US2116899A - Synchronous converter - Google Patents

Description

May 10, 1938.- J. KALSEY 2,116,899

SYNCHRONOUS CONVERTER Filed Feb. 12, 1956 5 Sheets-Sheet l INVENTOR.

MM as) A TTORNEYZ May '10, 1938. j KALSEY 2,116,899

SYNCHRONOUS CONVERTER Filed Feb. 12, 1936 5 Sheets-Sheet 2 INVENTOR.

A TTORNEY.

May 10, 1938. J. KALSEY SYNCHRONOUS CONVERTER Filed Feb. 12, 1936 5 Sheets-Sheet 4 INVENTOR. W W.

ATTORNEK May 10, 1938. J K L 2,116,899

SYNCHRONOUS CONVERTER Filed Feb. 12, 1936 5 Sheets-Sheet 5 I IQIXIL EN TO-R.

ATTORNEY.

Patented May 10, 1938 UNITED STATES PATENT OFFICE 15 Claims.

This invention relates to a rotary synchronous converter for the conversion or rectification of alternating current into direct current.

The object of the present invention is generally to improve and simplify the construction and operation of converters of the character described; to provide a rotary converter having a main and an auxiliary commutator with cooperating brushes to deliver alternating current thereto; to

10 provide means for relieving the brushes on the main commutator of current load while passing from one segment to another, or more specifically stated, to provide means for by-passing the current to the brushes of the auxiliary commutator 15 While the brushes on the main commutator are passing from one segment to another; to provide means, including a fixed and a variable resistance, for preventing short circuiting between the brushes of the main commutator and p the brushes on the auxiliary commutator; to provide automatic means for maintaining the converter in synchronous operation with the generator supplying the A. C. to be converted; to provide automatic means for selecting a predetermined current polarity and for maintaining the D. C. output of the commutator at the selected polarity; to provide means for automatically breaking the A. C. current supply to the converter if it fails to operate in synchronism 30 with the A. C. supply; to provide a converter of high efiiciency; and, further, to provide a con verter in which the D. C. output may be regulated and varied on the A. C. side of the circuit.

The converter is shown by way of illustration 35 in the accompanying drawings, in which- Fig. 1 is a diagrammatic view showing the converter whereby A. C. current is converted into D. C., said diagram also showing the synchronous motor whereby the converter is driven, and,

40 furthermore, showing automatic control apparatus for maintaining the driving motor in synchronism with the generator supplying the A. C. current and also for selecting the proper polarity,

said diagram, furthermore, disclosing means for automatically breaking the supply of current to the converter if the driving motor gets out of phase and, furthermore, showing means for vary-- ing the current flow to the converter;

tator B;

Fig. 3 is a cross section of the auxiliary commutator C;

Fig. 4 is a cross section of the commutator A; 55 Fig. 5 is a diagrammatic view showing the cur- Fig. 2 is a cross section of the main commurent flow in an alternating current supply circuit during one complete cycle of operation;

Fig. 6 is a diagram showing the pulsating D. C. current produced by the converter during one cycle of operation; 5

Figs. 7 to 11, inclusive, are diagrams showing the different positions assumed by the commutator B during one cycle of A. C. input;

Figs. 12 to 15, inclusive, are diagrammatic views showing one of the collector rings D, the 10 main commutator ring B, and the auxiliary commutator ring C, said view showing the relative positions assumed by the brushes H and H when passing from one segment of the respective commutators to the other;

Figs. 12, 13 14 and 15 are diagrammatic views showing how the resistance gradually increases and decreases through the brushes H and H while passing from one segment to the other;

Fig. 16 is a longitudinal section through the converter as actually built;

Fig. 17 is a cross section of the converter taken on line XVII-XVII of Fig. 16;

Fig. 18 is a cross section taken on line 2 XVIIIXVIII of Fig. 16

Fig. 19 is a perspective view showing the relative position of the commutators A, B and C and the collector rings D and E; and

Fig. 20 is a diagrammatic view showing the automatic control apparatus for maintaining the driving motor in synchronism with the generator supplying an A. C. current, also for selecting the proper polarity.

