US2194285A

US2194285A - Electrical voltage transformation apparatus

Info

Publication number: US2194285A
Application number: US70283A
Authority: US
Inventors: Edgar D Lilja
Original assignee: Barber Colman Co
Current assignee: Barber Colman Co
Priority date: 1936-03-23
Filing date: 1936-03-23
Publication date: 1940-03-19
Anticipated expiration: 1957-03-19

Description

E. D. LILJA March 19, 1940.

ELECTRICAL VOLTAGE TRANSFORMATION APPARATUS Filed March 23, 1936 3 Sheets-Sheet l INVENTOR Edgar .17. Lil a BY m f V ATTORNEY E. D. LILJA March 19, 1940.

ELECTRICAL VOLTAGE TRANSFORMATION APPARATUS Filed March 2:5, 193s 3 Sheets-Sheet 2 V N .r 4......

E. D. LILJA March 19, 1940.

ELECTRICAL VOLTAGE TRANSFORMATION APPARATUS Filed March 23, 1936 3 Sheets-Sheet 5 T '29 7. L/u: Var/m5 EXC/TAT/ON C URVES AT IVOLOAD TRA MsFoRMER NAL HANSFORMKR NORMAL OPERA TING V04. TA 6 E FULL Lona EXCIT'AT/ON In C l/RRENT PERCENT or J' g- FLux .DENSITF-EXC/TATION CURVE /MPROVD TRA NJFORMER Com :11 TIC/VAL TRA NJFORMA'R Alan/v41. UPERA TI/VG POINT INVENTOR Edgar D. Li lja BY ATTORNEYS W/WWWMMMMW I I I 100 200 PRIMARY AM PERE Tun/vs (Rms) Patented Mar. 19, 1940 UNITED STATES PATENT OFFICE ELECTRICAL VOLTAGE TRANSFORMATION APPARATUS Application March 23, 1936, Serial No. 70,283

7 Claims.

My invention relates to electrical voltage trans formation apparatus.

Electrical voltage transformation apparatus, embodying my invention, is particularly useful for supplying low voltage current to the control circuits of electrical temperature control units utilized to control oil burners or similar elements of household heating systems, although my invention is by no means limited thereto but is, on the other hand, susceptible of a wide variety of uses. In such installations, it has heretofore been the common practice to use an ordinary stepdown static transformer, as the principal element of the electrical voltage transformation apparatus, to supply alternating current at a relatively low voltage of approximately twenty volts; for example, from an ordinary household lighting circuit normally operating at approximately 110 volts.

Particular precautions must be taken in installations of this type in order to avoid fire hazards and the like resulting from abnormal circuit conditions either in the voltage transformation apparatus itself or in the secondary circuit to which the apparatus supplies low voltage current. I have found that installations of the type described using conventional transformers are seriously objectionable in that if the secondary circuit of the transformer is opened when fullload current or short-circuit current is flowing therethrough, the interruption or opening of the circuit may result in an arc of comparatively high ignition value. A dangerous fire hazard may thus be created if any of the conductors in the secondary circuit are broken. In addition, the secondary voltage may rise to 9. dangerously high value when the secondary circuit is opened or the primary current may become so large upon short-circuiting of the secondary circuit that dangerous overheating of the transformer results. My improved electrical voltage transformation apparatus is designed to overcome the dangerous conditions resulting from such abnormal circuit conditions.

Two general types of conventional transformers have heretofore been proposed for use in installations of the type noted. These types of transformers are commonly designated as low reactance and high reactance type transformers in. view of the respective inductive reactance characteristics thereof. It has been found, however,

that although the high reactance type transformers may not overheat when the secondary circuits thereof are short-circuited, severe arcing 55 does result upon opening of the secondary circuit when short-circuit current is flowing therethrough and the no-load voltage is frequently high. On the other hand, a low reactance type transformer will be badly overloaded and consequently overheated upon short-circuiting of its 5 secondary winding and severe arcing will occur upon opening of the secondary circuit when short-circuit current is flowing therethrough even though the no-load voltage of such a transformer is only slightly higher than the full-load l0 voltage. My improved voltage transformation apparatus is designed to obviate these objectionable operating characteristics of such high reactance or low reactance type transformers heretofore used.

It is an object of my invention to provide an improved electrical voltage transformation apparatus which efficiently supplies current at a desired higher or lower voltage than the voltage of the source of supply to a secondary circuit and 20 which at the same time effectually limits the voltage and current in the secondary circuit to predetermined safe-values upon the occurrence of abnormal circuit conditions within either the secondary circuit or the transformation apparatus itself.

A further object of my invention is to provide an electrical voltage transformation apparatus including an improved arrangement for limiting the current flowing in the secondary circuit to a safe value when the secondary circuit is shortcircuited.

