US3671814A

US3671814A - Electromagnet with a field-responsive control system

Info

Publication number: US3671814A
Application number: US136397A
Authority: US
Inventors: Heinrich Dick
Original assignee: Voith Getriebe KG
Current assignee: Voith Getriebe KG
Priority date: 1970-04-22
Filing date: 1971-04-22
Publication date: 1972-06-20
Anticipated expiration: 1989-06-20
Also published as: NL7105392A; DE2019345C3; DE2019345A1; CH530077A; NL164684B; FR2086243B1; JPS5532208B1; GB1324445A; AT306171B; FR2086243A1; DE2019345B2; SE361773B; NL164684C

Abstract

In an electromagnet, to generate a force which is independent from the armature position, an on-off control system for the excitation current is provided which operates as a function of the magnetic field intensity. The control system causes the excitation current to oscillate between two values and thus have a constant mean value according to a preset desired value. The timelag of the inductivity or its change between two close values is utilized for measuring the magnetic flux density and for a comparison with a desired value.

Description

United States Patent Dick [45] June 20, 1972 s41 ELECTROMAGNET WITH A FIELD- 3,165,675 1/1965 Shapiro ..317/010. 6 RESPONSIVE CONTROL SYSTEM 3,170,095 2/1965 Goldstein... 17/010. 6

3, 41, l [72] Inventor: Heinrich Dick, Heidenheim, Germany 2 002 3/ 966 Smith 317mm 6 [73] Assignee: Voith Getrieb KG, Heidenheim (Brenz), rima y E.\aminer-L. T. Hix

Germany AttorneyEdwin E. Greigg [22] Filed: April 22, 1971 ABSTRACT I. 6 7 [211 App N0 13 in an electromagnet, to generate a force which is independent from the armature position, an onoff control system for the [30] Foreign Application Priority Data excitation current is provided which operates as a function of April 22 1970 Germany 20 19 345] the magnetic field intensity. The control system causes the excitation current to oscillate between two values and thus have 521 u.s.c1 ..317/123, 317/148.5 R, 317/010. 6 a constant mean value according to a P desired value- [51 1 Int. Cl. ..H0lh 47/32 The timelag of the inductivity or its Change between two close [58] Fi ld of Se h 317/123, DIG, 6 values is utilized for measuring the magnetic flux density and for a comparison with a desired value. 56 R ierences Cited 1 e 7 Claims, 5 Drawing Figures UNITED STATES PATENTS 1,817,431 8/1931 Anderson ..3l7/D IG. 6

I Z I68 2 I65 5\ 7- 8 14a mun-30mm rm 3,671,814

SHEET 10F 2 INVENTOR.

SHEET 2 [BF 2 ELECTROMAGNET WITH A FIELD-RESPONSIVE CONTROL SYSTEM BACKGROUND OF THE INVENTION This invention relates to an electromagnet with a stationary, ironclad coil and a movable armature projecting through an open location of the iron cladding; said armature is drawn to the iron cladding by the magnetic field generated by virtue of current flowing through the coil. Assuming a constant excitation current, upon movement of the armature towards the iron cladding, the flux density increases. The electromagnet is further of the type that includes a force accumulator (gravitational force, spring, pressure cushion) urging the armature to move away from the iron cladding.

It is known to generate a distance-independent linear force by means of a plunger coil device which is characterized by a circular cylindrical magnetic field generated by a permanent magnet or by a direct current and having radially extending short magnetic field lines into which a thin-layer coil is axially immersed. Depending on the magnitude of the current flowing through the plunger coil, the latter is exposed to a greater or lesser axially orientated force which is independent form the position of the coil provided that all turns of the plunger coil are disposed in the undisturbed magnetic field. A plunger coil device of this kind, however, is capable of generating only comparatively small forces. Plunger coil devices designated for larger forces are unproportionately large and heavy. The best plunger coil devices are able to produce a force corresponding to approximately 0.4 times their own dead weight. It is also a disadvantage that the required control power is very high and that the coil constitutes the moving part. Apartfrom their large weight, plunger coil devices are very expensive due to their complex structure and the requirements for high precision in the manufacture of the coil.