Referring to the drawings in detail, and particularly Fig. 1, A, B and C indicate a series of commutator rings and D and E a pair of collector rings. All of the rings are secured on the shaft F mounted in suitable bearings and this is, in turn, driven by a synchronous motor, generally indicated at G, which derives its alternating current from the same source as the alternating current to be converted.

An alternating current synchronous motor is a motor whose speed is governed by the number of cycles in the alternating current energizing the same. Thus, in the case of a synchronous motor with a certain number of poles, and connected to the output of an alternating current generator having the same number of poles, the speed of both machines will be the same and at any given instant the relative position of both armatures will be identical. In the case where the number of poles on both machines are not the same,

the resultant motor speed will be an exact ratio to the speed of the generator.

Taking a two pole A. C. generator, one complete revolution of the same will produce one complete A. C. cycle. During one-half of this cycle the flow of electricity will be in a positive direction and during the other half the flow will be in a negative direction. These one-half cycles are known as alternations and are diagrammatically illustrated in Fig. 5.

Should a two pole A. C. generator revolve at a speed of sixty revolutions per second, it would produce a sixty cycle current. By increasing the poles and reducing the speed proportionately, the current would still be sixty cycles. In other words a sixty cycle alternating current is a current which reverses sixty times per second. Taking a two pole synchronous motor operating on sixty cycles A. (7., its speed will be 3600 R. P. M. If this motor has four poles the speed will be 1800 R. P. M. The motors armature motion is produced by what is known as a revolving magnetic field and in this process the armature locks in step with the current cycle,

From the foregoing it should be apparent that a predetermined point on the shaft of the synchronous motor will at all times coincide with a predetermined point on the A. C. curve; in other words, if a point on the shaft is in a certain position and the alternating current at that moment is in a negative direction and the value of the same bears a certain relation to the A. C. curve, when the shaft has made one complete revolution all the current conditions for that particular period will be repeated.

For the purpose of description let it be assumed that the commutator B, in Fig. 1, has four segments insulated from each other and the shaft F, and that the motor G driving the shaft is a four pole 1800 R. P. M. sixty cycle synchronous motor and, further, that segments 1 and 3 of the commutator B are connected to the collector ring D while the segments 2 and 4 are connected with the ring E. In addition thereto, let it be assumed that the shaft F is rotated in an anti-clockwise direction and that a sixty cycle alternating current is impressed on the segments by means of brushes H and J, which are positioned at a angle with relation to each other.

Under these conditions with the commutator revolving, the various forces diagramed in Fig. 5 will be impressed upon the several segments of the commutator as follows. Assuming that the motor is running at its proper speed and when at the zero point, or at a of Figs. 5 and '7, the brushes H and J will not be in contact with any of the segments, at which time there is no current flow in this circuit but when the segments 5 and 2 are moved, just making contact with the brushes H and J, there will be a positive flow of current starting in the circuit, and further rotation of the segments, as shown in Fig. 8, the positive flow of current in the circuit will be at its maximum value. The positive current flow now starts to decrease until it reaches point 19, shown in Figs. 5 and 9, (or 1/120 second of time) where there will be no flow of current and the brushes are out of contact with the segments, further rotation of the segments bringing the segments 4 and I in contact with the brushes H and J, respectively, and the current flow will be started in a reverse or negative direction through the circuit and increase in value until it reaches its maximum negative value, then starts to decrease until the zero point is reached when one cycle is completed (or 1/60 second of time). Under this arrangement the segments l and 3 always receive a positive flow of current and 2 and 4 always receive a negative flow of current; these segments being connected to their respective rings D and E will rectify the A. C. input or convert it to a direct current and the current will be a pulsating one as diagrammatically illustrated in Fig. 6. Condensers or filters of suitable type, not shown, will, however, be placed in the D. C. line to smooth out the current flow.

