US2399363A

US2399363A - Regulating system

Info

Publication number: US2399363A
Application number: US533083A
Authority: US
Inventors: Cyril C Levy
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1944-04-27
Filing date: 1944-04-27
Publication date: 1946-04-30
Anticipated expiration: 1963-04-30

Description

April 30, 1946. c. c. LEVY 2,399,363

REGULATING `SYSTEM Filed April 27, 1944 /CO. BY

Patented Apr. 30, 1946 UNITED "TENT OFFICE y 2,399,363 x REGULATING SYSTEM Cyril C. Levy, Plttsburgh,.Pa., assigner to Westinghouse Electric Corporation, East Pittsburgh. Pa., a corporation oi Pennsylvania Application april 27, 19H44', serial No. 533,083 s claims. '(ol. 314-64)4 Thisinvention relates to a l in particular to regulating systemsl for larc furnace regulators. L

Electronic regulating systems have been employed heretofore for controlling the operation o f reversible motorslin response to predetermined changes in operating conditions of the system with which the motoris employed. In practice, it has been found that in such systems utilizing a plurality of sets of electronic tubes for controlling the reversing ofithernotor, that although as ageneral rule one set of the tubes conducts while the other setk is nonconducting, there also develops a condition in which the conducting period is transferred immediately from one set of tubes to the other set of tubes, thereby causing hunting in the regulating system. Further, where such systems have been employed for arc furnace regulation, considerable overrun has often been encountered tothe detriment of the electrodes;

An object ofthis invention is to provide a electronic regulating system'havlng a controllable dead zone of operation for preyenjtlng hunting.

Another object trol of the operation of a'rk'aversible direct-current feed motor. in Laccordance with predetermined conditions in an associated alternatingcurrent load circuit while preventing hunting.

A further object of this invention is to provide an electronic regulatingf'nsystem having antlhunting characteristics and dynamic braklngfor controlling the operation `of ,a reversible motor.

Other objectsof this invention will -apparent from the following description when taken in conjunction with theaccompanylng drawing, the single figure of which is a diagrammatic view of an are furnace regulating system embodying the teachings of this invention.

Referring to the drawing, an electric furnace I is illustrated, the furnace comprising a receptacle i2 containing a bath of metallic material fI4 and three movable electrodes Il, i8, and 20.

control lsylltem' and I of this invention is to provide, a simple and highly sensitiye electric dischargej regulating system for eifecting a sensi tive-con-` 'control the position of the electrode lating system associated with each of the motors is of like construction and operates in the same manner, only one of the regulating systems and motors for controlling the positioning of the electrodes is illustrated.

In the drawing a reversible motor 38 comprising an armature 38 and a separately excited field winding 40 is illustrated for raising and lowering the electrode 2U. 'I'he armature 38 is mechanicaliy connected to the electrode 20 in any suitable manner for effecting movement thereof and in the embodiment illustrated is connected by the shaft 42, a winding drum 44 and a flexible conductor 46 which passes over the pulley".

In order to control the energy in the load circuit, suitable means may be provided for'controlling the direction and amount of current flowing through the armature 3l of themotor 3,6 to 10. '.In` the -embodiment illustrated the electric discharge deces lill, 52 and 54,' I6 are connected in paired elation in a well known manner so for .complete rectification of the alternating current from the source of supply", supply a unidirectional current to in y. predetermined direction.

Thus the anodes 5l and Bil of Ithe

discharge devices

50 and 52 are connected to the terminals of secondary winding 62 of former 64 and anodes B6 and 63 of the discharge devices 54 and 56 are connected to the terminals of secondary winding 10 of the control transL former, the primary winding l2 of which is connected across

supply conductors

30 and 32. The cathodes I4 and 16 of

discharge devices

50 and 52 are connected by conductor 18 to conductor lli and one side of the armature 38, of the armature being connected I2 through resistor I4 to the mid I6 oi the secondary winding 62,

32, and 34 to the motor 36 or center tap Likewise the and of

discharge devices

54 and 58 are connected by conductor 92 to conductor 32 and one side of the armature 38, the other side of the armature being connected by conductor Il through resistor 94 to the center tap 96 of the secondary winding l0.

