USRE21407E - Control system - Google Patents

Description

March 26, .1940.

common SYSTEM Origingl Filed Dec} 21;, 1934' INVENTOR. MAL/AM 7. 63005 ATTORNEY.

w. T. CRODEN Re. 21,407

Reiuuecl Mar. 26, 1940 I g V Re. 21,407

UNITED STATES PATENT OFFICE Original No. 2,086,120,. dated July 6,1987, Serial No. 758,628, December 21, 1934. Application for reissue May 8, 1939, Serial No. 272,278

1: Claims. (Cl. 471-214) My invention relates to alternating. current elated to be varied in accordance with the speedcontrol systems, either single or polyphase and load-voltage characteristic ofsuch motor. has particular reference to a system for con- It is also an object of my invention to provide trolling the voltage applied to alternating cura control system of the character set forth in the 5 rent motors having series characteristics in such preceding paragraphs in which a current transmanner that the speed of such motors is mainformer and rectifier associated therewith are emtained constant, irrespective of load. ployed to excite the direct current control wind- In certain machine tool applications it has been ing employed for saturating the saturable core refound desirable and oftentimes necessary to supactor. y

lo ply electric motors for their operation having ex- It is also an object of my invention to provide tremely high starting torqu and hi h .torque a. special current transformer for use in a control sensitivity. In these cases it is common practice system of the character setforth in the preceding to employ series type direct current motors, but para raphs whi h includes mea s fo p du the use of such motors i acco pani d by ra non-linear relationship between the changes in ous disadvantages. current flowing through the primary of. said cur- 15 The use of direct current seri motors ne rent transformer and the voltage induced in the sarily requires a source of direct current for transformer lfoperation. This is oftentimes diillcult or ex- It 8 8-150 an ject my invention to pr i e pensive to obtain, since it is common practice to s st m 01' t a t r s t th in th p dins go supply industrial and manufacturing establish- Paragraphs in which -8 W ctlflers are om ments with alternating current type of power. P y d in Combination with e P Shifting e-- u th th di t current series m t vice to providevthe desired regulation of voltage while supplying demands as regards torque h in accordance with the speed load voltage characteristics, has a badly drooping inherent speed acteristic of the motor with which the system is regulation curve resulting in wide fluctuations employed. 25 in motor speed t moderate changes in load It is a further object oi my invention to provide upon t t a control system of the character set forth in the As is well known constancy of speed is also a Preceding pa a raphs in whi h s mp d an desirable and oftentimes essential requirement adjustable means for establishin! the p d 0! go in the operation of machine tools, and for this the motorwhieh will be maintained y 8u-' w reason direct current series motors are, to some lilting equipment extent, unsatisfactory for this application. This Other i s d v ta es of my invention diillculty has been overcome in some applications will be apparent from Btudy the 0w 8' by employing t t speed motors of t specifications, read in connection with the accomduction or squirrel-cage type which have been Panymg drawing wherein: specially wound to provide the desirable torque s- 1 is a schematic representation of the syscharacteristics. Such motors are, however, very m of y invention as a p d to a Single Phase inemcient and require the use of a mechanical c t including n alternating current series transmission or gear change box in order to promiltor; 40 vide the necessary speed selection to permit dif- 2 15 schematic representatmn of a 0 ferent machining operations to be performed urable core reactor which \is Particularly with the machine tool with which such motor is Plicable use the systems illustrated EL Referring to the drawing. I have illustrated diagrammatically a regular alternating current It is therefore an object of invention to roi vide a control system which en combined v vith Service or source of power comprising line ductors I and 2, it being understood that a local a series type alternating current motor will pro-- enemtor is a com lete m 1 t Tr 1 vide the typical series type torque characteristics 8 p eq Va en ac n8 these leads, the line .I connects first thro and operate to maintain the speed of such motor V fi ld 3 and mature clan alternating f i 5o constant ir esp ti of load variations series motor which represents the load or point so It is a further object of my invention to proof utflization, vide a control system of the character set forth A primary indin 3| of a current transformer in the p ec p aph in which a saturable' 32, and a series winding 33 of a saturable core core reactor is employed to cause the voltage apreactor 34 are also connected in series with the 5 plied to the motor with which the system is assomotor 3-4 across the supply lines i-2. Direct current components of magnetic flux in the reactor core are supplied through a pair of rectifler tubes of the so-called grid-glow or "Thyraton" type. These tubes are supplied by a transformer whose primary It is bridged across the lines I and 2, and whose secondary 81 is connected between anodes 38 of the two rectiflers 35. A center tap it of the secondary winding 31 connects through a pair of series connected direct current control windings 40 and 4| of the saturable reactor 34 and thence back to cathodes 42 of the rectifier tubes 35. Grids 43 of these tubes connect to opposite ends of a primary winding 44 of a transformer 45, a center tap of which winding also connects with the cathodes 42.

