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Nov. 26, 1946.
2,411,744
E. E. MOYER
ELECTRIC CONTROL CIRCUIT
Filed Oct. 29, 1941
wants.“ 2958
33
____x_________
Inventor‘:
Elmo E Negev‘,
W
b5 His
'
Patented Nov. 26, 1946
anion
UNITED STATES PATENT OFFICE
2,411,744
ELECTRIC CONTROL CIRCUIT
Elmo
Meyer, Scctia, N. Y., assignor to General
Electric Company, a corporation of New York
Application October 29, 1941, Serial No. 416,974
6 Claims.
1
£31 invention relates to electric control cir
cuits and more particularly to improved electric
control circuits for accomplishing variable ener
gization of a load circuit, such as an electric mo
tor, by the use of electric valve translating ap
(Cl. 250-27)
2
It is another object of my invention to provide
a new and improved electric control circuit for
an electric valve translating apparatus for ef
footing variable energization of a load circuit
which provides for a predetermined initial ener
paratus.
gization of the load.
It is still another object of my invention to
In control systems employing electric valve
provide a new and improved electric control sys
translating apparatus for e?ecting variable ener~
tem employing self-synchronous type motion
gization of a load circuit and particularly in
systems where the load circuit comprises a motor 10 transmitting devices which insures smooth regu
lating action under all operating conditions.
which is to be maintained in some de?nite oper~
Briefly stated, in the illustrated embodiment
ative relation with respect to another motor, it
of my invention I provide a controlled electric
is desirable to insure that the output of the elec
valve system for variably energizing the arma
tric valve apparatus is returned to a predeter~
mined value each time the system is deenergized 15 ture winding of an electric motor to maintain
a predetermined speed relation between the mo
so that the initial energization of the load cir~
tor and another motor which is not energized
cuit is always the same. In systems of this char
acter employed for controlling the energization
through the electric valve means. The control
of one of a plurality of motors to maintain the
motors in de?nite speed relation by means of
synchronous motion transmitting devices driven
by the motors and operating to control the posi
tion of a movable element for varying the output
of the electric valve means, it has been found
that if the movable element is positively driven
in accordance with the relative positions of the
movable elements of the motion transmitting de
vices, uneven operation of the regulating system
results if the speed relation of the motors con
tinues to depart from the desired relation after
the movable element of the valve controlling
means has reached the limit of its movement. In
of the electric valve means to effect the desired
variable energization is accomplished by means
of motion transmitting devi es including a dif
ferential motion transmitting device which is
operatively connected with the rotor of a phase
shifting device for controlling the excitation of
nection between the motion transmitting devices
and the movable element of the tube controlling
means which permits the motion transnitting
devices to remain in synchronism after the mov
able element of the control device has reached
the electric valve means. The connection be
tween the rotor of the difjerential signal device
and the rotor of the phase shifting device is ac
complished by means of a torque clutch which
permits synchronous movement of the di?eren
tial device after the ro'cr or“ the phase shifting
device has reached the limit of its movement.
This provides for smooth functioning of the con~
trol at the time the desired speed relation is again
attained and operating of the movable element
of the phat e shifting device within its e?‘ective
range is resumed. In order to insure that the
initial output of he electric valve apparatus i
always the sa.
upon energization of the anode
the limit of its travel
cathode circuit thereof, I provide means for au
order to overcome this difficulty I provide a con
In this Way a continued
temporary departure of the desired relation be 40 tomatically returiing the rotor of the phase
shifting device to a predetermined position upon
tween the movable elements and the motors after
deenergizaticn of the anode-cathode circuit of
the movable element of the tube controlling
means has reached the limit of its travel per
mits the motion transmitting devices to remain
the electric valve means and also provide means
for preventing energization of the anode~cathode
circuit until the rotor has been returned to the
predetermined position. In order to improve the
tion approaches that desired and the motion
smoothness of action of the control system still
transmitting devices tend to rotate the movable
further I provide an improved anti-hunting cir~
element of the controlling means in the opposite
cuit including a resistor-capacitor combination
direction the control is picked up smoothly as
contrasted with the sudden changes which occur 50 responsive to voltages of the load circuit for
modifying the excitation voltages applied to the
if the motion transmitting devices temporarily
electric valves. The anti-hunting circuit in
lose synchronism with the rotating elements of
cludes a pair of parallel resistors each of which
the motors which are controlled.
is provided with a variable tap for varying the
t is an object of my invention to provide a
magnitude thereof so that it is possible to adjust
new and improved electric control circuit.
in synchronism and as soon as the speed rela
2,411,744
3
independently the time constant of the condenser
, circuit and the magnitude of the resistance in
cluded in the control circuit.