It should be evident that the commutator itself does not consume any energy and will convert one hundred percent of the current input. It should also be evident that the amount of energy and the pressure behind the same can be altered without changing the general arrangement as a variable transformer may be placed on the A. C. side of the circuit to vary the amount of energy and pressure. The synchronous motor G driving the commutator need only be large enough to provide the desired rotation and inasmuch as there is no electrical connection between the two it can be operated on the conventional voltage. It should be evident that the variations impressed or converted will not change the load on the motor. The only requisite is that both currents have identical periods and cycles. Inasmuch as the supply is derived from one generating system this is a practically assured condition.

The consumption of energy by the synchronous motor driving the converter is constant, hence the cost of conversion depends upon the amount of energy converted. The motor driving the converter in operation in my laboratory at the present time consumes about 200 watts per hour and my average use of D. C. approximates 10,000 watts. In this particular instance the efficiency of the converter is 98%. Regulation of D. C. voltage after delivery to a consumer is a complicated matter and in most cases is accompanied by the loss of considerable energy. Compared to this the regulation of A. C. supply is a simple matter consuming very little energy in the process. With the use of my converter the desired D. C. values are regulated at the A. C. side by means of the variable transformer shown, or the like.

The converter so far described would seem phase with the generator and the resultant curve were regular, then at the points a, b, c, where the flow reverses, there would be no voltage and hence no current. Thus if the brushes H or J had only the thickness of the imaginary zero line it could pass from one segment to the other at the time when the circuit is dead. However, a practical brush must have thickness and thus at some period the brush must contact both segments at the same time and though the energy at that instant would be very small there would be a momentary short circuit. Now, if the thickness of the insulation be increased a width slightly wider than the width of the brush this defect would be overcome but another condition would appear; that is, as the trailing edge of the segment pulls away from the brush there would be a momentary break in the circuit. The tendency for an interrupted current is to continue contact after a break and this condition is carried on through space, the result is a: disagreeable sparking which quickly pits both the copper and brush which, in turn, intensifies this phenomena until finally the device becomes useless.

In practice it is never possible to obtain an alternating current free from imperfections and we seldom have currents where all conditions are in phase. Thus, the zero line of the alternating current curve, see Fig. 5, or rather the relation of the same with respect to-the curve is never constant and hence there maybe considerable current in the circuit at the time the brushes cross the joint. The result is sparking, pitting and scoring which, in turn, causesfaulty contact and irregular commutation. Thus, in the case of a narrow joint we have short circuits, in wider joints we have sparking and flashing.

To overcome this defect I have developed a system whereby the load carried by the segments isgradually reduced as the joint comes under the brush and entirely ceases at the moment the trailing edge of the segment leaves the brush. At the same time the load is transferred to an auxiliary brush which continues to deliver the load to the circuit until the main brush has traveled across the insulated joint. This system is of prime importance for it allows a continuous delivery of energy and hence eliminates all sparking. The manner in which this is accomplished is best illustrated in Figs. 12 to 15, which are diagrams showing the relationship between the segments, brushes and current values at or near the A. C. zero line, said diagram indicating four different conditions while the brushes are crossing the joint between segments.

In these'diagrams D'represents one of' the collector rings which delivers the converted or direct currentto the'direct current side of the converter, B the maincommutator, and I andZ the segments thereon which receive the alternating current to be converted, said current being introduced by the brush H. In this case the insulation between the segments has been widened, the'width being slightly in excess of' that of the brush. In addition, the trailing edge of each segment is constructed of material 6', such as nichrome, or the like, having a high resistance to the passing of the'current. C indicates an auxiliary commutator containing conducting segments 1' positioned directly over the zeroline; or their position occupies the corresponding space of the insulation between the segments I andZ. The segments 1 shown in the diagram are insulated from the rest of the commutator but are'in electrical connection with segment I through wire 8. The segments! also have a trailing edge 9 of high resistance material. The brush H, which will be referred to as the auxiliary brush; is only onehalf the width of the brush H.- In these diagrams the auxiliary segment I- and segment I' are directly connected by the wire 8. The brush H is connected to brush H by means of a special resistance hereinafter to be described.

