US1635002A - Continuous-current transformer - Google Patents

Description

July 5, 1927. 1,635,002

- T. C LENNOX CONTINUOUS CURRENT TRANSFORMER Filed Aug. 20, 1925 3 Sheetg-Sheetl Tnventor': v Thomas C.L enno H L5 Attorney 1,635,002 y 5, 2 T. c. LENNOX CONTINUOUS CURRENT TRANSFORMER Filed Aug. 20, 1925 3 Sheets-Sheet 2 Tnventm: Thomas QLennOX,

HLs Attorneg.

July 5,1927. 1,635,002

T. C. LENNOX CONTINUOUS CURRENT TRANSFORMER Filed Aug. 20, 1925 s Sheets-Sheet 3 I, Irn/entor-z Thomas C. Lennox;

U Mf M(/.

His Attorney.

Patented July 5, 1927.

UNITED STATES 1,635,002 PATENT OFFICE.

THOMAS C. LENNOX, 0F PITTSFIELD, MASSACHUSETTS, ASSIGNOR TO GENERAL ELEG TRIO COMPANY, A CORPORATION OF NEW YORK.

CONTINUOUS-CURRENT TRANSFORMER.

Application filed August 20, 1925. Serial No. 51,494.

My invention relates to apparatus for transforming direct current energy from one voltage to another voltage.

The general scheme to be used is the well known one of converting direct current to alternating current'by i'nechanical means such as a commutator or some other device for making and breaking circuits in regular and appropriate sequence, transforming the alternating current to the desired voltage and rectifying it again to direct current by other suitable contact making devices.

The particular features constituting the subject matter of the present invention are means used in the above type of apparatus to cause the load current through the recti fying contacts to have substantially zero value at the instant the contacts open so as to avoid arcing or sparking, and means to bring the voltage between the contacts suiliciently close to a zero value when the contacts are about to close or have just opened, so as to avoid the formation of an arc in advance of actual contact, or after opening when high voltage is being commutated. If these conditions of current and voltage are achieved throughout the range of load to be carried by the apparatus, the design of successful direct current transformers becomes largely a matter of the mechanical design of suitable contact making devices.

The various features of my invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding ofmy invention reference is made in the following description to the accompanying drawings in which Fig. 1 is a representation of prior art devices which will be referred to in describing my invention; Fig. 2 is an illustration of myimproved commutating arrangements showing the means employed for preventing short circuit of'the coils undergoing commutation but which still permits the load current to flow uninterrupted; Figs. 3 and 4 and 5 are illustra tions showing various modifications of my invention provided with means for supply-- ing the necessary reactive kva. of the transformer apparatus; and Figs. (5 and 7 are curves explanatory of my invention.

In Fig. 1, l0 and 11 represent a trans former arranged in that form of circuit known as a ring or polygon. 10 may be considered the primary and 11 the secondary. As here represented the apparatus is 16 connected to supply a low voltage direct.

current circuit 18, bear on the commutator of the secondary. If now this apparatus is driven in any suitable manner so as to produce relative rotation between the commutators and their brushes, continuous current will tend to flow through the primary coils first in one direction and then in the opposite direction. Thusthe current, although continuous in the line, is alternating in the I coils 10 and hence it may be expected to be transformed by the apparatus and reappear in the secondary winding llat corresponding values of voltageand current as determined by the ratio of transformation where itmay be rectified by the brushes 16 to supply the low voltage source.

There are certain inherent reasons why this simple arrangement will not function successfully. It is seen that such a device will not successfully receive exciting current for the transformer from the direct current lines as such current, having to flow in inductive circuits, would resist interruption by the brushes and cause excessive sparking. It has heretofore been proposed to overcome this objection by driving'the rotating part of the rectifier by a direct current motor which is also arranged to act as a synchronous alternating current generator to supply the needed energizing current'for the transformer, that is, the motor would act as'a synchronous inverter.

Such apparatus as is outlined above forms the basis for many schemes for direct current transforn'iation and the. inventions claimed herein consist largely of improvements and modifications of this type of apparatus, it being understood that the physical form shown is of no consequence and may be replaced. by any arrangement which re 'tains substantiallythe same general arrangements of circuits and provides for making and breaking the circuit in the necessary sequence.