A further object of my invention is to provide an electrical voltage transformation apparatus including an improved arrangement for limiting the voltage induced in the secondary circuit to a safe value upon interruption of the secondary circuit when the secondary circuit is operating at either full-load or under short-circuit conditions.

A further object of my invention is to provide an electrical voltage transformation apparatus utilizing a capacitance connected in series relation in the primary circuit of the transformation apparatus and having a capacitive reactance substantially equal to the apparent fullload inductive reactance of the primary circuit for increasing the impedance of the primary circuit when the impedance of the secondary cir- 50 cuit is varied from its full-load value.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to the accompanying drawings in which,

Figure 1 is a side elevation of a voltage transformation unit embodying my invention, a portion of the front wall of the casing thereof being broken away.

Fig. 2 is a side elevation of the transformer included in the apparatus shown in Fig. 1.

Fig. 3 is a sectional view along the line 3-3 of the transformer shown in Fig. 2.

Fig. 4 is a wiring diagram of the apparatus shown in Fig. 1.

Fig. 5 is a wiring diagram of a modified form of voltage transformation apparatus embodying my invention.

Fig. 6 is a wiring diagram of another modified form of voltage transformation apparatus embodying my invention.

Fig. 7 is a graphic illustration of the no-load line voltage and excitation characteristics of the voltage transformation apparatus, shown in Fig. l, as compared to the corresponding characteristics of a similar apparatus provided with a conventional transformer.

Fig. 8 is a graphical representation of the excitation characteristics of my improved transformer, included in the apparatus shown in Fig. l, as compared to the excitation characteristics of a conventional transformer.

.Referring to the drawings, I have shown in Fig. l a voltage transformation apparatus embodying my invention. In the particular form illustrated, the parts of the apparatus have been designed for use in supplying low voltage current from an ordinary household. lighting system to the con trol circuits of electrical temperature regulating devices or the like. The parts of the apparatus have been arranged in a compact unitary structure, which is especially rugged in construction and protects them from damage as well as afiording protection to the user against accidental contact with any high voltage electrical elements of the apparatus.

In general, electrical voltage transformation apparatus, embodying my invention, includes as its principal elements a transformer having inductively coupled primary and secondary circuits and means including a capacitance associated with the primary circuit for minimizing the impedance of the primary circuit when the secondary circuit is subjected to full load, and for avoiding any substantial decrease in the impedance of the primary circuit when the impedance of the secondary circuit is varied from its full-load value. That is, the Value of the capacitive reactance of the condenser is preferably so chosen with respect to the apparent inductive reactance of the primary winding of the transformer at full-load that these two reactances substantially balance each other when the apparatus is operating at full-load. By the term apparent inductive reactance of the primary, I mean the total inductive effect on the primary circuit resulting not only from the self-inductance of the primary winding itself but also from the mutual inductance of the secondary winding as well, which in turn reflects the efiect of the load. Since the load connected to the secondary winding will, in conjunction with the impedance of the secondary winding itself, determine the current flowing through the secondary winding, this load will also afiect the mutual in aieaaes ductive efiect of the secondary winding and in turn the apparent inductive reactance of the primary winding. It will thus be seen that the apparent inductive reactance of the primary winding varies with changes in load imposed upon the secondary winding and that consequently a critical value of capacitive reactance is to be used which balances the apparent inductive reactance of the primary winding at full load.

When a condenser, having a capacitive reactance substantially equal to the apparent inductive reactance of the primary winding at full load, is connected in series therewith, the circuit will be tuned substantially to resonance at full load. That is, the reactance of the primary circuit will be substantially zero. As a result, when any change in the load on the system is made from the normal full-load value, the reactance of the primary circuit will be increased or at worst decreased only a small amount. For example, if the secondary winding of the transformer is short-circuited, the phase relation of the secondary current will be changed, thus shifting the phase relation of the magnetomotive force of the secondary winding, which is in opposition to the magnetomotive force of the primary winding, and thereby changing the net magnetomotive force producing flux in the core and consequently changing the apparent inductive reactance of the primary winding. Then, since the inductive reactance of the primary winding no longer exactly balances the capacitive reactance of the condenser, the primary circuit will have a comparatively large inductive or capacitive reactance and the impedance of the primary circuit is little, if any, lower than at full load. Similarly, when the secondary winding of the transformer is open-circuited, no current flows through the secondary winding and the flux induced by the current flowing through the pri mary winding is unopposed. As a result, the apparent inductive reactance of the primary winding is increased and exceeds the capacitive reactance of the condenser so that they no longer balance. In general, then, the impedance of the primary circuit is at or near its minimum when full-load current is supplied from the secondary winding of the transformer and the impedance of the primary circuit is usually increased, and

at worst decreased very little, thus limiting the current flowing therethrough, whenever the load imposed on the secondary circuit is either increased or decreased from its full-load value. The effect of so limiting the current on shortcircuit is to reduce the input power to a small part of its full-load value.

