US851828A - Alternating-current machinery. - Google Patents

Description

' BEST AvAiLABLE 0GP".

-A. s. MOALLISTBR. ALTERNATING CURRENT MACHINERY.

PATENTED APR. 30, 1907.

APPLIUATION FILED APR. 2, 1906.

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747% 6615 e a x nvemforz' THE mmms PETERS co., WASHINGTON, n. c.

I PATENTED APR. 30, 1907.

k A. S. MoALLISTER. I ALTERNATING CURRENT MACHINERY.

APPLIOATION FILED APR. 2, 1906.

2 SHEETS-SHEET '2 1m: NDRRIS PETERS co WASHINGTON n. c.

ADDAMS S. MGALLISTER, OF NEW YORK, N. Y.

ALTERNATlNG-CURRENT MACHINERY.

Specification of Letters Patent. 7

Lratented April 30, 1907.

Application filed April 2. 1906. Serial No. 309,481.

To It "II/72,0712, if; DI/(by concern.-

Be it known that I, ADDAMS STRATTON Mo- ALLISTER, a citizen of the United States, residing at New York, in the county of New York and State of New York, have invented certain new and useful Improvements in Alternating-Current Machinery, of which the following is a specification.

My invention relates to alternating current machines of the commutator ty e and its object is to provide a method and apparatus for preventing sparking at the commutator brushes.

Commutation in alternating current machines presents difliculties which are absent in direct current machines, since in an alternating current machine the coil,which is shortcircuited' at the instant of commutation, is subject to an alternating or pulsating field. Even when the armature is stationary there exists an appreciable electro-motive force between terminals of each coil under the brushes due to the transformer action of the field flux, the field acting as the rimary and the short-circuited armature coi s as the secondary of a transformer. When a brush passes from one commutator segment to the next, as the armature rotates, the electro-motive force is ordinarily short-circuited through the brush which completes theconnection between the segments. The current which flows through this short-circuit is of large value and produces an excessive heating of the brush, the segments and the coil. Moreover, the rupture of this current produces destructive arcing at the brushes, and its presence is, in general, detrimental to the perfect performance of the machine. To the evil effects of this localcurrent in the shortcircuited coils may be attributed the slow progress which has been made in the development of the commutator type of alternating current machines.

Various methods of reducing sparking have been devised. It has been pro osed to use resistance in the armature leads in order to limit the value of the local short-circuit current and motors embodying this idea have been constructed which have operated satisfactorily. It is evident, however, that the resistance in the armature leads to be of any value, must absorb an appreciable amount of energy, due both to the local current and to the normal useful current passing through the armature. Practical requirements de m and that these resistances shall be of limited cross-section, entailing the constant danger that they will burn out, and, furthermore, they must be laced in a position where it is diflicult to malie repairs or to replace them in case they are burnt out.

My present invention provides a method by which the sparking may be decreased to any desired extent, a minimum of ohmic resistance may be employed, and such resistance may be placed in an easily accessible location external to the machine.

My invention will be best understood by reference to the accompanying drawings, in which Figure 1 shows diagrammatically a dynamo or motor arranged to show certain features of the invention; Fig. 2 shows the same with the armature rotated to a different position; Fig; 3 shows the complete arrangement of my invention applied to a machine, the armature being in a position intermediate to that of Figs. 1 and 2; Fig. 4 shows a modification thereof, and Fi 5 shows a top plan view of a commutator w1th the brushes in cross-section.

' The machine may be bipolar or multipolar and may have the field winding F, arranged as a distributed or a concentrate winding and connected in any desired manner. The armature A, may be of any type such as those now well known in direct-current practice and the coils may be arranged in lap or wave winding, the terminals of the coils being connected directly to the commutator segments without the interposition of resistancein the leads. The live segments a, and the insulating segments b, of the commutator, may be approximately equal in width though this is not absolutely necessary.

Instead of using a single brush or set of inter-connected brushes, at each point of commutation as usual, I employ three independently insulated brushes for each set, each brush having a width somewhatless than the width of each dead or insulating segment. Two of the brushes, viz; the outer brushes 1,3, are connected to the terminals of a reactance coil L, while the middle brush 2, is connected to the middle point of the reactance coil and to a terminal of the machine by means of conductors Z, 1,.