Assuming theconverter is in rotation and in the direction'of arrow is, and again referring to Figs. 12 to 15', the diagrams show'four different positions of 'the commutator segments under the brushes andeach-position' indicates on the load transfer diagram depicted at the right just how the load is distributed; In the first position, Fig. 12, allofth'e load or current is delivered by the brushH to segment I'and thence to the collector ring'D by means of thewir'e' M. The auxiliary brush: at this moment is inactive. In the second position, Fig. 1 3, the brush H has partially left the segment I and is entirely bearing on the high resistance tip 6. Under this condition the H brushcan only deliver a fraction of the total load, meanwhile, the auxiliary brush H is bearing directly on the load transfer segment I and the main load or current is delivered from brush H through the special resistance, to be described, to H then to segment I through wire 8 to segment I and finally through wire l4 to the collector ring D. In this'p'osition the main load is delivered by the auxiliary segment.

As motion progresses brush H has diminishing contact with the resistance tip 6 of segment I until finally the segment becomes non-conductive until it reaches the position shown in Fig. 14 where'the flow through the segment has entirely ceasedi Meanwhile the load is being delivered. through the brush H but at this moment this brush begins to contact the resistance tip 9 of segment I and as motion continues and resistance offered by the segment becomes greater a small fraction of time after this condition and in the fourth position, see Fig. 15, brush H has contacted segment 2 and brush H has ceased to deliver current. Meanwhile the current has reversed so that the brush H is delivering current to segment 2 which is connected to the negative side of the system. In the fourth position before brush H has left the auxiliary segment the brush H has contacted segment 2 and also the current flow has been reversed. The condition of dual contact maintains only for a small fraction of time, the facts are that for that period, current is being delivered'by one leg to both sides of the circuit. Though the actual short-circuit is through resistance, the value of the current may be such as tonullify the advantages of the resistancetip of the auxiliary segment I.

To overcome this condition I have designed an automatically controlled variable resistance. Its function is to render brush-H conductive in proportion to the total value of theentire load impressed upon the system. By calculation and tests, the energy transferred to the brush H should be one-half of one percent of the total energy going through the converter. Inasmuch as the total energy varies according to the demand, an arrangement whereby the energy passing through brush H should at all times be in the proper proportion is necessary. Fig. 1 depicts the two brushes H and H with a fixed resistance l5 between'them. This resistance is permanently adjusted tota-ke care of a minimum requirement. In addition there is a variable resistance 16 in shunt with this arrangement. This resistance consists of'a unit plunger II'mounted within'the coil l8 of a solenoid. The coil inthe solenoid is fed'with D. C. current from the wires 19 and 20 and the amount of current passing through the same'governs the position of acontact arm 2|. The contact arm travels over the resistance coil [6 and hence controls the amount of current capable of passing.