Although relatively large forces may be generated by a small magnet of the kind mentioned heretofore, the attracting force on the armature depends to a large extent on its position. Thus, the attracting force increases hyperbolically as the armature approaches the coil core. Although, by a suitable design of the magnetic field (partial field line shunt) an approximately linear force/distance curve may be obtained in zones and thus the effect of distance within each zone is substantially eliminated, such design restricts the magnet to the zone of minimum power. To permit the generation of larger forces independently of the distance would necessitate the provision of a magnet of very large dimensions. The disadvantageous results are extensive space requirements, large weight and a high current consumption. Moreover, a strokeindependent attracting force can be achieved only along short distances of displacement caused by the attracting force.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide an improved electromagnetic device of simple and light-weight structure which is adapted to generate a relatively large, distance-independent linear force and with which the magnitude and the direction of said force may be altered in a rapid manner.

Briefly stated, according to the invention, there is provided an electromagnetic device of the aforeoutlined type which includes a means for regulating the excitation current. Said means comprises a transducer element which is responsive to the magnetic field intensity and which is disposed in the air gapbetween the armature and the iron cladding. The transducer element, which may be a Hall-generator or a field resistor, upon command by a desired value setter, regulates the excitation current to obtain a magnetic field excitation which is constant at least as far as its average value with respect to time is concerned thus resulting in a constant,'distance-independent magnetic force.

The invention will be better understood as well as further objects and advantages of the invention will become more apparent from the ensuing detailed specification of several ex- BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a circuit diagram of an embodiment of the invention, including an electromagnet in longitudinal section;

FIG. 2 is a circuit diagram of a further embodiment of the invention, including, in longitudinal section, an electromagnet designed as a solenoid valve;

FIG. 3 is a circuit diagram of still another embodiment of the invention, including an associated electromagnet in longitudinal section;

FIG. 4 is a circuit diagram of still a further embodiment of the invention, including an associated electromagnet in longitudinal section and g FIG. 5 is a circuit diagram of still another embodiment.

DESCRIPTION OF THE EMBODIMENTS Turning now to FIG. 1, there is shown an electromagnet generally indicated at 1, having a coil 2, and iron cladding, 3, 3' surrounding the coil 2 and an armature 4'axially movable therein. The radial face 4a of armature 4, together with a projection 5 integral with the iron cladding 3 in the coil core defines an air gap 6. A spring 7 is disposed between the projection 5 and the radial face 4a of the armature 4 to urge the latter outwardly thus tending to increase the air gap 6.

A field resistor 8, responsive to the magnetic field strength, is affixed (e.g..glued to the end face of the projection 5. The field resistor 8 may be constituted by a semiconductor element which alters its resistance in the same sense as the change of a traversing magnetic flux. Thus, the voltage drop across the field resistor is a direct measure of the attracting force of the armature. The two terminals of the resistor 8 are broughtout through a bore provided in cladding 3.

The electronic circuit associated with the magnet 1 comprises a regulator part9 and a switch part 10, which are connected through

conductors

12 and 13 to a voltage source such 1 as a battery 11. The regulator part incorporates a resistance bridge circuit formed of the field resistor 8, as well as a fixed resistor 14 and a

variable resistor

15, 16. Between the

resistors

14 and 8 there is disposeda measuring point 148, whereas another measuringpoint 165 is located between the two

resistor parts

16 and 15 of the variable resistor l5, 16. From the battery 11 a constant voltage is applied to a feed point 168 between the

resistors

16 and 8 and to a feed point 145 between

resistors

14 and 15. The two potentials, of which that at 165 maybe arbitrarily set, are compared with each other in the resistancebridge circuit. This tapped voltage is applied through a series resistor 23 to one input E of an amplifier V which has two inputs H5 and E and an output A. The measuring point 148 is connected through the series resistor 24 to the input i E. When the resistance of the field resistor is reduced assuming initial potentials at both measuring

points

148 and 165 the potential at point 148 will increase relative to that at point 165. As a result, the potential increasesat output A relative to the terminal 10 of the amplifier, assuming an initial state of identical potentials. The amplifier output A is connected to the base of a transistor T The increase of potential at the amplifier output A and thus at the base of the transistor T caused by a drop of the resistance of the field resistor 8, allows current to flow through the collector-emitter leg of the transistor T thus driving a power transistor 1,. As a result, the collector-emitter leg of the power transistor T connected in series with the coil 2 of the magnet 1 between the

feed conductors

12 and 13 is rendered conductive and thus the coil feed circuit is closed.

