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Energy-Conversion Properties of Induction
Machines in Variable-Speed Constant-
Frequency Generating Systems
M. RIAZ
MEMBER AIEE
N A LIST of more than 380 technical
problems affecting the defense of the
United S ta tes prepared by the National
Inventors Council (Office of Technical
Services, U. S. De par tme nt of Comm erce)
appears the following item under the
general heading of Power Supplies:
741.
AC Generators—Constant fre
quency, variable speed AC Generators .
This is bu t one indication of th e imp or
tance attached today to the problem of
constant-frequency a-c generation, partic
ularly with regard to air-borne applica
t ions.
In recent years, ma ny solutions
to this problem have been proposed, de
veloped and, in some instances, specific
constant-frequency generating systems
have been actually utilized in aircraft and
guided missiles.
Two basic approaches have been con
sidered: The initial approach had to do
with the development of constant-speed
drives capable of converting the variable
speed of a propeller or turbine prime
mover to a constant-speed output shaft
which drives an a-c generator. This sys
tem is presently employed in many
types of aircraft, the control of the con
stant-speed drives being normally effected
on a differential principle by hydraulic
means , a l though pneumatic and mechan
ical drives have also been suggested.
All these systems suffer from a complexity
and reliability standpoint, resulting from
the close manufacturing tolerances neces
sary to achieve the desired control at the
speeds involved, and from the auxiliary
equipment required to supply oil, high-
pressure air, and the like.
Despite the relative success of some of
these systems, attention has also been
given to the possibility of developing all-
electric constant-speed drives. Th e all-
electric constant-speed drive assumes the
configuration of a variable-speed genera
tor connected to a motor suitably con
trolled to run at constant speed. One
particular arrangement, initially proposed
by Pes tarmi,
1
using d-c metadyn es for the
generator and motor, offers some interest
ing possibilities, notwithstanding its large
weight and size and the complexities
associa ted with comm utators . Another
recent ly sugges ted arrangement
2
uses a
variable-frequency a-c generator to supply
the stator excitation of an induction
moto r, the roto r of which is excited by t he
difference between the unregulated stator
frequency and a standard frequency de
rived from a relatively low power source.
The mo tor then runs a t a cons tant speed in
synchronism with the standard controlled
frequency and drives a conventional a-c
generator. A basic feature of an all-
electric drive is the possible utilization of
the variable-speed generated power,
whether d-c or a-c, to supply certain
types of electrical loads in addition to the
constant-speed motor load.
All constant-speed drive systems essen
tially introduce an extra piece of equip
ment placed between an input varying
speed shaft and the a-c generato r. An
alternate approach to the problem of
constant-frequency generation is to de
rive this constant-frequency output
directly from the variable-speed rotating
shaft. Several system s hav e been sug
gested along this line of which two kinds
can be distinguished; one involves new
types of constant-frequency alternators,
and the other involves rectification of
variable-frequency a-c power followed by
its inversion to consta nt frequency. This
inverter-type system is now coming to the
fore as a consequence of the recent de
velopment of semiconductor devices ca
pable of operating at high power levels.
Furthermore, it provides an interesting
method of generating constant frequency
for those aircraft or missile systems in
which the prime source of power is not in
the nature of a shaft speed but in the
form of direct cu rrent.
The majority of the suggested con
stant-frequency variable-speed a-c gen
erators employs some form of induction
machine in conjunction with auxiliary
a-c or commutator machines which may,
in some cases, form an integ ral pa rt of t he
induct ion machine.
3
It is with these induction-machine sys
tems arranged to produce constant-fre
quency generation that this paper is
mainly concerned. The object is to de
termine the basic physical limitations and
advantages inherent ly present in the de
sign of such systems. The approach
adopted in this study is based upon con
sideration of the energy-conversion prop
erties of induction machines.
