US2959730A - Alternating current limiter - Google Patents

Description

Nov. 8, 1960 Filed May 26, 1955 C. M. DAVIS, JR., ET AL 2 Sheets-Sheet l A. c. SUPPLY CONTROL TRANSFORMER IO f l I4 24 SYNCHRO GENERATOR MAGNETIC SERVO AMPLIFIER MOTOR INVENTORS CHARLES M. DAVIS JR. EDWARD T. HOOPER JR.

*RTTORNEYS' c. M. DAVIS, JR., ET AL 2,959,730

ALTERNATING CURRENT LIMITER Nov. 8, 1960 2 Sheets-Sheet 2- Filed May 26, 1953 o 1r n\ RADIANS FICA.

'- .006 EDWARD T. HOOPER JR.

O .002 .004 CURRENT lN AMPERS I R \M Ho ATTORNEYS INVENTORS CHARLES M. DAVIS JR.

ALTERNATING CURRENT LIMITER Charles Mitchell Davis, Jr., Washington, D.C., and Edward 'll. Hopper, Jr., Hyattsville, Md., assignors to the United. States of America as represented by the Secretary of the Navy Filed May 26, 1953, Ser. No. 357,651

3 Claims. (Cl. 323-89) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to impedance elements and more particularly pertains to non-linear impedance elements.

The present invention relates to an alternating current impedance element which presents a low resistive impedance to AC. currents below a predetermined level and a relatively high resistive impedance to AC. currents above that level. The impedance element is particularly adapted for use in series with the control transformer of a servo system wherein it is desired to pass low level signals without appreciable attenuation, and wherein it is desired to attenuate higher level A.C. signals in order to prevent errors from being reflected into other servo systems connected to the control transformer and to prevent loading of the control transformer.

An important object of this invention is to provide an impedance element having a 10W impedance to AC. currents below a predetermined level and a high impedance to currents above that level.

Another object of this invention is to provide an im pedance element in accordance with the foregoing object for limiting currents above a predetermined level, which impedance element presents a substantially resistive impedance to the circuit. 7

Another object of this invention is to prevent loadin of the control transformer in a servo system under high level signal conditions and to apply relatively low level signals from the control transformer to the amplifier of the servo system without appreciable attenuation.

A further object of this invention is to provide a current limiting impedance in the control circuit of a magnetic servo amplifier which will not introduce a large time delay into the servo system.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is a schematic diagram of a servo system employing the non-linear impedance limiter;

Fig. 2 is a BH curve illustrating the hysteresis loop of a ferromagnetic material;

Fig. 3 is a set of curves illustrating the amplitude and phase relation between the applied voltage and exciting currents in the limiter;

Fig. 4 is a set of curves illustrating the voltage current relation for the non-linear impedance, using different numbers of turns in the winding.

It has been ascertained that saturable reactors having rectangular hysteresis loops exhibit the characteristics of very non-linear resistors. The action of such an element in a circuit, when operated below saturation, is that of a current limiter possessing low initial impedance.

The non-linear resistive nature of a winding dis-. posed on a core of saturable magnetic material having rates Patent a rectangular hysteresis loop characteristic will best be understood by the following theoretical development.

If a sinusoidal voltage is impressed upon a circuit con sisting of a series inductance L and resistance R, the applied voltage leads the current by an angle 0 where (1) 0=tan- The instantaneous power p can be expressed as:

(2) p=ie=I V sin wt sin (wt-0) Integrating Equation 2 over a half cycle of the power frequency, the energy dissipated W is:

m m (3) W 4f cos 6 When wL is much greater than R, 0 approaches and W approaches zero. The power absorbed by an inductance during a half cycle is therefore returned during the same half cycle. When R is much greater than 40L, 0 approaches zero and W approaches and all the power supplied during the half cycle is dissipated during the same half cycle. Thus, the smaller that 0 is, the more resistive the circuit appears.