The aforedescribed response to a change in the magnetic field strength in air gap 6 takes place practically without a time lag. Stated in different terms, the coil 2 is energized as Starting from the preceding low magnetic flux, a stronger I magnetic field will thus be progressively built up in the air gap 6 so that, among other effects, the armature '4 is slightly disemplary embodiments taken in conjunction with the drawing. placed towards the projection 5 against the force exerted by the spring 7. The increase of the magnetic field intensity causes an increase of the resistance of the field resistor 8. Such increase, in turn results in a dropping of the potential at the measuring point 148 relative to the potential at the measuring point 165. The dropping of the potential continues as long as the potential at 148 is smaller than the potential at 165. A negative input voltage will thus be applied to the amplifier V and accordingly, the potential at the output A will become increasingly negative relative to the zero point i 0. As the output potential passes the i point value in the direction of negative values at the base of transistor T,, the collectoremitter leg of the latter becomes non-conductive, whereby the power transistor T is cut off. This results in a de-energization of the coil 2.

By virtue of a bypass diode D connected parallel with the coil 2 in the direction of the preceding current flow, the magnetic field in the air gap 6 decays exponentially and relatively slowly. The simultaneous increase of the air gap due to the outward movement of the armature 4 as urged by spring 7, reduces the flux passing through the field resistor 8 and thus, the resistance of the latter drops. This reduction in resistance once again causes the coil 2 to be energized. This results in an increase of the resistance of the field resistor 8 which, in turn, causes the coil 2 to be de-energized, etc.

The aforedescribed regulation of the magnetic field intensity may be regarded as a two-point control which oscillates with a systemic frequency. This frequency comprises squarewave pulses of identical amplitude and represents the on-off switching frequency for the coil current. A magnetic field ex citation of greater or lesser intensity will be needed dependent upon the position of the armature 4 and the pulling force to be exerted by the magnet. Accordingly, a lower or higher frequency will be set by the system. This is governed by the voltage drop which is detemiined by the resistance of the field intensity-responsive resistor 8 and which is compared with a set (desired) potential difference. By virtue of the latter, it is possible in practice to compare and regulate the magnetic flux with another desired value.

A practical application of the magnet according to the invention is illustrated in FIG. 2. The control circuit shown therein is fully equivalent to that illustrated in FIG. 1. In the resistor bridge circuit of FIG. 2, instead of the

variable resistor

15, 16 of FIG. 1, two fixed resistors a and 16a are provided between the two

feed points

145 and 168. The amplifier input E is connected to the variable output of a function generator 14" which may be, for example, a sinusoidal generator adjustable with respect to frequency and amplitude or a generator adapted to supply from a given moment, upon receipt of a command signal, a defined ramp function with adjustable parameters. Or, a tacho generator may be used which produces an rpm-analogous potential difference at the feed point 145 with respect to the other feed point 168.

The magnet l of FIG. 2 is an electrohydraulic transducer wherein the armature is formed of a piston 4' of a pressure limiting valve generally indicated at 17. The displacement of the piston 4 in response to the magnetic field results in a greater or lesser restriction of the volumetric flow delivered by the pump through the throttle formed by the control lands 18 and 19. Depending on the attracting force of the magnet 1 a greater or lesser pressure is built up upstream of the throttle (i.e. in the delivery side of the pump 20). The pressure which is indicated by the pressure gauge 21 may be directed through the connecting conduit 22 to any desired loads and may be limited as to its maximum value by means of the pressure limiting resistor 24. The generated pressure is also transmitted to the radial end face of piston 4' in the air gap 6 through a radial and an axial bore provided in the piston 4'. In this manner, in the air gap 6 a pressure cushion is generated which acts against the attracting force of the magnet. The magnitude of said pressure cushion is immaterial, provided a counterforce is produced which will counteract the attracting force of the magnet. In order to ensure that the forces urging the piston 4 outwardly, and thus the attracting forces generated by the magnet, do not become excessive and that the generated pressure does not affect the entire cross section of the piston, there is provided a reducing pin 7 which is slidably disposed in the axial bore of piston 4' in a fluid tight manner and which,' exposed to the generated pressure, abuts the projection 5. The pressures which may be controlled by the electrohydraulic transducer 1', 17 are very large. Pressures of up to 50 kg/cm or more may be controlled with ease by means of an electromagnet having a weight of approximately 200 g. The oscillation superimposed on the entire system, enables the transducer to respond very rapidly and permits a corresponding output signal to follow with great rapidity the changes in the input values.