My invention will be better understood by ref
erence to the following description taken in con
nection with the accompanying drawing and its
scope will be pointed out in the appended claims.
In the drawing, the single ?gure is a schematic
representation of one embodiment of my inven
4
phase stator winding 35 of a phase shifting de
vice 36 having a three-phase rotor winding H.
The phase shifting device 36 is of a type well
known in the art and is similar in mechanical
construction to a three-phase wound rotor in
duction motor. The circuit between the three
phase terminals of the stator winding 35 and the
respective control members 25 of valves i9, 20
and BI is completed through current limiting re
tion.
10 sistors 33. The neutral terminal of the stator
Referring now to the drawing, I have shown my
winding 35 is connected to the cathode bus 3a of
invention embodied in a controlled electric valve
the valves I9, 26 and 2! through a portion of a
system for variably energizing the armature‘
resistor 39 determined by the position of the slider
windings of a direct current motor IE to main
46. Resistor 39 and a parallel connected resistor
tain a predetermined speed relation between the 15 4| form part of an anti-hunting circuit which is
motor H3 and another direct current motor H
operative to introduce by means of the portion
which has the armature l2thereof energized from
of resistor-39 included in the circuit of the con
a direct current supply it through ‘a suitable
trol members 25 a voltage dependent upon
motor starting and control circuit (not shown).
changes in voltage across the terminals of arma
The direct current motor ill has the terminals 20 ture Id of the machine 5 ii. The anti-hunting cir
of the armature M- thereof connected in the di
cuit includes a resistor 42 and capacitor £33 con
rect current circuit of an electric valve recti?er
nected in series across the terminals of the arma
illustrated generally by numeral £5. The motors
ture l4 and a capacitor Mi connected between the
iii and H each include a ?eld winding l5 ener
common point of resistor 112 and capacitor 43
gized from a suitable source of direct current I‘! 25 and the terminal of parallel resistors 39 and 4|
through a variable resistance it.
remote from the cathode bus 54. The condenser
45 for a given voltage of the armature M is
As mentioned above the armature winding of
charged to a predetermined value. Changes in
the motor H3 is energized from the direct current
the armature voltage result in a change in the
circuit of an electric valve recti?er l5. As illus~
charge of the capacitor lit and the charging cur
trated in the drawing the recti?er is a three~
rent ?owing through resistor 39 introduces a di
phase half-wave system comprising three electric
rect current bias voltage in the circuit of control
valves i9, 23 and 2! each preferably of the type
members 25 which a?ects the conductivity of the
comprising a container enclosing an ionizable
medium, such as a gas Or vapor, and within
which are mounted an anode 22, a cathode 23 and
valves in a direction to oppose the change in ar
ing so of an anode transformer 35 having a delta
cathode bus 34 and serve to minimize transient -
conneeted primary winding 32 connected to the
voltages on the control members and improve the
controlling action thereof.
As previously mentioned, the motor 56 is vari
ably energized to operate at a speed dependent
mature voltage. The magnitude of the bias volt
age for a given current ?ow through resistor 39
associated heater element 24, a control member
or grid 25 and a shield grid 25. The particular
is controlled by the position of the slider 46 while
the position of a short circuiting slider 45 on re
type of valve illustrated is not essential to the
present invention and any of the well known
sistor 4| determines the total resistance of the
types of controlled electric valves may be em 40 parallel resistance combination 39 and iii to de
termine the time constant of the discharge cir
ployed, if desired. The anode-cathode circuits of
cuit of the anti-hunting capacitor Mi. Capacitor
the electric valves are energized from a three
43 cooperating with resistor 152 operates as ?lter
phase alternating current supply circuit 2'5 which
to remove a portion of the ripple from the voltage
energizes an alternating current bus 23 under the
control of a manual switch 2%. The anodes of the 4.5 of the anti-hunting circuit. Capacitors 155a are
connected between the respective control mem
electric valves [9, 2E3 and 2! are connected to the
bers 25 of electric valves I9, 20 and 2! and the
end terminals of a Y-connected secondary wind
alternating current bus 28. The direct current
circuit of the recti?er is completed from the neu
tral connection 33 of the secondary winding 38
to one armature terminal of motor ill and from
the other armature terminal of the motor to the
upon the speed of the motor H. The arrange
ment for controlling the position of the rotor 37
cathode bus 34 of the electric valves I 9, 20 and 2! . 55 of phase shifting device 36 to accomplish this
As is well understood by those skilled in the art
the three-phase recti?er circuit described above
speed relation between motors l8 and I I will now
be described. A source of alternating current
control voltage is derived from the alternating
is effective to supply direct current to the arma
ture of the motor I 0 at a voltage dependent upon
current bus 28 by means of a control transformer
the energization of the control members or grids 60 46 having a primary network til comprising three
phase windings provided with taps which may
25 of the electric valves.
