US 2526671 A Abstract available in Claims available in Description (OCR text may contain errors) Oct. 24, 1950 w. KOBER 2,526,671 VOLTAGE COMPENSATOR Filed July 25, 1Q46 4 Sheets-Sheet 1 FIG.2. v PM A GENERATOR LOAD 22 E 1 O FTJH I FIG.3. FIG.4. INVENTOR. 5 WILLIAM KOBER Ot. 24, 1950 w. KOBER 2,526,671 VOLTAGE COMPENSATOR Filed July 25, 1946" 4 Sheets-Sheet 2 L5 T N INVENTOR. WILLIAM KOBER Oct. 24, 1950 w, KQBER 2,526,671 VOLTAGE COMPENSATOR Filed July "25, 1946 4 Sheets-Sheet s F l G. 9. INVENTOR. WILLIAM KOBER Oct. 24, 1950 w. KOBER 2,526,671 VOLTAGE COMPENSATOR Filled July 25, 1946 4 S heets-Sheet 4 I b Wa w,. I O E o I F|G.10. FIGJL INVENTOR. WILLIAM KOBER WQ/L Patented Oct. 24, 1950 J, VOLTAGE COMPENSATOR William Kober, Spring Lake, N. J., by Decree of Court to the United States of America as represented by the Secretary of the Army Application July 25, 1946, Serial No. 686,093 4 Claims. (01. 322-96 (Granted under the act of March 3, 1883, as The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon. The invention relates to voltage compensating and current limiting devices and particularly to such devices that are adapted to be used in conjunction with permanent magnet generators as disclosed in applicants copending application entitled Alternating Current Machines, flled July 6, 1944, Serial Number 543,744. Permanent magnet generators present an inherent inductive reactance, commonly termed armature reactance. The terminal voltage of the generator depends, among other things, upon the value of the voltage drop due to armature reactance under normal load conditions. It is, therefore, an object of the invention to provide means whereby the effect of the armature reactance of a generator upon the terminal voltage may be minimized, the said means being adapted to cancel the inductive reactance of the generator under normal load conditions. It is another object of the invention to provide means whereby the voltage regulation of a permanent magnet generator will be unaffected by deviations from unity power factor. As explained in applicant's application for patent entitled Alternating Current Machines, referenced above, permanent magnet generators must be protected against damage to the permanent magnets caused by heavy current flow under short circuit conditions. As explained therein, the transient is usually the only factor causing damage, since the armature reactance is large enough to limit steady state short circuit current to values below those capable of permanently affecting the magnets. It is, therefore, another object of the invention to provide, means whereby the effect of the armature reaction may be neutralized at normal load current conditions, but will not be neutralized at steady state short circuit conditions, whereby the current through the generator will be limited at steady state short circuit conditions. It is another object of the invention to provide means that will automatically revert to an armature reactance neutralizing status when the short circuit is removed. It is another object of the invention to provide a method to determine graphically the output characteristics of a permanent magnet generator-compensator combination. These objects, together with other objects and advantages of the invention which will be apamended April 30, 1928; 370 O. G. 757) parent to one skilled in the art, are achieved in one embodiment of the invention by introducing a negative reactance, whose value is approximately equal to the positive armature reactance. in series with the armature winding of the gen-. erator, thereby limiting the voltagedrop'across the terminals of the generator-compensator combination to the drop caused by the resistive components thereof. This,negative'reactance'is introduced by connecting the primary of a transformer in series with the armature and the outgoing line and by .connecting a condenser across the secondary. This arrangement, in its e1ementary. form, is old in the'art. However, to make its use possible with a permanent magnet generator, the following special performance features are essential: When the line current begins to exceed a predetermined value, as at short circuit, the compensating circuit must cease to function normally, thereby removing the negative reactance from the circuit. The current is then limited by both the armature reactance and by the reactance of the transformer. After the short circuit is removed, the compensator must automatically revert to normal operation, thereby again introducing a negative reactance into the circuit to neutralize the armature reactance. For a better understanding of the invention, reference is made to the following specification of one embodiment of the invention, the said specification to be read in connection with the accompanyin drawings, in which: Figure 1 is a schematic circuit diagram of a compensator embodying the invention and shown in connection with a permanent magnet generator and a load circuit. Figure 2 is a graph showing a magnetization curve of a compensator core with the secondary open and a curve showing the voltage-current relations in the compensator capacitor. Figure 3 is a graph showing the relationship between the voltage impressed across the compensator and the resistive current therein. Figure 