As illustrated, provision is made to control the bias of grids 93, |00 and 102, IM of

discharge devices

50, 52 and 54, 55 respectively in accordance with the current and voltage conditions of the load circuit to control the value and direction of current supplied to the motor 38 from the alternating-current source. For this purpose a current transformer |06 is disposed in conjunction with conductor 26 for applying to the control trans-y of the pairs a suitable rectifying means such as the rectii'ying bridge circuit a control voltage which is responsive to the current in the load circuit. The direct-current terminals of the bridge circuit |06 are connected to a control resistor ||0 so as to apply thereto the direct-current voltage. A rectifying means such as the rectifying bridge circuit H2 is connected to conductor 26 by conductor I4 and by conductors ||6 and |8 to the receptacle I2 whereby a control voltage which is a measure of the potential of the electrode arc is impressed thereacross. The direct-current terminals of the bridge circuit ||2 are connected to a control resistor |20 so as to apply the resulting direct-current voltage thereto.

A potential transformer |22 having its primary winding connected to the

supply conductors

80 and 32 is also connected to a rectifying bridge circuit |24 for applying a voltage proportional to the voltage of the constant supply circuit across a controlu'esistor |26 of the control circuit. Connected in series circuit with the series connected resistors ||0, |20, and |26 is another control resistor |28 disposed to have a voltage applied thereto from a rectifying device such as the tube |30 which is connected through potential transformer |38 across the

supply conductors

30 and 32. The direct-current voltage across the different control resistors ||0, |20, |26, and |28 are of diilerent predetermined polarity, the purpose of which will be explained more fully hereinafter.

The amplitude control circuit comprises a bridge formed of two impedances |50 and |52 and two saturable reactors |54 and |56 having their direct-current secondary windings connected in series, the bridge circuit being connected by conductors |58 and |60 to the

supply conductors

30 and 32 through the variable condenser I62. The direct-current secondary windings of the reactors |54 and |56 are connected by conductors |51 and |59 across the series connected control resistors ||0, |20, and |26 whereby the voltage derived therefrom is impressed across the secondary windings to control the impedance of the reactors. As illustrated, the bridge circuit is connected to the primary winding of a grid transformer |64, the terminals of the secondary winding of which are connected to the grids |44 and |46 and a center tap of which is connected through the parallel connected capacitor |66 and resistor |68 to the cathode |42.

A similar control circuit is provided for con-' trolling the bias of grids |02 and |04 of tubes 54 and 56 to control the operation of the motor 36. As in the case of

tubes

50 and 52, control resistors |10, |12, |14, and |16 are connected in series circuit relation with each other in the grid circuit of tubes 54 and 56. The control resistors |10, |12, |14, and |16 are disposed to have a directcurrent voltage impressed thereon in response to the electrode current, arc potential, constant source of supply, and the output of rectifying tube |18, respectively. For this purpose, current transformer |80 is disposed in conjunction with conductor 26 to supply the rectiiying bridge |62 and consequently the control resistor |10 with a voltage proportional to the current in the load circuit; the rectifying bridge |84 is connected across the electrode arc for supplying a directcurrent voltage to resistor |12 which is proportional to the electrode potential; and the resistor |14 is connected across a rectifying bridge |86 through potential transformer |88 to supply

conductors

30 and 32.

The direct-current voltage applied to control resistor |16 is determined by the output of tube which, similarly to tube |30, is provided with two anodes |90, |92 connected to the terminals of the secondary winding of a potential transformer |94, the center tap of which is connected to one end of the control resistor |16| a cathode |96 connected to the other4 end of control resistor |16 and two grids |98 and 200 disposed to control the output of the tube. The primary winding oi' the transformer |94 is connected across

supply conductors

30 and 32.