Referring to Fig. 1, it will be seen that if complete interruption of the direct current is to be avoided it is necessary for the brush to make contact with the commutator segment it approaches before leaving the previous one. As a result, the phase of the winding between the segments must be short cireuited for a short period. If the winding is receiving alternating current excitation from a synchronous machine, such a short circuit even though occurring during that part of the cycle when the voltage reverses and is conse uently small in value, will result in a considerableflow of current through the brush. If the voltage per phase is small, this current may be limited to av harmless value by using a brush of high resistance material, but if more than a comparatively few volts are to be used, this method is impracticable. Therefore it appears that the contacts must be arranged so that the partof the brush, or its equivalent, which touches the one segment must be separate from the part which touches the other so that we may interpose some device forlimiting the current due to the short circuiting of the transformer phase.

Fig. 2 represents such apparatus in which the. positive brushes 19 and 20 are connected to the primary of an alternating current transformer T and the negative brushes 21 and 22 are connected to the primary of a transformer T which will be called for convenience the brush transformers. It will also be observed that the primary brushes 19 and 20 are connected to one winding of the trans former T and the secondary brushes 23 and.

24 are connected to the other winding of the same transformer. The same arrangement is carried out with respect tothe negative brushes 21, 22, and 25, 26, and transformer T,. The direct current lines 17 and 18 are connected to middle points of the respective windings of the transformers T and T Now, if the transformers T and T have the same ratio of turns as the main transformer 10, 11, they will give substantially the desired result. That this is so, will be evident if we remember that the load current in the primary is in one direction representing the inflow of power and that in the secondary is in the opposite direction representing the outflow of power as represented by the arrows at T, whereas the current tending to flow due to the excitation of the main transformer is the same in each case representing a flow of power from the synchronous machine 27, provided to supply the excitation current of the main transformer, into the short circuit on the phase under the brushes, as represented by the arrows at T. Therefore, if the coils are properly opposed in the brush. transformer, the load currents will compensate for each other and only encounter the leakage reactance of the brush transformer while the short circuit currents will add to each other in causing induction in the core and thus encounter great reactance and be correspondingly limited in amount. In other words, the short circuit current will flow as exciting current in the brush transformer and the line current will flow as load current. Thus, if the exciting current and reactance of the brushtransformer are made small, a minimum of reactance will be inserted in the line circuits and a maximum in the commutator short circuit. This balance of primary commutator voltage against secondary commutator voltage is, in so far as I know, original with me and I consider it an important aspect of my invention.

It will be observed that exciting current for the brush transformers must through the brushes and'consequently even though it is very small in amount, it is desirable to bring it to zero value wheuthe contacts open. Considering those parts of the brush transformer windings between brushes 20 and 21 and the primary lines at the time the brushes 19 and 22 are out of contact, the voltage 20 to 21 is obtained from the synchronous machine 27 and is passing through its maximum value. The voltage between the primary lines is constant and obtained from the source of power 17. The coils of the brush transformer between brush and line are consequently subjected to a varying voltage representing the diiference between these two quantities. Nowif the value of the varying voltage from the synchronous machine is greater than the line voltage, current will flow from brush to line, whereas if it is less, current will flow from line to brush. If the brush is in the neutral position as regards the excitation from the synchronous generator, the voltage 20 to 21 due to the cynchronous machine, starting at a given value when the brush comes into contact with a segment will rise to a maximum, then fall to the first value as the brush loses contact. Now, if the line voltage has an intermediate value, current will first flow into the brush, then reverse and flow out and then reverse and flow inward. Consequently by properly adjusting the two voltages, the current can be so adjusted as to just come to zero on the last reversal as the contacts open. This is represented in the curves of Fig. 6 where line B represents the direct current line voltage with respect to a zero line 0; curve C represents the alternating current excitation voltage between brushes 20, 2l,as it appears rectified by the corresponding brushes; curve D represents the difference between the direct current line voltagc and the su erimposed alternating current voltage, w ich difference is what produces the exciting current for the brush transformer, which exciting current is represented-by the curve E. The alternating current excitation voltage producing the curve C should be so, proportioned with respect to the direct current line voltage as to bring the exciting current curve E to substantially a zero value when the circuit of this phase is opened at the points X. This may be accomplished by a suitable balance between the direct current supply voltage and the excitation voltage supplied by the synchronous generator 27, as

for instance by connecting the field coils 28 of the generator 27 across the direct current source, as shown, or 'by any other arrangement such that the excitation of' 'generator 27 is proportional to the direct current line voltage.