Referring now more specifically to the hysteresis loop illustrated in Fig. 2, the energy supplied to the core is expressed by the equation:

m m f From Equation 1, tan 6 is shown to be the energy stored 7 Area ACE'A: cos a per cycle divided by the energy dissipated per cycle, and

therefore Area CEDC' (8) tan Area ACEA also sin 0 (9) tan 0- 6 therefore sin 0 Area CEDC (10 cos 0 n? sin In the case of a closed loop core suchas toroidal core of a magnetic material having a rectangular'hysteresis loop, area ACEA is much greater than area CEDC and tan 0 is small. Therefore tan 0 approaches Zero and consequently 0 approaches zero. A saturable reactor comprising a winding on a core of magnetic material having rectangular hysteresis loop thus presents a substantially resistive impedance to an applied voltage when the core is operated below saturation. The voltage current wave forms for such a saturable reactor are illustrated in Fig. 3, and as is apparent, are nearly in phase.

One such magnetic material having rectangular hysteresis loop characteristics is Orthonol, having a composition of approximately 50% Ni 50% Fe and having the properties (11) Area CEDC= max and p20 is the permeability taken with the B at 20 gauss.

It has been ascertained further that the impedance of such a saturable reactor is very non-linear, as illustrated by the current-voltage curves shown in Fig. 4. The impedance of the reactor is low for small currents and increases sharply for currents large enough to place the core on the steep side of the hysteresis loop but not large enough to saturate the core. The core, therefore, has the characteristics of a current limiter which presents a substantially resistive impedance and possesses the advantage of a small initial impedance.

In order to achieve the aforementioned current limiting characteristics it is necessary that the core be operated below saturation. It is therefore necessary that the number of turns and the cross sectional area of the core be adjusted such that the flux density produced in the core by the maximum A.C. voltage to be applied to the winding is insufficient to drive the core into saturation. This condition can be fulfilled by designing the core in accordance with the following equation:

where N is the number of turns in the winding; A is the cross sectional area of the core in square centimeters; E is the full voltage to be applied to the winding; F is the frequency of the applied voltage and B is the flux density in the core, the value of which is chosen to be less than the residual fiux density Br of the core. Conveniently, B may be made equal to a value such as .85 Br. 7

As hereinbefore set forth, the limiting action of the impedance element occurs when the magnetic intensity in the core approaches the magnetic intensity at which the steep portion of the hysteresis loop occurs. However, the area of the hysteresis loop of a core of magnetic material and consequently the residual fiux density and coercive force varies as a function of the maximum flux density attained during the cycle, the flux density being dependent on the amplitude and the frequency of the applied voltage. Thus, a relatively high, constant amplitude sinusoidal voltage applied to the winding on a square loop core will be subjected to the limiting action during substantially the same portion of each half cycle of the voltage. For rectangular hysteresis loop core materials, the point at which limiting action occurs is very nearly equal to the A.C. coercive force of the material at the level of the applied voltage. It has further been ascertained that the A.C. coercive force for all except very low level voltages of rectangular hysteresis loop materials is approximately equal to the A.C. coercive force measured when the core is subjected to a magnetizing force sufficient to produce the maximum flux density in the core. For a material such as Orthonol, at a voltage which would produce a flux density which is only 20% of the maximum flux density, the coercive force is greater than of the coercive force measured when maximum fiux density has been attained in the core.

The impedance element when properly designed may thus be used to limit currents produced by a constant amplitude sinusoidal voltage, or to pass relatively low level currents and sharply attenuate currents above a predetermined level. The following equation which expresses the relation between the magnetic intensity and the ampere turns per unit length of the core may be utilized to design the reactor to obtain the desired limiting action.

Where N is the number of turns in the winding; 1 is the mean length of the core in centimeters; I is the value of current above which it is desired to limit, and He is the coercive force of the material. The value of He for convenience may be chosen to be the He measured when the core has been subjected to an A.C. voltage sufiicient to attain maximum flux density therein, subject to the errors introduced due to the minor hysteresis p eifect produced at lower voltages.

The specific application for which the limiter was designed was to prevent the control transformer in a servo system employing magnetic amplifiers from being loaded. Heretofore, a large resistance has been employed in series with the control transformer and the control windings of the magnetic servo amplifier. The impedance of the resistance had to be much larger than the impedance of the control winding in order to prevent large currents from flowing, and consequently when the sig nal from the control transformer is small, the signal applied to the magnetic amplifier control windings would be extremely low.