The amplitude of the grid voltage on the grids |98 and 200 of tube |80 is controlled by the amplitude control circuit 202 which comprises the bridge formed of two

impedances

204 and 206 and two saturable reactors 208 and 2|0, the bridge being connected through a variable capacitor 2|2 to supply

conductors

30 and 32. The secondary windings of the reactors 20B and 2|0 are connected in series and are connected by conductors 2|4 and 2|6 across the series connected control resistors |10, |12, and |14 whereby the differential voltage thereof is impressed on the direct-current windings of the reactors to control the impedance thereof. The bridge is also connected to a grid transformer 2|8, the secondary winding or which is connected to the grids |98 and 200 and the center tap of which is connected through the parallel connected capacitor 220 and resistor 222 to the cathode |96.

The direct-current voltage impressed across the different control resistors |10, |12, and |14 have different polarities depending upon the rectifying bridges used for translating the alternating current to direct current. In the embodiment illustrated the direct-current voltages which are proportional to the voltage of the supply and proportional to the arc potential, that is, the directcurrent voltage across resistors |14 and |12, respectively, are additive and oppose the directcurrent voltage across resistor |10 which is proportional to the current in the load conductor.

In the case of control resistors ||0, |20, and |26 in circuit with the grids of

tubes

50 and 52, the voltage across resistor ||0 which is proportional to the current in the load circuit is of opposite polarity to the voltage across resistor |20 which is proportional to the arc voltage and the voltage across resistor |26 which is proportional to the voltage of the constant supply. In the case of both control resistors |28 and |16, the voltages across which are controlled by tubes |30 and |18,respectively, the voltages are of the same polarity as those across control resistors |28 and |14, respectively, for placing a negative bias on the grids of each pair of

tubes

50, 52, and 54, 56 when the tubes |30 and |18 are functioning to impress a voltage across the control resistors |28 and |16, respectively.

In order to provide for braking the motor 38 to prevent overrun thereof, electronic dynamic braking circuits are provided. As illustrated, a tube discharge device 224 having an anode 226, a cathode 228 and a grid 230 and a tube discharge ...ai ik aasases #El device 282 having an anode 284, a cathode 266 and a grid 288 are connected in circuit relation with the

conductors

80 and 62 to give the required braking. The tube 224 has itsv cathode 228 connected to conductor 82 on one side of resistor 64, the grid 230 of the tube being connected through the battery 240 to the other side of resistor 84, the anode 226-being connected through resistor 242 to conductor 18 and from thence to conductor 80. Likewise, the tube 232 has its cathode 236 connected to conductor 80 on one side of resistor 94, the grid 230 of the tube being connected through the battery 244 to the other side of resistor 94, the anode being con nected through resistor 246 to conductor 62 and from thence to conductor 62. The operation of,

the dynamic braking circuits will be described hereinafter.

In operation, assuming that supply conductors 80. 62. and 84 are connected tc a suitable constant source of alternating-current power (not shown), transformer 26 for supplying the load is energized. At the same time transformer 64 for supplying power to the motor 86 and control transformers |22, |88, |94I and |88 areenergized. Since there is no current flowing in the load circuit, there ls no voltage across either of the control resistors or |10, nor is there voltage across either of the control resistors 20 or |12 since there is no arc. However, there is voltage across control resistors |26 and |14 proportional to the voltage of the constant source, the voltage across resistors |26 and |14 being so small by design, however, as to be considered as negligible with respect to voltages impressed under predetermined conditions across the other control resistors. The voltage across the resistors |28 and |14 is by design of such a polarity aS t0 place a negative bias on the grids of the pairs of

tubes

50, 52 and 54, 56, respectively.

