US2636158A

US2636158A - Magnetic saturation device

Info

Publication number: US2636158A
Application number: US68543A
Authority: US
Inventors: Burnice D Bedford
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1948-12-31
Filing date: 1948-12-31
Publication date: 1953-04-21
Anticipated expiration: 1970-04-21
Also published as: GB670838A; FR1007500A; BE493005A

Description

April 1953' B. D. BEDFORD 2,636,158

MAGNETIC SATURATIGN DEVICE Filed Dec. 31, 1948 2 SHEETSSHEET l Ihventor: i Buwnice D. BedFord, 120 I60 200 by His Attovney.

April 21, 1953 B BEDFORD 2,636,158

MAGNETIC SATURATION DEVICE Filed Dec. 31, 1948 2 SHEETS-SHEET 2 Fig.7

Inventor: Bummce D. BedFoTd,

His Attorn ey.

Patented Apr. 21, 1953 MAGNETIC SATURATION DEVICE Burnice D. Bedford, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application December 31, 1948, Serial No. 68.543

13 Claims.

1 This invention relates to magnetic saturation devices for stabilizing a variable electrical quantity and more particularly to devices of this kind which are suitable for use as electrical currentor voltage standards.

There is need, and a wide field of application, in industry, and in the products of industry, for a simple, rugged, low cost, high accuracy, longlife device which will produce an electrical output of constant Value which is suitable for use as a reference standard in electrical circuits such as regulating and measuring circuits.

In accordance with this invention there is produced such a device which operates on a novel principle that involves the periodic or alternate desaturation of a magnetic core leg (ora plurality of such legs) which has a sharp kneed saturation characteristic and which is normally saturated substantially above such knee with a permanent magnet.

An object of the invention is to provide a new and improved magnetic current reference.

Another object of the invention is to provide a new and improved constant current reactor.

A further object of the invention is to provide a new and improved electric stabilizer.

A still further object of the invention is to provide a novel permanent magnet saturated electrical regulator.

The invention will be better understood from the following description taken in connection with the following drawing, and its scope will be pointed out in the appended claims.

In the drawing Fig. 1 is a perspective view of a preferred form of the devicecombined with a diagrammatic representation of a preferred circuit, Fig. 2 illustrates the sharp knee'd saturation characteristic of the normally magnetically-saturated portions of the device and is useful for explaining the operation of the device, Fig. 3 illus trates the relation between an alternating input voltage to the device and its constant output current, Fig. 4 illustrates the action-of a rectifier in converting the constant alternating output current to unidirectional current, Fig. 5 illustrates the regulating characteristics of the device for a plus and minus per cent variation in input voltage frequency, Fig. 6 is a perspective view of a modification of the invention, Fig. '7 is a diagrammatic view of another modification of the invention, Fig'. 8 is a diagrammatic viewof a different form of frequency compensating circuit, Figs. 9, 10, 11, 12 and 13 are circuit diagrams illustrating modified ways of connecting the main'current carrying coils so as to obtain a unidirectional output reference current.

Referring now to the drawing and more particularly to Fig. l, the device is indicated generally at I and comprises a main magnetic core member consisting of two parts 2 and 3. These parts may be made from fiat stacked laminations of ordinary sheet or strip magnetic material such, for example, as hot or cold rolled silicon steel. The member 2 is generally L-shaped having an arm 4 and another arm 5 at right angles thereto. The member 3 on the other hand is generally T-shaped having a center portion 6 and a cross member I. The main magnetic circuit of the device is completed by two difierent kinds of magnetic members which close or bridge what would otherwise be gaps in the main magnetic circuit. Thus a permanent magnet 8 is inserted in the gap between the portions #2 and e of the core with one pole thereof adjacent the member 4 and the other pole thereof adjacent the member 6. This permanent magnet is preferably an Alnico (aluminumnickel-ccbalt alloy) magnet which is characterized by especially high retentivity and coercive force. Some forms of its composition are the subject matter of Mishima patents, 2,027,994 to 2,028,000 inclusive, which are owned by the present assignee. Extending between the adjacent ends of the portions 5 and 7 of the core are a pair of parallel laminated core leg members Sand [G which thus bridge the gap between the adjacent ends of the members 5 and'l. These legs, 9 and it, are of relatively small cross-section in comparison with the cross-sec tion of the rest of the core, and they are made of magnetic material which has a very abrupt bend or sharp knee'in its magnetization or saturation characteristic. A suitable magnetic material having such properties is known generally as nicaloi, which consists of approximately equal portions of nickle and iron.

