US2699525A - Voltage regulator employing saturable reactor - Google Patents

Description

Jan. 11, 1955 R. E. MORGAN 2,599,525,

VOLTAGE REGULATOR EMPLOYING SATURABLE REACTOR Filed Feb. 8, 1951 2 ShetsShe et 1 F121.

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by )Q/d w His Attorney.

Jan. 11, 1955 7 RE. MORGAN 2,699,525

VOLTAGE REGULATOR EMPLOYING SATURABLE REACTOR Filed Feb' 8, 1951 2 Sheets-Sheet 2 Ihvehtor Raymond E. Mov cgan,

b )QJ 4. 5:294,

His Attorney.

United States Patent VOLTAGE REGULATOR EMPLOYING SATURABLE REACTOR Application February 8, 1951, Serial No. 209,961

4 Claims. (Cl. 322-28) My invention relates to voltage regulators and, more particularly, to voltage regulators of the type which employ saturable reactors in conjunction with circuits that have become known as magnetic amplifiers.

An object of my invention is to provide a voltage regulator which is a completely static unit requiring no electron emitting components.

Another object of my invention is to provide a voltage regulator which requires little or no maintenance once installed and which is immediately available for use without Warm up.

Another object is to provide a static voltage regulator capable of maintaining extremely close regulation, such as less than 1.5% voltage variation between no-load and full-load condition on the apparatus whose output voltage is being regulated, and even to obtain a negative regu lation characteristic such as to produce an increase in this output voltage with an increasing load.

A further object of my invention is to provide a static voltage regulator having high speed of response and good reliability.

A still further object of my invention is to provide a voltage regulator which detects and compensates for small changes in wave shape of the voltage to be regulated and which utilizes such wave shape changes to improve both the degree and speed of the overall voltage regulation of the circuit.

In general, my invention comprises a stage of magnetic amplification of the self-saturating type, customarily called a magnetic amplifier, connected to receive its power supplying voltage from the voltage to be regulated, and means for rectifying the voltage to be regulated and for delivering this rectified voltage as unidirectional signal pulses to an amplification controlling element of the magnetic amplifier. In order to obtain a voltage regulating control signal, wave chopping means are included for varying the duration of these signal pulses from a reference point in accord with changes in the amplitude of the voltage to be regulated. These signal pulses have a direction and time phase such that a change in the duration thereof due to a chopping of the regulated voltage wave results in a corresponding change in the amplification of the magnetic amplifier. The voltage to be regulated may then be readjusted in a voltage amplitude compensating direction by a suitable voltage control means operating in response to the output voltage of the magnetic amplifier.

In a preferred embodiment of my invention, a bias circuit for the magnetic amplifier is also provided which detects changes in wave shape of the voltage to be regulated with changes in load, and which causes a variation in the amplification of the magnetic amplifier in a direction to compensate such wave shape changes. Phase shifting means are also preferably included in this bias circuit for displacing the phase of such detected wave shape changes relative to the phase of the magnetic amplifier power supplying voltage to a phase point providing greatest effect of the detected wave shape ghanges upon the amplification of the magnetic ampli- The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention .itself, however, together with further objects and advantages thereof can best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a schematic circuit diagram of j 2,699,525 Patented Jan. 11, 1955 ICC a voltage regulator embodying my invention; Fig. 2 is a family of wave shapes illustrating typical changes in output voltage of a wave chopping means included in the circuit of Fig. 1; Fig. 3 is a curve showing the eifect on the magnetic amplifier of change in the duration of signal pulses produced by the wave chopping means of Fig. 1 as a result of the change in the voltage to be regulated; Fig. 4 is a set of curves illustrating the effect on the output voltage magnetic amplifier of Fig. 1 produced by a change in the duration of a negative signal voltage pulse; Fig. 5 is a similar curve showing the effect on the output voltage of the magnetic amplifier of Fig. 1 which would be produced by a change in the duration of a positive signal voltage pulse; Fig. 6 illustrates a pair of curves representing the wave shapes of a typical voltage to be regulated when the apparatus producing the voltage is running under full-load and no-load conditions respectively; and Fig. 7 is a schematic diagram of a portion of the circuit of Fig. 1 illustrating a modification thereof which adapts the magnetic amplifier in the voltage regulator of Fig. l for use with positive signal pulses.