Referring to the drawings, I have shown in Fig. l a voltage transformation apparatus embodying my invention. The particular apparatus illustrated has been designed for use in supplying low voltage current from an ordinary household lighting system to the electrical control circuits of temperature regulating devices or the like. The voltage transformation apparatus shown in Fig. 1 includes a step-down transformer ill seecured to the detachable cover ll of a rectangular sheet metal casing 32 by through bolts l3. A transverse sheet metal partition 54 extends across the interior of the casing 22 and divides the same into compartments l5 and 36, respectively. The lower end of the transformer 50 extends into the compartment it through a suitable aperture formed in the cover ii and the current supply leads H and iii of the primary winding thereof are thus located within the compartment l5 of the casing l2. The lead. l1 connects the primary winding of the transformer III with one terminal l9 of a static condenser 20 which is secured in the bottom of the casing l2 by a strap 2|. The other lead [8 of the primary winding of the transformer l0 extends through an aperture 22 in the partition l4 and then through an insulating bushing 23 surrounding the edges of an aperture in the cover H. The lead l8 thus connects the primary winding of the transformer II! to one side of a suitable supply line such as a household lighting circuit. The other side of the supply line is connected to the voltage transformation apparatus by a lead 24 which also extends through the insulating bushing 23 and is connected to a terminal 25 of a suitable disconnecting switch 26 mounted in the compartment l6 of the casing 12. The switch 26 may be operated by a manual operating handle or button 21 to open and close the same and thus connect or disconnect the transformer IO to the supply line. The other terminal 28 of the switch 26 is connected to a second terminal 29 of the condenser 20 by a conductor 30 extending through an aperture 3| formed in the partition i4. A layer of insulating material l4 is preferably positioned on the inner side of the partition 14 to prevent short-circuiting of the condenser terminals i9 and 29 upon accidental contact thereof with the metal partition 14. It will thus be seenthat the parts of the voltage transformation apparatus shown in Fig. l have been arranged in a compact unitary structure which is especially rugged in construction and which protects such parts from damage as well as affording protection to the user against accidental contact with any high voltage electrical elements of the apparatus.

For reasons set forth hereinafter in greater detail, it is desirable that the transformer 10 used in my improved voltage transformation apparatus should have a magnetic core structure of such character that it will not be saturated during any conditions of operation likely to be encountered; that is, the flux produced in the core should bear a substantially constant ratio to the magnetizing current even during wide variations in the magnetizing current. In order to provide the desired magnetic characteristics in the core of the transformer I ll, I preferably provide an air gap therein, that is, a non-magnetic section, and I also utilize a core of sufficiently large cross sectional area that it operates at a comparatively low flux density.

Referring to Figs. 2 and 3, it will be seen that the transformer 10 includes a shell-type core 32 provided with a rectangular frame member 33 made up of thin substantially rectangular superposed laminations or core sections of magnetic sheet iron. Rectangular openings 34 are formed in the laminations and a vertical leg or core member 35 extends across the central portion of the openings 34. The leg 35 is made up of thin superposed laminations of magnetic iron of substantially the same thickness as the laminations in the frame member 33. The opposite ends of the leg 35 are v-shaped and the lower end 36 thereof is press fitted in a complementary V-shaped notch 31 formed in the frame 33. A similar V-shaped notch 38 is formed in the frame 33 on the opposite side of the opening 34 and receives the V-shaped end 39 of the core leg 35. The faces of the end 39 of the core leg 35 are cut off a small amount in order to provide a clearance v of approximately .002 inch between the upper end 39 of the leg 35 and adjacent surfaces of the notch 38 formed in the laminations 33. Thin shims 40 of brass or other non-magnetic material are inserted in the air gap or space between the adjacent surfaces of the end 39 of the core leg 35 and the notch 38 in order to prevent longitudinal displacement of the core leg 35.

The core leg 35 and shims 40 are press fitted into position within the notches 31 and 38 formed in the laminations 33 and the through belts I3 are inserted in holes formed-.,by complementary half-round grooves 4| and 42 formed at the base of the notches 31 and 38 and in the

ends

36 and 39 of the core leg 35, respectively, to hold the parts of the magnetic core 32 rigidly in position. These bolts l3 also pass through registering holes formed in. a cup-shaped sheet metal end shield 43 arranged on the upper side of the transformer and serve to hold the same in position thereon. It will be noted that the lateral edges of the end shield 43 register with the adjacent edges of the laminations 33.