The operation of the arrangement above described will be better understood byreferring to Figs. 1, 2 and 3. In Fig. 1, it will be observed that the only current whichean flow in the local circuit of the armature coil s I"? :2", Adi-nth under the brushes 1, 3, and the reactance coil L, is the exciting or magnetizing current of the reactance coil, which is practically a negligible quantity. The main power cur rent flowing through the armature passes differentially through the halves of the reactanee coil in opposite directions and hence causes no opposing reactance. That is, for the power or line current the coil L opposes no reactance but for the local short-circuited current it acts like a true reactance coil to choke or decrease the current to a negligible quantity. In Fig. 2, the brushes 1 and 3, bear upon insulating segments 7), and therefore receive no current whatever, while brush 2 is in contact with a live segment (1., and transmits the entire power or line current. It is evident that either in the position shown in Fig. 1, or that shown in Fig. 2, the machine is devoid of any short-circuiting effect and no abnormal heating will be produced at any point. Fig. 3 shows the brushes in a-position on the commutator intermediate between that of Fig. 1, and Fig. 2, which is the most disadvantageous position during the operation of the machine. Brushes 1 and 2, are in contact with one live segment while brush 3 contacts with another segment. Since brushes 1 and 3 are on different segments and are connected through the reactance coil L, no direct short-circuit exists between them, the electro-motive force of one armature coil being, however, impressed upon the terminals of the reactance coil. Since the brush 2 is connected to the middle point of the reactance coiLthrough conductors Z 1 there exists between brush 2 and said middle point an alternating electro-motive force having a value equal to one half of that of one armature coil, which would tend to circulate a current locally through brushes 1 and 2 and one half of the reactance coil. In order that this short-circuited current may be kept within proper limits, adjustable resistances 1",, 1' are inserted in the conductors Z 1 between brush 2 and the middle point of the coil L. It will be observed that these resistances are in series forthe local short-circuit current, but are in parallel for the normal useful armature or line current. These resistances are made of ample current carrying capacity for the maximum load of the machine and are, therefore, not subject to burn outs. Moreover, they are external to the armature and can be easily adjusted or repaired.

Although perfect commutation requires the resistances r 1-,, to have an appreciable ohmic resistance, it is to be noted that the leads between the armature and the usual controller C, necessarily possess an appreciable resistance, and that by the use of two leads as shown at Z Z which are always in series for the short-circuit current but are used either singly or in parallel for the nor mal armature current, the electrical efliciency of the motor equipment will compare favorably with that of a similar direct-current equipment where one lead is always in circuit and no spark-preventing devices are employed. In other words, it is preferably to employ, so far as possible, the necessary resistances of the cable leads and to insert only such additional amount of resistance as may be found necessary. The object of introducing resistances into the conductors leading to each brush being to decrease the shortcircuit current to a low and harmless value, it is evidently desirable to insert them in such a manner that the loss due to the passage of the normal load current will be small as possible. One method of accomplishing this result is shown in Fig. 3, wherein the adjustable resistances r 13, are placed in the local leads Z 1,, connecting one of the machine terminals T, with the middle brush. and the middle point of the reactance coil.

In the modification shown in Fig. 4, the reactance coil is divided and two leads are brought out from the middle, an adjustable resistance being inserted in each branch. It is obvious that the same effect will be produced if these resistances be placed between the brushes 1, 3, and the end terminals of the reactance coil. The reactance coils may be located at any convenient distance from the brushes of the machine and the connecting leads will serve as a resistance to limit the value of the short-circuit current, such additional resistance being inserted as may be found necessary and this may be varied at willduring the operation of the machine.

By referring to Fig. 5, it will be seen that the brushes 1, 2, 3, may be arranged in staggered relation so that they may not be crowded together as they bear upon the commutator.

' Mymethod of preventing sparking is applicable to any type of alternating-current commutator machine, whether motor or generator and may be used with a single set of brushes or with a plurality of sets of brushes.