Referring now to Fig. 1, I have shown one embodiment of my invention as it might be employed to regulate an output voltage V from an alternator 10 having a voltage controlling field winding 11. The output voltage V of alternator 10, is connected to input terminals 12 and 13 of a voltage regulator 14. A first stage of magnetic amplification shown as a bridge type magnetic amplifier 15, is connected through conductors 16 and 17 to receive its power supplying voltage from the voltage V to be regulated. Magnetic amplifier 15 is preferably of the type having at least one pair of load or reactance windings 18 and 19 respectively arranged on a saturable reactor 20 in series with current rectifying means such as full wave bridge rectifier 21 alternately to conduct a unidirectional current through a load circuit 22 when an alternating power supplying voltage is connected to the amplifier 15 through conductors 16 and 17. Load circuit 22 for magnetic amplifier 15 preferably comprises a current limiting resistor 23 connected in series with a control winding 24 of second stage of magnetic amplification 25 to receive the magnetic amplifier output voltage produced across output terminals 26 and 27 of bridge rectifier 21, as shown. Alternatively, load 22 for magnetic amplifier 15 may comprise alternator field winding 11, although I prefer to employ the additional stage of amplification 25 in order that smaller voltage sensing elements may be used and a closer voltage regulation and better speed of response obtained. The sec- 0nd magnetic amplifier 25 may be similar to that of the first stage 15, as indicated. The output voltage of this second magnetic amplifier 25 is connected through voltage regulator output terminals 28 and 29 to energize the alternator field winding 11. Second magnetic amplifier 25 preferably also receives its power supplying voltage from the voltage V to be regulated through conductors 16 and 17. The magnetic operating point of a saturable reactor 30 employed in the second magnetic amplifier 25 is adjusted for optimum sensitivity by means of a bias winding 31 which receives a proper biasing current from a biasing circuit 32 comprising a full wave bridge rectifier 33 connected across alternating current conductors 16 and 17 through current limiting resistors 34 and 35.

Referring now to the voltage regulating signal producing portion of Fig. 1, a saturation control winding 36 of saturable reactor 20 is connected to receive an input signal comprising the rectified output current pulses of a voltage responsive wave chopping means 37 which, in turn, is connected to be energized by the voltage V to be regulated. Voltage responsive wave chopping means 37 must be one which shifts the trailing edge of its output voltage wave in a predetermined direction, preferably leading, with an increase in the amplitude of an input voltage, and thereby functions to change the duration of the output voltage wave relative to an initial reference point thereof. Because of its convenience, economy and elficiency, I preferably employ a saturated or easily saturable transformer for wave chopping means 37. Transformer 37 preferably has a split easily saturable core 38 upon which is wonnd a primary winding 39 connected across input terminals 12 and 13 through a current limiting resistor 40; a secondarywinding 41, and a short circuited saturating winding 42. The output voltage of transformer 37, taken across, secondary winding 41, is connected to a full wave current rectifying means, such as bridge re'ctifier43, and delivered substantially unfiltered as negative current pulses to the control winding 36 of magnetic arnplifier 15. Fixed resistors 45 and 44 as well as a variable resistor 46, are connected in series with the current path from the transformer secondary winding 41 to control winding 36 and function to enable an ad ustment of the average magnitude of rectified current pulses supplied ,'to the control winding .36. A capacitor 47 is preferably alsojinclud'ed in the circuit across output terl minals 48 and 49 of full wave bridge rectifier 43 in order to provide a slight filtering of the current pulses supplied to control winding 36 and thus to prevent an overly rapid snapf regulation. The direction of rectification of bridge rectifier ['43 and the direction of control winding 36 are preferably such as to supply the rectifier output current pulses of transformer 37 to the magnetic amplifier as a negativeusignal, i. 'e. a signal producing fiux in the saturablereactor in opposition to the flux produced therein by the load windings 18 and 19. The rectifier output current pulses of transformer 37 may alternatively be supplied to amplifier '15 as positive signals, but this necessitates the use of 'an additional phase shifting network as will be more fullyexplained hereinafter.