From the above it is evident that the upper and lower halves of the brush transformers might constitute entirely separate devices connected from line to brush it the proper balance of exciting current is obtained such that it will always be zero as the circuit opens. The sum of. the voltages from brush 19 to line and from 20 to line where both brushes are in contact must of necessity equal the totalafrom 19 to 2() so that the original balance .oifcircuits is not disturbed.

It may not be desirable, however, to use entirely separate brush transformers for this purpose as when the winding, from 19 to line is magnetically coupled with that from 20 to line the one will serve to bring the other .to such value of voltage as will prevent any great voltage between the brush and the segments itap roaches. This is represented in Fig. 7 w ere ((1) indicates the rectified voltage waves of the two phases, the zero voltage line 0 and thedirect current line voltage B; curve (11) represents the voltage from brush to the direct current line for one phase; curve (0) represents the voltage from brush to the direct current line for the other phase; curve (d) represents curves (1)) and (c) as superimposed as they will be if the brush transformer windings are placed on the same core and will result in a compromise brush transformer voltage wave substantially as indicated in curve (6). Thus, in Fig. 7 (a), if one brush has a voltage 2 above the direct current line voltage B when the other lbrush comes into contact with its commutator segment at a point rep-' resented b the dotted line X the voltage from the rst mentioned brush to line Wlll cause a corresponding voltage from the other brush to line which will be equal and 0p posite as represented by the oint W to line B. This will lower the resu tant line voltage to a value below line voltage and approximately equal to that due to. the synchronous machine 27 at the instant of making or breaking contact which is represented by the distance of point 'Y from line B. As no load current will exist in the brush not in contact during this eriod, a close magnetic coupling of the coils will not be necessary to give the desired result. Moreover as the'total voltage brush to brush 19 to will start as amaximum in one direction then reverse and finish as an equal former T, it is obvious that they may be built as one transformer with separate windings on the same core. 1

From the above, it appears that we can obtain a complete balance of' voltages in such a systenrzvithout sparking. It will be observed that the balance between primary and secondary voltage is maintained regardless of the variation in brush voltage due to the change from phase to phase. In other words, the total secondary voltage is composed of the sum or difference of the instantaneous values of voltage in main and brush transformers and bears a constant relation to the primary voltage. We can consequently use a sma 1 number of phases without introducing a large ripple in the secondary voltage due to' brush action.-

Reducing the number of phases to a practical minimum we obtain a four-phase system in which each commutator has but two segments per pair of poles in the inverter or excitation generator). We may also use diametric connection if desired with good economy with so small a number of phases that all the windings will be active during a large part'of the cycle. This is shown in Fig.3. The two main transformer windings not being connectedtogether at the star points we may eliminate the brush transformer T and make T sufliciently large to transform the total brush voltage. 1

In what follows we will refer to the arrangement shown in Fig. 3 for the sake of simplicity although the principles involved will apply to any one of many combinations and number of phases.

Serious difficulty will still be encountered if a load current is drawn from such an apparatus. If the transformers could be built with zero value of leakage roactance the primary load current would flow through the apparatus in one direction and the secondary load current'inthe opposite direction without reactive effect and consequently would not tend to flash when interrupted by the commutator. As, however, the transformers must have some leakage reacta'nce, it is obvious that means must be provided to reduce the currents to zero at the opening of the circuits. The action of the commutator converts the direct currents into alternating current for transformation and the currents in the various windings are consequently all alternating or pulsatin The p11 satlng currents may be considere as alternating currents imposed on direct currents. rents, we know that their eifect will be to shift the phase angle of the secondary voltage away from the primary voltage causing a lag in secondary voltage. If we can overcome this tendency and keepthe secondary voltage in phase with the primary voltage, we will be enabled to commutate the currents without flashing. That this is so will be recognized if we remember that the lag in secondary voltage is/the result of the generation of an alternating voltage in quadrature withthe normal voltage and that this voltage is generated by .the reversal of the secondary current. If we oppose this voltage with an equal and opposite one so as to bring the primary and secondary voltages back into phase We will necessarily have provided the voltage required to reverse the current durin commutation. The means I propose for this purpose is a second syn chronous generator 29 driven by and in exact time phase relation with theexcitatiom motor inverter or generator 27 which may obviously be connected to the secondary of the main transformer as illustrated. The windings of this generator will be placed directly, or through suitable intermediate transformers, in series with either the primary or secondary or both coils of the main transformer. Voltage of the necessary value and phase position to overcome the reactance voltage of the main and brush transformers and to time bring about the desired balance will be generated in this machine 29. This may be done by providing the generator with fioldcoils 30 suitable for carrying the load current of the direct current lines and passing this current through this field. The generator will thus have an armature in series with the transformer windings and a field in series with the direct current line.