A secondary winding 48 of the transformer 45 is bridged by a condenser 41, and is connected between the line I and a series winding 4! of a second saturable reactor is, and thence backto the other conductor 2. V

' saturating windings i0 and ii of the reactor 49 are connected in series to be supplied by a bridge-type rectifier 52- from a secondary SI of the current transformer 32.

In this arrangement the current supplied by the current transformer 32 may be extremely small, since its function is merely to change the phase of the voltage supplied to the grids II of the grid-glow rectifiers. The actualv power drawn by these grids will be small, for the grids merely serve to determine at what portion 01' the cycle the tubes 35 shall start to draw. current. The control of rectiflers of the grid-glow type by change of phase of the grid voltage is suflicient- 1y well known so that it is unnecessary to .describe it here, it being obvious that the division of voltage between the transformer primary 48 and the reactor coil 4! will change in both phase and magnitude as the saturation of the reactor core 49 changes, and that through this means very large changes in the saturation of the reactor core 34 may be accomplished with relatively small changes in the power supplied by the current transformer 32. The voltage supplied acrom the motor may therefore be caused to vary very rapidly with slight changes in load, thereby providing extremely accurate and extremely rapid control of the speed of the motor I-4'.

The operation of the device will be understood by considering the motor as operating with a minimum load. Under these conditions the voltage supplied by the source l2 will be divided betweenthemotoi-Hand thereactorseries windings 83. Normal current through the primary winding ll of the current transformer 32 wiilcauseasmallvoltagetobei ducedinthe secondary winding 53. Current flowing as a resultofthissmallvoltagewillberectifledandim' duce a negligible D.-C. component in the con trol windings 50-". This results in the voltage which is applied to the grids of the grid-glow This would n'ormallycause an increased voltage drop in the'windings 33, but theincreased currentwillcalmea'largercurrenttoflowinthe rectifier circuit and-the control windings ll-Il,

the saturable reactor 4! and eachothersothatanincreaseddirectcurrcntis caused to flow in the control windings 40-. The voltage across the reactor windings 33 is therefore decreased instead of increased by the increased flow of current.

Attention is called to the fact that alternating current series motors have a series characteristic which requires an increase in terminal voltage to provide an increased power output when operated at a constant speed, and also that constant speed with changes in load cannot be accurately maintained with the regulator just described if the amount of direct current flowing in the direct current windings 4ll 4'l is made exactly proportional to the amount of current flowing through the primary of the current trans former 32. This is due, in part, to the particular characteristics of the series motor employed and due also, in part, to the relative proportions of the iron circuit employed in the saturable'reactor 34 and its relation to the windings placed thereon. For this reason the physical constants of the current transformer 32, saturable reactor 4|, and capacitor 41 are accordingly adjusted with respect to the saturable reactor 34, the characteristics of the tubes 35, and the particular series motor 3-4 with which the regulating equipment is to be used so as to provide the desired non-linearity of response; l have found that proper adjustment of these factors permits me to achieve substantially constant. speed regu- It is to be'understood that the drawing shows but one of a large number of modifications of my invention using rectifier tubes of the gridglow type, since there are many circuits using these tubes in either tull'wave or half wave relationship which. are well known in the art.-

It should also be noted that while the principal object of my invention is to provide an alternating current series motor and control equipment operating in combination therewith to maintain a substantially constant rotational speed of the but in attempting this due care should be exercised that unexpected voltages be not developed across the other windings. It is especially desirable that care should be given to the design of the saturated reactor since it is possible to-develop fairly high voltages in the various portions of the D.-C. windings even though the voltage across the terminals may be zero.

One desirable form of core for such reactors is illustrated schematically in Fig. 3. This core construction preferably includes three core legs It, II and I8 mutually interconnected each with the other by means 01' a triangular upper end yoke comprising portions II, I and ti and a similar lower yoke comprising members it. it and 64.