4 is a graph showing vectorially the relationship between current and voltage in the compensator, the values thereof being derived from the graphs shown in Figures 2 and 3. Figure 5 is a graph showing the relationships set forth in Figure 4 combined with the internal resistance and reactance of the permanent magnet generator. Figure 6 is a graph similar to Figure 5, and includes a circle representing the magnitude of the voltage generated in the permanent magnet generator. Figure 7 is a graph showing the voltage-current curve for a load at both unity and zero power factors, the said graph being derived from the graph shown in Figure 6. Figure 8 is a schematic diagram showing means to protect the compensator against overload. Figure 9 is a graph showing a modification of the operating conditions shown in Figure 6. Figure 10 is a graph showing the voltage-current curve for a load at both unity and zero power factors, the said graph being derived from the graph shown in Figure 9. Figure 11 is a graph illustrating an operating characteristic of the invention. Figure 12 is a schematic diagram illustrating the invention applied to a polyphase system. Figure 13 is a circuit diagram of a compensating transformer wound as an autotransformer. Referring to the drawings and particularly to Figure 1 there is shown a compensator ll consisting of an iron core transformer i2 having s a primary l4 and a secondary ll, and a capacitor l8 connected across the said secondary II. The primary H of the transformer I2 is connected in series with a permanent magnet generator 20 and a load circuit 22. As heretofore stated, it is the function of the compensator ID to neutralize the inductive reactance of the permanent magnet generator 20 under conditions of normal load current and to limit current flow under short circuit conditions. Both of these functions require further explanation in theoretical terms. First, it is necessary to examine the performance of a permanent magnet generator under various conditions of load. I For a permanent magnet generator designed according to the principles set forth in applicants copending application entitled Alternating Curwhere R is the equivalent resistance and X is the equivalent reactance of the generator, quantities that depend on design factors. If the load current lags the load voltage by an angle 0, the current may be written as I=Io (cos 0-1 sin 0) (2) where I0 is the numerical value of the current, and I is its vector value. Vectorially, E=V+IZ (3) where E is the generated voltage and V is the terminal voltage; Substituting (2) for I and (1) for Z in (3) E=V+Ic R cos 0+X sin 0) +7'Io(X cos 0R sin 0) (4) squaring to obtain magnitudes, (E+V)=2VIu(R cos 0+X sin 0)+Io (RH-X) (5) approximately, E=V, hence, setting E+V=2V, 4 the terminal voltage drop under load E-V will be =1. R cos o+x sin awfi s It will be noted that even at unity power factor loads, (sin 0:0, cos 0:1), the value of I, by its effect on Z, appears in the voltage drop. As power factor is reduced, the term X sin 0 increases rapidly before cos 0 shows much deviation from unity. If R and X are equal, the maximum drop will take place at 0=45, or at 0.707 power factor. If X is greater than R, and it is often of the order of 10 times as great, the maximum drop is encountered near zero power factor. It is often desired. in non-regulated generators of the type discussed, to kee the regulation unaffected by deviations in power factor from unity. This may be accomplished according to the invention by neutralizing the effective reactance X of the generator by an externally inserted reactanoe of -X. This neutralization of the X component will leave R only, with an added voltage drop due to the effective resistance of the neutralizing circuit. For exact balance, the regulation will then be represented to a good approximation by E-V=IoR cos 0 ('7) since my 2V is usually very small. The effective power output is of course Power=I V cos 0=(EV)% (EV)=power- (8) Since R and V are relatively constant in the working range of any given machine the drop (E-V) is also constant for a given power developed, regardless of power factor. It may be noted that not only is the effect of power factor on regulation removed, but the resu lation, including that at unity power factor loads. is reduced by the term In constructing a permanent magnet generator power unit, driven by an engine, it is desirable to use an engine governor controlled by the voltage of the generator, because it is possible then to allow the speed of the engine to increase under load. Thus, a permanent magnet generator when used with an electric governor having properly chosen characteristics and having a 5% regulation at constant speed, (and of course a voltage substantially proportional to the speed), may result in a unit having output voltage regulation of 2 /2%, and a speed regulation of 2 /2% (increasing). Obviously, if the generator regulation depends on the power factor of the load, the speed regulation of this governor will be variable, and with low power factor loads, may be excessive. If, however. the generator regulation is substantially independent of power factor, as it may be made according to the invention, this ype of governor will have a definite frequency regulation