energy-conversion properties of induction machines in variable-speed constant- frequency generating...

8/18/2019 Energy-Conversion Properties of Induction Machines in Variable-Speed Constant- Frequency Generating Systems

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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 = (


2/6

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


3/6

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


4/6

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


5/6

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


6/6

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

neers,

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.

E. Fitzgerald» Charles Kingsley, Jr. McGraw-

Hill Book Company, Inc., New York, N . Y., 1962.

5.

A L T E R N A T I N G C U R R E N T G E N E R A T I N G

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

no

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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