In order to bias magnetically the saturable reactor 20 of magnetic amplifier 15 as well as to provide voltage compensation for wave shape changes in the voltage to be measured, I'provide aphase shifting bias circuit designated generally by numeral 50. A bias winding 51 of saturabre reactor 20 is connected through a full-wave bridge rectifier 52 to receive unfiltered unidirectional current pulses rectified from the voltage V to be regulated. The polarity of bridge rectifier 52 is preferably such that the current pulses in the bias winding 51 are positive, i. e, of a direction to aid the saturating flux in the saturable reactor 20 produced by load j'winding's 18 and '19. A phase shifting network comprising 'a capacitor 53 and a resistor 54 are connected in the path of the rectified biasing current supplied to the bias winding 51 in order to shift the phase of the biasing pulses a predetermined degree relative to the power supplying voltage delivered to the magnetic amplifier 15 as will be more fully explained hereinafter. I

The operation of the voltage regulator of Fig. 1 can best be understood by reference to the wave'shap'es and curvesof Figs. 2, 3, 4, Sand 6. In Fig. 2 I have illustrated a family of typical wave shapes of the output voltage produced across the secondary "Winding 41 of saturable transformer 37 after rectification by current rectifying means 43 for a plurality of different alternating voltages supplied to the primary winding 39 of transformer 37. Until saturation is reached, as indicated'for example by the 75 volt wave shape and the 90 volt wave shape, the output voltage developed across the transformer secondary winding 41 merely increases in amplitude without appreciable phase shift. As the primary voltage continues to beincreased beyond the saturating point, how ever, the peak secondary voltage continues to increase but the duration of each alternation of secondary winding output voltage shortens as the lagging edge of the half cycle wave moves toward the leading edge, as indicated by the 150 volt and 200 volt wave shapes. As a consequence, the average current as measured by a direct current meter remains substantially constant but the trailing portion of the secondary winding voltage alternation is shifted leading as the primary voltage is increased beyond the saturating voltage point. The average direct current supplied to the control Winding 36 with an increasing voltage supplied to the primary of the saturated transformer 37, is represented by the typical curve X of Fig. 3. As illustrated by this curve X, the average current as measured by a direct current meter remains substantially constant for all voltages above the saturatingi voltage illustrated as being in the neighborhood of 90 v0 ts.

In accord with my invention, however, I have found that the amplification of the magnetic amplifier stage 15 is determined not only by the average value of the current supplied to the control winding 36 but is also dependent to a great extent upon the duration and time of 4 occurrence of the rectified control signal voltage pulses applied thereto relative to the tune of occurrence of a particular most sensitive portion of a power supplying voltage cycle delivered through conductors 16 and 17 to magnetic amplifier 15. This power supplying voltage derived from the voltage to be measured determines, or course, the phase of the voltage supplied across reactance windings 18 and 19 of the saturable reactor 20 of magnetic amplifier 15. With each alternation of the voltage to be measured delivered as a power supplying voltage to magnetic amplifier 15, one or the other of the load windings 18 and 19 delivers current to the load circuit 22 for an interval of time determined by the time during each alternation of the power supplying voltage at which the saturable reactor 20 reachessaturation. The time of saturation of saturable reactor 20, in turn, may be controlled by the amount and phase of the magnetic flux produced in the saturable reactor 20 by signal current pulses in control winding 36. Where the direction of flux produced by control winding 36 is such as to appose the effect of the flux produced inthe reactor 20 by the load windings 18 and 19, this control winding'signal is considered to be negative; and conversely, where the direction of control winding flux is suchas to 'aid the flux produced in the reactor 20 by reacta'ncle winding flax, such control winding signals are considered :positii e. I

The effect of a shiftin the *trailingf'e'dge df a pulsating negative signal supplied to control Winding 36 upon the amplification of magnetic amplifier 15, is shown in fthe curve of Fig. 4; while the eflectof a shift in the trailing edge of a pulsating positive signal supplied to thecontrol 36 is shown in 'Fig. 5. In Figs. 4 and 5, curve V'represents the voltage to be regulated (which "is employed "as the power supplying voltage for magnetic amplifier 15); curves A, B, and C plotted along a separate time axis below curve V represents the output signal voltage pulses of transformer 37 in the voltage rangeoverwhich regulation is desired; dashed curves An, BR, and Citrepresent typical voltage waves produced across one 'of the reactance windings 18 or 19 during a completecycle ofpower supplying voltage as a result of correspondingly lettered signal pulses; while curves AL, Br, and Cr. 'representtypical magnetic amplifier output voltages produced across load circuit 22 as a result'of correspdndirigly'letteredsignal ulses.