regardless of power factor changes. Another type of governor control similar to the the regulation of the generator, even at unity power factor, is improved by the addition of the compensator. Permanent magnet generators are usually designed to have a certain specified regulation. This requires a certain weight of generator to produce the necessary equivalent armature impedance, Equation 1, to obtain this regulation as explained in the above mathematical analysis. The armature resistance, R, may be reduced by increasing the copper cross section of the winding, which means only an increase in the slot depth, and usually only a moderate increase in weight for major decreases in R. However, X, to be reduced, requires an increased field, or a. generator built on a larger scale, involving proportionate increase in weight. Even if the regulation is defined at unity power factor, the most favorable operating condition, the reactance X must still be kept within limits. If, however, the reactance is neutralized accordin to the invention, the entire design picture is shifted to a more favorable position. A compensated permanent magnet generator will be built with no more regard for X than is implied in the compensator weight, which is roughly proportional to X. Thus, a design balance between generator and compensator weight results with an entirely different low R. and moderately high K in the equivalent circuit. Figure 1 shows a schematic diagram of such a device. As far as the provision of the desired component X is concerned, the operation is described in the following, If the capacity of the condenser is C, the frequency F, and the primary and secondary turns are Np and N5, and the primary and secondary voltages and currents are Vp, Vs and Ip and Is, then it can be shown that the effect of the capacity C across the secondary is the same as that of a capacity across the primary. This condition, with the secondary load referred to the primary, leaves an equivalent circuit of an inductance, represented by the compensator primary winding and the iron core, shunted by a condenser of capacity 6 tical axis) is plotted against current (horizontal axis). Curve OABCD is a magnetization curve of the compensator core, taken with the secondary open. Straight line OD shows the relation between voltage and current in the capacity Thus, at voltage E, a current OH flows in the condenser, and a current OF in the-inductance. Since these currents are opposite in phase, the difference FH (equal to Al) represents the current flowing into the parallel combination. Since the capacitive current is the greater, the current Al is capacitive in phase. In the region 0 to B before the knee of the magnetization curve, this resultant current is very nearly proportional to the voltage. As saturation sets in, the current increases more slowly; reaches a maximum value, and then decreases, reaching zero at D. Beyond D, the current again increases, but it is; now inductive instead of capacitive. If C, as shown, is the point of tangency to the curve OAD of a line parallel to line OD, it is clear that current C3 is the maximum current. The curve of Figure 2 shows onl the relations of the reactive current component. A resistive current component, representing core loss and copper loss in the compensator, is shown in Figure 3. This curve may be calculated from core loss and winding data. The information in Figures 2 and 3 cannot be used directly. In the art, point C has mistakenly been described as the breakdown point. A most noticeable characteristic of a compensator as applied to a generator in the invention is the great diiference in operation when working into loads of difierent power factors. Since, as yet. no considerations connected with load power factor have entered the picture, it is obvious that, in itself, point C has no critical importance. In fact, the actual meaning of a breakdown point must be found in far more complex considerations. It will be necessary to work up to these considerations step by step. First, from the data of Figures 2 and 3, a vector relationship between voltage and resultant current may be plotted, as in Figure 4. Here, V: is voltage along the axis of reals, (resistive), V7 voltage along the axis of imaginaries (reactive). Both voltages are reversed, as their use will appear as a voltage drop in an impedance. The direction of the current vector is always vertically upward along the Vr axis, but its magnitude of course is variable, each point on the curve corresponding to a different value of current. It will be noted that the resultant voltage OL always increases as the curve progresses in the direction of the arrows. The current, however, increases to a certain point (approximately 03 of Figure 2), and then decreases, swinging in phase rapidly until (at point D, Figure 2) it is all resistive. Beyond this point, the phase changes and current again increases. \It will be seen that Figure 4 is in reality a three dimensional graph, the two dimensions of the plane of the figure showing the two components of the vector voltage, and the third dimension, vertically above the plane, showing the magnitude of the current. This third dimension is shown indirectly by the numbers showing relative current values on the