A Two-Package Generator of Combined Excitation

Yu. N. Kroneberg, V. B. Gomzyakov, A. S. Zhibinov

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

The problem of regulating magnetoelectric machines in recent years has increasingly been solved by the use of combined excitation. The absence of a universal method for combining excitation windings and permanent magnets in a common magnetic system, as well as the variety of requirements, has led to a trend toward the development of a broad range of such machines, each of which has its own advantages under particular requirements.

Below we consider a generator (Fig. 1), the stator of which consists of two wound packages 1, an annular excitation winding 2, and a yoke 3. The rotor has two pole systems, consisting of alternating magnetically soft poles 5 and bars 4 of permanent magnets (all magnets of one package being only of north polarity, and of the other only of south polarity), fastened on a magnetically soft sleeve 6. With winding 2 de-energized, the fluxes of magnets 4 are partly closed through poles 5 (as in ordinary alternating-pole machines), and partly through yoke 3 and sleeve 6 (as in two-package inductor machines). With the corresponding

Fig. 1. Two-package generator of combined excitation. 1 — stators; 2 — excitation winding; 3 — external magnetic circuit; 4 — poles of the permanent magnets; 5 — poles of electromagnetic excitation; 6 — magnetically soft sleeve.

switching-on of winding 2, its m.m.f. acts counter to the flux of yoke 3 and sleeve 6, which only slightly reduces the flux of magnets 4, but greatly increases the flux of poles 5 and, correspondingly, the e.m.f. It is essential that for both packages only the constant component of the flux is common. Consequently, phase asymmetry and nonlinear distortions in the winding of one package should not cause analogous phenomena in the winding of the other, which would be observed if the windings were placed in the slots of one stator package.

package. To a lesser extent, fluctuations in the load of one package should also affect the voltage of the other.

Such a generator retains all the principal advantages of machines with combined excitation. Its use is advisable primarily where the advantages of a two-package design can manifest themselves. A common case is autonomous systems whose consumers of electric power impose such contradictory requirements on the type of current, frequency, voltage, number of phases, etc., that they cannot be met by a single generator even with separate windings, and the use of two separate machines becomes necessary.

In a two-package generator, each package may be regarded as a separate machine; therefore it can provide different frequencies, voltages, numbers of phases, and different emf waveforms; the weight of the structural parts is considerably less than for two separate machines; the problem of contactless operation is solved simply and naturally; only one voltage regulator is required (this is both an advantage and a drawback).

Experimental verification was carried out on a model generator with the following basic data: stator bore diameter—12.5 cm; length of one package—3 cm; number of magnetically soft poles (for each package)—4; number of permanent-magnet bars (per package)—4; rotational speed—3000 rpm; there is no slot skew.

When operating as a single source, the windings of the packages were connected in series. Tests showed (Fig. 2) that the no-load emf has a maximum typical of inductor machines and an initial emf approximately equal to half of this value. Thus reliable self-excitation and a sufficiently wide control range are ensured. The change of the field in the air gap as the excitation current increases explains the principle of regulation. Harmonic analysis of the winding emf showed that, in the tested machine, the 5th, 15th (tooth), and 17th higher harmonics predominate. When the excitation current is varied from 0 to 1.5 A, they vary, respectively, within the ranges 3.1–3.25%, 6.8–1.0%, and 2.4–2.5%, while the nonlinear distortion factor does not exceed 8%. A feature of the short-circuit characteristic is the presence of an initial armature current caused by the magnet flux.

The established symmetrical operating conditions of the generator are represented by a family of external characteristics (Fig. 3), obtained under active load.

For prolonged operation and thermal loading (the machine is ventilated) permissible for class A insulation, the rated power of the tested specimen should be 1.6–2 kW (specific power 100–130 W/kg) at a voltage of 310–330 V.

For voltage stabilization from no-load to full load, the ratio of excitation-current variation will be approximately 1.5–2; and the maximum excitation power will not exceed 2.5–3.5% of the rated output power. The depth of no-load voltage regulation (the regulation range relative to the rated voltage) will be 50–60%.

Thus, the generator tested at a rotational speed of 3000 rpm has good specific indicators and entirely satisfactory controllability.

When the generator operates as two sources, the automatic regulation system can stabilize the voltage of only one package, while the emf of the other will vary somewhat in accordance with the change in the dc component of the flux. The stabilizing influence of the regulation system for the second package is manifested in the fact that, when the load of the first package changes, the emf of the second changes

Fig. 2. No-load and short-circuit characteristics.
1 — no-load characteristic; 2 — short-circuit characteristic; 3 — variation of the coefficient of nonlinear distortion of the output voltage

less intense than under no-load operation of both packages and variation of the excitation current within the same limits. Accordingly, the dependence of the voltage of the unregulated package on its load current (the excitation current is varied in order to ensure voltage stability of the regulated package at its constant load) is also flatter than the external characteristic. For example, at a voltage of the regulated package of \(150\ \mathrm{V}=\mathrm{const}\) and a load of \(2.5\ \mathrm{A}=\mathrm{const}\), the voltage of the other package, when its load is increased from 50 to 100%, changes by only \(\pm 2.5\%\), whereas the voltage change according to the external characteristic (with constant excitation current) is \(\pm 5.7\%\).

Consequently, when the windings of the packages are connected to separate loads, the regulation system can provide complete stabilization of one voltage (with more stringent requirements for stability)

Fig. 3. External characteristics

and partially the other. It is also possible to use a regulator that stabilizes the average voltage of the windings.

The experiments fully confirmed that phase unbalances (up to single-phase loading) and nonlinear distortions (even with a rectifier load) in the winding of one package are not reflected in the corresponding indicators of the other package.

Thus, the investigations carried out showed that a two-package combined-excitation generator can be successfully used as a regulated power source when both packages operate into a common load; moreover, its use in autonomous systems for supplying loads with different requirements for frequency, number of phases, and voltage waveform is highly attractive.