be adjusted to provide the desired magnitude of
The control circuit for energizing the control
control voltage. The secondary network 128 pro
members 25 to effect variable energization of the
vides a source of energizing voltage for the rotor
motor Ml will now be described. In the arrange
ment illustrated the motor H3 is intended to op 65 winding 3'! of the phase shifting device 35. The
erate at a ?xed speed relation with respect to the
transformer is also provided with secondary
windings 49 and 5d. Winding 135 provides a
motor H as may be desired in many commercial
applications such, for example, as in the case of
source of energy for heating the cathode heaters
a cable making equipment where motor IE, for
24 of electric valves l9, 2!! and 2! and the sec
example, may be the reel driving motor and the 70 ondary 156 provides a source of voltage for ener
gizing the rotor windings 5i and 52 of angular
motor H may be the ?yer motor or, in other
motion transmitting devices 53 and 54 respec~.
words, the motor which controls the twist or lay
of the cable. The conductivities of the electric
tively. The devices 53 and Ft are provided with
valves l9—2l are controlled in accordance with
three phase Y-connected stator windings 55
the voltages of the phase terminals of the three
and 56 respectively with the phase terminals of '
2,41 1,744
5
stator winding ‘,55 connected with the phase ter
minals .01’ the three phase rotor winding 57 of a
differential motion transmitting device 53 While
6
the valves 59, 20 and 2| are controlled by the
movable contacts 55 of an anode contactor 61
having an operating electromagnet 68 including
an operating coil 69,. The coil SSis connected to
be energized from one phase of the transformer
secondary network 48 through a manually con
phase terminals of the winding 56 are connected
‘with the phase terminals of the polyphase stator
winding '59 of the differential motion transmit
ting device .58. Capacitors ‘653 are connected
trolled switch 79 and a pair of spaced ?xed con
across the stator terminals of the differential
tacts ‘ii which are bridged by a conducting mem
device and are of such a rating that they take a
ber '52 when the phase shifting device is in the
leading excitation current equal to the lagging 10 position of minimum output of the electric valves.
excitation current required by the differential de
The cco._erating contacts ‘ii and 12 also pro
vice with the result that the exciting current
vide a mechanical stop for the rotor of the phase
‘which must be supplied to the diilerential by the
shifting circuit. A mechanical stop '13 is pro
device 53 for example, is equal only to the power
vided to limit the movement of the rotor wind
component of the current which is in general
ing ill in the opposite direction. It is apparent
only a small portion of the total excitation cur
that these stops may be adjusted in angular po
rent.
'
sition to determine the minimum and maximum
As illustrated in the drawing, 1e rotor wind
output of the electric valve means. A variable
ing 5| of device 53 is coupled to the shaft of
contact ‘ill actuated by the relay S8 completes a
motor ID by means of suitable gearing designat 20 circuit in parallel with the contacts ii to provide
ed by the numeral 6! and the rotor
of device
a holding circuit for the coil 68 after it has moved
the contacts 555 to closed position.
54 is mechanically coupled to the shaft of motor
The rotor 3'! of the phase shifting device 36 has
H by suitable gearing 62. This gearing is ar~
ranged so that the speed of rotation of windings
5i and. 52 are equal when the desired speed relation between the motor iii and the motor it
exists. As is ‘well understood the Sel'syn sys
tem including devices
54
‘558 operate to
produce a torque on the rotor 5? dependent on
the
For example,
displacement
if windings
between5iwindings
and 52 are
til in
andequi
librium position no torque is produced on \v'nd~
an inherent tendency to rotate in a direction to
reduce the output of the electric valves I9, 20
and 2i. In order to utilize this tendency to
restore the phase shifting device to the position
of minimum output when the anode-cathode cir
cuit or electric valves are deenergized, I provide
ns for loading electrically the stator winding
, ipedance elements such as resistors 15 in
y se to opening movement of the contacts 66.