curve. For simplicity, 1.0 has the meaning of a rated current, 1.5 of 1.5 times rated current, etc. Figure 4 shows the vector relation between 7 voltage and current of the compensator. For an analysis of the operation of the generator-compensator-load group of elements, it will be found convenient to draw from Figure 4 the derived curve of Figure 5. In Figure 5, the values of Figure 4 are combined with the internal resistance and reactance of the generator, r and :c. Any resultant voltage OM is the drop in the generator and compensator impedances combined. Such a voltage as OM is not measurable directly on an actual machine, since the generator impedance drops are internal, and always appear combined with the generated voltage E. The ' usefulness of the concept of combining these drops appears, however, from the fact that besides the information given by Figure 5, only the generated voltage E, which is by definition always of constant value, and the load voltage V remain, and of course these related in the manner here the sign of W is plus because curves 4 and 5 were drawn in a negative sense, as previously explained. It is now possible to pass on to the final diagram, Figure 6, from which the performance of the compensated generator may be plotted. Figure 6 is like Figure 5, except that from O, a circle of radius ON (6TJ=E) has been drawn. The vertical line is the direction of the actual current, and we see that for a load of 1.5, E6 is the generated voltage in phase and magnitude, N P the voltage across the load in phase and magnitude, and OP the voltage drops in the compensator and generator internal impedance in phase and magnitude. Of the three voltages, the phase of only one is fixed, and this is the phase of $35, which is determined by the vector impedance of the load, which is of course fixed by th character of the load. The direction of NP and the value of current (here 1.5) fix the triangle ONP and complete the solution. Note that for another load QP, which differs in power factor from NP but draws the same current 1.5, the qu te different triangle OPQ results. It is now readily perceived that, along with the vector voltage-current characteristic of the compensator, the following factors are of major importance in determining the characteristics of the generator-compensator-load combination. 1. The magnitude of the generated voltage E. 2. The internal impedance of the generator. 3. The power factor of the load, as well as its non-vectorial impedance. FromFigure 6, all the characteristics of the generator-compensator-load combination may be predicted. However, some care is necessary in this procedure. First, it is noted that a line drawn from the E voltage circle NT to the impedance curve WT always represents the voltage in phase and magnitude across the load, and the current value along the curve OPT at which the line strikes curve OPT represents the current in the load. Thus, a series of vertical lines drawn between the two curves gives a series of voltage and current values for loads of unity power factor. It will be noted that the intersection of circle NT and curve OPT represents a short circuit (V=0). For unity power factor, the above series of lines gives the current-voltage curve for the load shown in solid line in Figure 7. This curve may be looked upon as a simple generator-compensator output characteristic holding only for unity power factor. However, its simple form and the ease of analysis of critical load values from it justifies its preparation as a derived curve from Figure 6. Thus, a line such as OW represents by its slope the conductance (I/E) of a certain load. The impedance of the load, which is the reciprocal of the conductance, will often be referred to in the following. The point of intersection W of this line and the curve WE represents the actual voltage and current taken by that load. Since in the case shown, for any angle OW in the first quadrant of the graph (its entire working region), there is only one possible point of intersection, the generator-compensator characteristic at unity power factor is single valued, This means that for any given resistance placed across the output terminals, only one possible condition of operation can result. Such a curve shows no breakdown, or presents no sudden changes in operation as certain critical loads are passed. Suppose now, in Figure 6, a series of horizontal lines is drawn from circle IE to curve OPT. Since the lines must start on the circle, and go to the left it is found that at first the lines move downward from T, then lengthen and swing back past T, finally reaching P and 0. The E, I relations for zero power factor are shown in dotted lines in Figure '7. The curve reveals several new important types of operation. Thus, a zero power factor load (lagging) of admittance given by the slope of line OW; intersects the dotted curve in three places. Hence, the generator-compensator characteristic at zero power factor is not single valued. This means that a number of possible points of operation exist for this load OW. Of these, the only desirable ones (normal operation) are on the cusp curving up from E before the point of tangency of We. Any other points represent enormous voltages and currents in the compensator, leading to