p Although I have illustrated the effect produc'eil uplon the magnetic amplifier 15 by fa'change in signal pulse duration in connection with only'one rea'ctan'ce winding of the magnetic amplifier, it "will b'e appreciated thata similar effect is produced upon the magnetic amplifier due to the remaining 'reactance winding'durihg'alteriiate half cycles. of the power supplying -voltage. In Figs, *4 and 5 the initial'negatlv'e half cycle maybe 'c'onsidered the inactive.fperiod of the reactance winding whose effect is being illustrated, while the succeeding halfcy'cle may be considered the active period'of thereact'an'ce Winding.

Referring nowto Fig, 4,'it has been found tliatan'ga tive signal pulse supplied to controlwindin'g 36 ofthagnetic amplifier 15has the ,"greatesteifect uponthearnplification thereof during the time interval defined by vertical lines N1 and N of the inactive .periodbf tlie reactance winding involved. This strongly atfectingtiihe interval usually occurs intheregion of'inaxilnuminstantaneous supply voltagein the'halfcyclepreeedingsaturation in the core leg of'thereactanc'e winding'involyed. The reason for the greater'efiect oflrega'tivesigti-al pulses during this time interval than'du'ring btherintervalsbf time during'the'supply voltagec'ycle'is that there is rio saturation of the reactance winding core leg during i this inactive period and the supply voltage reverses"diiring this period. Consequently, there is novolt'age drop aerbss the ieactancewindin'g "since the reactanCC Winding flax has substantially completely collapsed. The negatives'i'gnal pulse occurring at this time can, therefore, cause the 'magnetic properties of the 'core to 'inbve "thtb'u'gh a minor hysteresis loop and end at a ditfer'entm'agnetically biased condition which will affect the time of reactance winding current conduction,during the"sueceetling half cycle of supply voltage. During this "succeeding positive alternation, the reactance yvinding"core 'lg"will be in a magneticallysaturated con'ditionand small negative sig'nalpulses duririg'this alternation will, therefore, havelittlefefie ct upon the eur're'nt'flowiiig 'through the reactanee winding.

The effect of varying the duration of the negative signal pulses so that the trailing edge of these pulses passes through this most sensitive negative signal receiving time interval N1N2 is illustrated by the curves A, B and C and the corresponding reactance voltage curves AR, BR. and CR, and the output voltage curves A1,, B1. and CL- As the trailing edge of the signal voltage pulse is moved in a leading direction, such as from curve A to curve C, the time of collapse of the voltage across the reactance winding is also shifted slightly in a leading direction such as from curve AR to curve CR and the time of saturation during the succeeding positive half cycle is accelerated such that an increase in output voltage, illustrated by the relative curves AL and CL, will appear across the load circuit 22 due to the corresponding increase in current through the reactance winding involved. It is to be understood that the time interval representing the region on the supply voltage cycle of greatest effect for a negative signal, is that region which results in a minimum signal output from the magnetic amplifier. Therefore, as the signal voltage pulses move away from this most sensitive region due to the wave chopping action of transformer 37, these signal pulses will have less effect, and the output voltage of the magnetic amplifier will increase to approach the no signal level.

Referring now to Fig. 5, it has been found that a time interval defined by vertical lines P1 and P2 and repre-- senting the region of the supply voltage wave immediately preceding the active alternation thereof, comprises the region of maximum sensitivity of the magnetic amplifier to a positive signal. If the output voltage pulses produced by transformer 37 are supplied to control winding 36 as positive signals D, E and F occurring at a time phase relative to the power supplying voltage V of the magnetic amplifier such as indicated inFig. 5, then the trailing edge of these positive signal pulses will pass through this region P1P2 of maximum effect in a manner similar to that described in connection with Fig. 4. It should be noted however, that a shift in the trailing edge of the positive signal voltage through this most sensitive region P1P2 in a leading direction functions to retard the time of saturation of the reactance winding core leg as indicated by the curves DR, ER and FR of Fig. 5, and thus to reduce the output voltage of the magnetic amplifier, as indicated by the corresponding curves D1,, EL and FL. In order to obtain the proper time relations between the power supplying voltage and the positive signal pulses as illustrated in Fig. 5, it is necessary that the phase of the signal voltage pulses be shifted in a lagging direction by an amount in the neighborhood of 60 depending upon the range of voltage regulation desired. A phase shifting network suitable for this purpose is shown in Fig. 7 and may comprise a capacitor 6%) and a resistor 61 connected across voltage regulator input terminals 12 and 13. The primary winding 39 of the wave chopping transformer 37 is then connected across capacitor 60 in order to produce the requisite phase shift of the transformer output voltage wave.