Electromechanical Energy
Conversion in the Induction
Machine
Although the theory of the induct ion
motor is now undoubtedly well es tab
lished and may be found in almost any
textbook on rotating electric machines,
4
it is desirable to describe briefly the
fundamental characteristics of an in
duction mach ine. Thi s review will serve
to i l lus tra te the nomenclature and conven
tions adop ted in the analysis. To em
phasize the electromechanical energy-
conversion aspect, the induction machine
as shown in Fig . 1 will be viewed from
its three terminal parts through which
power is either delivered or received,
depen ding. on t he part icular mode of
operation. Th e relationships between the
terminal quantities are given by the per
formance equations of the induction ma
chine. For stead y-sta te polyphase opera
tion, these equations may be written on a
per-phase basis as:
V
r
=jsaX
m
I
s
+ Rr+jsX
r
)Ir 2 )
03
S
where
Vs ,
r = stator and rotor terminal voltage
phasors
7
S
,
IT
= stator and rotor curr ent phasors
R
s
,
R
r
= stator and roto r resistances
a = equivalent ro tor-to-stator turn s ra tio
5 = s l ip = (
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I
s
STATOR V
e
s
S H A F T
1
I N D U C T I O N
M A C H I N E
I
r
r
V
r
ROTOR
Fig. 1 . Block diagram of the induction
machine as viewe d from its terminals
x
s
= stato r leakage reactance = X
s
—X
m
x
r
= rotor leakage reactance
X
r
—a
2
X
m
It is often convenient for numerical
calculations to express the performance of
an induction machine in terms of an
equiva lent circuit. Th e particula r form
of the equivalent circuit illustrated in F ig.
2 may be derived directly form equations
1 and 2 using the concept of an ideal in
duction machine having a voltage ratio of
as/I and a curren t ratio of I/a. This
equivalent circuit made up of two sub
networks is convenient because it sep
arately exhibits each of the stator and
rotor circuits and so conforms with the
terminal representation given in Fig. 1.
The polarities and directions of the volt
ages,
currents, and powers are indicated
in Fig. 2. Pow er will be tak en to be posi
tive when delivered to the induction
machin e. In order to stress the power
conversion characteristics of the induc
tion machine, the stator and rotor resist
ances are assumed to form part of the
external circuits connected to the m achine
and attention is focused on the electrical
terminals defined by the voltages
E
s
and
E
r
in Fig. 2. Th e per-phase power ex
pressions are defined as
n
s
=P
s
-\ls\
2
Rs =
(5ie[I
s
*Es]
= | I , | | Ä | C O S * . (4)
n
r
= Pr-\lr\
2
RT = Ke[Ir*Er]
= | /
r
| | E
r
| c o s 0
r
(5)
Q
s
= 3rn[I
s
*E
s
] = \l
8
\\E
s
\
sin Θ, 6)
Qr=3m[Ir*E
r
] = \lr\\Er\
sin
$
r
7)
P
m
= - 7 >
m
= - r e ( l - s )
r
P
(8)
where P and Π indicate real or active
power and Q denotes reactive pow er.
Carrying out the operations implicit in
the foregoing definitions through the use
of equations 1, 2, and 3 yields
n
g
= aX
m
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The power factor angle
is
,
r
. 0 0
(D)
0 < s < l
l U )
T
e
< 0
Fig.
4
Phasor diagrams corresponding
to the
different modes
of
operat ion
of an
induction
machine
power supplied to the stat or, slip-fre
quency power
is fed to the
rotor.
For a
fixed slip frequency,
th e
motor speed
is
nearly independent
of
load.
By
shifting
the phase
of the
injected ro tor v oltages,
the power factor
at
which
th e
m otor
op-
erates
can be
controlled. This
can be
seen from the relation.
0
s
=
tan
l
—=tan
x
-n
r
/s
in which
50) of the
loads
as
well
as the
reactive power Q
0
of th e
machine mu st
be
supplied
by
suitable external circuits
which usually assume the form
of
capaci
tors
or
overexcited synchronous gene
rators conected in parallel
to
the induction
generator.
The conventional induction generator
has
a
short-circuited roto r
(Π
Γ
= I
r
2
Rr) . I t
is characterized
by a
load
and
power fac
tor operat ion that
is
completely deter
mined
by the
value
of
slip.
If op-
erated
in
parallel with synchronous
ma-
chines,
th e
induction generator
has its
voltage an d frequency fixed by these ma -
chines.
If
used
in an
isolated system,
the
induction generator can provide its own
excitation when
a
suitable capacitor bank
is connected across
it s
terminals, th ereby
establishing a resonant circuit condition.
The p hasor diagram governing this s i tua
tion is shown in Fig. 4(B) . Only the ac-
t ive component
of
s ta tor current
I
s
cos 0
S
(neglecting stator resistance effects) can
be supplied
by the
generator
to a
load
while
the
lagging reactive comp onent
of
s ta tor current
has to be
furnished
by ex-
ternal cap acitors or synchronous machines.
A factor expressing t he efficiency
of
power
conversion in the induction generator ma y
be defined
a s
lg**
stator power output
_
—
II
S
mechanical power input P
m
- Π , 1
- l - j ) n , 1-5
(*0
This
is the
case
of the
induct ion motor
operating below synchronous speed.