Reference is now made more specifically to the servo system illustrated in Fig. 1 of the drawings. The servo system comprises an output shaft 1%, an input shaft 12, an error detector comprising the synchro generator 14 and the control transformer 16 to measure the angular difference between the input and output shaft and a con troller comprising the magnetic amplifier 18 and servo motor 20 to regulate the motion of the output shaft in accordance with the magnitude and direction of the error signal. The synchro generator 14 may be coupled to one shaft such as the input shaft 12 and the control transformer coupled to the other shaft 10. The servo motor 20 is suitably coupled to the output shaft iti so as to drive the latter in a direction and at a rate determined by the error signal from the control transformer.

It has been ascertained that improved servo system response can be achieved by passing the low level control signals to the magnetic amplifier 13 without attenuation and by attenuating or limiting higher level A.C. signals from the control transformer 16 to prevent the overloading of the control transformer. In accordance with the present invention a saturable reactor limiter comprising a core 22 of rectangular hysteresis loop material having a winding 24 thereon was utilized, the win ing 24 being connected in series with control transformer 16 and the control windings of the magnetic servo amplifier 18. As hereinbefore set forth, the saturable reactor limiter using a core material having rectangular hysteresis loop characteristics, when properly designed, presents a low impedance to A.C. currents below a predetermined level determined by the design, and presents a high impedance to AC. currents above that level.

7 Although the impedance of the limiter using rectangular hysteresis loop core material is primarily resistive, the limiter does present a small inductive impedance which causes the current flowing therethrough to lag the voltage is illustrated by the wave form curves in Fig. 3. The magnetic amplifier is a carrier type system. The measured phase angle in one application between the current and voltage of the 400 cycle carrier was 9. This represents a phase shift of less than between the current and voltage envelopes of the carrier modulated with a 100 radian per second signal. This delay can be disregarded in a servo system,

A comparison between a two stage magnetic amplifier with no additional impedance added to the control circuit; with a 10,000 ohm resistance added, and with a limiter added is shown in the table. The values of voltage gain with an input signal of V; volt, and the current which flows with 57 volts out of the signal source for each of the above named cases is given.

From the above data it can be seen that the limiter when connected in series with the control transformer and the control winding of the magnetic servo amplifier serves to allow the gain of the magnetic amplifier to be increased while limiting the current drawn from the signal source. This is achieved without introducing an appreciable time delay'into the servo system.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In a circuit having a source of voltage, means responsive to said voltage connected to said source for applying a load thereto, a current limiting non-linear impedance connected in series between said source and said means, said impedance comprising a core of magnetic material having rectangular hysteresis loop characteristics, a winding on said core, the cross sectional area of the core and the number of turns in said winding being such that the voltage applied to said winding is insuflicient to drive the core to saturation, the number of turns in said winding and the length of said core being such that the magnetic intensity in the core is approximately equal to the coercive force of the core, when the current through said winding reaches the desired limited value.

2. In a circuit, a source of alternating current, means responsive to said alternating current connected to said source for applying a load, a non-linear impedance element connected in series between said source and said means for limiting the A.C. current flowing therethrough in response to the application of an AC. voltage comprising a core of saturable magnetic material having rectangular hysteresis loop characteristics, a winding on said core, the number of turns in said winding and the cross sectional area of said core being proportioned so that the magnetic flux density produced in the core by the AC. voltage applied to the winding is less than the satu rated flux density of the core, the number of turns in said winding and the length of said core being proportioned so that the magnetic intensity in the core is approximately equal to the coercive force of the core, when the current through said winding reaches the desired limited value.

3. In combination with a servo system including a control transformer and a magnetic servo amplifier, said magnetic amplifier having a control winding, an alternating current limiter including a core of saturable magnetic material having rectangular hysteresis loop characteristics, a winding on said core, and means conmeeting said winding in series with the control winding of said magnetic amplifier and said control transformer, the cross sectional area of said core and the number of turns in said winding being such that the full output voltage of said control transformer applied to the winding of said limiter and said control winding produces a flux density in said core below the saturated flux density thereof, the number of turns in said limiter winding and the length of said core being such that currents in said limiter Winding below a predetermined level produce a small change in flux density in the core and currents above that level produce a large change in flux density in the core.

References Cited in the file of this patent UNITED STATES PATENTS 1,920,618 Zierdt Aug. 1, 1933 2,184,371 Thompson Dec. 26, 1939 2,375,609 Zuhlke May 8, 1945 2,569,468 Gaugler Oct. 2, 1951 2,661,452 Curry et al. Dec. 1, 1953 2,666,151 Rojchman et al Jan. 12, 1954

Images