The device is so proportioned that the legs e and [0 are saturated to a point considerably above'the knee of the saturation characteristic by preferably only a small portion or" the total flux of the permanent magnet 8. The rest of the flux'of the permanent magnet 8 by-passes the legs 9 and It through a pair of parallel related gaps H and [2 which are formed by the membersi and 3 so 'asto be in shunt relation to magnet 8. Thus gap H is formed between the adjacent ends of the portions 4 and l and gap it is formed between the endof the portion 6 and the center of the portion 5. The magneti cross section of the main part of the core is sufficiently large so that it is not saturated.

Mounted respectively on the legs 9 and it are a pair of serially connected main windings i5 and it. These windings are connected with their polarities reversed with respect to the direction of saturating unidirectional flux from the permanent magnet so that any current which flows in the circuit containing both windings causes one winding to produce a magnetizing force in the same direction as the saturating flux and causes the other winding to produce a magnetizing force in the opposite direction to the saturating flux. By reason of the magnetic paths through the gaps H and [2 which are in shunt with the legs 9 and la the main windings i3 and I4 have negligible coupling or mutual inductance and the flux in each one is independent of the other.

The circuit for the device illustrated in Fig. 1 comprises any convenient source of alternating current l5 which is connected through a transformer it of suitable ratio to a circuit 51 in which the windings l3 and hi are serially connected. In orde to obtain a unidirectional output from the device, the circuit ll may be connected to the input terminals of a bridge connected rectifier 18 whose output terminals are connected to a load device such as a resistor l9 through which it is desired to pass a constant current. A choke 28 is connected in the output circuit of the rectiher for smoothing the current. Another load d vice 2! across which it is desired to have a constant reference voltage may be connected across the resistor 19 with an adjustable connection 22 so that voltage of the device 29 may be selected for any desired value,

For compensating the device for variations in both frequency and magnitude of input voltage, a pair of compensating coils 23 and 2e are mounted respectively on the legs 9 and Ill, preferably concentric with, and outside, the windings l3 and it. These compensating windings are serially connected in a compensating circuit which includes a frequency compensating portion comprising a capacitor 25 and an adjustable resistor 26 connected across the secondary winding of the transformer 16. The input terminals of a rectiher 21 are connected across the capacitor 25 and the output terminals of the rectifier are connected to the windings 23 and 2 1 in series through a reactor 28 and an adjustable resistor 29. The magnetizing effect of the

compensating windings

23 and 24 is in the opposite direction to the flux from the permanent magnet 8 through the saturated legs 9 and ID. It will, therefore, be apparent that the two windings, 23 and 26, are the equivalent of a single compensating winding around both the legs 9 and it which would have a magnetizing efifect in the same direction.

The operation of the device illustrated in Fig. l is as follows: Fig. 2 is a curve 30 which shows the relationship between the flux in the legs 9 and 18 measured Vertically and the magnetomotive force applied to those legs measured horizontally. As the magnetomotive force and flux increase in a positive direction from zero, the relationship is linear up to a point B at which the material in the legs 4 and iii saturates abruptly and beyond this point there is very little further increase in flux as the magnetomotive force increases. The shape of the curve is, of course, similar for flux and magnetomotive force in the reverse or negative direction. The abrupt knee of the curve in that case being at D. It will further be noted that the curve is very steep below saturation, that is to say, between the points B and D so that very small changes in magnetomotive force will produce very large changes in flux,

