US2760142A

US2760142A - Equalizing series resistors for parallel circuits in rectifiers

Info

Publication number: US2760142A
Application number: US452030A
Authority: US
Inventors: Richard C Hitchcock
Original assignee: Syntron Co
Current assignee: Syntron Co
Priority date: 1954-08-25
Filing date: 1954-08-25
Publication date: 1956-08-21
Anticipated expiration: 1973-08-21

Description

Aug. 21. 1956 R. c. HITCHCOCK 2,760,142

EQUALIZING SERIES RESISTORS FOR PARALLEL cmcums IN RECTIFIERS Filed Aug. 25, 1954 2 Sheets-Sheet 1 AC 7 7 /7 Fig.5

P \J v A c 30 a r If INVENTOR. Q1 cHAeo CH/TcHcOcK H15 A 7'T'OEMEY 21. 1956 R. c. HlTCHCOCK 2,750,142

EQUALIZING SERIES RESISTORS FOR PARALLEL CIRCUITS IN RECTIFIEIRS Filed Aug. 25, 1954 2 Sheets-Sheet 2 QQF M) INVENTOR. ,Q/cHA 20 C. H/TCI-ICOCK H s ATT'OQME'V Fig.4,

United States Patent EQUALIZING SERIES RESISTORS FOR PARALLEL (IIRCUITS IN RECTIFIERS Richard C. Hitchcock, Indiana, Pa., assignor t0 Syntron Company, Homer City, Pa., a corporation of Delaware Application August 25, 1954, Serial No. 452,030

6 Claims. (Cl. 321-27) This invention relates generally to rectifiers and more particularly to equalizing series resistors for use in parallel paths of connected rectifiers. Rectifiers, and particularly those of the dry disc type, have a forward nonlinear current characteristic. This nonlinear variation in voltage drop for different currents in the forward direction varies for different rectifiers of the same type and size. Some rectifiers may be matched at a specific voltage drop for a specific current value but a different current value may give an entirely different voltage drop in these rectifiers.

When rectifiers are connected in multiple to increase the rectified current rating some of these rectifiers may be evenly matched in their voltage drop for a specific current or current range but if the current load varies from this current or current range, the rectifier having the lowest voltage drop will assume most of the load and eventually will break down, causing destruction of substantially the whole circuit.

The principal object of this invention is the provision of an equalizing series resistor in each multiple path of the parallel connected rectifiers whether they are tubes, vibrators or dry disc type of rectifiers to limit the current flow in any one path and thus attain equalization of the current to such a degree that the rectifier of the lowest voltage drop will not assume most of the load and burn out. By preventing the failure of one rectifier of a parallel group the circuit and the rectifiers are maintained in service for a materially longer period of time.

In the case of a dry disc type of rectifier the whole stack is saved because the low voltage drop of only one cell was prevented from taking enough current to burn it out.

In dry disc rectifiers having many multiple paths and a few or no cells in series this problem is much more serious. The greater the number of multiple paths, the more acute the problem.

This is readily solved by inserting a resistance in series with each multiple path and thus limiting the final distribution of the current.

Other objects and advantages appear hereinafter in the following description and claims.

The accompanying drawings show for the purpose of exemplification without limiting the invention or claims thereto certain practical embodiments illustrating the principles of this invention wherein:

Fig. 1 is a diagrammatic View of a plurality of mercury arc rectifiers connected in multiple and each having an equalizing series resistor.

Fig. 2 is a diagrammatic view of a plurality of diodes connected in multiple and each has an equalizing series resistor.

Fig. 3 is a diagrammatic view of a plurality of dry disc rectifiers connected in multiple and each having an equalizing series resistor.

Fig. 4 is a diagrammatic view of a plurality of full wave rectifiers of the dry disc type connected in multiple "ice and each leg of each rectifier has an equalizing series resistor.

