US2399695A

US2399695A - Follow-up system

Info

Publication number: US2399695A
Application number: US371331A
Authority: US
Inventors: Howard A Satterice
Original assignee: Submarine Signal Co
Current assignee: Submarine Signal Co
Priority date: 1940-12-23
Filing date: 1940-12-23
Publication date: 1946-05-07
Anticipated expiration: 1963-05-07

Description

May 7, 1946. H. A. SATTERLEE FOLLOW-UP SYSTEM Fild Dc. 23, 1940 INVENTOR Howard ,4 J0 rfer/ee BY ATTORNEY nected to the remote by one end to the anode ll of ing 9 is connected by one Patented May 7, 1946 uurrsn STATES PATENT OFFICE FOLLOW-UP SYSTEM Howard A. Satterlee, Dayton,

Submarine Signal Company corporation of Maine Application December as, 1946, Serial No. 371,331 7 Claims.

The present invention relates to follow-up systems.

Various arrangements have heretofore been proposed for the purpose of causing a remote object to take up a predetermined position in response to the setting of a control indicator to a corresponding position by an operator at a control station. Among the difllculties which arise in such systems arethe dimculties of reducing to a minimum the time lag between the setting of the control indicator and the corresponding motion of the remote object, the elimination of hunting and the obtaining of suflicient accuracy in the final position of the remote ob- Ject. It is an object of the present invention to provide a follow-up system which will provide accurate and close coupling of the remote object with the control device, without hunting and which moreover will have the desirable characteristics of simplicity and reliability.

This and other objects of my invention will best be understood from the following description, taken in connection with the accompanying drawing which schematically illustrates the invention.

As indicated in the drawing, the driving motor I which may be geared or otherwise conobject is preferably of the direct current type having a commutator type armature 2 and a field winding 3 separately ex-- cited from a source of direct current. Two grid controlled rectifiers of the gaseous electron discharge type 4 and 5 provide the armature operating current. Anode potential for the two tubes is provided by a transformer 6 having a primary winding 1 connected to a source of alternating current and two secondary windings 8 and 9. The secondary winding 8 is connected tube 4 and by the other end through conductors l2 and I 6 to the armature 2 and from the latter by conductor II to the cathode l4 of tube 4. The secondary windend to the anode ii of the tube 5 and by the other end by conductors l8 and I! to the armature 2 and thence by conductor it to the cathode i9 or tube 5. The grids and 2| of the two tubes are respectively connected through current-limiting resistors 22 and '23 to the

movable contacts

28 and 28 of

potentiometers

32 and 33 and through the potentiometers to the

extremities

30 and 3| of a centertapped resistor 24. The motor armature 2 is shunted by a center-tapped resistor l3, the center tap of which is connected through a b ing battery to the center tap of the resistor 24. High frequency by-

pass condensers

26 and 21 are also provided between the cathodes and grids of the tubes 4 and 5, respectively.

The potentiometer 32 is connected across a source of alternating current of the same frequency as that which supplies the anodes of the rectiflers 4 and 5 but of a phase angle lagging the potential of anode II by approximately 90. Similarly, the potentiometer 33 is connected across a source of alternating potential of the same frequency as that supplying the anode circuits of the tubes but of a phase angle lagging the potential of anode I! by approximately 90. The magnitudes of the two alternating potentials thus introduced into the grid circuits of the rectifiers are adjusted to be of a very small value, depending upon the characteristics oi. the gaseous discharge tubes 4 and 5. The maximum values of these alternating grid potentials may, for example, be of the order of the magnitude of the maximum negative potential of the critical grid voltage curve for the tubes in question at the anodev voltage used. The critical grid' -voltage curve is to be understood as the graph'of the grid-to-cathode voltage which is Just sufllcient to initiate conduction through the tube at each instant in the positive hali' cycle of the anode Potential.

It will be understood, of course, that the type of tubes referred to is that in which when conduction has once been started, it will continue until the anode potential reverses its polarity, the grid having, control only prior to the time conduction commences in any particular positive half cycle 01' anode potential.

The grid circuit of tube 4 may thus be traced from the grid 20 through the resistor 22 the potentiometer 32, the upper half of re tor 24, the biasing battery 25 and the upper half of resistor l3 to the cathode i4. Similarly, the grid circuit of tube 5 may be traced from the grid 2| through the resistor 23, the potentiometer 33, the lower half of resistor 24, the biasing battery 25 and the lower half of resistor l3 to cathode It.