The magnetomotive force and flux in the legs 9 and ill produced by the permanent magnet 8 is represented by the line A-A and it will be observed that this is well above and beyond the knee of the saturation curve. Referring now to Fig. 3, the sine wave 3i represents the voltage of the source i5 in Fig. 1. The substantially fiat topped wave 32 represents the current in the circuit ll which contains the two main windings i3 and I4. Due to the inductance of these coils the current in them lags the voltage by substantially degrees. As the current rises from zero in what may be called the positive direction, it will be assumed that the coil 53 tends to increase the magnetization of the leg 9, and, consequently, the coil i l will tend to demagnetize the leg l9. However, as the legs 9 and it are saturated little change of'fiux can occur, and, consequently, the coils i3 and Hi have very low impedance and the current rises rapidly from zero to the point E. At this point the magnetomotive force of the coil M is sufiicient to reduce the flux in the leg H] to the value B in Fig. 2 which corresponds to the knee of the saturation curve. Consequently, any further increase in current in the coil It will desaturate the leg ill so that there will be a very large change in flux in the leg is for very small further increases in current in the winding Ill and, consequently, the winding i l will immediately have a large value of self-inductance and the increase in current will be abruptly limited. The point F in the current wave corresponds to the zero point of the voltage wave and it is the maximum or crest value of current in the circuit [1. This may be taken to correspond to a value of flu C in the leg i8. Beyond this point the current begins to fall, but initially it decreases very slightly because the leg I6 is desaturated. At point G the leg I!) saturates abruptly again and the current then falls rapidly to zero. The negative half-waves of voltage and current have the same shape and are produced in the manner explained above except that due to the reversal of current it is the leg 9 which becomes desaturated by the current in the coil l3 so that it is the coil !3 which abruptly limits the current rise and causes an abrupt increase in current again after the leg 9 desaturates.

Fig. 4 shows the effect of the rectifier 18 in converting the flat topped current wave 32 into a substantially continuous direct current. The effect of the choke 20 is to smooth out the wave 4 and practically eliminate the dip indicated at 33.

It will be seen from the above that the eifective value of the current in the circuit I1 is determined primarily by the amount of flux which the permanent magnet 8 passes through the legs 9 and ii), that is to say, it is determined by the location of point A in Fig. 2. This may readily be adjusted by controlling the air gaps II and I2 so as to control the amount of saturating flux which passes through the legs 9 and ill.

Variations in supply voltage will merely raise and lower the point (3 between the points B and D without having any substantial effect on the magnitude of the current in the circuit ll. This will be particularly true if the unsaturated impedance of the coils l3 and i is high in comparison with the load impedance and it is, therefore, desirable to have that relationship in the device. In fact, a typical relationship is such that the unsaturated impedance of the coils l3 and i4 is individually about twenty times the load impedance and the load impedance is about twenty times the saturated impedance of the coils l3 and I4 individually.

The current through the load resistor I9 will, therefore, be substantially a constant value over a considerable range of variation in voltage of the input source I5. If it is desired to have a constant output voltage instead of a constant output current, then a load device such as 2I may be connected in shunt to the resistor l9. Alternatively, if it is desired to have a constant current through the device 2|, then the resistor I 9 may be omitted.

Due to the slope of the magnetization curve, the output increases slightly as the input voltage is raised. To eliminate this slight increase and produce a constant output independent of voltage variation, the compensating windings 23' and 24 provide a unidirectional magnetomotive force which varies directly with the inputvoltage and which opposes the magnetomotive force of the permanent magnet. In other words, the compensating coils 23 and 2t tend to shift the point A slightly to the left in Fig. 2 as the input voltage increases so that the two effects tend to neutralize each other and the current remains constant.

The fundamental circuit including the main coils I3 and I4 is comparatively insensitive tofrequency variation because the output is determined primarily by the position of point A in Fig. 2. However, to reduce the effect of frequency change, to a minimum, the

frequency compensating elements

25 and 26 are provided. As the frequency increases and the output tends to decrease the impedance of the capacitor 25 decreases thus lowering the voltage applied to the compensating

windings

23 and 24, and, therefore, reducing the amount of compensation. With a decrease in frequency, the output voltage tends to rise. However, the impedance of the capacitor increases, causing a higher voltage to be applied to the compensating windings which in turn reduces the output current. The amount of frequency compensation can be adjusted by the rheostat 26. Likewise the amount of voltage compensation can be adjusted by the rheostat 29.

In order to minimize the effect of the alternating current through the main windings I3 and I4 on the permanent magnet 8, this permanent magnet may be provided with a low resistance short-circuited turn 34 around it. This acts to resist any change in flux to the permanent magnet.

Fig. 5 illustrates the opera-ting characteristics of the complete compensated circuit shown in Fig. 1 when used as a voltage reference; thus, the vertical values represent the output voltage and the horizontal values represent input voltage, and it will be seen that over a range of input voltage between I80 and 228 that the output voltage is practically constant at a value in the neighborhood of 7 and three-fourths volts. It will, of course, be understood that these particular values of voltage are merely representative of one size device and its setting and that various other values can of course be obtained. Inother words, Fig. 5 is intended primarily toindicate qualitatively rather than quantitatively the high accuracy of the device. The center curve is for an input of 60 cycles and the two outer curves show per cent variations in frequency to 54 and 66 cycles respectively, and it will be seenfrom this figiue that over the range of constant output voltage that a 10 per cent plus or minusvariation in frequency is hardly noticeable.