The transformer I0 is provided with inductively coupled primary and

secondary windings

44 and 45 which are arranged concentrically about the core leg 35 and mounted thereon. A layer of insulation 46 is interposed between the adjacent inner surfaces of the secondary winding 45 and the lateral surfaces of the core leg 35. Also a layer of paper 41 or similar insulating material is interposed between the adjacent superposed surfaces of the primary and

secondary windings

44 and 45. The secondary winding 45 is provided with a relatively smallnumber of turns as compared with the primary winding 44 so that when a relatively large voltage is impressed on the primary winding 44, a relatively small voltage will be induced in the secondary winding 45.

The primary winding 44 is provided with leads l1 and 18 through which electrical energy is supplied thereto from a suitable source of alternating current such as a domestic lighting system. As is shown in the wiring diagram in Fig. 4, the condenser 20 is connected in series relation with the primary winding of the transformer Ill. The value of the capacitive reactance of the condenser 29 is so designed during the manufacture of the apparatus that it is substantially equal to the apparent inductive reactance of the primary winding 44 of the transformer l0 when full-load current is flowing through the second-' ary winding 45 thereof. In other words, the impedance characteristics of the transformer and condenser are so related that a condition of series resonance will be had at full-load, as was described above. In the particular voltage transformation apparatus illustrated, which is adapted to supply approximately 50 volt amperes at 25 volts and at a lagging power factor of about 55 percent from a volt 60 cycle supply circuit, I have found that satisfactory operation is had if a condenser of approximately 5 microfarads tion and disconnection of the load from the secondary winding I have found that if a condenser is connected in series relation with the primary winding of a'conventional transformer in such manner that the primary circuit is in series resonance when the apparatus is operating under full load, undesirable or abnormal voltage and current conditions are likely to occur during certain conditions of operation of the apparatus. Because these abnormal voltage and current conditions arise from the effects of certain transient electrical phenomena, their existence would not be readily apparent from a theoretical analysis of the steady state conditions prevailing in such an apparatus. This abnormal operation may best be understood by considering the operation of an apparatus diagrammatically illustrated in'Fig. 4 when a conventional transformer is substituted for my improved transformer in therein. Such a conventional transformer might be similar to the transformer ill except that no air gap would be provided therein and the cross-sectional area of the core would be relatively smaller so that the flux density therein would be relatively high.

The excitation characteristics of such a conventional transformer are indicated by curve 55 in Fig. 8, which shows the relation of the peak flux density in kilolines per square inch to the primary ampere-turns of the transformer. The normal operating point for such a transformer is indicated at 55 on the curve 55 showing that the flux density is approximately 70 kilolines per square inch and comparatively close to the saturation point of the transformer core. It should be noted that the bending of the curve 55, as the primary ampere turns are increased, indicates that after a flux density of approximately kilolines per square inch is obtained, comparatively little increase in flux density is had even though the magnetizing current or primary ampere turns are greatly increased.

As the capacitive reactance of the condenser, to be connected in series relation with the pri mary winding of the hypothetical conventional transformer, is designed to equal the apparent inductive reactance of the primary winding at full load, the primary circuit will be substantially in series resonance at full load. Consequently, whenever the loadcurrent is increased or de= creased, the reactance of the primary circuit will be increased, thus limiting the current flowing therethrough. I have found, however, that if the secondary circuit of the conventional transformer is open circuited when full-load or shortcircuit current is flowing therethrough, the primary current and secondary voltage of the conventional transformer may rise to dangerously high values, these values being sustained so long as the primary circuit remains energized.

This action of the conventional transformer upon opening of its secondary circuit under load may best be understood by referring to Fig. 7 as well as Fig. 8 noted above. Curve 5? in Fig. 7 shows the relation of the linevoltage to the excitation current in percent of full-load current when the apparatus is operating at no-load, that is, with the secondary circuit open. From an inspection of curve 5?, it will be seen that the circuit exhibits negative impedance characteristics from points 58 to 59. Between these points the line voltage required to maintain a given current decreases as the current increases. Beyond point 55}, an increase in voltage is again required to increase the current This negative greases impedance characteristic is probably due to the fact that the impedance of the conventional. transformer decreases from points 5% to 59 upon an increase of current flowing therethrough. The decrease in impedance of the transformer results from magnetic saturation of the transformer core, for upon referring to Fig. 8 it will be seen that when the excitation current is in creased above the normal operating point the flux induced in the transformer core increases a comparatively small amount. Thus, since the reactance of the primary winding of the transformer depends upon the amount of flux induced in the transformer core, the reactance of the transformer will decrease after saturation of the core has been reached. The saturation of the transformer core is accompanied by a distortion of the wave form of the alternating current flowing through the primary circuit so that high fre quency harmonics are introduced into the current. The capacitive reactance of the condenser varies inversely as the frequency of the current flowing therethrough and consequently offers little impedance to the high harmonic currents resulting from the distortion in wave form of the primary current. It is believed that this decreased reactance of the condenser is responsible for prolonging the negative impedance effect initiated by the conventional transformer. Beyond point 59 on curve 57, the impedance of the primary circuit again increases because the substantially non-distorting resistance and leakage reactance of the transformer form a greater part of the total impedance thereof and predominate over the effect of the core-flux reactance in determining the transformer impedance and exciting current-wave form.