The voltage of each of the series generator phases must be in quadrature with the normal voltage of the transformer phase to which it is connected. If connected in the primary it must lag, if in the secondary it must lead and if in both it must lag in the primary and lead in the secondary. The voltage of this generator may then be ad justed by means'of an inductive resistance shunt 31 across the field until a balance is obtained between transformer reactance voltage and enerated voltage such that commutation is successful. In order that this balance may be maintained throughout a suitable range of load current, it is necessary to have the relation between field ampores and armature volts in the generator similar to the relation between coil amperes and reactance volts in the transformers. Thus if the leakage fluxes in the transformer occur in material predominatelynonmagnetic the air gap or non-magnetic por- Considering these alternating cur-' ator, that is, one which if mounted on the same shaft will have double the number of poles. This generator may be wound for single phase alternating current and be placed in series with the brush circuits either directly or through intermediate transformers I. Such an arrangement is of intermediate transformer I so that the direct currents are balanced within the transformer and saturation of the core by unidirectional flux avoided. The direct current is kept out of the series generator coils altogether by this method. Thevoltage in the series generator 29 should be in quadrature with the normal voltage in the brush transformer T.

Fig. 5 shows a modification employing three rectifying devices and three phase excitation, The brush transformer T in these cases becomes three phase zig-zag connected.

In accordance with the provisions of the patent statutes, l have described the principle of operation of, my invention, ,together with the apparatus which I new consider to represent the best embodiment thereof; but I desire to have it understood that the apparatus shown and described is only illustrative and that the invention may be carried out by other means.

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

1. Apparatus for converting direct ourrent of one voltage to direct current of a diiferent voltage comprising a main polyphase, transforming apparatus having mechanical rectifying devices associated with its primary and secondary windings such that the circuit between the cooperating rectifying contacts are completely opened at the point of reversal of the alternating current wave, a brush transformer having its primary winding connected between the primary rectifying contacts of the same polarity and its secondary winding connected between the secondary rectifying contacts of the same clarity, said brush transformer having t e same ratio of transformation as the main transformer, whereby the voltage of commutation of the primary is balanced against the voltage of commutation of the secondary. I

2. Apparatus for converting direct current of one voltage to direct current of anrepresented in Fig. 4': showing a preferred means of arranging this circuit by means other voltage comprising a main polyphase transforming apparatus arranged to be connected between direct current circuits by means of mechanical rectif ing devices respectively associated with tie primary and secondary windings of said main transformer, an auxiliary brush transformer having its primary winding connected across the primary rectifying contacts of one polarity, and its secondary connected across the [secondary rectifying contacts of the same polarity, said brush transformer having the same ratio of transformation as the main transformer and terminals at neutral points of said brush transformer windings for connection to direct current lines of corresponding polarity.

3. Appar'atus'as claimed in claim'2, characterized by the provision of an auxiliary synchronous alternator driven in synchronism with the rectifiers for'producing the exciting current for said transformers.

4. Apparatus as claimed in claim 2 char- I mechanical rectifier associated with its primary winding, a mechanical rectifier associated with its secondary winding and a brush transformer. having the same ratio as the main transformer inductively connected between the direct current primary and secondary rectifying contacts of the same polarity for balancing the primary voltage of commutation against the sec-.

ondaryyvoltage of commutation.

In witness whereof, I have hereunto set my hand this 11th day of August 1925 THOMAS C. LENNOX.

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