I claim as my invention:

l. The combination with an alternatingcurrent machine, having a rotor provided with a commutator, of means for preventing sparking comprising a plurality of separate brushes at each point of commutation, connections between said brushes having means for producing a counter electro-motive force, and a plurality of leads from said brushes non-inductive at all times with respect to the normal current of the machine: substantially as described.

2. The combination with an alternatingcurrent machine, having a rotor provided with a commutator, of means for preventing sparking comprising a plurality of separate brushes, connections between the brushes having self-contained means for producing a LIO ' with a commutator, of means for preventing counter electro-motive force and providing a plurality of inductive local circuits for the current due to the electro1notive force induced in the rotor coil short-circuited by the brushes; substantially as described.

The combination with an alternatingcurrent machine, having a rotor provided with a commutator, of means for preventing sparking including a plurality of separate brushes, connections between the brushes providing a plurality of local circuits for the current due to the electro-motive force in cluded in the coil short-circuited by the brushes, means in said connections whereby reactance is opposed to the passage of current in the local circuits while offering a free passage to the normal current of the machine, said connections affording a plurality of leads from each point of commutation for the normal current of the machine; substantially as described.

l. The combination with an alternatingcurrent machine, havinga rotor provided sparking including a lurality of separate brushes, connections etween the brushes providing a plurality of local circuits for the current cue to the electro-motive force in duced in the coil. short-circuited by the brushes, an inductive resistance in one of said local circuits, and a combined inductive and non-inductive resistance in another local circuit between said brushes; substantially as described.

5. The combination with an alternatingcurrent machine, having a rotor provided with a commutator, of means for preventing sparking including separate brushes connected through a reactance device and another brush connected to an intermediate point in said reactance device, all of said brushes being arranged to convey the normal current of themachine; substantially as de scribed.

6. The combination with an alternatingcurrent machine, having a rotor provided with a commutator, of means for preventing sparking including separate brushes connected through a reactance device and another brush connected to an intermediate point in said reactance device through a noninductive resistance; substantially as described.

7 The combination with an alternatingcurrent machine, having a rotor provided with a commutator, of means for preventing sparking including separate brushes connected through a reactance deviceand another brush connected to an intermediate point in said reactance device through adjustable non-induetive resistances, said con- N am.

.nections being so arranged that the noninductive resistances are in series for the local currents and in parallel for the normal current of the machine; substantially as described.

8. The combination with an alternatingcurrent machine, having a rotor provided with a commutator, of means for preventing sparking including a plurality of sets of separate brushes connected to the leads of the machine, each set having a reactance device connected between two of the brushes, and a non-inductive resistance connected between an intermediate brush and an intermediate point in said reactance device; substantially as described.

9. The combination with an alternatingcurrent machine, having a rotor provided with a commutator, of means for preventing sparking including a plurality of sets of separate brushes connected to the leads of the machine, each set having a reactance device connected between two of the brushes, and non-inductive resistances connected between an intermediate brush and an intermediate point in said reactance device, and a con troller connected to said brushes throu h said non-inductive resistances; substantial y as described.

10. The combination with an alternatingcurrent machine, having a rotor provided with a commutator composed of alternate live and dead segments, and sets of brushes therefor arranged to convey the normal working current of tlie machine, each set ineluding separate brushes locally connected through a reactance device and another brush connected "to an intermediate point of the reactance device; substantially as described.

11. The combination with an alternating current machine, having a rotor provided with a commutator composed of alternate live and dead segments, of three separately insulated brush units at each active point ofcommutation and a reactive connection between any two of said brush units; substantially as described.

12. The combination with an alternatingcurrent machine, having a rotor provided with a comm itator composed of alternate live and dead segments, of three separately insulated brush units at each active point of commutation, each brush having a width less than that of a dead segment and a reactive connection between any two of said brush units; substantially as described.

In testimony whereof I afiix my signature in presence of two Witnesses.

ADDAMS S. MCALLISTER.

l/Vitnesses: BERTHA M. PARKER, EUGENE C. BROWN.

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