US2389507A - Commutation means for rotary rectifiers - Google Patents

Description

Nov. 20, 1945. K L, HANSEN 2,389,507

COMMUTATION MEANS FOR ROTARY RECTIFIERS Filed Aug. 30,' 1943 INVENTOR. Km; 1. 62mm B 2 mzT Patented Nov. 20, 1945 COMMUTATION MEANS FOR ROTARY RECTIFIERS Klaus L. Hansen, Milwaukee, Wis.

Application August 30, 1943, Serial No. 500,511

'4 Claims.

This invention relates to commutation means.

A special form of commutator is the mechanical rectifier which in its simplest form consists of two semi-cylindrical metal segments which are connected to a source of alternating current with two brushes arranged to bear on the commutator at diametrically opposite points, such brushes being connected to a load circuit. The rectifier segments are rotated in synchronism with the alternating current.

A rectifier constructed in this manner is subject to severe sparking which is most emphasized for all loads other than small resistance loads. Various methods have been employed to lessen the sparking, one of which is to connect a resistance from segment to segment. If this resistance has a high value, it is not effective in preventing sparking and when it is too low, a

. great deal of current fiows through the resistance and thus markedly lowers the efficiency of the apparatus.

Another method which has heretofore been used to a limited extent is to divide the segments into several small sub-segments near the points at which commutation takes place, resistances being provided between the main segments and the several sub-segments. Various modifications of this last scheme have also been tried in which there may be a gradual change of the material of the segments so as to effect an increase in resistance towards the ends of the segments. The number of segments must necessarily be relatively small or the time of direct contact between the alternating current supply and the direct current load circuit is shortened to such an extent that the efficiency is seriously impaired.

In addition to this there are several other defects among which may be mentioned the fact that if a few segments only are employed, it is necessary to use a. high resistance between them in order to efiectively limit the circulating current.

Thus in the prior devices there is a choice between good commutation and low efflciency or high efiiciency and poor commutation.

This invention is designed to overcome the above noted defects and objects of this invention are to provide novel commutating means which is very eflicient, which avoids sparking to a marked degree, and which can be operated to commutate the alternating current without excessive or detrimental sparking whether or not the commutation occurs at the instant of zero current and zero voltage.

In greater detail, objects 01' this invention are to provide a parallel path between the main brushes at the instant of commutation, which parallel path is established prior to the time that a main brush leaves a receding main segment and is maintained until after a main brush has engaged an approaching segment, whereby excessive sparking is avoided.

Further objects are to provide an additional parallel path between the main segments which has interposed therein a variable resistance that cation, in which either short-circuit rectification or open circuit rectification may be employed, that is to say, in which either the main brushes may bridge the main commutator segments at the instant of commutation or may leave one main commutator segment before engaging the succeeding commutator segment during rectification, in both cases means being provided whereby the load circuit remains unbroken and whereby a bridging path is provided between the main brushes at the instant of rectification.

Further objects are to provide a construction in which relatively wide brushes may bear on the main segments, thereby allowing the axial length of the segments to be shortened although a relatively high current i handled by the device, the invention being designed to provide proper commutation whether short-circuit rectification or open circuit rectification is employed, the bridging circuit or bridging circuits coming into play to either directly or partially short-circuit the segments by providing a bridging path at the instant a brush leaves a segment, engages an approaching segment, or bridges successive segments.

Advantage is taken of the fact that the brush voltage drop does not vary linearly with the current but remains approximately constant thou it increases very slightly with increase in current. The voltage drop is relatively high for small currents and is relatively lower for large currents.

This invention utilizes this phenomenon and further objects of the invention are to provide a commutator in which advantage is taken of the variation in brush voltage drop to assist in obtaining substantially sparkless commutation, the arrangement being such that several brush voltage drops are arranged in series at the instant of commutation.

Further objects are to provide a rectifier which secures substantially sparkless commutation though providing an eflicient apparatus.

An embodiment of the invention is shown in the accompanying drawing, in which the single figure isa view showing a synchronous rectifier.

The invention in its broadest aspects is directed to commutating means. However, for the purpose of illustration the invention has been shown in the form of a synchronous rectifier.

Referring to the drawing, it will be seen that a synchronous motor indicated generally at I has been provided and is supplied from the single phase alternating current mains 2. The synchronous motor may be provided with a direct current field 3 which is connected to a source of direct current hereinafter described and whose excitation may be varied by means of a rheostat 4-if desired. The rotor 5 of the synchronous motor isconnected to a pair of slip rings'fi and 'l and to a main commutator indicated generally by the reference character 8; The main commutator is provided with a pair of main segments 9 whichare respectively connected to the slip rings 6- and l. The main commutator is also provided with apair of auxiliary segments H] which are directly connected by means of a bridging circuit H. Main brushes l2bear on the main commutatorand these main brushes are connected to the-'loa-d'indicated at'i3. Direct current is also supplied from the main brushes l'2'to the field 3 of the synchronous motor as shown.