rapid overheating and damage. Suppose a variable inductance is placed across the generator-compensator terminals, starting with infinite impedance and decreasing progressively. The operating point (intersection of a line such as W. with the dotted curve in the normal region) moves up the cusp until the impedance corresponding to We is reached. A slight increase beyond this point finds only intersection, We, and operation immediately shifts to point W1, which is in a highly abnormal region for the compensator. The critical load impedance W is defined as the breakdown load, and has corresponding breakdown current and voltage values. If new the impedance is again increased, operation at We is not restored; instead, the working point moves from W: to We to Wr. As the slope of the line (WI drops below the value OWr, there again is a sudden change to the only possible point of operation at Wh in the normal region. The critical load impedance Wn is defined as the recovery load, with corresponding recovery voltage and current values. Now it has been stated that the compensator, to operate satisfactorily, must be capable of normal operation at full load and even at reasonable overload. Switching transients or load fluctuations often produce greatly increased saturation in the compensator core, and will often produce breakdown well below OW of the curve. Recovery will not take place unless the load is less (higher impedance) than the recovery impedance OWr. Thus, for reliable operation, it is acaaari necessary to design the generator-compensator sothat for all the loads and. power factors in down impedance is not the design limit, instead, the recovery impedance is. this limit. In testing a completed unit, recovery is best tested for by putting the maximum load atminimum power factor in circuit, short circuiting the machine, and then removing the short circuit. The machine must recover normal operation out of the short circuit into its load. I Since the machine may still be overloaded to its damage, a circuit breaker should be provided. Referring to Figure 8, this breaker should have a delayed or thermal overcurren't' release to protect the generator, and also have a delayed or thermal overvoltage release to protect the compensator. sator, caused by overload, is always characterized by high voltages across both primary and secondary, and these voltages are the ones to be. connected to the delayed overvoltage release equipment. Any devices of this type known to the art may be used in obvious fashion. Further discussion of Figure 6 may now be undertaken. 'It will be noted that the dividing line between performance like the solid curve of Figure 7 and that of the dotted curve takes place for a load having the phase angle of the tangent to the circle NQT at T of Figure 6. It is obvious that the solid curve operation is in general preferable to the dotted curve operation, making a smaller and lighter compensator for a given recovery load value. This indicates the desirability of keeping 0G approximately equal to E, the generated voltage, as then the intersection will give an approximately horizontal tangent and the favorable condition of the solid line curve of Figure 7 is extended to very low power factors. However, for this condition the starting point at T, Figure. 6, of the V, I, dotted curve, of Figure 7 may be at quite low currents, limiting the slope of the recovery line OWr, so that, in general, actual complete analysis for the design is required for best results. Roughly, however, OG=E is a useful rule of thumb. For loads of fairly high power factor, it may be seen that the value of I in Figure 'l for V=0 should be kept reasonably high for best results, indicating a point T (Fig. 6) having a fairly high current value. However, here too, OG==E is a rough indication of desirable performance. This means simply that in Figure 2, the-point of intersection D should roughly equal the generated voltage of the generator. I For further discussion-Figure 9 shows a condition like Figure 6 in which 0G is less than E. At unity power factor, a curve Figure 10, like the curve of Figure 7 results. Here, an impedance line We. has three intersections with the curve, and breakdown and recovery conditions We and W1 exist. It is'instructive to review the above design considerations in more detail. In the practical Abnormal operation of the compen-' 1 1o core steel. a. magnetization curve for the core dimensions and material being figured can be plotted, as OABCD in Figure 2. In plotting this curve, it is most convenient to plot the ordinates as volts per turn and the abscissa as ampere turns. The ratio of the transformer is obtained from the relation between-the reactance X of the generator to be compensated and the reactance p m of the condenser, where f is the frequency and c the capacity in rnicrofarads. The ratio is then v 21m v It should be noted that regardless of the ratio of transformation, the volt ampere rating of the condenser at full load should be at least equal to 1 x, the volt amperes being compensated. ' Generally, for a given volt ampere rating, soondenser is smaller and cheaper as the rated voltage increases, so that higher voltagesare preferred until bushing and transformer insulation become a problem. The