It will thus be seen that a voltage transformation apparatus embodying a transformer and a condenser connected in series with the primary thereof has a negative impedance characteristic of such form that there is a comparatively large range of line voltage over which the no-load primary current may have more than one value;

for example, a conventional transformer in. such an apparatus might, under no-load conditions, operate at either points 60 or 6| on curve 61. If current is first supplied to the primary winding of the conventional transformer with the secondary winding open-circuited, operation would ordinarily take place at point 60 and the primary winding would draw a no-load exciting current corresponding to the value indicated at point 6% on curve 51. On the other hand, if the conventional transformer is operated at full load and the secondary circuit open-circuited during such time, the primary winding may in the subsequent no-load operation draw an exciting current corresponding to the value indicated at either point 60 or point 6| indicated on curve 57. If the alternating exciting current happened to be passing through the zero point at the instant the load was removed from the secondary circuit, no-load operation would probably be had with a primary exciting current corresponding to the i value indicated at point 60, which may conveniently be termed the normal no-load exciting current. If, however, the instantaneous primary current at the time of interruption of the sec ondary circuit is comparatively large, no-load 70 operation would probably occur with an abnormal primary exciting current of a value corresponding to point (H on curve 51. This result probably obtains because the magnetomotive force produced by the load component of the primary current is momentarily unopposed by the magnetomotive force produced by the secondary current, the latter having been cut off upon open-circuiting the secondary winding The unopposed flux thus produced by the load component of the primary current saturates the transformer core and establishes no-load operation at the abnormally high no-load exciting current value indicated by point 6| on curve 51. Consequently, a high secondary voltage would appear. and severe arcing would take place during the opening of the switch 54. It will be seen that the value of the exciting current at point 6| on curve 51 is more than five times the value of the full-load current and the transformer would, therefore, soon be overheated. In addition, a dangerous over-voltage would be imposed on the condenser connected in the primary circuit since the circuit is at or near resonance and the current is abnormally high so the voltage across the condenser may be several times as great as the line voltage. Moreover, the high secondary voltage appearing at the terminals of the secondary winding of the transformer, because of the abnormally high primary exciting current, may result in a hazardous condition.

The improved transformer, which I have illustrated in connection with the preferred embodiment of my invention described above, is particularly effective in obviating the abnormal voltage and current condition encountered with the conventional transformer described above. Although it embodies other improvements in design and construction, the two fundamental characteristics of my transformer, which are especially effective in obviating the defects of operation described, are that my improved transformer normally operates with a comparatively low flux density in its core and draws a substantially unf distorted magnetizing current because of the air gap in the core.

In general, the low flux density of the core is the most important factor and if it is had, im proved operation will result even though the air gap is omitted. The excitation characteristics of curve 55 for a conventional transformer.

my improved transformer are indicated by curve 53 in Fig. 8. The greater reluctance of the ma netic core used in my transformer causes the curve 63 to fallsomewhat below the excitation That is, a greater excitation is required to produce a flux of the same density in the core. Also, the comparatively large cross-sectional area of the magnetic core used in my improved transformer makes it possible normally to operate the same under full load at the comparatively low flux density of approximately 30 kilolines per square inch, as indicated at point 54 on curve 63. The altered magnetic characteristics of my improved transformer as compared with those of a conventional transformer improve its operation at no load and at normal full load, and-also prevent the abnormal conditions referred to above in connection with the operation of a conventional transformer.

The improvement in operation effected by the change in magnetic characteristics in my improved transformer at normal full load results from the fact that the transformer is operated at a lower flux density and therefore, with lower core losses than a conventional transformer of the same general size and rating. In addition, the copper losses in my improved transformer are lower since the number of turns in the windings is decreased. The no-load losses in my improved transformer are low because at no load, the voltage impressed on the transformer is considerably below that at full load and the flux density is correspondingly reduced below the normal full-load value of 30 kilolines per square inch.