'I'he transformers |38 and |64 are connected to supply power to the rectifier tubes |80 and |18,

respectively, the output of which is controlled by the impedance of the amplitude bridge circuits |48 and 202, respectively. As illustrated, with the bridges connected across the source of supply and when the system is initially energized, the only source of supply for the direct-current secondary windings of the reactors of the bridge circuits is the small negligible direct-current voltage across resistors |26 and |14. Under these conditions, the impedance of each of the amplitude bridge circuits is a maximum, with the result that the volt--v age on the grid transformers |64 and 2|8 is negligible, and the altemating-current component applied to the grids of tubes |30 and |18 is negligible, and the tubes operate at maximum output to give a large direct-current voltage across control resistors |28 and |16, respectively. The voltage thus applied to resistors |28 and |16 is of the same polarity as that across resistors |26 and |14, rpectively, and, therefore, co-operate therewith to positively place a negative bias on the grids of

control tubes

50, 52 and 54, 56 to prevent operation of the motor 36.

When the electrodes I6, I8, and 20 are manually lowered to a position where an arc is struck, the maximum arc potential is established and a direct-current voltage which is proportional thereto is impressed across control resistors |20 vand |12. Since the polarity of the direct-current voltage across resistor |20 is the same as that across resistors |26 and |26 when the arc is irst established, a large negative bias is applied to the grids 98 and |00 of

tubes

50 and 52 through the circuit extendingfrom the control grids 98 and |00 through conductor 99, control resistors |10, |20, |26, |28 and condutcor |0| to the

cathodes

14 and 16. Since the bias of the grids is negative, the tubes and 52 are blocked.

, However, the polarity of the direct-current voltage across resistor |12 opposes that across re sistors |14 and |16. Since the direct-current voltage across resistor |12 is larger than thatJ across resistor |14, the latter voltage 'being negligible as stated hereinbefore, a direct-currentvoltage which is the differential of the voltages across resistors |12 and |14 is impressed across the direct-current secondary windings of the reactors 2|0 and 206 of the amplitude bridge con trol circuit 202 to decrease the impedance of the bridge circuit and consequently increase the voltage on the grid transformer 2|8 connected in circuit therewith. This increase in voltage across the primary windings of the grid transformer 2|6 increases the amplitude oi' the alternating-current component of the voltage on the grids |98 and 200 of tube |80 to decrease the output therefrom and decrease the direct-current voltage applied across control resistor I16. Since the directcurrent voltage applied to the secondary windings of reactors 2|0 and 208 is a maximum under the conditions described when the arc is first established, the amplitude bridge control circuit functions to control tube |16 to substantially prevent the impressing of a direct-current voltage across resistor |16. Under these conditions the differential of the direct-current voltages across the control resistors |10, |12, |14, and |16 is such that the bias of grids |02 and |04 of motorcontrol tubes 54 and 56 is rendered lessnegative through the circuit extending from the control grids |02 and |04 through conductor 248, control resistors |10, |12, |14, |16 and conductor 250 to the cathodes 86 and 90. The control grids |02 and |04 thus being made positive with respect to the

cathodes

88 and 90, the tubes 54 and 56 arc alternately rendered conductive when their anodes are positive with respect to their cathodes.

When tubes 54 and 56 are thus rendered conductive, a fully rectified direct current is supplied to the armature 88 of motor 86 through the clrcuit extending from the center tap 66 of the secondary winding 10 of the transformer 64, through resistor 84, conductor 80, armature 38, conductor 82, conductor 62,

cathodes

88 and 90 and anodes 66 and 68 of the tubes 54 and 56, respectively, back to the terminals of the secondary winding 10. The feed motor 35 then operates to feed the electrode 20 towards the metal 4 at a relatively high speed as the voltage applied to the grids |02 and |04 is a maximum.