In the modification shown in Fig. 6 the axis'rof the permanent magnet 8 is arallel to the, saturated legs 9 and I0 and the main portion of the core comprises similar upper-and

lower members

35 and 36. A magnetic shunt member 3'I is inserted between the members- 35 and 36 and preferably air gaps are provided in this magnetic shunt path. The main windings I3 and I4 and the compensating

windings

23 and 24 are the same as in Fig. 1.

Fig. 7 shows still another form of the core which in general is similar to Fig. 1 except that the core is made symmetrical by placing the saturating legs 9 and III on opposite sides of the portion of the core which contains the permanent magnet 8. The compensating windings have been omitted from Fig. '7 for the sake of simplicity.

Fig. 8 shows a compensating circuit with a modified form of frequency compensation comprising a. reactor 38 inparallel with a rheostat 39 in one of the leads leading to the input of the rectifier 21. The action is essentially the same as the capacitive frequency compensating circuit shown in Fig. 1 except that with the inductive type of frequency compensation shown in Fig. 8 the reactor is serially connected with the input of the rectifier 27, whereas the capacitor 25 is shunt connected to the input of the rectifier. Therefore, the inverse eifectsof changes in frequency on a reactor and on a capacitor will have the same enect on the, compensating circuit because of the change from the parallel to series connection. The rheostat 39 is used to adjust the amount of frequency compensation which is produced by the reactor 38.

In the compensating circuits'which have been described, the current inthe compensating coils is direct current; it is, however, possible to use alternating current in the compensating coils for compensation. In that case sufficient inductance should be present in the circuit to make it an inductive circuit. Also the'voltage which is applied to the compensating coils must be large enough to overcome the voltage induced in the compensating coils by the main coils.

Fig. 9 is a modified connection of the main coils I3 and I4 for eliminating-the resistance of one of those coils in the load circuit. Thus, the coils I3 and I4 are connected in parallel circuit relation, each with a half-waverectifier 40 and M connected in series circuit relation with it. The rectifiers are reversely connected so that for halfwaves of current in one-direction the load current Will only flow through one of the main windings, and for half-waves of current in the opposite direction,- the load current will flow only through the other main winding. In this manner all of the load current flows through only one of the main windings at any one time.

Fig. 10 isanother modification which may be described as a bi-phase circuit which is for the purpose of reducing the number of rectifiers required in the circuit shown in Fig. 9. Thus the secondary winding of thetransformer I6 has a mid-tap and the connection is such that half the voltage of the winding I6 is applied to the main winding I3 through arectifier 40 and the load I9, and likewise the other half of the voltage of the secondary winding of the transformer I6 is applied through the rectifier AI, and the main winding I 4 to the load I9. In order to prevent the inductance of the main windings I3 and I4 from extending the current pulses through them beyond a half cycle and thus causing an overlapping of the currents from the two coils in the load which would resultin an output eur rent. increase. as .the supplyvoltage increased;

additional windingsfl and 4 3 connected respectively in series with the main windings l3 and l4,-but mounted on the opposite core legs, are provided. Those coils induce a voltage in the main coils on the opposite leg which counteracts the current carry-through effect which has been described. Fig. 11 illustrates another way of compensating for the carry-through effect mentioned above in connection with Fig. 10. In this case the additional compensating

coils

42 and 43 are connected in the series in the common portion of the circuit and they are bridged by an additional half-wave rectifier 44. Fig. 12 is the same as Fig. 1 but without the additional rectifier.

Fig. 13 is a bridge circuit in which the main coils l3 and 14, with their individual serially connected half-wave rectifiers 40 and 4|, are connected in a bridge circuit with two additional half wave rectifiers 45 and 46. In this circuit the load current flows through each coil for a half cycle only and the output current in the load i9 is remarkably free from ripple.