ing 51.
trated in the drawing, this is accomplished
ins of a switching device '16 having mov
on
direction
windingof5'!the
dependent
relative ondisplacement
the magnitude
of the
windings 5i and
As illustrated in the draw
ing, the rotor winding
is: mechanically coupled
by means of. a shaft
‘c- a torque clutch illus
trated schematically at 513 which has the out
put shaft
thereof connected ' Tith the rotor of
The device ‘Z6 is provided with an actu
oil "z‘la connected to be energized from one
p ase of the control power supply transformer
secondary 558 through a circuit including a mov
able contact l'8 on the anode contactor 61. From
an inspection of the circuit of coil l1 it is seen
that it is electrically connected with the trans
the phase shifting selsyn 33v
torque cou
pling provides what under normal operation is a 45 former secondary winding 68 whenever the anode
positive drive of the rotor of
‘chase ehift~
contacts iii)
closed. In this way the resistors
ing device
but which no
led rota~
‘55 are automatically disconnected from the sta
tor winding of the phase shifting device 36 when
shifting rotor has reached the ii.
of its travel.
the anode leads of the electric valves !9, 20 and
This arrangement allows the differential selsyn
are closed by closure of contacts 65. When
to continue to rotate
the event that the de~
the rotor 31 of the device 36 is energized from
parture from the desired speed relationship be
the supply circuit 28 this loading of the stator in
tween machines l9 and ii is not
{nediately
c. ases lire torque tending to rotate the phase
corrected by control of the electric valves £9 to
shifting device to the position of minimum out
‘st before the winding 3? reaches
of its ex
put of the electric valves and to maintain the
treme positions.
_
With the motor control system illustrated it is
desired that the electric valves
25
that
have the
theircontrol
ranode~cathode
circuit therefor
circuits beenergized
adjusted so
that the output voltage is insuf?cient to rotate
the reel motor iii until after the motor ii has
been started. Preferably the minimum output
of the valves is suf?cient to produce considerable
torque in the reel but insuf?cient to rotate it. To
this end means are provided for' insuring that
the rotor 3? of the phase shifting device
is
returned to the position of minimum output of
the electric valves when the anode-cathode cir
same in that position with the cooperating con
tacts l'i
'12 operating as a mechanical stop
against which the rotor is held.
Although the operation of the various elements
of the system described above has been described
during the description, it is believed that the fea—
tures and advantages of the present invention
will be acre apparent from a brief consideration
, _. ~aeration of the system as a whole.
Let it
be assumed that switches 29 and ‘w are open
that both motors Hi and I! are at standstill.
If it is desired to start the system, switch 29 is
first closed energizing transformer 46 and the
rotor w ding of phase shifting device 35. The
and Eli are also energized and the
cathode heater elements of the electric valves
l9,
26 are energized. The contacts 11 are
normally closed and resistors '45 are thereby con;
nected across the terminals of the stator wind
70 selsyns
for
cuitspreventing
of the electric
the energization
valves are deenergized
of the anode»
cathode circuits until the rotor has been returned
to the position correspondingr to minimum cute
put voltage of. the electric valve recti?er. Refer
ring again to the drawing, the anode circuit of
ings 35 of the phase shifting device 36, thus in
2,411,744
7
creasing the torque produced on the rotor to in
8
mined amount are presented in a divisional ape
sure that it is rotated to the position of minimum
plication, Serial No. 477,031, ?led February 25,
‘output and that movable contact 72 is closed on
1943.
contact 7!. After the cathode heater elements
In order to prevent hunting of the regulating
are at operating temperature, preferably insured
system the circuit including resistor 42, capacitor
by a. time delay relay (not shown) having a con
‘is and parallel resistors 39 and ii are connected
tact in circuit with switch 70, the switch ‘HI is
across the armature terminals of the motor ID.
closed and coil 33 of relay 68 is energized from
The capacitor it tends to have a charge depend
one phase of winding it through the contacts ‘H
ent upon the voltage of motor iii. Any change
and 72 of the limit switch associated with the 10 in this voltage produces a change in the con
phase shifting device 36. As soon as relay 68
denser charge at a rate dependent upon the
picks up, contact 18 is closed to complete a circuit
time constant of the condenser resistor circuit.'