A control potential for the grids of the tubes is impressed across resistor 24. The circuit for producing the grid control voltage which is to be impressed across resistor 24 will be described subsequently.

It is' convenient at the present time to consider the operation of the grid controlled recti-,

- tier circuit which has so far been described.

Considering, first, the tube 4, it will be noted that the anode circuit of the tube contains a source of alternating current and. the armature 2 of motor I. It will also be noted that the grid circuit of the tube 4'contains four sources of potential in series, namely, the alternating potential provided by potentiometer 32, the direct potential drop across the upper half of resistor 24, the direct potential provided by the biasing battery 25, and the direct potential drop across the upper half of resistor 12.

The drop across the resistor 24, as will be described later, is arranged to vary in polarity with If, now, it be assumed that the motor I is at rest and that a potential is impressed across resistor 24 of sufiicient magnitude and of such a polarity that the point i positive and the point 3| is negative, the grid 20 will become positive with respect to its cathode, thereby permitting anode current to fiow through the tube 4 and the armature 2 during the half cycles of anode potential or portions thereof in which the anode I I is positive with respect to its cathode. The armature 2 being immersed in the continuous magnetic field provided by the winding 2 will therefore commence to rotate under the influence of the rectified current impulses flowing through it. As the armature increases its speed, it will'generate a back E. M. F. which is exactly proportional to the armature speed and is available for control of the rectifier grid at all times when no current flows through the tube. This back E. M. F. being applied across the resistor It, causes a current to fiow through this resistor which therebyintroduces a potential into the grid circuit of tube 4 which opposes the potential due to the drop across the upper half of resistor 24. when the back E. M. F. across the upper half of resistor i3 reaches a magnitude substantially equal to the potential drop across the upper half or resistor 24, the potential of grid 20 will automatically be varied to cause the discharge in each positive halt cycle of anode potential to take place at the right instant in each cycle to prevent the motor from increasing or decreasing its speed beyond that which corresponds to the magnitude of the potential then existing across resistor 24. The motor speed is thereby accurately controlled. It should also be noted that when the polarity of the potential impressed across the resistance 24 is such as to make the point 30 positive, the point Si is negative and the grid 2| of tube 5 becomes negative so that when the tube 4 is operating, the tube 5 remains inactive. Similarly, if the potential across resistance 24 be reversed so that the point 3| becomes positive and the point 30 negative, tube I will become active, causing the motor armature 2 to rotate in the reverse direction. The tube 4 then remains inactive.

If, after the motor has been running in one direction, it should be desired to stop it, the controlling potential across resistor 24 is reduced to zero, so that the bias battery 25 is effective to hold both grids to cut-oft potential. However,

should the reduction of the potential on resistor 24 be sudden and should the motor tend to continue'rotation. the tube opposite to the one which was driving the motor will be made active by the E. M. 1". generated by motors motion, tending to reverse the motor. This action amounts to a form of dynamic braking which rapidly brings the motor to'a stop.

Let us now consider the circuit for producin the necessary control voltage for the grids of the rectifiers 4 and 5. This circuit is shown in the upper portion of the drawing. It is arranged to impress a potential across the terminals 30 and ii of the resistor 24 which has a polarity corresponding to the direction of the required displacement of the driven object and a magnitude which may be proportiona1 to the magnitude of the desired displacement, or may reach a maximum value even for small required displacement of the driven object. To this end a handwheel [0 may be arranged to drive a pointer 34 which is mounted adjacent to a scale 35 in such a manner that by rotating the handwheel the pointer may be rotated into a position corresponding to the desired position of the driven object. The handwheel shaft is also connected mechanically to the rotor or polyphase winding 38 of a selfsynchronous motor 31. The motor 31 is also provided with a single-phase winding 30 which is connected to a source of alternating current of a high frequency. By this I mean a frequency which is high compared to the 60 cycle frequency heretofore used, say approximately 1000 to 2000 cycles per second; but I prefer to use a frequency in the neighborhood of 1500 cycles per second. The rotor 35 is electrically connected in parallel with the rotor 39 of a similar self-synchronous motor 40 having a single-phase stator windin M. The rotor 39 is mechanically connected to the driven object. When the rotor 38 is in the same angular position with respect to its stator winding 38 as the rotor 38 is with respect to its stator winding 4|, there will be no voltage induced in the winding 4|. On the other hand, when the rotor 36 is displaced relative to the rotor 32 there will be a voltage induced in the winding 4|. The polarity or relative phase of this volt ge depends upon the direction of the displacement and the magnitude of the voltage depends upon the magnitude of the displacement.