In many D. C. uses itis' of the utmost importance that the fiow'of current is always in a predetermineddirection therein; in other words, that the polarity is never reversedl Inasmuch as the synchronous motor may lock in step with either the top or the bottom half'of theA. C. wave when started, or after interruption ofservice, a controlling device to force the commutator to deliver D. C. with the desired polarity is essential. This control is'shownin Figs. Land- 20'. In these figures, G indicated the synchronous motor whereby the converter as a whole is driven. Rotating with the converter is the commutator A. A section of this commutator is shown in Fig. 4. In this case it will be noted that there are two segments 23 and 2G oppositely disposed with relation to each other but electrically connected. Between them are disposed two insulating segments 25 and 26. Two brushes 2? and 28 make contact and the current supply is taken directly from the A. C. feed line indicated by the numerals 3i! and 3!; the current value being controlled by the transformer M. The action is such that when a commutator is in the position shown, it will allow current to pass through. When the commutator makes onequarter of a revolution no current can pass. Inasmuch as the commutator A is synchronous with the motor G, the resultant current flow through the commutator is a direct half-wave pulsating current. The numeral 33 indicates an evacuated tube containing a metallic dome electrode M. In addition thereto, there is a ring-shaped electrode 35, the lower portion of which is shielded with a non-conducting material as shown at 36. A third electrode Si is also employed. It consists of a small concave disc, the concave side of which is covered with a photoelectric sensitive material. The tube contains a very small amount of inert gas, such as neon. The phenomenon here is that all surfaces within the evacuated tube will attract this gas and following certain physical laws will spread evenly over the surfaces. The amount of gas introduced is slightly more than what is necessary to effect this covering, making the space between the electrodes slightly conductive for small electric charges. The conduction of the electrical charge in this tube is brought about by the movement of electrons. These electrons can only move from a negative surface to a positive surface. The amount of energy actuating the two electrodes 34 and 35 is just enough to allow electronic movement between them. As the electrons impinge upon either of the electrodes the gas deposited will emit a glow on the surface of the dome. Should the flow be from the ring to the dome 34 the top interior surface will glow. Should the flow be from the dome to the ring, the ring surface will glow. In the first case the glow would be transmitted to the disc 37. In the second case any downward glow from the ring would be stopped by the ring shield 36 and the upward glow toward the dome would be insufficient to cause any reflection from the dome, thus whether the disc 31 would be energized or not would depend upon the direction of current flow passed by the commutator. The disc 31 is in circuit with a relay magnet 38 which, if energized, will cause the two contacts 39 and 4|] to separate and hence will break the current to the synchronous motor G.

Assume when the motor G is started that the commutator will pass the current in such a direction as to cause the dome interior 34 to glow. If that is the case it will energize the photoelectric disc 31 and cause the current to flow through the transformer N and the relay 38 and break the motor contacts 39 and 40 and thereby slow the motor down. When the motor has slowed down one-quarter of a revolution the relative position of the commutator has been changed and will now pass the current in the opposite direction. This will keep the domes interior dark and the motor will remain locked in the desired position. One important feature of the control system just described is that it operates entirely on the same circuit as the motor G making it independent of the energy passing through the converter.

If for any reason the synchronous motor should not function in synchronization with the current to be converted and while the D. C. current is in use, the result would be an introduction of A. C. current into the D. C. system and the relationship between the A. C. converter and the D. C. would be upset; also the current ratio for the auxiliary brushes would be destroyed which might result in damage or total destruction of the converter. To prevent this a circuit breaking relay has been developed. This is shown in Fig. 1 and presents a system of passing a current to any standard circuit breaker only when the motor is out of synchronization with the alternating current. This system, furthermore, allows the same current actuating motor G to operate the relay and also the breaker, thus making the entire control independent of the energy across the converter. The circuit connected to the breaker magnet 45 is open at the contact points 22, the current entering the magnet 43 passes through the commutator A and also through a condenser 44. The principle here is that a direct current will not pass through a condenser whereas an alternating current will.

Now as long as the commutator passes direct current the contact points 42 will remain open but as soon as the motor G is out of step the commutator A will pass alternating current, thereby energizing the relay 43 and permitting the current to pass through the contacts 42 and the breaker magnet 45, causing the same to open the circuit breaker 46.

The break in the circuit feeding the converter is, of course, only momentary as the tube 33 performs two functions first that of selecting the proper polarity and, secondly, that of maintaining the motor G in synchronism with the A. C. generator supplying the system. That is, if the commutator A is off phase alternating current will not only pass through the condenser 44 but also through the tube 33, causing a momentary break between the contacts 39 and 40, thereby breaking the circuit through the motor G and slowing it down sufficiently to not only maintain it in phase but also in proper polarity.

The converter shown in Fig. 1 is intended for a large output of energy and in some instances an energy of very high voltage. It is for this reason that the transformer M and the third commutator are used as a direct current of comparatively low voltage; for instance, 1.10 volts are required for the operation of the control apparatus such as the synchronous motor G, the control tube 33, the circuit breaker 46, etc. If the converter is used for a low voltage output of fixed value the current for the control apparatus may be taken directly from the D. C. line or stepped down through a transformer, and in that case, the commutator A may be eliminated.