condenser should be capable of enduring a short circuit condition until the protective device opens the circuit. It should therefore have at least a 5 minute life at a voltage equal to the generator rated voltage times the transformer step-up ratio, which is approximately the voltage applied to its terminals during short circuit. A new graph (Fig. 11) is now set up, with actual primary voltages for the compensator as ordinates and actual primary current as abscissa. The capacity referred to the primary design of a compensating transformer, the fol-, (again equal to X), gives the line OD. The primary now has N turns and the ordinates are N times the volts per turn of the preliminary curve, and the abscissa times the ampere turns. It is obvious that the voltage corresponding to the point D depends on the choice of N. A value of N is now selected which gives a voltage at the intersecting point D roughly equal to the generator rated voltage. The transformer is now completedexcept for choice of the maximum wirevcross sections that will fit in the windows of the transformer core. Curves, like 3 through I may now be calculated, and the generator-compensator output characteristics calculated. The result may suggest a shift to a somewhat different value of N, according to the principles previously explained in connection with these curves. Finally, a complete check of the transformer design for heating, lass, etc., is made and a final design with possible changes in transformer core area, etc., results. At this time, slight corrections to the turns ratio caused by loss in the transformer may be made, to secure nearly exact compensation for X at full load on the machine. The loss in the transformer increases the effective resistance of the generator-compensator combination, and is naturally kept as low as is possible without entailing excessive weight. In a polyphase permanent magnet generator, polyphase compensation may be obtained by one compensator in series with each phase or by a polyphase transformer, for example of the three leg three phase core type, or a shell type, since fluxes from the three primaries combine in the well known way and afford a, saving in total iron over three separate single phase transformers. The three windings are each in series with a phase of the generator, and proper polarity must be observed. Each secondary may have its own condenser, and the secondaries may be quite separate, or connected in star or delta. Obviously, an auto transformer connection is also available in polyphase applications. For many machines, the compensator also works well on unbalanced load for the impedance of a generator phase loaded alone is usually not too far removed from that of the impedance per phase for a three phase load. If it is desired to make a three phase or other generator reconnectible, theimpedance of each winding section may be balanced by an individual compensator, before the connecting panel is reached, with the result that all possible connections are automatically compensated. If two or more coils appear in each phase, they may be combined as a split primary in the same core, since series or parallel connections will always he made in such a way that the fluxes in the transformer add. As an illustration, Figure 12 shows a three leg three phase compensating device having secondary winding 50 and capacitor 52 and used with a six coil 120 parallel, 208 Y parallel, 240 series, 416 Y series machine of a well known type. A possible objection to the separate compensation of each generator coil is the presence of excessive circulating currents. These, it is well known, are limited by the impedances of the coils, which have now been neutralized. However, the impedance is neutralized only at the fundamental frequency. At the third harmonic, the reactance of the generator is three times as great as at the fundamental, while the reactance of the compensator is that at the fundamental. Thus, /9 of the original reactance remains at third harmonic frequencies to cut down circulating currents. The th harmonic similarly shows /25 of the original reactance. As for circulating currents at fundamental frequency, normal balance of the windings holds these to negligible values. Figure 13 shows a compensating transformer wound as an autotransformer. Some saving in weight over a separate winding transformer may be obtained for the well known reasons applying to any autotransformer design. It will be apparent that a compensator of the above type may be used as a current limiting device in any alternating current circuit. Figure 13 shows the addition of a choke 54 to a compensator 55, whereby the reduced short circuit current flowing in a. compensator may be used to act as current limiter in a circuit. Where the capacitive reactance of the compensator is not desired, this is neutralized fully, or in part, or even overneutralized by the choke 54. A similar additional choke may be used with a compensated generator to shift the compensator design to a more desirable region, although usually the generator would be designed to have a suitable reactance inherently. There has thus been disclosed a compensator adapted to neutralize the armature reactance of a permanent magnet generator under normal load conditions, to