Fig. 5 is a diagrammatic view of a plurality of full wave rectifiers of the dry disc type connected in multiple and each leg has one or more rectifier cells in series with an equalizing resistor.

Fig. 6 is an actual showing of a plurality of full wave rectifier cells of the dry disc type stacked and connected in multiple for a three phase system and each cell is connected through a series resistance to their respective circuit line to limit and to equalize the current flowing therethrough.

Referring to Fig. l of the drawings the

mercury arc rectifiers

1, 2 and 3 each have two electrodes 4 and 5 and are connected by the filament 6. Each of the electrodes 4 and 5 are connected in multiple and the total is connected in series with the load L. The electrode 4 of each rectifier is provided with a resistor such as indicated at 7. These resistors 7 may be as much as fifty per cent of the resistance to the flow of current through the operating rectifier. The filament 6 of each rectifier provides sufficient heat to vaporize the mercury sufficiently to carry the are between the electrodes 4 and 5. However, the resistance of the path through the filament is so much higher than that of the ionized vapor between the electrodes that very little current passes through the filament when the rectifier is operating. The resistances 7 are likewise so much lower than the filament that they are negligible but they may range from ten per cent to fifty per cent of the resistance of the gap which is sufficient to equally distribute the load between the

rectifiers

1, 2 and 3.

A similar circuit is shown in Fig. 2 using

diodes

8, 9 and 10 each having the usual cathode 11 and the anode 12. In series with each of these rectifier circuits a resistor 13 is placed which may be of the magnitude of from ten per cent to fifty per cent of the resistance of the average diode of the same size and characteristics. Here again the load L is placed in series with the multiple paths formed between the three diodes.

In Fig. 3 a group of three dry disc type rectifiers are placed in multiple as shown at 14, 15 and 16. These may be any type of dry disc rectifier such as copper oxide or selenium or any of the other types of rectifying elements. Each rectifier cell shown in this view is provided with a series resistor 17 and the Whole of the pulsating current flow from an alternating current source passes through the load L to complete the circuit.

In the circuit illustrated in Fig. 4 the group of dry disc rectifiers are connected into three complete full wave bridge circuits 18, 19 and 20. These three bridge circuits are in turn connected in parallel.

The direction of current flow or low voltage drop is indicated by the direction of the arrow and the resistance in this direction must be limited to avoid ununiform distribution of current through these rectifier cells. A limiting resistance 25 is placed in each leg of each bridge to limit the flow of current therethrough and to equally distribute this current load. This current limitation may be as low as twenty per cent of the resistance of each cell or as high as fifty per cent of the voltage drop in the direction of flow of the high current through the cell. This voltage drop may be in the nature of .35 volt to .50 volt for cells having a voltage range of 1.1 to 1.3 volts which approximates i g=26.8% or =45.5%

The load current flowing through these series resistors reduces the available voltage for the load and it is this small voltage drop which is sensitive to changes in for- Patented Aug. 21, 1956'- Each bridge circuit 1 has a rectifier in each of its four

legs

21, 22, 23 and 24..

ward resistance of the cell and thus has the tendency to equalize the current flow between the several cells in parallel. This equalization minimizes the variations in the cells themselves and properly distributes the current between the cells regardless of their matching. This distribution remains constant as the lower the cell resistance, the higher the current and the higher the resistance drop to correct this fault. Thus the cells do not have the tendency to become overloaded which causes them to burn out.

If the cells are stacked with several in series as Well as in parallel the equalizing series resistor still functions to produce a more uniform distribution of current in the parallel fields. This is illustrated in Fig. 5 Where the full wave rectifiers 26, 27 and 28 are connected with the series resistance 30 in the same manner as that illustrated in Pig. 4 except the

cells

31 and 32 in each rectifier leg represent two or more cells in series with each other.

Fig. 6 illustrates the actual rectifier stack as made up of rectangularly shaped cells approximately twelve bysixteen inches in which there are sixty cells to the complete stack. This rectifier stack is assembled for operation on three phase alternating current.