In the use of direct current it is important in many instances that the polarity is in the correct direction; for instance, in the use of Xray apparatus, electroplating apparatus, high frequency actuated dust or gas precipitators, in battery charging, etc. Wherever alternating current is converted into direct current by synchronous conversion, manual operation to insure the correct polarity is usually necessary. By using the apparatus shown in Fig. 20 correct polarity is automatically insured, and while this and other features of the present invention are more or less specifically described, I wish it understood that various changes may be resorted to within the scope of the appended claims. Similarly, that the materials and finish of the several parts employed may be such as the manufacturer may decide, or varying conditions or uses may demand.

Having thus described my invention, what I claim and desire to secure by Letters Patent is:-

1. In a. converter of the character described, a main and an auxiliary commutator, and a pair of collector rings connected therewith, each of said commutators having a plurality of conductor segments spaced circumferentially with insulating material and the conductor segments on the auxiliary commutator being aligned with the insulating space between the segments on the main commutator, a pair of brushes for impressing alternating current on the segments of the main commutator, a pair of brushes engaging the segments on the auxiliary commutator, a common feed wire connecting the first-named and secondnamed brushes whereby alternating current is passed through the second-named brushes while the insulating spaces between the segments on the main commutator are passing under the firstnamed brushes, and a fixed resistance in the connection between the first and second-named brushes.

2. In a converter of the character described, a main and an auxiliary commutator, and a pair of collector rings connected therewith, each of said commutators having a. plurality of conductor segments spaced circumferentially with insulating material and the conductor segments on the auxiliary commutator being aligned with the insulating space between the segments on the main commutator, a pair of brushes for impressing alternating current on the segments of the main commutator, a pair of brushes engaging the segments on the auxiliary commutator, a connection between the first-named and second-named brushes whereby alternating current is passed through the second-named brushes while the insulating spaces between the segments on the main commutator are passing under the first-named brushes, and a fixed and a variable resistance in the connection between the first and second-named brushes.

3. In a converter of the character described having a main and an auxiliary commutator and a pair of collector rings connected therewith, a synchronous motor for driving the converter, said motor being supplied with alternating current from the same source as the A. C. to be converted, a third commutator on the converter also supplied with alternating current from the same source, and means actuated by the current output of the third commutator for breaking the A. C. supplied to the converter if the synchronous driving motor gets out of synchronism.

4. In a converter of the character described having a main and an auxiliary commutator and a pair of collector rings connected therewith, a synchronous motor for driving the converter, said motor being supplied with alternating current from the same source as the A. C. to be converted, a third commutator on the converter also supplied with alternating current from the same source, and means actuated by direct current output of the third commutator for automatically selecting and maintaining the direct current output of the converter at a fixed polarity.

5. In a converter of the character described having a main and an auxiliary commutator and -a pair of collector rings connected therewith, a

synchronous motor for driving the converter, and means actuated by the direct current output of the converter for varying the alternating cur- 'rent input to the auxiliary commutator.

6. In a converter of the character described having a main and an auxiliary commutator and a pair of collector rings connected therewith, a synchronous motor for driving the converter, said motor being supplied with alternating current from the same source as the A. C. to be converted, a third commutator on the converter also supplied with alternating current from the same source, means actuated by current output of the third commutator for breaking the A. C. supplied to the converter if the synchronous, driving motor gets out of synchronism and other means for automatically selecting and maintaining the D. C. output of the converter at a fixed polarity.

7. In a converter of the character described having a main and an auxiliary commutator and a pair of collector rings connected therewith, a synchronous motor for driving the converter, said motor being supplied with alternating current from the same source as the A. C. to be converted, a third commutator on the converter also supplied with alternating current from the same source, means actuated by the current out-- put of the third commutator for automatically selecting and maintaining the D. C. output of the converter at a fixed polarity, and means for varying the A. C. input to the converter to vary the D. C. output.