Turning now to FIG. 3, in the control system for regulating the magnetic force, a so-called Hall generator 8 is used which, similarly to the field resistor 8 of FIGS. 1 and 2, is also disposed in the air gap 6. The generator 8 requires a constant feed current which is supplied by a voltage source 25. At the two output terminals of the generator 8' there appears a voltage which, assuming a constant feed current, is proportional to the magnetic flux traversing the generator. If the magnet coil fed directly by the power amplifier through a diode D is energized, the Hall generator will supply a voltage which increases with the inward movement of the armature and the corresponding increase of flux density. The amplifier inputs HE, E are connected to two circuits in which current flows in opposite directions. One circuit, formed by the lower resistor part 15' of a variable resistor 15', 16', a series resistor 23' and the amplifier input, is adjustable at will to set the driving potential difference by varying the location of the tapping point The outer circuit is formed by the Hall generator 8' and a series resistor 24. The polarity of the Hall generator in the circuit must be such that the Hall voltage opposes the driving potential difference across the resistor part 15'. When the Hall voltage exceeds the potential difference across the resistor 15, the potential of the point 168' shifts towards the negative range so that an input signal of a polarity in accordance with the terminal designation appears at the amplifier input E, +E. The input signal causes a corresponding amplified potential increase with respect to $0 at the amplifier output A. Because of the blocking effect of the diode D,, the shift of the amplifier output into the positive range causes the magnet coil to be de-energized. In response to the now decreasing magnetic field and the outward movement of the magnet armature 4 as urged by the spring 7, the Hall voltage will drop. At one moment during this process the Hall voltage will become smaller than the voltage increase across the resistor l5, and the point 168' will become positive relative to the other measuring point 148. An input signal with a polarity opposite to that of the temtinal designation will then appear at the amplifier input --E, +E resulting in the appearance at the amplifier output A of a correspondingly amplified powerful potential drop relative to fl) so that the magnet coil 2 is energized through the diode D,. The aforedescribed energization and de-energization is repetitive similarly to the embodiment described in connection with FIG. 1. Here too, a systemic switching frequency will appear.

The desired value of coil excitation for the magnet according to FIG. 3 (i.e. the force to be exerted by the magnet) may be adjusted on the variable resistor 15', 16 or may be preset by a function generator provided instead of the variable resistor similarly to FIG. 2. Or, the auxiliary voltage source 25 may be replaced by a function generator of the kind heretofore described for setting the desired value for coil excitation. The Hall voltage generated by the Hall generator is proportional to the product of its feed current and magnetic flux so that the magnetic intensity can also be affected by the control current which flows through the Hall generator.

By means of the embodiment illustrated in FIG. 4 a voltage responsive to the magnetic field intensity is generated in a different manner. The magnet system is provided with an auxiliary winding 2" disposed within the coil 2'. This auxiliary wind ing may be regarded as the secondary winding of a transformer, the secondary voltage of which depends on the change, with respect to time, of the field line density of the surrounding magnetic field. As already described, during the control of the excitation current a systemic oscillation takes place. The exciter coil 2' is supplied practically only with the positive half waves of a square-wave voltage whose mean value with respect to time is equal to the excitation current required for the specified armature pull. This means that the magnetic field is continuously increased and then decreased through the bypass diode D. The said magnetic field is detected by the auxiliary coil 2" on the terminals of which a voltage appears which is proportional to the change of magnetic flux with respect to time. Since it is desired, however, to ob tain a voltage which is proportional to the flux itself, the voltage delivered by the coil has to be integrated with respect to time. For this purpose there is provided an amplifier V,, the inputs of which are connected with the output terminals of the auxiliary winding 2" and which is associated with a feedback capacitor C. The capacitive feedback of the amplifier output to one of the amplifier inputs gives the amplifier its integrating characteristics. Thus, between measuring points 148" and 168" of the resistance bridge circuit a generator is provided which delivers a voltage proportional to the magnetic flux in the magnet 1". The effect of this generator and the mode of operation of this embodiment is equivalent to that of the precedingly described embodiment.