for coil Tia to operate contacts ‘E1 to open cir
The resistor 39 is connected in the circuit of the
cuit position and disconnect one terminal of each
control members 25 so that the polarity of the
of the resistors ‘it from the stator winding 35. 15 voltage caused by a change in the condenser
The contacts 66 in the anode leads are closed
charge is in a direction to oppose the change
and the contact ‘M in parallel with contacts ‘H
in voltage of the armature machine Hi causing
and ‘i2 is closed to complete a holding circuit for
the change in the condenser charge. In this
the coil 69. The tube circuit is now in operating
way overshooting of the system is prevented and
condition and will function to impress a variable
a smooth regulating action obtained. The slider
voltage on the armature winding of motor I!)
as controls the magnitude of resistance in cir
dependent upon the relative position of windings
cuit with condenser 55 and in this way controls
5i and 52 of the devices 53 and 54 respectively.
the time constant of the condenser circuit. The
The next step in the operation is the starting
slider 46 controls the portion of resistor 39 in
of the motor H ‘which is brought up to oper
the control circuit and in this way controls the
magnitude of the anti-hunting voltage for a
ating speed by any suitable starting circuit (not
shown).. As motor I! starts to rotate winding
given current through resistance element 39.
52 will move with respect to winding 5| and in
Condenser 43 cooperates with resistor 422 to pro
this way produce a torque on winding 51 which
vide a ?lter to ‘remove some of the ripple from
the anti-hunting voltage. The tendency of the
is transmitted to rotate the movable element of
phase shifting device 36 in a direction to increase
regulating system to huntmay also be decreased
the output voltage of the electric valve recti?er
by the introduction of reduction gearing between
and in this way cause the reel motor it to start
the rotor 57 and rotor 37 of the phase shifting
rotating. The torque imparted to the winding
device. The gearing may be incorporated in the
51 and its associated structure is always in a : clutch unit 64 if desired.
direction to rotate the movable element 3'! in
When it is desired to shut down the motors,
a direction to effect the change in the impressed
motor H is ?rst deenergized and brought to a
Voltage'on the armature of motor it necessary
standstill by dynamic breaking if desired and
to change its speed in the proper manner to
with the control circuit for motor is functioning
bring the elements 5| and 52 into the corre it) in this way the‘ motor it follows motor II to
spondence and in this way to maintain the de
standstill at which time switch ‘it is opened.
sired speed relation between motors iii and il.
Upon opening of the control switch ‘ill coil 69
Inasmuch as movement of the rotor 31 of the
of the anode contactor operating relay ‘6B is
phase shifting device 36 to one of its extreme po
opened to deenergize the anode-cathode circuits
sitions may fail temporarily to restore the de
of the electric valves i9, 23 and 2!. Contact 18
sired ‘positional relation between the movable el
is also opened to deenergize the winding 77a of
ements‘ of motors it) and II, it is desirable to
the relay 16 thus allowing contact Ti to close and
provide for relative movement between the wind
connect resistors '55 across the stator winding 35
ing 5? of the differential motion transmitting de
of the phase shifting device 36. As described
vice 58 and the rotor 3?. To this end the torque
above this insures that the winding 37 is returned
clutch i545 interposed between the winding 57 and
to the position of minimum output against the
the winding 3'?‘ permits relative movement of
stops provided by contacts ‘ii and I2 and places
these windings when the torque required to 1'0
the system in position to be again operated. If
tate winding 3? exceeds a certain value. The
the shutdown is for a long period the contact 29
clutch is adjusted so that the windings 3? and 5?
may be opened and the system completely de
are maintained in ?xed relation to each other as
energized.
-
'
long as winding 37 is not against the stops pro
vided by contacts 'H and 12 at one limit of travel
While I have described what I at present con
sider the preferred embodiment of my inven
A and the mechanical stop 73 at the other limit of
tion, it will be obvious to those skilled in the art
travel. In this way the system including motion 60 that various changes and modi?cations may be
transmitting devices 53, 54 and 58 are allowed to
made without departing from my invention, and
remain in synchronous relation so that as the
I, therefore, aim in the appended claims to cover
elements of the motors iii and H approach the.
all such changes and modi?cations as fall within 7
desired relationship and the phase shifting device
the true spirit and scope of my invention.