The winding 4| is connected across a centertapped resistor 42 whose extremities are connected to the grids of two high

vacuum threeelectrode tubes

43 and 44. The cathodes of these tubes are connected together and to the movable contact 45 of a potentiometer resistance 48 whose extremities are connected across the same source of high frequency alternating potential mentioned above. The phase of the potential across resistance 48 is therefore the same as that across the winding 48. One end of the potentiometer resistance 46 is connected to the center tap of resistance 42 and the otherend is connected to the common point 41 or the anode circuit of the

tubes

43 and 44. The anodes of these tubes are connected to the point 41 through filters 4| and 48 which are of suitable value to filter out the rectified high frequency alternating component introduced by the potential drop across resistance 40. Each filter may comprise, for example, a resistance and a parallelly connected condenser as shown. The anode circuit of tube 43 may thus be traced from the anode of the tube through the filter 40 to the terminal 41 through a portion of resistance 46 to movable contact 45 to the cathode of the tube. Similarly, the anode circuit of tube 44 may be traced from the anode the grid of each of the tubes 43 of the tube through the filter 43 to the terminal 41 through the resistance 43 to the movable contact 45 and to the cathode of the tube. Since the portion of resistance 43, which lies between the terminal 50 and contact 45, is in series with and 44, it provides an alternating bias potential for the grids of these tubes. This potential is adJusted to be of such a value that anode current flows through both of the tubes even though there is zero voltage induced in the winding 4|. It will be observed that the voltage induced in this winding by relative displacement between the two

rotors

35 and 39 appears across the resistance 42 and adds or subtracts to the grid voltage of the

tubes

43 and 44, depending upon the direction of the displacement. If the displacement is in one direction, the potential drop across resistance 42 will be out of phase with the bias voltage on one of the grids but in phase with the bias voltage on the other grid. One tube will therefore pass more current while the current of the other will be reduced. If the relative displacement of the two rotors 35 and 33' be in the opposite direction, the effect on the anode currents of the two rectifiers will, of course, be reversed.

The result of the anode current flow in the anode circuits of the two

tubes

43 and 44 will be to produce a potential difference across the terminals of a center-tapped resistor 5| which is connected between the two anodes. The ends of the resistor 5| are also connected to the grids of two amplifying

tubes

52 and 53 which may be operated as Class A amplifiers. The cathodes of the

tubes

52 and 53 are connected together and to the movable contact 54 of a potentiometer resistance 55 which is connected across a source of direct potential. The negative terminal of the resistance 55 is connected to the center tap of resistance 5|, and the positive terminal of the resistance 55 is connected to the center tap of a center-tapped resistor 55 Whose extremities are connected to the anodes of the

tubes

52 and 53, respectively. Any potential appearing across resistance 5| is thus amplified and will appear across resistance 56 and also across

terminals

30 and 3| which lie in the grid circuit of the grid controlled gaseous rectifiers 4 and 5.

It will now be evident that when the position of the driven object corresponds to the position of the pointer 34 so that the position of rotor 35 with respect to its stator 38 corresponds to the position of rotor 33 with respect to its stator 4|, no voltage will be produced across the winding 4| and equal currents will flow in the anode circuits of

tubes

43 and 44. Since these currents are in opposite directions, there will be no voltage across the resistance 5| and therefore no voltage across the

terminals

30 and 3 I. Th motor 2 will therefore remain at rest. When, however, the handwheel I is rotated to set the pointer 34 into a position corresponding to a desired new position of the driven object, a voltage, corresponding in phase to the direction of the position change which was made, will appear across winding 4| and resistance 42, causing the anode currents of

tubes

43 and 44 to change. A potential difference will therefore exist across resistance which will correspond in polarity to the direction of the displacement of the pointer 34. This potential will be amplified by the

tubes

52 and 53 and will appear across the

terminals

30 and 3|, causing either the rectifier 4 or 5 to energize the motor 2 and to rotate the driven object in the desired direction as above described. Rotation of the driven object also which it is being in such a direction into positional correspondenc with the rotor 36 and thereby ag reducing the voltage in the winding 4| to Thus when the driven object reaches the desired position, the motor 2 will stop.