As the electrode 20 approaches the metal I4, the arc potential decreases and consequently the direct-current voltage applied to control resistors |20 and |12 decreases while the current in the load conductor 20 increases with a resulting increase in the direct-current voltage applied to control resistors ||0 and |10. As the differential oi' the direct-current voltages across resistors |10, |12, and |14 approaches zero, the differential voltage impressed across the direct-current sec#v ondary windings of reactors 2|0 and 206 also approach zero to effect an increase in the impedance of the amplitude bridge circuit 202 and control the operation of tube |80 to impress a direct current voltage across control resistor |16, the direct-current voltage being of the polarity, as referred to hereinbefore, to place a negative bias on the control grids |02 and |04 of

tubes

54 and 66 to block them from passing current. The amplitude bridge control circuit 202 thus anticipates the correct positioning of the electrode 20 to prevent hunting,

During the period of time that motor control tubes 64 and 56 are conducting current to effect the operation of the motor 36 to lower the electrode 20l the current ows through the resistor 04 and the voltage drop thereacross is sufilcient to overcome the effect of battery 244 and render the bias of grid 238 of discharge device 232 negative and block the tube from conducting. However, when the tubes 54 and 56 are blocked and current ceases to ow through resistor 34, the battery 244 functions to render the bias on grid 236 positive and permit the tube 232 to pass current. Under these circumstances, the counterelectromotive force of the motor 36 is utilized to provide dynamic braking for the motor 36, current flowing from the armature 38 through conductor 60, cathode 236, anode 234, resistor 246, conductor 62 and conductor 62 to the other side of the armature 36. The dynamic braking thus provided quickly stops the motor 66 and prevents further lowering or overrun in positioning the electrode.

Under conditions of equilibrium where the electrodes are positioned for ideal operation, the direct-current voltages across resistors and which are proportional to the current owing in the conductor 26 and the direct-current voltages across resistors |20 and |12 which are proportional to the arc voltage are in substantial balance and the direct-current Voltage across each of resistors |28 and |16 is sufcient yand of the correct polarity to prevent firing of either of the

pairs

50, 52 and 54, 66 of motor control tubes. In fact, in approaching such ideal positioning of the electrode 20, the voltages across resistors |28 and |16 are of such magnitude as to effectively block the

control tubes

50, 62 and 54, 56. The amplitude bridge control circuits |48 and 202 thus provide a positive dead zone in the operation of the

pairs

50, 52 and 54, 56 of motor control tubes, thereby preventing the immediate transfer of ring from one set or pair of tubes to the other except in cases where a positive short circuit such as where a cave-in of the metal I4 about and in contact with the electrode 20 is encountered.

If a short circuit is encountered as referred to while the motor is operating to lower the electrode 20, the direct-current voltage applied to control resistors I0 and |10 immediately becomes a maximum and that applied to control resistors |20 and |12 becomes a minimum with opposite effects on the operation of the

pairs

50, 52 and 54, 56 of motor control tubes. Under these conditions, the direct-current voltage across resistor |10 is of the same polarity as the voltages across resistors |`|4 and |16 to overcome the directcurrent voltage, if any, across resistor |12 and place a negative bias on the grids |02 and |04 to block the tubes 54 and 56 from conducting.

When the direct-current voltage across resistor l0 is increased as described, the differential voltage of the direct-current voltages across control resistors I0, |20, and |26 is impressed across the direct-current secondary windings oi.' the reactors |54 and |56 oi' the amplitude bridge control circuit |48 to decrease the impedance of the bridge circuit and consequently increase the voltage on the grid transformer |64 connected in circuit therewith. This increase in voltage functions to increase the amplitude of the altemating-currentI component of the voltage on the grids |44 and |46 of tube |30 to decrease the output therefrom and decrease the direct-current voltage applied across control resistor |28. As the direct-current voltage applied to resistor I0 is a maximum and the direct-current voltage applied to resistors |20, |26 and |26 are at a minimum and of opposite polarity to that across resistor ||0, a positive bias is applied to the grids 96 and |00 of

motor control tubes

50 and 52, respectively, through the circuit extending from the control grids 68 and |00 through conductor 89, control resistors ||0, |26, |26, |28 and conductor |0| to the

cathodes

14 and 16. The control grids 96 and |00 thus being made positive with respect to the

cathodes

14 and 16, the

tubes

50 and 52 are alternately rendered conductive when their anodes are positive with respect to their cathodes.

