Basic Principles of Designing Combined Excitation Systems

Yu. N. Kroneberg

(Presented by the seminar of the Department of Electrical Machines and General Electrical Engineering)

The absence of effective methods for acting on the flux of permanent magnets (PM) substantially complicates the regulation of magnetoelectric machines. This problem has accompanied their entire development, but at present its importance has sharply increased thanks to major successes in the field of hard-magnetic alloys [1, 2].

At the present time, the principal methods are electromagnetic methods of regulation (by means of special windings placed in the machine). There are two fundamentally different mechanisms by which regulating windings act on the working magnetic flux. In the first case, it plays the role of an excitation winding (EW) and, having a direct inductive coupling with the armature, can induce an emf even with demagnetized PM. In the second, it only magnetizes nonlinear magnetic cores, which is to some extent equivalent to changing the magnetic resistance. With demagnetized PM, no emf is induced. Analysis shows [3] that, on the whole, machines of the first type are more promising, i.e., those with combined excitation (CE).

The number of known CE machines is already sufficiently large and requires systematization. In doing so it is necessary to take as a basis such classification features as would reflect the essence of the excitation systems themselves and would make it possible to disregard incidental circumstances.

In our opinion, the fundamental feature (Fig. 1) is the structural level of the machine at which the action of the PM and the EW is combined. Below we shall explain this in more detail. Figs. 2 and 3 show certain externally very different designs of known CE machines. All of them, however, have one feature in common: the flux of each pole is equal in magnitude and is created by the combined action of the mmf of the PM and of the EW. In this they differ significantly from machines (Figs. 4, 5) in which some of the poles are excited by PM, and the rest by the EW. Finally, there exists a class of machines characterized by the presence of two or more inductors (usually arranged along the axis), some of which are excited by PM and some by the EW. If each inductor excites a separate armature, then it is more convenient to speak of separate machines; however, more often the armatures are made common, magnetic coupling of the inductors is observed, and then the system must be considered as a whole.

Thus, there are three structural levels at which the actions of the PM and the EW are combined; accordingly, machines of the first class may be called machines with combined poles, of the second—with com-

Fig. 1. Classification diagram of combined-excitation machines

with combined inductors, and the third—with combined magnetic systems. At present, the machines of the first two classes are of primary importance; we shall therefore consider them in greater detail.

Analyzing machines with combined poles, one may note that the excitation winding (EW) is linked either with the full flux of the machine (Fig. 2), or only with the flux of the magnetic circuit connected in parallel with the permanent magnet (PM) (Fig. 3). In the first case the dc excitation winding and the PM are connected in series; in the second, in parallel. In both cases a gradual increase in the EW current leads to an increase in the armature flux; the flux of the magnetic shunt first decreases, then disappears, and then increases again in the opposite direction. The difference is that with a series CE the dc EW prevents demagnetization of the PM, since it is connected in accordance with its dc component (the stabilization mode corresponds to the minimum EW current), whereas with a parallel connection it facilitates it. This circumstance is undoubtedly in favor of series CE. But it is far from always decisive. Other factors are also important: the rationality of the arrangement of the active elements of the inductor, assurance of contactlessness, structural simplicity, etc. Therefore, no unambiguous preference can be given to either group. At present, ac machines are made predominantly with parallel CE, and dc machines with series CE.

Machines with combined inductors differ in the construction of the pole systems and are divided into two groups. The first (Fig. 4) includes machines in which the poles of one polarity are excited by the PM, and those of the other by the EW (more precisely, this refers only to the predominant influence of the dc sources, since the magnetic system is coupled), i.e., two homopolar systems are combined. The second group (Fig. 5) includes machines whose inductors are formed by combining heteropolar electromagnetic and magnetoelectric pole systems.

Machines with combined inductors formed by combining homopolar systems are distinguished by a specific distribution of the armature leakage flux (Fig. 6a): it is alternating in sign, as in alternating-pole (alternative) machines, but in the general case it has a constant component typical of inductor (pulsating) machines. In order for it to be able to close, it is necessary

Fig. 2. Electrical machines with series combined excitation [2]:
a — direct-current machine; b — synchronous machine with a claw-shaped rotor; c — inductor machine.
1 — magnet; 2 — magnetic shunt; 3 — excitation winding

Fig. 3. Electrical machines with parallel combined excitation:
a — salient-pole synchronous machine [4]; b — synchronous machine with an externally closed regulating flux [5]; c — synchronous machine with an internally closed regulating flux [6].
1 — magnet; 2 — magnetic shunt; 3 — excitation winding

