turns of the AC winding in one of the half-periods with a delay by the angle $\alpha_k$ relative to the beginning of the period of the mains voltage, and automatically “opens” them when the load current passes through zero (the operating diagrams are shown in Fig. 4, c). In its regulating properties and characteristics, such a magnetic-valve regulator differs little from a semiconductor symistor with natural commutation. The phase of all harmonics of the load voltage here changes substantially during regulation (see $U_{\Gamma 1}$ in Fig. 4, c), which is considered inadmissible for a number of electrical installations.

MVUs with artificial commutation are more interesting [13]. Here the switch “closes” at the beginning of each period of the supply voltage for a regulated time $\alpha_k$, with a duration of not more than a half-period, after which it is forcibly “opened” and remains in this state until

the beginning of the next period (the diagrams in Fig. 4, g). Such regulators provide a voltage in the load all of whose harmonics have no phase shift relative to the mains voltage (see the first harmonic \(U_{r1}\) in Fig. 4, g). This is very important for many ac loads and for the construction of voltage stabilizers in multiphase systems.

The technical implementation of the effect of regulating the load voltage without phase shift is obtained on MVU considerably more simply than on symistors or on antiparallel-connected valves. In known variants, MVU prove to be not only simpler, but also 1.5–2 times lighter and smaller in overall dimensions. The external characteristics of PC FMR with artificial commutation of the key element are shown in Fig. 3, v. Their drawback is the absence of current limiting, which is also characteristic of MVU with natural commutation and of symistor systems.

By the present time, a large body of research has been carried out on operating modes, parameters, and characteristics of MVU, and methods for their calculation have been developed [10, 11]. In terms of their regulating properties, they are inertialess voltage regulators, providing, under artificial commutation of the keys according to [13], a constant shift of the fundamental (and of any other) harmonic of the output voltage. It is interesting that the output voltage of an MVU with artificial commutation contains both odd and even harmonics, whose relative weight depends substantially on the commutation angle of the key element. The first, second, and third harmonics may predominate. This makes it possible, on the basis of MVU, to create inertialess frequency multipliers.

Among specific electrical systems based on artificially commutated MVU, powerful single-phase and three-phase ac voltage stabilizers are now being tested [4].

There is reason to believe that, with pulsed methods of controlling ferromagnetic devices of the IP FMR and PC FMR classes, promising possibilities are created for highly effective practical application.

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