FIG. 5 shows a practical application of the invention wherein the magnet is void of any separate magnetic field-sensitive transducer. The role of the transducer necessary for the regulation of the excitation current is taken over by the magnet coil itself which is shown as an inductance L and as an ohmic resistance R L is the momentary inductance of the magnet system depending on the position of the armature of the magnet and the coil size, while R is the ohmic resistance of the copper windings. The circuit system is based on the principle that the excitation current in the magnet system can be measured as a voltage drop across a measuring resistor R which is serially connected to the coil L, R,,. This current or the measuring voltage taken from the terminals of the measuring resistor R contains a constant direct voltage component resulting from the voltage drop across the two ohmic resistances R and R in addition to a voltage component which is proportional to the product of the induction and the change of the excitation current and which varies in accordance with the buildup and decay of the magnetic field. The aforenoted constant direct voltage component of the measuring signal initially obtained is first suppressed by means of a differentiating circuit formed of a capacitor 19 and a resistor 30 and is then integrated in a first integrating stage V,, C The output volt age of the latter is proportional to the change of flux in the magnet system L, R This signal is again integrated in a second integrating stage V C to provide a voltage which is proportional to the flux density prevailing in the magnet. Since the circuit is based on a voltage which is proportional to the product of the momentary induction and the momentary excitation current, it follows that the signal obtained from the output of the second integrating stage is, too, dependent upon the position of the armature. Thus, similarly to the previously described embodiments, the armature position too is taken into consideration in the measurement. The signal finally obtained is compared with a desired potential adjustable at a variable resistor 31, and, depending on whether the signal or the desired potential predominates, the coil L, R is connected to or disconnected from the current supply through the switching amplifier V and the two transistors T and T Although the circuit according to FIG. 5 is more complex from a technological point of view, it is advantageous in that the inventive principle can be practiced with conventional electromagnets without any modification of the magnet system itself.

In the different embodiments described hereinabove, the momentary magnetic field strength in the magnet is measured in four different ways and the work coil is energized or deenergized depending on whether the signal characterizing the magnetic field strength is greater or smaller than an adjustable desired value. Since the buildup of a magnetic field or the diminishing of an existing magnetic field are phenomena which have a timely course and since the magnetic field excitation, with preset and closely adjacent values, requires a certain period of time, this period, as proposed by the invention, may be utilized for signal measurement and for comparison with desired values. Depending on the results of the comparison, corrective measures are taken. The inertia inherent in the inductance provides sufficient time for a twopoint regulation which is a feature utilized in accordance with the invention.

It is thus seen that the transistor circuit connected to the output of the amplifier in the embodiments is, in fact, a solidstate on-off power switch which converts the regulating system for the excitation current into a two-point control system. This feature involves two advantages. in the first place, the on-off control limits the tum-on period of the power transistor in the power dissipation range to the required minimum (due to the steep voltage increase at the transistor the range of power dissipation is very rapidly traversed), so that the losses at the power transistor are maintainedat a very small value. In the second place, the magnet armature is subjected to small oscillations which eliminate static friction and hysteresis effects.

What is claimed is:

1. In an electromagnet of the type that includes (a) a magnet coil, (b) an iron cladding surrounding said magnet coil, (c) an armature movable within said coil and defining an air gap with a part of said cladding, (d) means for supplying an excitation current to said coil to generate a magnetic field passing through said air gap and exerting an inwardly directed attracting force to said armature and (e) means exerting an outwardly directed force on said armature; the intensity of said magnetic field being dependent upon the position of said armature from said cladding, the improvement comprising a circuit means for regulating said excitation current; and circuit means including A. a magnetic field intensity-responsive means disposed within said cladding and responding at least indirectly to the intensity of said magnetic field,

B. setting means for obtaining signals corresponding to a desired value of magnetic force,

C. comparator means for comparing the output signals of said magnetic field intensity-responsive means with those of said setting means and A D. switching means for regulating the admission of said excitation current to said magnet coil in response to the output signals of said comparator means for providing in said air gap a magnetic field being of constant magnitude at least as to a mean value with respect to time and being independent from the position of said armature.

2. An improvement as defined in claim 1, wherein said means defined in (A) responds directly to the intensity of said magnetic field and is disposed in said air gap.

3. An improvement as defined in claim 2, including A. a field resistor constituting said magnetic field intensityresponsive means,

B. means for applying the voltage drop across said field resistor to said comparator means for comparing said voltage drop with a desired potential difference prevailing at said setting means and C. an amplifier having input means for receiving the output signals of said comparator means, said amplifier having output means connected to said switching means for supplying the latter with an amplifier output current which is at least decreased when said voltage drop exceeds said desired potential difference and which is increased when said desired potential difference exceeds said voltage drop.

4. An improvement as defined in claim 3, including A. a resistance bridge circuit containing 1. said field resistor,

2. a variable resistor constituting said setting means,

B. means for connecting two diagonal measuring points of said resistance bridge circuit to two inputs of said amplifier,

C. a first transistor having a base to which the output signals of said amplifier are applied; said first transistor having a collector-emitter leg,

D. a second, or power transistor having a base to which the signals of the collector-emitter leg of said first transistor are applied; said second transistor having a collectoremitter leg; said first and second transistors forming part of said switching means and E. a direct voltage source connected to diagonal feed points of said resistance bridge circuit and, through the collector-emitter leg of said second transistor, to said magnet coil.

