STEWART

Performance of a

Ten Kilowatt Rotary

Converteri

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PERFORMANCE OF A 10-KILOYIATT ROTA.RY CONVERTS.

BY MILES VINCENT STEWART

TRESIS FOR THE 0E8REE OF BACHELOR OF SCIENCE I

ELECTRICAL ENGINEERING

IN THE

COLLEGE OF ENGINEERING

UNIVERSITY OF ILLINOIS

PRESENTED JUNE, 1901

no!St 4-1

UNIVERSITY OF ILLINOIS

May 51, 1901 x BO

THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY

Myles Vincent Stewart

ENTITLED PerforuiHTiCG of a Ten Kilowatt Synchronous Converter..

1- APPROVED BY ME As FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE

of Bachelor of Science in Electrical Engineering

HEAD OF DEPARTMENT OF

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preface.| j

a

The rota*y converter is a very important machine in a'lie? ^voing

current distribution to-day. My object in this thesis was tp;co£p]pp--

rate by laboratory experiments, some of the laws govern:' ay the working

of this type of inachine. The "fest inghouse polyphftse rotary was used,

bein,*, taken as a general type of modern construction in this line.

The different uses to which this machine may be applied were tak-

en up in the best manner possible, under the circumstances, and consid-

erable care was taken in working out the results. The number of in-

struments required and the lack of thr - ^-phase power from the Univers-

ity plant rendered it necessary to limit the investigation almost en-

tirely to single-phase working.

The work was done alone in general, except lag in those cases where

the lines of investigation were taken up by the' class in the regular

laboratory exercises.

In the course of the investigation it was found that:

(1) The heating of this machine was abnormal for full load and

over

.

(c) The no-displacement curve on the excitation base was very

nearly a straight line.

(S) This rotary was slightly over-compounded for about half

-

load.

(4) The true efficiency did not 3xceed 75. per cent.,for single-

phase working, as a direct rotary.

(5) Hunting was, apparently; not entirely done away with by the

copper damping shoes^or grids., under the pole-tips.

(6) This rotary could not be started from the slip-rings on ac-

count of the low resistance of the copper grids.

June 1901. Miles Vincent Stewart.

o

Department of Electrical Engineering.

University of Illinois.

Syllabus of Lectures. ROTARY (SYNCHRONOUS )CONVERTERS.

1. DEVELOPMENT OF THE SYNCHRONOUS CONVERTER.

1. - On the conversion and the transformation of electrical energy.

2. - Derivation of synchronous converter from direct-current generator.

"A converter is a rotary device transforming electric energy

from one form to another without passing it through the inter-

mediary form of mechanical energy." A.I.E.E. Rep. Com. on Stan-

dardization.

3. - Historical development; early and late types of machines.

II. THE'CclY AND PRINCIPLES.

1. - General principles underlying action of machine as converter.

2. - Relation of synchronous converter, A.C. and D.C. features, to: (a)

Electric generator; (b) Electric motor.

Armature inductance and reactions; as motor, as generator.

III. CLASSIEICATION.

1. - General: Synchronous commutating machines, Tjhich .comprise:

( asynchronous (rotary ) converters, A.O. to D.C, or vice versa.

( b )Couble-current generators, producing both A.C. and B*Gf.

(c) Phise converters, A.C. to A.G.,same frequency, different .phase,

(d frequency converters, A.C. of one frequency to A.C. of an-

other frequency , with or without change o/Aphase. j

2. - Subdivision: As to excitation: (a)self -excited; (b )sepi:rately ex-

cited. As to kinds of field windings: (a)Shunt-wound; (b)com-

pound wound.

IV. ELEMENTS OF DESIGN.

1. - Al lowances, constants, coefficients, general and special.

2. - Electric and magnetic circuit of machines; winding diagrams.

3. - Mechanical and structural details and accessories.

V. CONSTRUCTION OF MACHINES.

1- Electrical details: windings; connect ions A.C. end, single and poly-

phase circuits; relations of phase; number of poles;.insula.tion.

2. - Magnetic considerations:lamination of armature cores and poles.

3. - Mechanical and structural details and accessories.

4. - Limit3tions upon frequency and voltage im. osed by construction.

5. - Description of machines in commercial use.

VI. INSPECTION. See schedule 0.2, of the Manual.

VII. OPERATION OP MACHINES. See A. C. Schedule 220.(1,2 and A).

1. - Startin5:as C.C. motor; as A. 0. induct ion motor*?independent motor.

2. - Functional, working rreversability of machine; direct , inverted.

3. - Best operating conditions; unity power factor.

4. - ?aulty working and remedies: "pumping", hunting.

5. - Faulty working due to speed variation of engine driving the gene-

rator.

<?.- Influence of frequency upon operation.

7. - Regulation control . of output direct -current voltage:

(a Compounding; (b)induction regulator; (c)A.C. series of re-

actance.

IX. PERFORMANCE AND TESTING.

1. - Principles, methods; errors,. precautions, precisions cjr measurments.

Effect of variables on performance; voltage, frequency, excitation,

character and amount of load, wave form, etc.

3. - Voltage regulations and ratios, theoretical and actual, A. 0. and D.C.

4. - Phase relations and characteristics, influence cf excitation.

5. - Efficinecy of, and losses cf machine in various kinds of service,

c.- Magnetic determinations, leakage, distribution and magnetization.

?. - Armature resistance rocicup.noo , inductance and reactions.

8. - Output influenced by heating and number of chases, various combin-

ations.

9. - Special studies of machine., in various relations and services'.

X. APPLICATIONS. - Elictric Lighting and Railway Service.

Syllabus of Lectures. Rotary Converters. (xvi)

TABLE OP CONTENTS,

Page

List of Experiments .. .. /

Index of curves 3

Index of tables...!