While there has been shown and described a particular embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention, and therefore it is aimed in the appended claims, to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a magnetic core having a winding leg, a permanent magnet for magnetically saturating said winding leg substantially beyond the knee of its magnetizing characteristic, a winding on said leg, and a circuit including said winding in series, said circuit being arranged to send pulses of current through said winding in such a direction and of such magnitude as to desaturate said winding leg, the saturated impedance of said winding being low in comparison with the total impedance of said circuit and the unsaturated impedance of said winding being high in comparison with the total impedance of said circuit.

2. In combination, a magnetic core having a winding leg of magnetic material which has a very high permeability for values of flux density below an abrupt saturation point and has a very low value of permeability for values of flux density above the abrupt saturation point, a permanent magnet for magnetically saturating said winding leg substantially beyond said abrupt saturation point, a Winding on said leg, a rectifier, and a circuit including said winding and said rectif er in series, said circuit being arranged to send pulses of current through said winding and rectifier in such a direction and of such magnitude as to desaturate said winding leg, the saturated impedance of said winding being low in comparison to the total impedance of said circuit and the unsaturated impedance of said winding being high in comparison with the total impedance of said circuit.

3. In combination, a magnetic core having a winding leg, a permanent magnet for magnetically saturating said winding leg substantially beyond the knee of its magnetizing characteristic, and a circuit, having alternating current input terminals and output terminals, said winding being connected in said circuit so as to carry pulses of current in such a direction and of such ma nit as to desaturate said winding leg, the

saturated impedance of said winding being low in comparison to any impedance connected across its output terminals and the unsaturated impedance of said winding being high in comparison with any such impedance connected across said output terminals.

4. In combination, a magnetic core having a winding leg, a permanent magnet for magnetically saturating said winding leg substantially beyond the knee of its saturation characteristic, a main winding on said leg, a circuit including said winding in series, the saturated impedance of said Winding being low in comparison with the total impedance of said circuit and the unsaturated impedance of said winding being high in comparison with the total impedance of said circuit, and a compensating winding on said winding leg connected to be energized in shunt circuit relation to said circuit and to apply a unidirectional magnetomotive force to said winding leg in opposition to the magnetomotive force of said permanent magnet.

5. In combination, a magnetic core having a winding leg, a permanent magnet for magnetically saturating said Winding leg substantially beyond the knee of its magnetization characteristic, a main winding on said leg, a circuit having alternating current input terminals, said Winding being series connected in said circuit, said circuit having load terminals, said winding having a saturated impedance which is low in comparison to the load impedance of said circuit and having an unsaturated impedance which is high in comparison with the load impedance of said circuit, a compensating winding on said winding leg, and means including a reactive device and a rectifier for energizing said compensating winding with a unidirectional current which varies in versely with the frequency of said input terminals and which is of such polarity as to oppose the magnetomotive force of said permanent magnet.

6. In combination, a magnetic core having a winding leg, a permanent magnet for magnetically saturating said winding leg substantially beyond the knee of its magnetization characteristic, a main winding on said leg, a compensating winding on said leg, a circuit having alternating current input terminals and load terminals, said main Winding being connected in said circuit, said main winding having a saturated impedance which is low in comparison with the load impedance of said circuit, said main winding having an unsaturated impedance which is high in comparison with the load impedance of said circuit, and means including a reactive device and a rectifier for energizing said compensating winding with direct current which is proportional to the magnitude of the voltage of said input terminals and which is inversely proportional to the frequency ofsaid input terminals, the magnetizing elTect of said compensating winding being in opposition to the permanent magnet.

7. In combination, a magnetic core having a winding leg, a permanent magnet for magnetically saturating said winding leg substantially beyond the knee of its magnetization characteristic, a unidirectional conducting device, a circuit including said winding and said unidirectional conducting device in series, the polarity of said unidirectional conducting device being such that the current through said winding desaturates said winding leg, the saturated impedance of said winding being low in comparison with the impedance of the rest of said circuit and the un- 9 saturated impedance of said winding being high in comparison with the impedance of the rest of said circuit.

8. In combination, a winding leg of magnetic material, a permanent magnet, a magnetic core for magnetically interconnecting said winding leg and permanent magnet, said core having a flux path which is magnetically in shunt relation to said winding leg and to said permanent magnet, a major portion of flux of said permanent magnet being diverted through said flux path and a minor portion of the flux of said permanent magnet passing through said winding leg, said winding leg being normally saturated substantially above the knee of its magnetization curve by said minor portion of the flux of said permanent magnet, said knee being a sharp bend and said magnetization curve being characterized by large changes in flux for small changes in magnetomotive force below saturation and by large changes in magnetomotive force for very small changes in flux above said knee, a winding on said winding leg, and a circuit having alternating current input terminals and output terminals with said winding serially connected therein, said winding acting to limit a current through it in the direction which opposes said permanent magnet to a value which is predetermined by the degree of presaturation of said winding leg by said permanent magnet.