provide a reactance that is additive to the armature reactance under conditions of excessive load, and that is designed to revert quickly to an armature reactance neutralizingstatus when the excessive load is removed and conditions of normal load obtain. There are also disclosed methods whereby the various characteristics of a permanent magnet generator-compensator combination may be readily determined. It will be apparent that many modifications and changes may be effected in the invention disclosed herein without departing from the spirit thereof. It is therefore intended that the specification herein be descriptive of but one embodiment of the invention. The full scope of the invention herein is pointed out in the attached claims. What is claimed is: 1. An alternating current permanent magnet generator supplying a load which may become excessive and a compensator in combination therewith comprising a transformer having a ferromagnetic core dimensioned to be saturated by excessive load currents, a primary winding and a secondary winding, said primary winding being connected in electrical series arrangement between said generator and said load, said secondary winding being connected in electrical series arrangement with a capacitor, said transformer having a turns ratio substantially equal to the square root of the ratio of the capacitor reactance to the reactance of the generator to neutralize the synchronous reactance of the gen erator under normal load conditions; said transformer having sufficient impedance to provide a reactance that is additive to, and greater than the reactance of the generator under conditions of excessive load and to revert to a generator reactance neutralizing condition when normal load conditions are resumed. 2. A substantially constant speed alternating current permanent magnet generator supplying a load which may become excessive and a compensator in combination therewith comprising a transformer having a ferromagnetic core dimensioned to be saturated by excessive load currents, a primary winding and a secondary winding, said primary winding being connected in electrical series arrangement between said generator and said load, said secondary winding being connected in electrical series arrangement with a capacitor, said transformer having a turns ratio substantially equal to the square root of the ratio of the capacitor reactance to the reactance of the generator to neutralize the synchronous reactance of the generator under normal load conditions; said transformer having sufficient impedance to provide' a reactance that is additive to the reactance of the generator under conditions of excessive load and to revert to a generator reactance neutralizing condition when normal load conditions are resumed. 3. An alternating current permanent magnet generator supplying a load which may become excessive and a compensator in combination therewith comprising a transformer having a ferromagnetic core dimensioned to be saturated by excessive load currents, a primary winding and a secondary winding, said primary winding being connected in electrical series arrangement between said generator and said load, said secondary winding being connected in electrical series arrangement with a capacitor, said transformer having a turns ratio substantially equal to the square root of the ratio of the capacitor reactance to the reactance of the generator to neutralize the synchronous reactance of the generator under normal load conditions; said transformer having sufficient impedance to provide a reactance that is additive to the reactance of the generator under conditions of excessive load and to revert to a generator reactance neutralizing condition when normal load conditions are resumed, and an overload relay means connected in series with said generator, compensator, and load for disconnecting said generator from said load in response to an excessive load condition. 4.A substantially constant speed alternatinz current permanent magnet generator supplying a load which may become excessive and a compensator in combination therewith comprising an auto-transformer having a ferromagnetic core dimensioned to be saturated by excessive load currents, and a winding having primary and secondary portions, said primary portion of said winding being connected in electrical series arrangement between said generator and said load, said secondary portion of said winding being connected in electrical series arrangement with a capacitor, said transformer having a turns ratio substantially equal to the square root of the rational the capacitor reactance to the reactance of the generator to neutralize the synchronous reactance of the generator under normal load conditions; said transformer having sumcient impedance to provide a reactance that is additive to, and greater than the reactance oi the generator under conditions of excessive load and to revert to a generator reactance neutralizing condition when normal load conditions are resumed. WILLIAM KOBER. REFERENCES CITED The following references are of record in the file of this patent: UNITED STATES PATENTS Number Name Date 1,513,633 Scheller Oct. 28, 1924 1,676,312 Alexanderson July 10, 1928 Patent Citations
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