By the use of the above arrangement a tight coupling is obtained between motion of the handwheel l3 and motion of the driven object so that there is substantially no time lag between them.

The maximum voltage impressed across resistor 24 should preferably be substantially equal to the full-speed back E. M. F. of the motor impressed across resistor l3. While it is often convenient to have the motor operate at a speed proportional to the required displacement of the driven object, this need not always be the case. Where, for example, a high sensitivity is desired and consequent rapid follow-up action, it may be preferable to arrange the amplifying circuit so that full control voltage is impressed across resistor 24 even for small displacements between pointer 34 and the driven object.

I have found that the gaseous grid controlled rectifier circuit described above operates in this follow-up system most satisfactorily when the controlling potential impressed across resistor 24 is a substantially pure direct potential without alternating components. It is necessary, therefore, to provide a suitable filter, as above described, to remove any alternating components which may otherwise be present. However, when filters are introduced into a follow-up system there is likewise inevitably introduced a time delay between the motion of the driven object and the motion of the controlling indicator. Where sensitivity and rapidity of response of the controlled object to motions of the controlling device are required, such time lag is, of course, objectionable, for it causes the system to act as though an elastic coupling member had been introduced between the controlling and the controlled mechanisms. According to my invention, however,

such elasticity is substantiall entirely eliminated because of the fact that I use a high frequency in the self-synchronous motor circuit for the production of the controlling potentials. Filters for such high frequencies can readily be constructed which substantially eliminate the high frequency components from the control voltage output circuit and which at the same time have only a negligible time constant.

Having now described my invention, I claim:

1. In a follow-up system for driving an object into positional agreement with a controlling object, a power motor for said driven object, a gridcontrolled rectifier circuit for supplying power to said motor in one direction or the other in response to a direct potential which varies in polarity in accordance with the direction of positional disagreement between said objects and means for generating said potential comprising a self-synchronous transmitter motor having a stator winding energized by alternating current of a high frequency abov approximately 1000 cycles per second and a rotor winding adapted to be rotated by the operator into a position corresponding to the desired position of the driven object, a self-synchronou receiver motor having rotor electrically connected in parallel with that of the transmitter and mechanically connected to the driven object and a stator winding adapted to have a voltage induced in it upon positional disagreeas to bring the rotor 33 larity corresponding to the direction of said disagreement. rectifying means, means impressing said induced voltage on said rectifying means, the latter being adapted to produce a direct potential having a polarity corresponding to the direction of said disagreement and containing an alternating component of said high frequency, a filter for removing said high frequency component and means for impressing the resulting rectified and filtered voltage upon said grid-controlled rectifier circuit.

2. In a iollow-up system for driving an object into potential agreement with a controlling object, a power motor for said driven object, a gridcontrolled rectifier circuit for supplying power to said motor in one direction or the other in response to a direct potential which varies in polarity and magnitude in accordance with the direction and magnitude of positional disagreement between said objects and means for generating said potential comprising a self-synchronous transmitter motor having a stator winding energized by alternating current oi a high frequency above approximately 1000 cycles per second and a rotor winding adapted. to be rotated by the operator into a position corresponding to the desired position of the driven object, a self-synchronous receiver motor having a rotor electrically connected in parallel with that of the transmitter and mechanically connected to the driven object and a stator winding adapted to have a voltage induced in it upon positional disagreement between the two rotors and their respective stators, said voltage having an instantaneous polarity and a magnitude corresponding to the direction and magnitude of said disagreement, rectifying means, means impressing said induced voltage on said rectifying means, the latter bein adapted to produce a direct potential having a polarity and magnitude corresponding to said direction and magnitude of said disagreement and containing an alternating component of said high irequency, a filter for removing said high frequency component and means for impressing the resulting rectified and filtered voltage upon said gridcontrolled rectifier circuit.