The most important effect of the change in magnetic characteristics utilized in my improved transformer is in preventing the conditions of abnormal operation referred to above with respect to the operation of a conventional transformer. This improved operation may best be understood by reference to Fig. '7 in which the relation between line voltage and excitation current expressed in percent of full-load current is illustrated by curve 65. Upon reference to this curve it will be seen that my'improved transformer also displays a negative impedance characteristic. For example, between points 66 and 61 on curve 65 the excitation current increases as the line voltage is decreased. There is one important difference, however, in that the low point 61 is now of a value greater than the normal operating or line voltage applied to the transformer. In fact, upon reference to Fig. '7 it will be seen that the point 61 represents a very much higher voltage than the corresponding point 59 on curve 51 for the conventional transformer. This difference in operating characteristics probably results primarily from the fact that the lesser density of flux in the transformer core reduces the drop in transformer impedance. It also results in less current wave distortion and consequently minimizes the change in capacitive reactance of the condenser. Furthermore, when the secondary circuit is opened under full load, for example, the transient counter electromotive force or voltage induced in the primary winding of the transformer during the opening of the secondary circuit is higher and more effective in reducing the primary current from its fullload value to its normal no-load value indicated at point 62, in view of the fact that the transformer core does not become saturated even when the primary current is greatly increased. The subnormally saturated transformer core, which I have provided, thus constitutes a means utilizing a transient counter electromotive force generated in the primary winding upon disconnection of the load from the secondary winding to prevent the sustained flow of an abnormal primary exciting current of substantially greater value than the normal no-load primary exciting current.

My improved voltage transformation apparatus is also especially effective in limiting the primary current to a safe value when the secondary circuit is either accidentally or deliberately short-circuited. Thus if the secondary winding 45 of the transformer I0 is short-circuited, the phase relation of the secondary current will be changed and the apparent inductive reactance of the primary winding will be correspondingly changed. As a result, the capacitive reactance of the condenser 20 is no longer balanced by the inductive reactance of the primary winding 44 of the transformer so that the total impedance of the primary circuit is increased, thus limiting the primary'current to a comparatively low value when the secondary circuit is short-circuited. Consequently, if a short circuit occurs in the secondary circuit of the voltage transformation apparatus described above the power supplied to the apparatus is decreased,

or similar protective devices.

through leads 172 and 73.

It is also unnecessary to provide any fuses or other protective devices for my improved voltage transformation apparatus to guard against possible harmful results, caused by a failure of any of the elements of the primary circuit. That is, if either the primary winding fi l of the transformer ll] of the condenser are open-circuited, no current will iiow through the primary circuit and the apparatus will simply become inoperative. If, on the other hand, the primary Winding dd of the transformer it is short circuited the capacitive reactance of the condenser will no longer be balanced by the apparent inductive reactance of the primary winding of the transformer and the resulting increase in total impedance of the primary circuit will limit the current flowing therein to a comparatively small value. Conversely, if the condenser 29 is short-circuited the apparent inductive reactance of the primary winding dd will no longer be balanced by the capacitive reactance of the condenser 2D and as a result, the current flowing in the primary circuit will be limited to a comparatively small value, due to the increase in total impedance of the primary circuit.

Moreover, my improved voltage transformation apparatus improves the power factor of the circuit to which it is connected. This improvement in power factor is brought about by the leading current drawn by the condenser 20 and in this respect, my improved voltage transformation apparatus is particularly desirable as compared to the conventional transformer supply arrangement, which draws a lagging current. The improvement in power factor is especially important when several voltage transformation units are used to supply a variety of control circuits in any particular installation.

In Fig. 5 I have illustrated the wiring diagram of a modified form of voltage transformation apparatus embodying my invention. The arrangement shown in r'g. 5 includes a conventional step down transformer provided with a primary winding and a secondary winding 710, inductively coupled through a magnetic core 7]. The primary winding is provided with a relatively large number of turns as compared to the secondary winding id, in order that the secondary voltage induced in the winding "1U will be relatively small as compared to the voltage impressed on the primary winding Elem trical energy is supplied to the primary winding til through a suitable source of alternating current, such as a household lighting circuit, A suitable manually operable switch id is inserted in the lead in order to facilitate connection and disconnection of the voltage transformation apparatus from the supply circuit.

A condenser id is connected to the lead 13 in series relation with the primary winding lid of the transformer 68. The capacity of the condenser 15 is so chosen that the capacitive reactance of the condenser is substantially equal to the apparent inductive reactance of the primary winding 69 when the apparatus is operating under full, load. Consequently. the impedance of the primary circuit is near its minimum at full load and is decreased little, if any, whenever the load on the secondary circuit is varied from its full-load value as was described above with respect to the apparatus shown in Figs. 1-4, inclusive.