It will be appreciated that if the unidirectional output pulses of the transformer 37 are supplied to magnetic amplifier 15 as a negative signal, it is necessary that the output voltage of the magnetic amplifier 15 be passed through a phase inverting means such as an additional stage of magnetic amplification in order that the direction of voltage change in the voltage supplied to the alternator field winding be such as to compensate for alternator output voltage changes. In other Words, an increase in output alternator voltage V when supplied as negative voltage pulses of decreasing duration will produce an increasing output voltage from the magnetic amplifier 15 which is amplified and inverted by magnetic amplifier 25 so that the output voltage of magnetic amplifier 25 supplied to field winding 11 is correspondingly decreased. On the other hand, if the output voltage of transformer 37 is supplied to the control winding of magnetic amplifier 15 as positive signals, there is no need to include an additional phase inverting means before the output voltage from the magnetic amplifier 15 is supplied to the field winding 11 although an additional stage of non-inverting amplification may be employed if desired. In order to convert the second magnetic amplification stage 25 from an inverting to a non-inverting stage it is necessary only to reverse the connections to the control winding 24 and bias winding 31 of the second magnetic amplifier 25 and thereby to reverse the direction of the control signal in reactor 30.

In the operation of the biasing and wave shape compensating circuit 50, capacitor 53 and resistor 54 comprise a phase shifting circuit which functions to shift the phase of the wave shape changes detected by the current rectifying means 52 to the most sensitive phase point for a signal direction which varies the output of magnetic amplifier stage 15 in a direction to compensate such wave shape changes. Typical wave shape changes which occur in an alternator output voltage with a change in load from full-load to no-load is illustrated in Fig. 6. As can be seen from the voltage waves of Fig. 6, there is a slight rise in the wave shape in going from no-load to full-load during the initial of each alternator voltage half-cycle while there is a slight dip in the wave shape in going from no-load to full-load during the second or lagging 90 of the alternator voltage half-cycle. The rise in voltage due to the wave shape change during the leading 970 occurs during a time phase that is, of course, leading with respect to the phase of the entire alternator output voltage wave. Conversely, the dips in the wave which occur during the second 90 are during a time phase that is lagging with respect to the phase of the alternator voltage wave.

In the voltage regulator 14 of Fig. 1, the changes in wave shape during the lagging portion of the half-cycle, in other words, the changes due to the dips in the wave in going from no-load to full-load, are employed to obtain the wave shape compensation desired. The polarity of current rectifying means 52 and the direction of the bias winding 51 are such as to supply rectified current pulses to the bias winding 51 as a positive signal. The phase of wave shape changes represented by dips in the wave during the lagging portion of the alternator voltage half cycle is displaced by phase shifting network comprising capacitor 53 and resistor 54 to occur during the time interval defined by vertical lines P1 and P2 of Fig. 5, which comprises the most sensitive region of the magnetic amplifier 15 to changes in a positive signal. Since these wave shape changes comprising the dips in the wave normally occur approximately'90 lagging with respect to the entire wave to be regulated, the values of capacitor 53 and resistor 54 are chosen to be such as to shift the phase of these dips approximately 90 leading in order to reach the above-mentioned most sensitive positive signal region for the magnetic amplifier 15.

In the magnetic amplifier 15, an increase in positive signal bias will, of course, hasten the time of saturation of the saturable reactor 20 and produce an increase in the amount of current passing to output circuit 22 through load windings 17 and 18. Conversely, a decrease in the bias current will cause a corresponding decrease in the output current through output circuit 22. Consequently, as the alternator load changes from no-load to full-load, the dip in the wave shape produced thereby will be detected by circuit 50 and function in the same manner as a decrease in bias current through bias winding 51, and thereby to decrease the signal supplied to the control windmg 24 of second magnetic amplifier 25 through output circuit 22. Since magnetic amplifier 25 is connected as a phase inverting stage of amplification, the output voltage of second magnetic amplifier stage 25 will be correspond- .ingly increased to increase the strength of the alternator voltage controlling field produced by alternator field winding 11. Therefore, with a change from no-load to fullload upon the alternator, the resultant dips in the wave shape are employed to off-set the tendency of the output voltage to be reduced by the effect of the change in load.