3'! ‘is to be moved away from its extreme position
What I claim as new and desire to secure by
th'e'devices 53, 5d and 58 are functioning smoothly
Letters Patent of the United States is _:l
and the winding 5? is not operating unevenly as
1. In combination, a supply circuit, electric
it would tend to if it were resynchronizing after
translating apparatus energized from said cir
having been out of step with the remainder of
cuit and including eiectric valve me‘ans'having
the motion transmitting system. Claims includ 70 a control member for controlling the conductivity
ing this feature of interconnecting the motion
thereof, a control circuit for controlling the
transmitting system with the phase-shifting de
energization of said control member including
vice by means which permits relative movement
phase shifting means including an element mov
therebetween when the force required to move
able to vvary'the phase of the output voltage
the ,phase-shiftingpdevice exceeds va predeter 75 thereof and thereby to vary the output-of said
2,411,744
10
electric valve means, means for deenergizing said
electric translating apparatus, and means re
trically loading one of said windings to increase
the tendency of said windings to rotate to a posi
sponsive to the deenergizing operation. of said last
tion corresponding to the minimum output of said
mentioned means for returning said movable ele
electric valve means.
ment to a position corresponding to the minimum Ci
5. In combination, a supply circuit, a load cir
output of said valve means.
cuit, electric translating apparatus interconnect
2. In combination, a supply circuit, electric
translating apparatus energized from said circuit
ing said circuits and including electric valve
means having a control member for controlling
and including electric valve means having a con
the conductivity thereof, a, control circuit for con
trolling the energization of said control member
including a phase shifting device having rela
tively movable windings movable between extreme
positions for varying the output of said electric
trol member for controlling the conductivity
thereof, a control system for controlling the ener
gization of said control member including a phase
shifting device having an element movable to
vary the phase of the output voltage thereof
thereby to vary the output of said valve means,
valve means between a minimum and a maxi
means operative to return said movable element
to a position corresponding to the minimum out
put of said valve means in response to deener
mum, at least one of said windings being a poly
phase winding, impedance means, circuit con
trolling means for connecting said impedance
means with one of the windings of said phase
gization of said translating apparatus, and means
preventing energization of said translating ap
paratus when the movable element of said phase
shifting device is in any but the position corre
sponding to minimum output of said electric valve
the circuit between said translating apparatus
and said supply circuit, said circuit controlling
means.
shifting device, switching means for interrupting
means being operable in response to operation of
said switching means to open circuit position to
connect said impedance means with said winding
3. In combination, a supply circuit, a load cir
and thereby causing said relatively movable wind
cuit, electric translating apparatus interconnect
ings to move to a position corresponding to mini
mum output of said electric valve means.
6. In combination, a supply circuit, a load cir
ing said circuits and including electric valve means
having a control member associated therewith for
controlling the conductivity thereof, a control cir
cuit for controlling energization of said control
member including phase shifting means having an
element movable between extreme positions to
cuit, electric translating apparatus interconnect
ing said circuits and including electric valve
means having a control member for controlling
the conductivity thereof, a control circuit for con
vary the output of said electric valve means be
tween a minimum and a maximum, switching
means for controlling the connection of said
translating apparatus with one of said ?rst two
named circuits, and means operated in response
trolling the energization of said control member
including a polyphase phase shifting device hav
to the circuit interrupting position of said switch
ing means for returning the movable element of
said phase shifting means to the position corre
sponding to minimum output of said electric valve
maximum, and a plurality of resistance elements,
switching means for connecting said resistance
elements between the phase terminals of one of
means.
ing means for controlling the energization of said
electric valve means from said supply circuit, and
ll. In combination, a supply circuit, a, load cir
cuit, electric translating apparatus interconnect
ing said circuits and including electric valve
means having a control member for controlling
the conductivity thereof, a control circuit for con
trolling the energization of said control member
including a phase shifting device having relatively
rotatable windings operable to extreme positions
for varying the output of said electric valve means
between a minimum and a maximum, said rela
tively rotatable windings having an inherent
tendency to rotate in a direction to reduce the
output of said electric valve means to a minimum, 55
and means responsive to a change in an operating
condition of said translating apparatus for elec
ing relatively movable windings movable between
extreme positions for varying the output of said
electric valve means between a minimum and a
the windings of said phase shifting device, switch
means operable in response to operation of said
second named switching means to a position to
deenergize said electric valve means to operatively
connect said resistance elements to one winding of
said phase shifting device to electrically load said
device and cause said windings to move to a posi
tion corresponding to the minimum output of said
electric valve means, and means preventing opera
tion of said second switching means to energize
said valve means except when said phase shifting
device is in a position to produce minimum output
of said electric valve means.
ELMO E. MOYER.
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