3. In a follow-up system for driving an object into positional agreement with a controlling object, a. power motor for said driven object, a gridcontrolled rectifier circuit for supplying power to said motor in one direction or the other in response to a direct potential which varies in polarity in accordance with the direction oi positional disagreement between said objects and means for generating said potential comprising a self-synchronous transmitter motor having a stator winding energized by alternating current of a high frequency or approximately 1500 cycles per second and a rotor winding adapted to be rotated by the operator into a position corresponding to the desired position of the driven object, a self-synchronous receiver motor having rotor electrically connected in parallel with that of the transmitter and mechanically connected to the driven object and a stator winding adapted to have a voltage induced in it upon positional disagreement between the two rotors and their respective stators, said voltage having an instantaneous polarity corresponding to the direction of said disagreement. rectiiying means, mean impressing said induced voltage on said rectifying means, the latter being adapted to produce a direct potential having a polarity corresponding to ment between the two rotors and their respective stators, said voltage having an instantaneous pothe direction of said disagreement and containing an alternating component oi said hlghirequency, a filter for removing said high irequency component and means for impressing the resulting rectified and filtered voltage upon said grid-controlled rectifier circuit.

4; In a follow-up system ior driving an object into positional agreement with a controlling object, means for producing a direct potential varying in polarity and magnitude in accordance with the direction and magnitude oi positional disagreement between said objects comprising selfsynchronous transmitter and receiver system. means exciting the same with alternating current oi a high irequency above approximately 1000 cycles per second, means mechanically connecting said system with said controlling object and said driven object for producing an alternating potential of said high irequency having a magnitude and an instantaneous polarity corresponding to the magnitude and direction of said positional disagreement, rectifying means for said high frequency potential adapted to produce a direct potential having a polarity and a magnitude corresponding to the direction and magnitude oi said positional disagreement, and having an alternating component of said high frequency, and a filter for removing said high frequency component.

5. In a follow-up system for driving an object into positional agreement with a controlling object, means for producing a direct potential varying in polarity and magnitude in accordance with the direction and magnitude oi positional disagreement between said objects comprising selfsynchronous transmitter and receiver system, means exciting the same with alternating current oi a high frequency above approximately 1000 cycles per second, means mechanically connecting said system with said controlling object and said driven object for producing an alternating potential oi said high frequency having a magnitude and an instantaneous polarity corresponding to the magnitude and direction of said positional disagreement, rectifying means ior said high frequency potential comprising a pair of oppositely connected vacuum tube rectifiers having cathode, anode and grid electrodes, a source of anode potential for said tubes, means connect ing said tubes in opposite relation to each other, means impressing a derivative of said high frequency potential upon the grids of said tubes in opposite instantaneous polarity, means biassing both of said grids with an alternating potential of said high frequency of the same phase as the potential of said sen-synchronous system exciting current and of a magnitude to cause both tubes normally to pass current, whereby for one instantaneous polarity oi said produced potential the currentpassed by one tube will increase and the current passed by the other tube will diminish and vice versa for the opposite instantaneous polarity of said produced potential, means in the anode circuit of each tube for separately filtering out the high frequency alternating component, means opposing the .filtered rectified outputs of the two tubes, means deriving a resultant potential corresponding to said outputs and means responsive to said resultant potential ior altering the position of said driven object to remove said positional disagreement.

6. In a follow-up system for driving an object into positional agreement with a controlling object, means for producing a direct potential varying in polarity and magnitude in accordance with'the direction and magnitude of positional disagreement between said obiects comprising self-synchronous transmitter and receiver system, means exciting the same with alternating current of a high frequency above approximately 1000 cycles per sec d, means mechanically connecting said syste with said controlling object and said driving object for producing an alternating potential oi said high frequency having a magnitude and an instantaneous polarity corresponding to the magnitude and direction of said positional disagreement, rectifying means for said high frequency potential comprising a pair of oppositely connected vacuum tube rectifiers having cathode anode and grid electrodes, a source of anode potential for said tubes, means connecting said tubes in opposite relation to each other, means impressing a derivative of said high frequency potential upon the grids of said tubes in opposite instantaneous polarity, means biassing both of said grids with an alternating potential of said high frequency of the same phase as the potential of said self-synchronous system exciting current and of a magnitude to cause both tubesv normally to pass current, whereby for one instantaneous polarity of said produced potential the current passed by one tube will increase and the current passed by the other will diminish and vice versa for the opposite instantaneous polarity of said produced potential, means in the anode circuit of each tube for separately filtering out the high frequency alternating component, a resistor connected between the anodes of the two tubes so that a direct current will fiow through said resistor which is the resultant of the opposed rectified and filtered outputs of the two tubes and means responsive to the polarity and magnitude of the resulting potential drop across said resistor for altering the position of said driven object to remove said positional disagreement.