8. In a converter of the character described, a main and an auxiliary commutator and a pair of collector rings connected therewith, each of said-cornmutators having a plurality of conductor segments spaced circumferentially with insulating material and the conductor segments on the auxiliary commutator being aligned with the insulating space between the segments on the main commutator, brushes for impressing alternating current on the segments of the main and auxiliary commutators, said brushes being connected, and a material having a high resista'nce to the fiow of a current forming the trailing end of each segment.

9. In a converter of the character described, a

ain and an auxiliary commutator and a pair collector rings connected therewith, each of 5 id commutators having a plurality of conductor segments spaced circumferentially with insulating material and the conductor segments on the auxiliary commutator being aligned with the insulating space between the segments on the main commutator, brushes for impressing alternating current on the segments of the main and auxiliary commutators, said brushes being connected, a material having a high resistance to the flow of a current forming the trailing end of each segment, and a fixed and a variable resistance in the connection between the brushes.

10. In a converter of the character described having a main and an auxiliary commutator and a pair of collector rings connected therewith, a synchronous motor for driving the converter, said motor being supplied with alternating current from the same source as the alternating current to be converted, a third commutator on the converter also supplied with alternating current from the same source, means actuated by the current output of the third commutator for breaking the alternating current supplied to the converter if the synchronous driving motor gets out of synchronism, and means for varying the direct current output of the converter by varyingjthe alternating current input.

11. In a converter of the character described ha ving a main and an auxiliary commutator and a pair of collector rings connected therewith, a synchronous motor for driving the converter, said motor being supplied with alternating current from the same source as the alternating current to be converted, a third commutator on the converter also supplied with alternating current from the same source, means actuated by the current output of the third commutator for automatically selecting and maintaining the direct current output of the converter at a fixed polarity, and means for varying the direct current output of the converter by varying the alternating current input.

12. In a converter of the character described a main and an auxiliary commutator and a pair of collector rings connected therewith, each of said commutators having a plurality of conductor segments spaced circumferentially with insulat:

7 ing material and the conductor segments on the auxiliary commutator being aligned with the in;- sulating spaces between the segments on the main commutator, brushes for impressing alternating current on the segments of the main and auxiliary commutators, said brushes being connected, a material haying a high resistance to the flow of a current forming the trailing end of each segment, and a fixed resistance in the connection between the brushes.

13. In a converter of the character described a main and an auxiliary commutator and a. pair of collector rings connected therewith, each of said commutators having a plurality of conductor segments spaced circumferentially with insulating material and the ".conductor segments on the auxiliary commutator being aligned with the insulating spaces between the segments on the main commutator, brushes for impressing alternating current on the segments of the main and auxiliary commutators, said brushes being connected, a material having a high resistance to the flow of a current forming the trailing end of each segment, and a variable resistance in the connection between the brushes.

14. In a rotary converter for converting alternating current to direct current, a synchronous motor for driving the converter in synchronism with the alternating current to be converted, means for automatically breaking the current to the alternating converter if the converter gets out of synchronism, said means including a switch in the alternating current supply circuit of the converter, a commutator driven by the motor and supplying alternating current to the control circuit to open the switch when the converter is out of synchronism, said commutator supplying direct current when the converter is in synchronism, and a condenser interposed between the commutator and the control circuit to render the control circuit inoperative when direct current is supplied thereto.

15. In a rotary converter for converting alternating current into direct current, a synchronous motor for driving the converter in synchronism with the alternating current, means for automatically selecting the polarity of the convcrters curr'nt output and for automatically maintaining the cirect current output at the polarity selected, said means comprising a com-- mutator driven by the synchronous motor, said commutator supplying current to a control circuit, a switch adapted to be opened or closed by the control circuit, said switch controlling the alternating current supply to the synchronous motor, and a photoelectric relay in the control circuit, said relay preventing current of a selected polarity from passing through the control circuit but passing current of an opposite polarity, said current of opposite polarity actuating the control circuit to open the switch to momentarily break the alternating current circuit through the synchronous motor and thereby change the polarity of the output of the converter to the selected polarity.

' JOHN KALSEY.

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