In
general,
th e
rotor slip-frequency power
may supply,
in
addition
to the
rotor
I
r
2
R
r
losses, some external active
or
passive
circuits which
are so
adjus ted
as to
con
trol speed and
power factor.
The
phasor
diagram
for
this mode
of
operation
is
given in Fig. 4 C).
The ordinary induct ion motor
has a
short-circuited rotor (E
r
=— I
r
Rr).
Its
power conversion efficiency is defined
as
Vm~
mechanical power outp ut
stator power input (neglecting stator
losses)
e
s
— tan
^QÌ^ ÌJQL
u
x
= tan
= tan
l
——
Ir
2
Rr
(s>0)
C A S E E : S > 1 ; T
e
> 0
This also defines
a
motor opera t ion
complete ly equivalent
to
Case
C,
with
the
role
of
s ta to r
and
rotor interchanged.
Assuming the same sequence of polyphase
excitation,
th e
motor
in
Case
E is
excited
through i ts rotor and turns in the opposite
direction
to
the motor opera t ing as
in
Case
C with stato r excitation. Because of this
reversal
of
rota t ion,
th e
slip being cus
tomarily defined with respect t o the s ta tor
becomes greater tha n unit y. Clearly
the
slip with respect to the rotor is 1/s
C A S E
D :
0 < S < 1 ;
T
e
<
Here , input power
is
delivered mechan
ically to the shaft and electrically to the
rotor while
th e
stator supplies electric
power to an external load. The phasor
diagram representing these conditions
is given
in Fig. 4 D).
This generator
opera t ion
is
characterized
by an
energy
balance
in
which s-per-unit power
is
delivered electrically
to the
rotor circuit
and 1 — per unit is supplied mechan ically
to the shaft to produce one-per-unit stator
output power. Power amplification
is
therefore achieved as determined by the
ratio
A
=
electric power outp ut
_
— n
s
_
1
electric power input
Π
Γ
s
=
1 -5 5>0)
which
is
also
th e
frequency-transforma
tion ratio
of
the induction machine (equa
tion 14).
C A S E F : S > 1 ;
T
e
< 0
This case also refers
to a
generator
operation similar
to
Case
E but
with
the
roles
of
s ta to r
and
rotor interchang ed.
Because the rotor must now be driven
in a
a direction opposite
to
t h a t
of
Case
E, it
therefore rotates
in
opposite direction
to
the rotating field
of the
s ta tor , thereby
establishing
a
slip 5 greater th an unit y.
Th e resulting power amplification is de
fined by
th e
ratio
Λ = — = 5
Induction-Machine System s for
Constant-Frequency Generation
A variable-speed constant-frequency
generating system using induction
ma-
chines can
b e
viewed from
an
energy-con
version standpoint as constituting essen-
M A R C H
1959
Riaz—Energy-Conversion Properties
of
Induction Machines
27
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tially the inverse of a variable-speed a-c
moto r system. Wh eras the problem of
speed control of a-c motors has been
studied from the earliest days of a-c power
systems, it is only recently th at engineers
are becoming interested in the use of
induction machines for generation, espe
cially in air-borne-type applications.
One general and basic method for con
trolling the speed of an a-c motor consists
of inserting voltages of the appropriate
frequency, magnitude, and phase in the
rotor circuits of the moto r. The principle
underlying this method of induction ma
chine operation has been discussed under
Cases A and C in the previous section. It
has led to the invention of numerous con
figurations know n by such nam es as
Kramer, Leblanc, Scherbius systems
which have been applied successfully to
large polyphase induction motor installa
tions as encountered, for example, in roll
ing mills and wind-tunnel drives. These
systems make use of auxiliary commuta
tor-type machines which act as frequency
changers to supply or recover the rotor
slip-frequency power without incurring
excessive ohmic losses. The Schräge
brush-shift motor is another example of a
machine system in which frequency
changer and main motor are combined in
one frame.
All these induction-motor systems can
be operated in an inverse manner to sup
ply constant-frequency power when driven
from a variable-speed source. How ever,
when considering their use for air-borne
applications, the fundamental laws relat
ing to the flow of power in the induction
machine should be borne in mind as dis
cussed under Cases B and E or F. Opera
tion above and below synchronous speed
can be realized; however, the larger the
speed variations (or slip), the larger the
size required for the auxiliary apparatus.