A transformer indicated generally at M supplied from the alternating current mains 2 has opposite ends of its secondary l5 connected to brushes l6 and i"! bearing on the'slip rings 6 and i respectively. 7

Obviously ifdesired the synchronous motor could lee-connected to the slip rings and the revolving main commutator by meansof gearing if dBSlTSCL'PI'OVldBd the main commutator was altered: so as to have-a suitable number of main and auxiliary segments and brushes.

It isobvious that asthe' main commutator rotates synchronously, it will alternately connect the load with opposite sides of the terminals of thesecondary I5 of the transformer and will thus rectify the alternating current.

It is to be noted that the auxiliary segments Ill are directly connected by a substantially shortcircuiting bridging circuit ll so that as the main brushes IZare leaving the main segments, they will contactthe auxiliary segments l0 and thus a bridging circuit will be established directly between the main brushes without involving the main commutator segments. Thus the energy of the current that seeks to continue to flow in the load circuit due to inductance or capacity 'for instance is dissipated prior to disconnecting the load circuit from the main segments then engaged bythe main brushes". "Also if at the instant, of commutation the alternating current supplied the main segments is not passing through the zero point, the bridgingcircuit will provide means for connecting the main segments together through the auxiliary segments and the main brushes. There is a double voltage drop in this circuit for each brush, namely, that between the main commutator segment and thebrush and between the brush and the auxiliary commutator segment. Upon further rotation of the commutator, the

main brushes will bridge the gap between the main segments and will also continue to engage the auxiliary segments and as the main brushes leave the receding main segments and engage the approaching main segments, the main brushes will still be in contact with the auxiliary segments.

This arrangement and this mode of commutation provides for substantially sparkless commutation.

It is to be noted that by the arrangement described the load circuit is never broken nor is the circuit including the secondary of the supply transformer ever broken. Instead a bridging path is established prior to the time that the l brushes leave the main segments and this bridging circuit is maintained while the brushes are engaging the approaching main segments.

This construction also utilizes the peculiar characteristic of the voltage drop at the brushes. It is well known that the voltage drop does not vary linearly with the current but remains approximately constant, increasing only 'very slightly with increase in current. The voltage drop is relatively high for small currents and is relatively lower for large currents.

It is apparent, therefore, that there is not a material voltage drop in the normal operation of the main brushes on the main segments of the commutator while the load circuit is being supplied with maximum current. However, during commutation the current flow is relatively small and consequently the voltage drop at the brushes is relatively high and, from the discussion given hereinabove, it will be seen that as the main brushes are leaving the main commutator segments, there is a double voltage drop at each brush for any remaining small current'flow. This same condition exists when the' main segments are bridged by the brushes and also exists when the brushes are engaging, the approaching main segments; It will; be seen, therefore, that substantially sparkless commutationresults though an extremely simple arrangement is provided;

It is to be noted that the revolubl'y, mounted unit. consists ofthemain commutator previously described and indicated generally at 8 and an auxiliary commutator indicated generally at [8.

The auxiliary commutator i8 is, provided with a plurality of segments IS which are connected by a closed resistor formed of a plurality of sections indicated at 26,21 and 22,. Only a few of the sections and only a few ofthe segments are shown,

but as many segments and as many sections. of the resistor as desired may be provided. Two diametrically opposite segments. of the auxiliary commutator are respectively connected to. the adjacent main segments 9.. These. two segments of the auxiliary commutatorliron adiameter at. right angles to thediame'ter' join-ingthe auxiliary segments iii of the main commutator. It is to-be noted that the sections 29 of the resistor have minimum resistance and that. the sections 22have maximum resistance and thatv the intermediate;

ratio. Obviously any other mode of variation is comprehended by this invention, provided the resistors 20, 2| and 22 gradually and successively increase in value.

A pair of auxiliary brushes 23 bear on the auxiliary commutator and are connected by a bridging circuit 24. This bridging circuit is preferably a substantially short-circuiting circuit. 7

The operation of the apparatus has been partially described hereinabove and in addition it will be seen that a bridging circuit is formed between the main segments of the commutator through the medium of the auxiliary commutator, the auxiliary resistor and the bridging circuit. This interposed path varies from a minimum value of resistance preferably approximating short-circuit during commutation to a maximum value of resistance at periods farthest removed from the period of commutation, that is to say,

when the main and auxiliary commutators have rotated 90.

The reason for having the sections of the closed resistor the lowest value and the sections 22 the largest value with the intermediate sections varying between these two limits, is to provide,for the gradual insertion of resistance in the bridging circuit at approximately the time commutation is taking place so as to maintain the voltage difference between the main segments of the main commutator a minimum at this instant and to gradually build up this resistance at a substantially increasing rate as the period most widely spaced from commutation is approached so that there will be substantially no sparking during commutation and also so that there will be the minimum loss or waste through the resistor at other times. In fact with this arrangement there issubstantially sparkless commutation at the auxiliary commutator also.

Even if the zero point for both the voltage and current does not occur at the instant of commutation, nevertheless the auxiliary circuit is not suddenly broken but instead resistance is gradually interposed in this auxiliary circuit which resistance increases to a maximum and thereafter decreases as hereinabove. described.