With the

tubes

50 and 62 conductive, a fully rectified direct current is supplied to the armature 38 of motor 36 through the circuit extending from the center tap of the secondary winding 62 of the transformer 64, through resistor 84, conductor 82, armature 36, conductor 60, conductor 18,

cathodes

14 and 16 and anodes 68 and 60 oi the

tubes

50 and 62, respectively, back to the terminals of the secondary winding 62. The feed motor 36 then operates to raise the electrode away from the metal |4 to a position where the direct-current voltage across the control resistors |I0, |20, |26 and |10, |12, |14 are again in substantial equilibrium and the direct-current volt age across each of control resistors |28 and |16 effects a negative bias on the grids of

control tubes

50, 52 and 64, 56.

In-raising the electrode 20 as Just described, dynamic braking is also provided when

tubes

50 and 52 cease to conduct, and current ceases to flow through resistor 84. When this happens, the battery 240 places a positive bias on the grid 230 of tube 224 to render it conductive whereby the counterelectromotive force of the motor 36 effects the dynamic braking. With the tube 224 conductive, a circuit is' completed extending from the armature 38 through conductor 82, cathode 228, anode 226, resistor 242, conductor 16 and conductor back to the armature 38 to effectively brake the motor 36.

The system of this invention is very sensitive as to changes in operating conditions and provides for a quick correction in the positioning of the electrodes. By utilizing the amplitude bridge control circuits in the manner described, provision is made for the prevention of hunting in the operation or firing of the motor control tubes, the bridge circuits ensuring a period of nonconductivity in transferring the control circuits from one pair of control tubes to the other for all operations except that where a direct short circuit is encountered. Further, the provision of dynamic braking as described prevents an overrun in the positioning of the electrodes thereby preventing excessive operation of the motor to adjust the position of the electrodes.

While this invention has been described with reference to a particular embodiment thereof, it is, of course, not to be limited thereto except in so far as is necessitated by the scope of the appended claims.

I claim as my invention:

1. In a control system, in combination, a reversible motor, a plurality or pairs of electric valves disposed to selectively connect the motor to source of alternating current to control the speed and direction of operation of the motor, a control each of the control circuits including a pair of sources of control voltages variable in opposite senses and a source of control voltage variable in response to changes in the pair of control voltages of opposite senses, the variable control voltage being of a polarity to render the valves associated therewith non-conducting when the pair of variable control voltages oi opposite senses are in substantial balance.

2. In a control system, in combination, a reversible motor, a plurality of pairs of electric valves having grids for controlling the conductivity thereof disposed to selectively connect the motor to a source of alternating current to control the speed and direction of operation o! the motor, a control circuit associated with each pair of electric valves for controlling the bias of the grids of the valves, each oi the control circuits includins a pair of sources of control voltages variable in opposite senses and a source of control voltage variable in response to changes in the pair of control voltages of opposite sense, the variable control voltage being of a polarity to render the valves associated therewith non-conducting when the pair o! variable control voltages of opposite senses are in substantial balance.

3. In a control system for. regulating the feeding of an electrode for producing and maintaining an electric arc, the combination comprising, a motor which is to be regulated, a plurality of pairs of electric valves having grids ior control ling the conductivity thereof disposed to selectively connect the motor to a source of alternating current to control the speed and direction of operation of the motor, a control circuit associated with each pair of electric valves for controlling the bias of the grids of the valves, each of the control circuits including a pair of variable circuit associated with each pair oi' electric valves,

sources of control voltage, the voltage of one oi' d0 the pair oi. sources being of opposite polarity to the voltage of the other one of the pair of sources,

and means including a variable control voltage which is responsive to and controlled by the difierential of the pair oi' sources of control voltages to render the valves associated therewith nonconducting when the control voltages of the pair of sources are in substantial balance.