9. In combination, a magnetic core having a plurality of winding legs of magnetic material having a sharp knee bend in its magnetization characteristic, a permanent magnet for saturating said legs substantially beyond the knee of their magnetization characteristic, a separate winding on each leg, and a circuit including said windings having alternating current input terminals and load terminals, successive half waves of current in said circuit being respectively limited to a constant value by the desaturation of a different one of said winding legs.

10. A constant current reactor comprising, in combination, a magnetic core having two legs, means having a constant unidirectional magnetomotive force for magnetically saturating said legs substantially beyond the knee of the magnetization curve of the material from which said legs are made, a separate winding on each of said legs, a variable voltage alternating current input circuit and a load interconnected by said windings by such a way that during all the odd numbered half cycles of input current all of the load current is carried by at least one of said windings with its magnetomotive force in opposition to said constant unidirectional magnetomotive force so as to desaturate its core leg during a substantial portion of said odd numbered half cycles, and during all of the even numbered half cycles of input current all the load current is carried by at least the other of said windings with its magnetomotive force in opposition to said constant unidirectional magnetomotive force so as to desaturate its core leg during a substantial portion of said even numbered half cycles.

11. A constant current reactor comprising, in combination, a permanent magnet, a pair of winding leg members, a magnetic core for connecting said winding leg members magnetically in parallel between the poles of said magnet, said winding leg members being made of magnetic material which has a sharp bend at the knee of its magnetization curve, the flux profit) duced in said winding leg members by said permanent magnet being substantially above the knee of said curve, a separate winding on each of said legs, a pair of alternating current input terminals and a load, said windings being serially connected with each other and with said load between said input terminals, the instantaneous magnetomotive force of said windings being such as respectively to aid and oppose the magnetomotive force of said magnet, the strength of the magnetomotive force of said windings when they oppose the magnetomotive force of said permanent magnet being sufiicient to carry the iiux in their legs substantially below the knee of said magnetization curve.

12. In combination, a silicon steel core member having a main magnetic circuit with two serially related gaps in it and having a shunt magnetic circuit with a gap in it, an aluminum-nickelcobalt alloy permanent magnet which bridges one of the gaps in said main magnetic circuit for magnetizing said core, two parallel nickeliron alloy leg members which bridge the others gap in said main magnetic circuit, said leg members having a saturation curve with a sharp knee bend in it and being saturated substantially above said knee by said permanent magnet, a pair of windings on each of said legs, a load, a pair of input terminals for receiving a. variable magnitude alternating voltage, a winding on one leg being serially connected with a winding on the other leg and with said load across said input terminals, said serially connected windings respectively aiding and opposing said permanent magnet so that during each half cycle of input current one of them produces a flux in its leg substantially below the knee of said curve, said remaining windings being connected across said input terminals through a frequency compensating network and a rectifier so that they oppose said permanent magnet.

13. In combination, a silicon steel core memher having a main magnetic circuit with two serially related gaps in it and having a shunt magnetic circuit with a gap in it, an aluminumnickel-cobalt alloy permanent magnet which bridges one of the gaps in said main magnetic circuit for magnetizing said core, two parallel nickel-iron alloy leg members which bridge the other gap in said main magnetic circuit, said leg members having a saturation curve with a sharp knee bend in it and being saturated substantially above said knee by said permanent magnet, a winding on each of said legs, a load, and a pair of input terminals for receiving a variable magnitude alternating voltage, the winding on one leg being serially connected with the winding on the other leg and with said load across said input terminals, said serially connected windings respectively aiding and opposing said permanent magnet so that during each half-cycle 01' input current one of them produces a flux in its leg substantially below the knee of said curve. BURNICE D. BEDFORD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,875,020 Kouyoumjian Aug. 30, 1932 2,363,857 Crever et a1. Nov. 28, 1944 2,435,062 Walsh Jan. 27, 1948 2,465,451 Hedstrom Mar. 29, 1949