5. An improvement as defined in claim 1, wherein said means defined in (A) responds indirectly to the intensity of said magnetic field.

6. An improvement as defined in claim 5, including A. an auxiliary winding disposed inside said magnet coil and constituting said magnetic field intensity-responsive means; said auxiliary winding generates output signals induced therein by the excitation current flowing in said magnet coil and Y B. an integrating circuit connected to said auxiliary winding; said integrating circuit is connected to said comparator means for applying thereto its output signals.

7. An improvement as defined in claim 5, wherein said magnetic field intensity-responsive means is constituted by said magnet coil itself; and improvement further includes A. a differentiating circuit connected to said magnet coil to receive therefrom output signals that include a voltage component due to the self-induction in response to the excitation current; said differentiating circuit is adapted to suppress a direct voltage component of the coil output signal due to the ohmic resistance of said coil,

B. a first integrating circuit connected to said differentiating circuit for delivering a voltage proportional to the change of the magnetic flux in said air gap and C. a second integrating circuit connected to said first integrating circuit for delivering a voltage proportional to the product of the momentary value of the excitation current and the momentary value of the inductance of said magnet coil; said last named voltage is applied to said comparator means including said setting means.

Claims

1. In an electromagnet of the type that includes (a) a magnet coil, (b) an iron cladding surrounding said magnet coil, (c) an armature movable within said coil and defining an air gap with a part of said cladding, (d) means for supplying an excitation current to said coil to generate a magnetic field passing through said air gap and exerting an inwardly directed attracting force to said armature and (e) means exerting an outwardly directed force on said armature; the intensity of said magnetic field being dependent upon the position of said armature from said cladding, the improvement comprising a circuit means for regulating said excitation current; and circuit means including A. a magnetic field intensity-responsive means disposed within said cladding and responding at least indirectly to the intensity of said magnetic field, B. setting means for obtaining signals corresponding to a desired value of magnetic force, C. comparator means for comparing the output signals of said magnetic field intensity-responsive means with those of said setting means and D. switching means for regulating the admission of said excitation current to said magnet coil in response to the output signals of said comparator means for providing in said air gap a magnetic field being of constant magnitude at least as to a mean value with respect to time and being independent from the position of said armature.

2. a variable resistor constituting said setting means, B. means for connecting two diagonal measuring points of said resistance bridge circuit to two inputs of said amplifier, C. a first transistor having a base to which the output signals of said amplifier are applied; said first transistor having a collector-emitter leg, D. a second, or power transistor having a base to which the signals of the collector-emitter leg of said first transistor are applied; said second transistor having a collector-emitter leg; said first and second transistors forming part of said switching means and E. a direct voltage source connected to diagonal feed points of said resistance bridge circuit and, through the collector-emitter leg of said second transistor, to said magnet coil.

3. An improvement as defined in claim 2, including A. a field resistor constituting said magnetic field intensity-responsive means, B. means for applying the voltage drop across said field resistor to said comparator means for comparing said voltage drop with a desired potential difference prevailing at said setting means and C. an amplifier having input means for receiving the output signals of said comparator means, said amplifier having output means connected to said switching means for supplying the latter with an amplifier output current which is at least decreased when said voltage drop exceeds said desired potential difference and which is increased when said desired potential difference exceeds said voltage drop.

4. An improvement as defined in claim 3, including A. a resistance bridge circuit containing

6. An improvement as defined in claim 5, including A. an auxiliary winding disposed inside said magnet coil and constituting said magnetic field intensity-responsive means; said auxiliary winding generates output signals induced therein by the excitation current flowing in said magnet coil and B. an integrating circuit connected to said auxiliary winding; said integrating circuit is connected to said comparator means for applying thereto its output signals.

7. An improvement as defined in claim 5, wherein said magnetic field intensity-responsive means is constituted by said magnet coil itself; and improvement further includes A. a differentiating circuit connected to said magnet coil to receive therefrom output signals that include a voltage component due to the self-induction in response to the excitation current; said differentiating circuit is adapted to suppress a direct voltage component of the coil output signal due to the ohmic resistance of said coil, B. a first integrating circuit connected to said differentiating circuit for delivering a voltage proportional to the change of the magnetic flux in said air gap and C. a second integrating circuit connected to said first integrating circuit for delivering a voltage proportional to the product of the momentary value of the excitation current and the momentary value of the inductance of said magnet coil; said last named voltage is applied to said comparator means including said setting means.