4. In a control system, in combination, a reversible motor, a plurality oi' pairs of electric valves disposed to selectively connect the motor to a source of alternating current to control the speed and direction ot operation of the motor, a control circuit associated with each pair of electric valves, each of the control circuits including a pair of sources of control voltages variable in op- Dsite senses and a source of variable control voltage, the source of variable control voltage including an electric valve, a saturable reactor` connected in circuit relation therewith, the saturation ci the reactor being responsive to the diiIerential oi.' the pair oi control voltages of opposite senses to control the output from the control voltage electric valve, the variable control voltage being oi' a polarity to render the valves associated with the control circuit non-conducting when the pair of variable control voltages of opposite senses are in substantial balance.

5. In a control system for regulating the feeding of an electrode for producing and-maintaining an electric arc, the combination comprising, a motor which is to be regulated, a plurality or pairs of electric valves having grids for controlling the conductivity thereof disposed to selectively connect the motor to a source ot alternating current to control the speed and direction of operation of the motor, a control circuit associated with each pair of electric valves for controlling the bias of the grids oi' the valves, each of the control circuits including a pair of sources of control voltage variable in opposite senses and a secondary source of variable `control voltage, the secondary source of variable control voltage including an electricvalve, a saturable reactor connected in circuit relation therewith, the saturation of the reactor being responsive to the diierential of the pair of sources of control voltages Vto control the output from the control voltage electricvalve, the secondary source of variable control voltage being of a polarity to render the valves associated with the control circuit nonconducting when the pair of variable sources of control voltages of opposite senses are insub\- stantial balance.

6. In a control system, in combination, a reversible motor, a plurality oi' pairs of electric valves disposed to selectively connect the motor to a source of alternating current to control the speed and direction of operation of the motor, a control circuit associated with each pair of electric valves, each of the control circuits including pair ot sources oi' control voltages variable in opposite senses and a source oi control voltage variable in response to changes in the pair of control voltages of opposite senses, the variable control voltage being of a polarity to render the valves associated therewith nonconducting when the pair of variable control voltages of opposite senses are in substantial balance, and means comprising an electric valve associated with each pair of control electric valves and connected in circuit relation with the motor to provide dynamic braking for the motor when the associated pair of control electric'valves are rendered nonconducting to stop the motor.

7. In a. control system, in combination, a reversible motor, a plurality oi' pairs of electric valves disposed to selectively connect the motor to a source oi' alternating current to control the speed and direction oi operation of the motor, a control circuit associated with each pair of electric valves, each of the control circuits including a pair oi' vsources oi' control voltages variable in opposite senses and a source of variable control voltage, the source of variable control volta/ge including an electric valve, a saturable reactor connected in circuit relation therewith, the saturation of the reactor being responsive to the differential of the pair of control voltages of opposite senses to control the output from the control voltage electric valve, the variable control disposed to selectively connect the motor to a source of alternating current to control the speed and direction of operation of the motor, a control circuit associated with each pair of electric valves, each of the control circuits including a pair of sources o! control voltages variable in opposite senses and a source of control voltage variable in response tc changes in the pair of control voltages of opposite senses, the variable control voltage being oi e polarity to re'nler the valves associated therewith noncomuctirig when the pair of variable control voltages of 'opposite senses are in substantial balance, and e, pair of dynamic braking circuits for the motor, each of the dy narnic braking circuits including a. resistor connected in the motor circuit between the motor and e. pair of the electric valves to have e control volt-- ege thereacross when the wir or electric vaives are conducting" and another electric valve associ ated with the pair of electric valves disposed to be connected across the motor and to be controlled by the control voltage across the resistor, said another electric valve being rendered nonconducting when the pair of electric valves are conducting and conducting when the pair of electric valves are nonconducting to thereby reverse the current flow to the motor and effect the dynamic braking thereof when the pair of electric 10 valves are rendered nonconducting.

CYRIL C. LEVY.