From the above discussion it will be seen that substantially sparkless commutation takes place at both the main commutator and the auxiliary commutator.

It is to be noted particularly that this invention will take care of very adverse conditions even when there is considerable phase displacement between the voltage and current curves. The auxiliary brushes cooperating with the auxiliary commutator and with the resistor automatically produce a zero potential or neutral zone at that part of the cycle where the main brushes bridge the segments of the main commutator regardless of the conditions of the load circuit.

It is to be noted that the synchronous motor may be relatively small as it does not transmit the power load of the load circuit but merely serves to rotate the several rotary parts in synchronism with the alternating current.

It is to be noted further that the main brushes may be made circumferentially as wide as desired and that the gap between the segments of the main commutator may also be made as wide as desired. Thi permits increasing the circumferential width of the main brushes and correspondingly reduces the axial length of the commutator for the same current density. In other words,

the axial length of the current carrying segments may be materially shortened.

It is to be noted that in the description of the invention chosen for illustration short-circuit commutation has been illustrated and described as the main brushes bridge the main segmentsv during commutation, but it is to be distinctly understood that open circuit commutation, as described hereinbefore, could be-employed if so desired. Even with open circuit commutation, before the main brushes leave the main commutator segments, the short-circuit bridging path is established and this short-circuit bridging path is maintained after the main brushes arrive at the succeeding commutator segments. However, the main brushes in open circuit commutation would never bridge the two main segments. Even for open circuit commutation as described and for short-circuit commutation as shown and described substantially sparkless commutation is obtained by following the practice hereinabove set forth.

It will be seen that a, novel means of commutation has been shown and described. It is to be distinctly understood that although the showing and description have been directed to synchronous rectifiers, nevertheless the principles of commutation hereinabove outlined may be applied in other ways and the invention except as recited in the claims is not to be limited to rectification. Therefore, .although this invention has been described in considerable detail, it is to be understood that such description is intended as illustrative rather than limiting, as the invention may be variously embodied and is to be interpreted as claimed.

I claim:

1. A device for commutating electric current comprising a revolubly mounted unit including 49 main and auxiliary commutators, said main commutator having main segments and auxiliary segments located between said main segments, said auxiliary commutator having segments, said unit including a resistance, a bridging circuit connecting the auxiliary segments of said main commutator, said auxiliary commutator having its segments connected to spaced points of said resistance and having spaced segments connected to the main segments of said main commutator, main and auxiliary brushes bearing on said main and auxiliary commutators respectively, and a bridging circuit connecting said auxiliary brushes.

2. A device for commutating electric current comprising a main commutator having main segments and having auxiliary segments located between the main segments and an auxiliary commutator having segments, main and auxiliary brushes bearing on said main and auxiliary commutators respectively, means for leading current to all of the main segments of said main commutator, a continuous closed resistor connected at spaced points to the segments of said auxiliary commutator, certain segments of said auxiliary commutator being connected to th main segments of said main commutator, the value of the resistance between the segments of the auxiliary commutator adjacent the segments thereof connected to the main commutator being a minimum and the value of the resistance between segments of the auxiliary commutator farthest spaced from the segments of the auxiliary commutator connected to the main segments of the main commutator being a maximum and the intermediate resistances varying progressively therebetween, a bridging circuit connecting the auxiliary segments of said main commutator, and a bridging circuit connecting said auxiliary brushes.

3. In a device for rectifying alternating current, a main commutator having main segments arranged to be supplied from a source of a1ternating current and having auxiliary segments located between said main segments, a bridging circuit connecting the auxiliary segments of said main commutator, an auxiliary commutator having segments, said main and auxiliary commutaters being designed to be driven in synchronism with the alternating current, spaced seg ments of said auxiliary commutator being connected to the main segments of said main commutator, a resistor having spaced points connected to the segments of said auxiliary commutator, auxiliary brushes bearing on said auxiliary commutator and electrically connected by a bridging circuit, and main brushes bearing on said main commutator and adapted for connection to a load circuit.

4. In a device for rectifying alternating current, a main commutator having main segments arranged to be supplied from a source or alternating current and having auxiliary segments lo=- cated between said main segments; a bridging circuit connecting the auxiliary segments of said main commutator, an auxiliary commutator having segments, said main and auxiliary commue tators being designed to be driven in synchronism with the alternating current, spaced segments of said auxiliary commutator being connected to the main segments of said main commutator, a closed resistor formed in successive sections and being connected to the segments of said auxiliary commutator between successive sections, the sections. adjacent the segments of the auxiliary commutator which are connected to the main segments of the main commutator having a minimum value and the successive sections of the resistor increasing to a maximum value at points spaced farthest from the Segments of the auxiliary commutator which are connected to the i'nain segments of the main commutator, auxiliary brushes bearing on said auxiliary commutator and electrically connected by a bridging circuit, and main brushes bearing on said main commutator and adapted for connection to a load circuit. I H

KLAUS L. HANSEN.

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