It will thus be seen that I have provided a voltage regulating circuit which employs a simple, saturated transformer as the voltage controlling element thereof. Since saturable reactors are employed throughout the circuit to achieve the voltage regulation, I have provided a completely static unit which requires little or no maintenance once installed and which is immediately available for use without warm up. Furthermore, since the change in the alternator field which provides the regulation does not depend directly upon the amplitude change in the output voltage, but rather upon the sensitivity of the magnetic amplifier stage 15 to changes in duration of a signal pulse supplied thereto, regulation to almost any desired degree of accuracy can be achieved even to the point of overcompensation by merely adjusting, for example, the

assigns amount of filtering of the rectified output voltage pulses of the transformer 37.

While I have shown a specific embodiment of my invention it is to be understood that many modifications may be made, and I intend by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

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

1. A voltage regulator circuit comprising a magnetic amplifier adapted to receive its power supplying voltage from a voltage to be regulated, means operating in response to the output voltage of said magnetic amplifier for controlling the magnitude of the voltage to be iregulated, said magnetic amplifier including a control'winding for varying the amplification thereof responsive to variations in the duration of electric signal pulses supplied thereto, and amplitude responsive wave chopping and rectifying means connected to said control winding for supplying to said control winding rectified pulses varying in duration in response to amplitude variations of the voltage to be regulated.

2. A control circuit comprising .a saturable reactor having at least one load winding and a saturation control Winding, a source of alternating voltage, a series circuit connected across said source comprising a load, 'a rectifier and said load winding, a control winding circuit comprising a saturable transformer having a secondary winding and a primary winding connected to receive a voltage derived from said source, and current rectifying means connected between said secondary winding and said control winding and polarized to deliver electric pulses to said control winding as a positive signal having a duration varying in response to the amplitude of said source voltage, and phase shifting means associated with said control winding circuit for shifting the phase of said electric pulses relative to said alternating voltage.

3. A voltage regulator circuit for regulating the output voltage of an alternator having a voltage controlling field winding comprising a magnetic amplifier connected to receive its power supplying voltage from saidalternator voltage and having its output voltage connected to energize said field winding, said magnetic amplifier including a saturation control winding, a saturable transformer having a primary winding-connected .to be energized by a voltage derived from said alternator voltage and havin'giasecondary winding connected to said control winding, and currentrectifying means in :circuit with said secondary winding and said control winding and polarized to deliver the voltage across said secondary winding as negative signal pulses to said control winding, said transformer effecting a shift in the trailingedge of said negative signal pulses in response .to amplitude variations of said alternator voltage to vary the amplification of said magnetic amplifier accordingly;

4. A voltage regulator circuit for regulating the output voltage of an alternator having a voltage controlling field winding comprising 'a magnetic amplifier connected to derive its power supplying voltage from said alternator voltage and having its-output voltage Vconnectedto energize said field winding, said magnetic amplifier having a control winding and a bias winding, a wave chopping transformer having a primary winding connected to be energized by a voltage derived from said alternator voltage and having a secondary winding, a rectifier connected to rectify and deliver the voltage across said vsecondary winding as electric pulses to said control winding, said transformer effecting a variation in the duration of said pulses responsive to variations in the amplitude of said alternator voltage, and a rectifying bias circuit for said bias'winding connected to ,rectify and supply said alternator voltage to said bias winding, said bias circuit including phase shifting meansfor shifting the :phase of said bias winding rectified alternator voltage relative to the phase 'of said alternator voltage whereby the effect of said :bias Winding rectified alternator voltage on the ramplification of said magnetic amplifier varies in response to changes in the wave shape of said alternator'voltage.

References Cited in 'the file of this patent UNITED STATES PATENTS 2,287,,755 Barth .June23, 1942 2,641,820 'Ker Feb. 13, 1951 2,558,572 Logan June 26, 1951 2,611,889 Huge Sept. 23, 1952 FOREIGN PATENTS 341,903 .Great' Britain May 21, 11-930

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