Furthermore, the use of auxiliary com
mutator machines may present severe
problems in the kind of operations and en
vironments encountered in air vehicles.
To eliminate these problems, considera
tion should be given to the developm ent of
new static frequency-conversion equip
ment employing high-power switching
transistors and magnetic cores which
could be used in conjunction with induc
tion machines to produce constant-
frequency generation.
A different arrangement for providing
rotor slip-frequency power to an indu ction
machine without recourse to commuta
tor or rectifier-type apparatus makes use
of another induction mach ine. Th e con
figuration bui lt in a single frame m ay b e
designated as a double-induction-ma
chine system.
5
This generating system,
consisting of two series-connected, me
chanically coupled rotors, ma y be regarded
as the coun terpa rt of the self-synchronous
system comprising two uncoupled induc
tion motors with interconnected rotor cir
cuits . This machine system running
above synchronous speed (s
8/18/2019 Energy-Conversion Properties of Induction Machines in Variable-Speed Constant- Frequency Generating Systems
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ri
rx
n
(A) - '- ~ (B)
Fig.
5. Equivalent circuit of double induction machine (stator connected in series)
A—Phase shifter in rotor
B—Phase shifter in stator
b e s e t b y i n h e r e n t p h y s i c a l l i m i t a t i o n s ,
may s t i l l prove to pos ses s def in i t e ad
v a n t a g e s in s o m e a i r - b o r n e e l e c t r i c s y s
t e m s .
Ap p en d ix . Analysis of the
Double Induction Generator
The double- induct ion genera tor cons i s t s
of two mechanical ly coupled induct ion
machines wi th the i r s t a tor s connected in
ser ies (or in paral lel wi th the poss ible inter
posi t ion of a var iable rat io t ransformer) and
their roto rs in ser ies . An app rop r iate regu
lator including a capaci tor exci tat ion sys tem
is assumed present in the s tator ci rcui ts to
cont ro l t e rminal vol t age and fr equency. A
phase shif t between rotor impressed vol tages
is int roduced by displacing the rotor wind
ings relat ive to each other whi le maintaining
the s t a tor windings in phase . Thi s phase
shif t can also be obtained by a s tator dis
placement with the rotors in phase or by a
combinat ion of both s t a tor and ro tor d i s
p l aceme nt s . These d i spl acement s can be
made ei ther mechanical ly or electr ical ly by
means of addi t ional windings , usual ly of the
so-cal l ed s a tura t ing type .
The s t eady- s t a t e equat ions of t he double-
induct ion generator are f i rs t wri t ten assum
ing tha t t he two induct ion machines ar e not
connected e i ther mechanical ly or e l ec t r i
ca l ly . These equat ions therefore as sume
the form given in equ at ion s 1 and 2 and are
best expressed us ing matr ix notat ion as
[V]=[Z]X[I]
Rsi+jXsl
jaSiXmi
jaXmi
Rri+jSiXn
R&-\-jXs2
jbStXmi
The turns r a t ios ar e denoted by a for ma
chine 1 and b y
b
for machin e 2.
The next s tep in the analys is is to deter
mine the cons t r a in t s es t abl i shed by con
nect ing the two induct ion machines to form
the given sys tem conf igurat ion. These
cons t r a in t s r educe to equat ing the s l i ps i n
the two machines and to wri t ing the fol low
ing r e l a t ion between the o ld var i ables (un-
pr imed) and the new var i ables (pr imed) :
[I] =
or
Γ «Ί
In
I$2
In
[C]xir]
~
1 0
0 1
1 0
0 -e
j
-fit«
Ir
(16)
mu tual coupl ing . S ince th i s coupl ing may
be var i ed by changing the turns r a t io b/a
and the phase sh i f t a, i t provides an added
degree of flexibility which can be utilized to
achieve cons t ant - f requency var i able-speed
system operat io n. Th is is in con tras t to the
s ingle- induct ion generator capable of oper
at ing at only one value of s l ip for any given
load.
The solut ions of equat ion 17 are
The cons t r a in t mat r ix [C] is es tabl ished on
the as sumpt ion tha t t he s t a tor s ar e in phase
and connected in ser ies whi le the ser ies-
connected rotors have a phase shif t of a.