In order to achieve the desired balancing of fluxes so as to prevent generation of unexpected or undesired voltages in a direct current control twindinmitlsessentialthatthismagnetic m imums-taxman series A.-C. windings 85 and 68 are connected, either in series or in parallel, so as to excite a magnetic circuit comprising the legs-51, 58 and the end' yoke members 59 and 6,2 .of the core. Under these circumstances the magnetomotive force that is developed across the leg 56 is extremely small, since if the device be truly symmetrical the magnetomotive forces across its ends are such as to balance out. It is upon this leg 56 that a saturating or D.-C. control winding 61 is disposed, the magnetic circuit of this winding speed of said motor, irrespectiveof load variations thereon.

It will also be noted that the system of my invention is adapted to use with relatively large motors of this type and that with such a system the amount of power required by the controlling mechanism is relatively small so that even when combined with series type alternating current motors of relatively large horsepower, the emciency of the system is not seriously impaired.

While I have shown and described the preferred embodiment of my invention, I do not desire to be limited to any of the details of construction shown or described herein, except as defined in. the appended claims.

I claim: I 1. In an alternating current system comprising a source of power and a load, means for regulating the power supplied to said load comprising a current transformer and -a saturable reactor in series with said load, a rectifier system including rectifier tubes of the grid-glow type connected to supply a direct current component to' said saturable reactor, a circuit including a second saturable reactor connected to supply a control voltage to said grid-glow tubes, and a rectifier supplied by said current transformer and connected to supply a direct current component to said second reactor.

2. In an alternating current system comprising a source of power and a load, a saturable reactor and a current transformer connected in series with said load, means connected across said source for supply a saturating current for said reactor, a rectifier system supplied by said current transformer, and means controlled by the output of said current transformer and includirig a second saturable reactor for controlling the saturating current supplied to said first mentioned saturable reactor.

3. In an alternating current system comprising a source of current and a load, means for regulating the power supplied to said load from said source comprising an impedance and a current transformer in series withsaid load, a rectifier system connected to the output of said current transformer, a saturable reactor connected to derive its saturating current from said rectifier system, means including a tube of the grid-glow type for controlling said impedance, and a control circuit for said grid-glow tube. including said saturable reactor.

4. In combination with an alternating current motor having series characteristics, means for increasing with increasing loads the voltage supplied to said motor from a substantially constant source, comprising a saturable reactor and a cur-f rent transformer connected in'series with said sourceand motor, a rectifier-system connected to supply saturating current tosaid saturable re actor, and means controlledby current from said transformer and comprising a second rectifier and saturable. reactor for controlling the-current supplied by said rectifier system.

5. In combination with an alternating current motor having series characteristics, means for increasing with increasing load'the voltage supplied to said motor from a substantially constant po tential source comprising a saturable reactor and a current transformer connected in series .with said source and motor, a grid glow tube rectifier system connected to supply saturating current to said saturable reactor, and means controlled by current from said transformer and comprising a second rectifier and saturable reactor for controlling the current siipplied by'said rectifiersystem.

a source of alternating current; an electric motor having a series characteristic connected to said 6. In an electric circuit, the combination of:

source of alternating current; and a regulator.

interposed between said source of current and said motor comprising a saturable reactor having an impedance winding connected in circuit with said motor and a direct current control winding, means for supplying direct current to said control winding, and means responsive to changes in load current of said ,motor for simultaneously tric motor having a series characteristic connected.

to said source of alternating current; and a'regulator interposed between saidsource of current and said motor comprising a current responsive means connected in series with said motor, a saturablereactor havinsan impedance winding connected in circuit with said motor, and a direct current control winding. control means connected between said current responsive means and said control winding for supplying direct current to said control winding to vary the impedance of said impedance winding oppositely relative to changes in current flowing through said current responsive means, and means associated with said current responsive means for controlling the amount of direct current in said control winding to vary the terminal voltage of said motor noniinearly relative to the current flowing in said current responsive merans in accordance with said series characteristic to maintain said motor speed at a substantially constant value independently, of variations in load on said motor.

8. In an electric circuit, the combination of: a'source of alternating current; an electric moto:- having a series characteristic connected to said source of alternating current; and a regulator interposed between said source of current and said motor comprising a saturable reactor' secondary winding to supply direct current to said impedancewinding oppositely relative to current transformer primary winding in accordsaid control w m, the impedance of linearly relative tothe current flowing in said changes in current flowing through said current ance with said series characteristic to maintain transformer primary winding, and means'associsaid motor speed at a substantially constant value ated with said secondary winding for controlling independently of yariations in load on said motor.

the amount of direct current in said control wind- T ingto vary the terminal voltage of said motor non- WILLIAM T. CRODEN.

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