The rectifier cells 31 are single discs and the forward direction of every two discs is in the same direction, the o direction of alternate pairs of discs is the same, and the direction of adjacent pairs of discs is opposite. These discs are separated by connection plates or straps 32 which may be for the A. C. terminals as well as the plus and minus D. C. terminals. These discs and plates, because of their size, have six uniformly positioned holes to receive the six stacking bolts 33. These bolts are tacked together at one end by a mounting plate 34 having a bracket 35 at one end. The bolts are insulated and as the plates are stacked thereon in the proper direction as indicated by the arrows, they are spaced by current carrying washers 36 but are insulated at their ends by the insulating washers 37.

The rectifier discs 31 as shown have a conducting metal base such as aluminum which is coated with selenium and an oxide is provided on one face of this disc. The metallic side of the disc has adjacent thereto an ordinary conducting washer but the selenium side of the washer has a special spring type collecting washer 38. The forward direction of this selenium cell is from the metallic side of the disc to the oxide side against which the collecting washer engages. Thus these collecting washers show the direction of the forward current flow to the plus of the D. C. It will be seen from Fig. 6 that there is only one cell in each leg and that the stack ismade up into full wave rectifier bridge circuits each having four legs with one cell per leg. These are fifteen such bridge circuits in this single stack and they are all in parallel.

The connection plates or straps 32 on the D. C. side are provided with the resistors 40 that connect them to the bus lines 41 and 42 of the plus and minus terminals of this rectifier.

On the A. C. side the straps 32 are connected directly 4 to three bus lines, A. C. 1, A. C. 2, and A. C. 3 which represent the three alternating current phases of a three phase source.

There being two rows of three assembly bolts 33 to make this stack, it is preferable to employ a connector plate or strap 32 on each row of bolts for each connection. It is then necessary to connect the corresponding straps 32 from the same positions. This is done by the bus bars 43 on both the A. C. and D. C. sides of the bridge circuits. However, only one resistor 40 is placed in each leg as a multiple of resistances may create another problem of current unbalanced conditions in the resistors themselves.

I claim:

1. A rectifier circuit comprising a plurality of rectifiers connected in a closed four legged bridge circuit, a plurality of said bridge circuits connected in parallel to produce full wave rectification from alternating current, and a series resistor in each leg of each parallel path, said resistors having substantially the same resistance.

2'. A single stack of rectifying cells, a series of connector plates intermediate selected of said rectifier cells, a plurality of resistors each having one end connected to each of said connectors, and connections with the other ends of said resistors placing selected of the cells in multiple.

3. A full wave rectifier stack comprising a series of groups of rectifier cells mounted in uniform directional patterns, a series of connector plates stacked between the rectifier cells, a resistor connected at one end to each connector plate, and circuit connectors attached to the other ends of selected resistors to complete a full wave multiple path rectifier.

4. The structure of claim 3 characterized in that said resistors are all substantially equal to each other.

5. The structure of claim 3 characterized in that said resistors provide a voltage drop of from twenty per cent to fifty per cent of the forward voltage drop of the rectifier in series therewith.

6. A full wave rectifier stack having alternating current and direct current connections comprising a plurality of rectifier cells mounted in parallel full wave four legged bridge connection, connector straps between the cells forming the independent legs of each bridge circuit for connection with the alternating current and direct current connections, and a resistor connected between each direct current connector strap of the stack and the direct current connections of the stack, each of said resistors having substantially the same resistance.

References Cited in the file of this patent UNITED STATES PATENTS 1,110,550 Hewitt Sept. 15, 1914 1,723,525 Ruben Aug. 6, 1929 2,088,474 Haller July 27, 1937 2,188,436 Hugus Jan. 30, 1940 2,444,255 Hewlett June 29, 1948 2,521,687 Cameron et a1. Sept. 12, 1950.