The impedance mat r ix [Ζ ' ] descr ib ing the
actual sys t em i s t hen ob ta ined f rom t he
equat ion
[Z ]-[C«*]X[Z]X[C]
where Ct* means the conjugate t r anspose of
C. After carry ing out the foregoing ma tr ix
opera t ions , t he per formance equa t ions of t he
double induct ion machine are f inal ly wri t ten
a s
Rrr+jsXr
Vt R
ss
Rrr — s XssXrr—Xmm
2
)-l·
j(R
rr
Xss~\- sR
ss
Xrr)
(19)
and
I
r
_bsX
m
2
sin
a+j(bsX
m
2
co s
a— asX
m
i)
Vt
RssRrr —
s{
XssXrr — Xmm
2
)~l·
j R
rr
Xss + sR
ss
Xrr )
where
Xm m
2
= a?Xm\
2
+b
2
X
m
2
2
— 2ab COS aX
m
iX
m
2
M,
ss+jXss
jaXmi—jbX
m
2e
joc
iasXmi -jbsX
m
2e~
ja
R
TT
+jsX
rr m
(17)
where
R
S
s = Rsl +
Rs2J
XsS = -X «
l
H~ Xs2
R
rr
=
R
r
i+Rrt;
X
rT
= X
r
i-\-X
r
2
V
t
= terminal vo l t age = V
s
i+ V
s
t
The e l ec t romagnet i c t orque i s g iven by
T
e
=
-(Re KjaXmi -jbX
m2
e-
ja
)I
s
*Ir]
18)
ω
3
(15)
I t i s i n t er es t ing to note th a t t h e eq uat ions
of the double- induct ion machine are s imilar
to those of a s ingle- induct ion machine given
in equ at ion s 1, 2, and 3. Th e ma in effect of
the double connect ion appears in the off-
d i agonal t e rm represent ing the s t a tor - ro tor
The s tator power is therefore
P
s
= (Re[Is*Vt] =
VtKR
S
sRTT
2
+ sRrTXmm
2
+ S
2
R
ss
XrT
2
)
jbXml
Rn-^-jSiXri
X
Γ/βΊ
In
Ia\
\jn\
[R
8s
Rrr
—s(XssXrr— Xm m
2
) ]
2
+
RrrXss
+ sRssXrr)
2
and the rotor power is
Pr=Ir
2
Rr
Vt
2
RrrS
2
X
mm
2
=
[RssRrr-s X
ss
X
rr
-X
m
m
2
)]
2
+
RrrXss + sRssXrr)
2
(20)
(21)
I t i s clear from e qua t ion 2 0 tha t , i f 5 is
negat ive , t he s t a tor power may be negat ive
indicat ing the del ivery of a net power out of
the double- induct ion genera tor . I f s t a tor
r es i s t ances ar e neglec t ed , t he s t a tor power
express ion becomes
Vt'sRrrXmm*
Π
* sKXssXrr-Xmm^+Xss Rr,
and the power f ac tor angle
(22)
MARCH 1959
Riaz
—
Energy-Conversion Properties
of
Induction Machines
29
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sXrr
e
s
=
tSLn
1
— t a n
1
X
R
TT
Rn
—S [Xrr — Xmm /Xss)]
Hence by modifying X
mm
control can be
effected on the power output or power factor
for changing slips. Howev er, the efficiency
of the double-induction generator (neglect
ing stator losses) is still
a
direct function of
slip since, from equat ions 21 and 22,
-U s _ 1 j < 0 )
:
-n
s
+I r
2
Rr~l-s (Rs = 0)
23)
which is the identical relation satisfied by a
single-induction generator.
Another procedure for deriving the per
formance equations of the double-induction
generator can also be developed with the use
of the equivalen t circuit shown in Fig. 2.
By combining two such equivalent circuits
so as to satisfy the constraints of intercon
nection (series stator and rotor circuits), a
complete equivalent circuit for the double-
induction generator can be obtained as
illustrated in Fig. 5. The two circuits
shown are completely similar except for the
location of the phase shifter. Th e perform
ance equations can then be obtained from a
direct inspection of these equivalent circuits
and must check with equation 17.
A very similar analysis can be carried o ut
for the case of
a
double-induction generator
in which the stator windings are connected
in parallel instead of the series connection
discussed here.
The
characteristics
are
substantially th e same for both types of con
nections.
References
1 . E N G I N E E R I N G D E S I G N S T U D Y F O R A T W O U N I T
ELECTRICAL) CONSTANT SPEE D
85
H O R S E P O W E R
A L T E R N A T O R D R I V E
J. M.
Pestarmi . Technical
Report 55-236, Wright Air Developm ent Center,
Dayto n, Ohio, June 1955.