Fig. 4. Electrical machines with combined inductors (superposition of like-pole systems):
a — two-stack salient-pole machine [7]; b — single-stack claw-shaped machine [8].
1 — pole excited by a magnet; 2 — pole excited by an electromagnet; 3 — magnet; 4 — excitation winding

Fig. 5. Electric machines with combined inductors (combination of heteropolar systems):
a — salient-pole with normal alternation of magnets,
b — salient-pole with disturbed alternation of magnets,
c — contactless with an externally closed flux of electromagnetic poles [9];
1 — magnet, 2 — electromagnetic pole, 3 — excitation winding

axial magnetic cores and either a two-stack design (Fig. 4a) or an additional air gap (Fig. 4b). When the excitation winding is switched off, the PM flux closes through its poles and the axial magnetic cores (Fig. 6a, solid lines). With a gradual increase in the excitation-winding current, the flux of the electromagnetic poles (Fig. 6a, dashed line) and the emf of the armature conductors located opposite them at the given moment also increase. The axial flux (the constant component of the armature flux) first decreases, then disappears, and subsequently increases in the opposite direction. Machines of this group, also called alternative-flux machines, are among the promising types.

Machines whose inductors are formed by combining heteropolar systems also have a specific distribution of the flux under the armature bore (Fig. 6b), characterized by the presence of subharmonic components. When the excitation winding is switched off, the PM flux closes predominantly through its poles (Fig. 6b, solid lines). With a gradual increase in the excitation-winding current, the flux of the electromagnetic poles (Fig. 6b, dashed line) and the emf of the corresponding conductors of the armature winding also increase. In principle, the magnetoelectric poles may be arranged in any order (the number of north and south poles must, of course, be the same). But for better utilization of the steel, it is desirable that the normal alternation of the magnetoelectric poles be observed (i.e., north—south—north, etc.); otherwise the yoke and the armature back become overloaded when the excitation winding is disconnected (Fig. 5a, b), and their cross sections have to be increased. Another additional requirement for the arrangement of the poles is symmetry, which is necessary to ensure magnetic balancing of the rotor irrespective of the magnitude of the excitation-winding current.

Figure 6. Fields of machines with combined inductors:
a — when like-pole systems are superposed,
b — when unlike-pole systems are superposed.

The classes and groups of CE systems considered are the basic ones. In principle, hybrid systems are also possible, representing a combination of two or several simple systems (for example, one can imagine machines with a combined inductor, some of whose poles are made combined). However, at present no successful solutions are known either among hybrid-type machines or among machines with combined magnetic systems. If necessary, hybrid types of CE machines can easily be classified according to the proposed scheme by introducing intermediate categories.

As follows from Fig. 1, the number of basic types of CE systems is relatively small. However, each of them has broad possibilities for application, since in principle any magnetoelectric or electromagnetic machine may be regarded as a particular case of a CE machine, i.e., may serve as the basis for developing several derivative variants differing in the construction of CE systems. This is already observed in the examples given, the number of which can easily be multiplied. On this basis, further detailing of the classification scheme is also possible. For example, according to the kind of current (direct, alternating), the type of poles (radial, salient, claw-shaped, non-salient), the type of armature (cylindrical, face-type), etc. All these features, like many others, are very important for each specific machine, but they have no direct relation to the excitation systems and therefore are not expedient for inclusion in the given classification scheme.

It is not difficult to see that the proposed systematization is based on fundamental features reflecting the essence of the course of electromagnetic processes. The affiliation of a group of machines with any classification category determines the similarity of the patterns of field distribution in the armature bore and of the equivalent magnetic-circuit substitution schemes. Hence follows the commonality in calculating the principal internal parameters (coefficients of magnetic fields, inductive reactances, steel losses, etc.) and output characteristics in steady-state and transient processes.

REFERENCES

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2. L. M. Palastin. Regulated electric machines with excitation from

permanent magnets. TsINTI of the Electrical-Engineering Industry and Instrument-Making, 1962.

1. A. S. Zhibinov, Yu. N. Kroneberg. On the effectiveness of regulating magnetoelectric machines by premagnetizing nonlinear magnetic circuits. In: Power-Supply and Electric-Drive Devices of Low Power. Vol. 2. “Energiya,” 1970.

2. G. Moresse, R. Dechet. Inductor of an electric machine. French Patent No. 1238273, 4.7.60.

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