The secondary winding it is connected to a suitable load through leads l6 and Ti. In Fig. 5

arouses this load is diagrammatically illustrated as an electric motor provided with 2. held winding "13 and a rotor A manually operable switch til is inserted in the lead 'l'l to facilitate connection and disconnection of the load from the secondary winding it.

As was pointed out in detail above, if a conventional transformer is utilized with a condenser connected in series relation with the primary winding thereof, the primary circuit being adjusted for series resonance at full load, dificulty may be encountered in the operation of the apparatus when the secondary circuit is interrupted if full-load or. short-circuit current is flowing therethrough. For the reasons pointed out above, the primary current as well as the secondary voltage may rise to dangerously high values under such circumstances. In order to avoid such abnormal operation I have provided a resistance, which is associated with the primary cir uit.

In the arrangement shown in Fig. 5 a resistance 3] is preferably connected in series relation with the condenser 75 and primary winding of the transformer :63. troduces an additional voltage drop in the primary circuit and thereby lowers the voltage impressed across the condenser 15 and primary winding respectively. Since the voltage drop across the resistance SJ is the product of its resistance and the current flowing therethroug this voltage drop increases proportionately as the primary current increases. That is, the voltage impressed on the primary winding '39 and condenser is progressively decreased as the primary current increases. Turning now to curve in Fig. l, which represents the relation of line voltage to excitation current for a conventional transformer having a condenser connected in series with the primary thereof, it will be seen i that the effect of the resistance :Si is to move the point on the curve 57 upward and to the left. That is, the steadily increasing voltage drop across the resistance 8] with increasing primary current tends to counterbalance the negative impedan e characteristic of the primary cire cult shown in curve between the points 53 and Consequently, the resistance may be given a sirfficiently large value so that the point is raised to a value greater than the normal operating voltage applied to the primary circuit. Consequently, abnormal nc-load operation will he entirely prevented when such a value of resistance is used. In other words, when such a resistance is inserted in the primary circuit, the

no-load primary exciting current will be limited to a value substantially equal to that indicated at point ti) on curve 57, even when the secondary circuit is open-circuited either under load, or under short-circuit conditions.

I have shown in Fig. 6 a second modified form of voltage transformation apparatus embodying my invention and utilizing a conventional transformer. The arrangement shown in Fig. 6 is very similar to that shown in Fig. 5 with the oneimportant difference that a resistance is con nected in parallel with the condenser in the primary circuit rather than in series therewith. The same numerals have been used to indicate identical parts in Fig. 6 corresponding with those shown in Fig. 5. Thusa conventional step-down transformer 68 is provided, having primary and secondary windings B9 and lid, respectively, inductively coupled through a magnetic core H. Electrical energy is supplied to the primary wind- The resistance in- 1 N A condenser 15 is connected in series relation with the primary winding 69 of the transformer 68. As was described above with respect to the apparatus shown in Figs. 1-4, inclusive, as well as Fig. 5, the capacity of the condenser is so chosen with respect to the apparent inductive reactance of the primary winding that the primary circuit is in series resonance at full load.

In order to avoid abnormal voltage and current conditions upon interruption of the secondary circuit when the apparatus is either shortcircuited on the secondary side or is operating under full load, a resistance 82 is connected in parallel relation with the condenser 75 and thus shunts the same. The effect of the resistance 82 shown in Fig. 6 is very similar to the effect of the resistance 8| shown in Fig. 5; that is, the circuit is less sharply resonant. The resistance r 82 is made small enough to decrease the negative impedance characteristic of the primary circuit to such an extent that the point 59 on the curve 51 in Fig. 7 is moved upwardly and to the left until it attains a value greater than the normal operating voltage for the circuit. As a result, abnormal no-load operation is entirely prevented so that when the secondary circuit is open-circuited, the primary exciting current will be limited to its normal small value indicated by the point 60 on curve 51 in Fig, 7 even though full-load or short-circuit current is flowing through the secondary circuit at the instant of interruption.

The arrangements shown in Figs. and 6 are each effective in limiting the primary current to a safe value when the secondary circuit is short-circuited. If the secondary circuit is shortcircuited for any reason the change in phase relation of the current flowing therein and thereby changes the apparent inductive reactance of the transformer primary winding. As a result, the capacitive reactanceof the condenser is no longer balanced by the apparent inductive reactance of the transformer primary. The impedance of the primary circuit is consequently increased and the primary current is thus limited to a safe value. It will be seen, however, that the arrangements shown in Figs. 5 and 6 are not so effective as that shown in Figs. 1 to 4, inclusive, in limiting the primary current in case of a short circuit in the secondary because the resistances inserted in the primary circuit make up a portion of the total impedance thereof. This portion of the primary circuit is unaffected by changes in the inductive or capacitive reactance of the circuit so that the tuning of the primary circuit is less sharp and the impedance shows less relative change with changes in the inductive or capacitive reactance of the circuit. The currents are consequently greater, upon short-circuiting the secondary circuit, in the apparatus shown in Figs. 5 and 6 than in that illustrated in Figs. 1 to 4, inclusive.