2. AIRCRAFT MOTO R GENERATOR WITH SECONDARY
S T A N D A R D F R E Q U E N C Y O U T P U T , L. J. Johnson,
S. E . Rauch . Transactions, Professional Grou p
on Component Parts, Institute
of
Radio Engi
neers, New York, N. Y., vol. CP-5, no. 1, Mar.
1958,
pp . 28-31.
3. FUTU RE AIRCRAFT ELECTRICAL SYSTEMS PR E
DICTED abridgement of D I R E C T E N G I N E - D R I V E N
U N C O N V E N T I O N A L E L E C T R I C S Y S T E M S ) , K.
Martinez. Journal, Society of Autom otive Engi
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New York, N. Y., Jan. 1958, pp. 68-70.
4 . EL E C T R I C M A C H I N E R Y :
AN
I N T E G R A T E D
T R E A T M E N T OF A-C AN D D- C M A C H I N E S book),
A.
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A R
RANGEMENT FO R C O N S T A N T F R E Q U E N C Y , K.
Polasek, K. A. Jonsson.
U. S. Patent no. 2,747,107,
M ay 22 , 1956. Also, British Patent
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700,036,
Sept. 18, 1951.
6. ARRANGE MENT ALLOWING THE ELIMINATION OF
B R U S H E S I N E L E C T R I C A L M A C H I N E R Y ,
M. E. Rémy.
Revue Générale de VElectricité, Paris, France, vol. 64,
no. 3, Mar. 1955, pp. 113-18.
Communication Systems
for
Railway
Traffic Control
H. C. SIBLEY
MEMBER AIEE
C
O N T R O L
of
railw ay traffic from
a
central location requires the transfer
of large amounts of information between
the central location and many points
along the railroad. Th e num ber of way
side information points depends
on the
t ype of signaling installed, which in turn
depends on the type and a m o u n t of
traffic handled by the railroad. Cer
tainly, effective communication is re-
quired with al l locations where track
switches and controlled signals are
situated . Inform ation mu st also be
available from a sufficient number of
points to keep the dispatcher informed
abou t the location and progress of all
trains.
Additional information of a specialized
na tu re is rapidly becoming a significant
pa r t
of
traffic control.
An
example
of
this special information is the signal
picked up by hot box detectors. Voltages
proport ional to the tempera tu re of each
journal box are t rans mi t t ed to the dis
patcher's office as freight tr ains pass
Paper 59-249, recommended by the AIE E Land
Transportation Committee and approved by the
AIE E Technical Operations Department for presen
tation a t the AIE E W inter General Meeting, New-
York, N . Y., February 1-6, 1959. Man uscript
submitted October 23 , 1958; made available for
printing December
10,
1958.
H. C. S I B L E Y is with General R ailway Signal
Company, Rochester, N. Y.
remote detector locations
a t
normal
speed.
The Genera l Rai lway Signal Company
designs and builds many communication
sys tems for railroad signaling. Thr ee
of these systems will
be
described
in
this paper.
Centralized Traffic Control
Centralized traffic control (CTC)
re-
quires reliable 2-way communication
between a control office and a number of
wayside locations. A typical applica
tion of CTC may extend over 200 miles
with 40 or more field locations. The
p lan t a t a field location m ay comprise
a track switch and signals a t the end of
a passing siding or m a y be a fair sized
interlocking, that is , a group of switches
and signals such as m ay be found at a
te rminal or at a junction, previously
controlled by a local operator in a tower.
A
C T C
system enables
the
dispatcher
to operate track switches and signals a t
any location. I t also provides informa
tion as to the position of all wayside
appa ra tus and the location of trains.
C O N T R O L SY ST E M
The control system is the pa r t of a
CTC sys tem which provides communica
tion outbound from the con tro l office
to an y of the wayside locations. The
information input to this system is
supplied by levers or pus hbu t tons
opera ted by the dispatcher to control
track switches or signals. The se levers
or pushbut tons are generally located on
a control machine or console, placing an
entire division of a railroad virtually a t
the dispatchers fingertips. A typical
control machine is shown in Fig. 1. The
o u t p u t of th e control s ystem positions
relays at the field locations.
One type of CTC control is a synchro
nous system using relays and transmitting
a polar code. In this system a control
code comprises 15 equal time interva ls
of direct voltage line energization of
ei ther polari ty . When the system is a t
rest
the
line
is
held energized with
negative polarity. Mechanical oscilla
tors provide the time base for the system.
Fig.
1 Typical control ma-
chine
30
Sibley— Communication Systems for Railway Traffic Control
M A R C H
1959
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