7. In a follow-up system for driving an object into positional agreement with a controlling object, means for producing a direct potential varying in polarity and magnitude in accordance with the direction and magnitude of positional disagreement between said objects comprising selfsynchronou's transmitter and receiver system, means exciting the same with alternating current of a high frequency above approximately 1000 cycles per second, means mechanically connecting said system with said controlling object and said driven object for producing an alternating potential of said high frequency having a magnitude and an instantaneous polarity corresponding to the magnitude and direction of said positional disagreement, rectifying means for said high frequency potential comprising a pair of oppositely connected vacuum tube rectifiers having cathode, anode and grid electrodes, means connecting said tubes in opposite relation to each other, means impressing a derivative of said high frequency potential of the same phase as the potential of said self-synchronous system exciting current upon the anodes of said tubes in the same instantaneous polarity, means biassing both of said grids with an alternating potential of said high frequency and of instantaneous polarity and magnitude to cause both tubes normally to pass current, and means impressing a derivative of said produced high frequency potential upon the grids of said tubes in opposite instantaneous polarity, whereby for one instantaneous polarity of said produced potential the current passed by one tube will increase and the current passed by the other tube will diminish and vice versa for the opposite instantaneous polarityoi' said produced potential, means in the anode circuit of each tube for separately filtering out the high frequency alternating component, means opposing the filtered rectified outputs of the two tubes, means deriving a resultant potential corresponding to said outputs and means responsive to said resultant potential for altering the position of said driven ob- .iect to remove said positional disagreement.

8. In a follow-up system for driving an object into positional agreement with a controlling obiect, means for producing a direct potential varying in polarity and magnitude in accordance with the direction and magnitude of positional disagreement between said objects comprising selfsynchronous transmitter and receiver system, means exciting the same with alternating current of a high frequency above approximately 1000 cycles per second, means mechanically connecting said system with said controlling object and said driving object for producing an alternating potential of said high frequency having a magnitude and an instantaneous polarity correspondlng to the magnitude and direction of said positional disagreement, rectifying means for said high frequency potential comprising a pair oi oppositely connected vacuum tube rectifiers having cathode, anode and grid electrodes, means connecting said tubes in opposite relation to each other, means impressing a derivative of said high frequency potential of the same phase as the potential of said self-synchronous system exciting current upon the anodes of said tubes in the same instantaneous polarity, means biassing both 01' said grids with an alternating potential of said high frequency and of instantaneous polarity and magnitude to cause both tubes normally to pass current, and means impressing a derivative of said high frequency potential upon the grids of said tubes in opposite instantaneous polarity, whereby for one instantaneous polarity 01 said produced potential the current passed by one tube will increase and the current passed by the other will diminish and vice versa for the opposite instantaneous polarity of said produced potential, means in the anode circuit of each tube for separately filtering out the high frequency alternating component, a resistor connected between the anodes of the two tubes so that a direct current will flow through said resistor which is the resultant of the opposed rectified and filtered outputs of the two tubes and means responsive to the polarity and magnitude of the resulting potential drop across said resistor for altering the position of said driven object to removesaid positional disagreement.

9. In a follow-up system for driving an object into positional agreement with a controlling object, a self-synchronous transmitter and receiver system, means energizing the same with alternating current of a high frequency above approximately 1000 cycles per second, rectifier and filter means operatively associated with said system for delivering a direct current, said filter being adapted to removealternating current components having a frequency not substantially less than said high frequency, motor means for driving said object and means-for energizing said motor, said energizing means being adapted to be activated by said direct current, whereby time lag between operation of said self-synchronous ystem and said motor is reduced.

10. In a follow-up system for driving an object into positional agreement with a controlling 6 aaoaeos object, a sell-synchronous transmitter and receiver system, means energizing the same with alternating current of a high irequeney above approximately 1000 cycles per second. rectifier and filter means operatively associated with said system for delivering a controlling direct current, said filter being adapted to remove alternating current components having a frequency not substantially less than said hiah frequency. a motor and a source or rectified alternating current of relatively low frequency for operatin: the same, said source having grid control means to which said controlling direct current is applied, whereby time lag between operation 01' said selfsynchronous system and said motor is reduced.

, HOWARD A. SA'I'I'ERIEE.