It will be understood that the parts of the voltage transformation apparatus shown either in Fig. 5 or Fig. 6 may be mounted in a compact unitary structure or a unit substantially like that shown in Fig. 1 if so desired.

Although I have shown certain specific embodiments of my invention which are particularly adapted for use in supplying energy to temperature control units for household heating systems, it will be understood that I do not desire to limit my invention to the particular construction shown and described, but intend, in the appended claims, to cover all modifications within the spirit and scope of my invention.

I claim as my invention:

1. An electrical; voltage transformation apparatus comprising, in combination, a primary circuit, means for connecting said primary cir-.

cuit to a source of alternating current, a secondary circuit, means including a magnetic core associated with said primary and secondary cir-. ,cuits for inductively coupling the same and for minimizing the open-circuit voltage induced in said secondary circuit, said magnetic core having ample cross-sectional area to operate at relatively low flux density when all load values of current from zero to full load are flowing through said secondary circuit, and means including a capacity associated with said primary circuit for minimizing the impedance thereof when said secondary circuit is operating at full-load and for automatically preventing a substantial decrease in the impedance of said primary circuit when the impedance of said secondary circuit is varied from its full-load value.

2. An electrical voltage transformation apparatus comprising, in combination, inductively coupled primary and secondary windings, means for connecting said primary winding to a source of alternating current, means for supplying alternating current from said secondary winding to a load, and a condenser connected in series relation with said primary winding, said condenser having a capacitive reactance substantially equal to the apparent inductive reactance of said primary winding .when full-load current is supplied from said secondary winding to the load, and means for limiting the open-circuit voltage induced in said secondary winding to a value substantially less than the full-load value of said voltage.

3. An electrical voltage transformation apparatus comprising, in combination, a transformer provided with inductively coupled primary and secondary windings, means including a condenser associated with said primary winding for minimizing the short-circuit voltage induced in said secondary winding, and means for minimizing the open-circuit voltage induced in said secondary winding.

4. An electrical voltage transformation apparatus comprising, in combination, juxtaposed primary and secondary windings, means for connecting said primary winding to a source of alternating current, means for supplying current from said secondary winding to a load, means including a magnetic core extending axially through said windings and forming a substantially continuous magnetic path about the same for inductively coupling said windings, means including a narrow non-magnetic section formed means including a condenser connected. in series relation with said primary winding for minimizing the impedance thereof when said secondary circuit is operating at fu1l-load and for automatically preventing a substantial decrease in the impedance of said. primary circuitwhen the impedance of said secondary circuit is varied from its full-load value, said condenser having a capacitive reactance substantially equal to the apparent inductive reactance of said primary winding when full-load current is supplied from said secondary winding to the load.

,5. An electrical voltage transformation ap paratus comprising, in combination, inductively coupled primary and secondary windings, means for connecting said primary winding to a source of alternating current, means for supplying current from said secondary winding to a load, a condenser connected in series relation with said primary winding, said condenser having a capacitive reactance substantially equal to the apparent inductive reactance of said primary winding when full-load current is supplied from said secondary winding to the load, and means utilizing a transient counter electromotive force generated in said primary winding upon disconnection of the load from said secondary winding for preventing the sustained flow of a primary exciting current of substantially greater value than the normal no-load primary exciting current.

6. An electrical voltage transformation apparatus comprising, in combination, a magnetic core, inductively coupled primary and secondary windings mounted on said core, means for conmeeting said primary winding to a source of alternating current, means for supplying current from said secondary winding to a load, a condenser connected in series relation with said primary winding; said condenser having a capacitive reactance substantially equal to the apparent inductive reactance of said. primary winding when full-load current is supplied from said secondary winding to the load, and means including a substantially non-inductive resistance associated with said primary winding for minimizing the no-load exciting current of said primary wLnding.

7. An electrical voltage transformation apparatus comprising, in combination, a primary winding, means for connecting said primary winding to a source of alternating current, a secondary winding, magnetic means coupling said primary and secondary windings, said magnetic coupling means comprising a metallic core having a cross-sectional area to provide a relatively low flux density in said core for all conditions of operation from zero to full load, and a condenser connected in series relation with said primary winding having a capacitive reactance substantially equal to the apparent inductive reactance of said primary winding when full load current is supplied to the load by said secondary winding.

EDGAR. D. LILJA.