Index of diagrams SIndex of {hotographa ...........6

Extension of Dewey Decimal System „„...,.. 7

Biblj bgraphy . , , ,• fO

Chapter I-Development of the Rotary Converter.

irt .1. Rectifier type. ......//

1st. 3. Induct or ium / /

Art. 3. Mot CP-Generator .//

Art. 4. Dynamotor //

Art. 5. Rotary Converter //

Art. 6. Rotary Converter vs. Motor generator /?*

Art. 7. Rotary Converter in Street railway Work£...,/^

Art. 8. Mot o e -Cenara t or for Lighting /3

Chapter II-Theory of the Rotary Converter.

Art s. 9510. Discussion of Alternating Current JU/*f

Art .11. Discussion of Rotary.

Art .1'"

. Inverted Rotary. 15~

Ar t . 13. Direct Rotary /fArt. 14. Self -starting of Rotaries isArt . 15. Machine in hand l(o

Art . 16. Voltage ratio I (o

Art. 17. Table of Voltages and Capacities

Art .13. Power -factor regulation /7

Chapter Ill-Classification.

Art . 19. Classification }Q

Art. 550. Machine in hand /8

Art. 21. Inverted Rotary . IB

Chapter IV-Elemenbs of Design

Art

.

22, Compared to direct -current generator

Page

tort . 23. ftrmat are react ions L . . . 1 .1.' /3

Art. 34. Tables of outputs and Voltages )3

Art. 25. Size of machine fixed Zl

Art. 26. Maximum allowable output 2'

Art. 27. Frequency £ /

Art.°3. Voltage. Z t

Art. 29. Variation of voltage for accumulators 2./

Art. 30. Poles 2* /

Art. 31. Peripheral speed ^ '

Art .32. Armature windings. . . .

Art ,33.8.M.F.per slot and slots.. 2.*.

Art. 34. Heating U. 2.2.

Art. 35. Self -induct ion in armaturel 2.2*

Art .38. Density of magnetic flux. .1 .1 .1 . ... 2. 2x

lr£.S7»Iieakage V, L. 2.3

Art. 3S. Values of 3 2. 3

Art. 33. Breadth of Machine .* 2?>

Chapter V. electrical and yechanical Data.

Art. 40. yechanieal data z4Art. 41. Electrical data zb

Chapter VI. Description.

Art. 42. Standard frequencies :

2 7

Art. 43. Voltages and phases 2.7

Art . 44 . Mach i ne i a hand Z 7

Art . 45. Starting devices Z 7

Art. 48. Self-starting. .1 . . ..

Art. 47. Exciters for inverted rotarias 2. r

Art. 48. Inverted rotaries £7Art. 49. Use of inverted notaries.... £.7

Art. 50. Speed of direct ritaryU 2.8

Art. 51. Speed of inverted rotary...... 2-8

Art. 52. Speed regulation of inverted rotaries £8Art. 53. Parallel-running for inverted rotaries L. &9Art . 54. Transformers in transmission........ 2.3

Art. 55. Six-phase system £9Art. 56. Par dialing of direct 3-v;ire rotaries 3°Art. 57. Pressure regulation 3 OArt.58.Eftfeat on feeding pressure. 3

Art. 59. Adaptations of regulators 3^Art. 60. Hunting 3 1

Chapter VII. Performance. Pag

Art. 61. As direct-current generator 3^Art. 62. As alternating current generator 3 £

Art. 63. Internal character 1st ios 3 2.

Art . 64. Double-current generator...! 33Art . 85. Short -circuit characteristic 3 3

Art. 6o. As direct-current motor 3 3

Art. 67. As single-phas.3 rotary converter 33

Art. 68. As inverted rotary 33

Art. 69. As single-phase synchronous notor. ... 3^Art. 70. No load characteristics 3^Art. 71. Mo load regulation 3*7

Curves ^jy

Tables 2>5

Diagrams 6 £s

Photographs 6Q

LIST OF EXPERIMENTS PBRltSMEB.

Experiment l.Direc'o -current generator. For regulation characterises.

Experiment 2. Alternating-current generator. One and owe - phase. For

regulation and short-circuit characteristics

Ixperiment 3. Double-current generator. Direct current and single-phase

alternating current. For internal and dynamic character-

istics.

Experiment 4. Direct-current mototr* Performance and efficieacy.

Experiment 5. Direct rotary. Single-phase. Performance and efficiency.

Sxperiment 6. Inverted rotary. Performance and efficiency.

Experiment 7. Synchronous Motor.Single-s jhase; Performance and efficiency

Tabic I -Direct -current generator

Regulation eharacterisb ics, Plate I

Table II-Three-phase alternator

Fower -f act oriSO, Plate II, Curve III

Table Ill-Three-phase alternator

Power -fact or=1.00

Table IV-Single-phase albernator

Bo .irinduotive load, Plate II, Curve I

Table V -Single-phase albernator

Short-circuit characteristic, Plate V

Table VI-Alternating and direct -current

Internal characteristics, Plate III

Table VII-Double-current generator

Dynamic characteristics, Plate IV

Table ¥111-Direct current motor

PerforBance and efficiency, Plate VI

Table IX-Single-phase direct rotary

Phase character isb ics

Performance and Bff iciency, Plates VII and VIII

Table X -Inverted rotary

Performance and efficiency, Plates IX, X and XI

Table XI-Single-phass synchronous motor

Phase characteristics

Performance and efficiency, Plates XII and XIII

Table XH-3ingle-phase rotary

No load phase characteristics, Plate XIV

Table XXFJ-S i n 5 Is -phase rot ary

l!o load regulation characterist ics, Plate XV

Table XHC-Resi stance data

IHDSX OF CURVES.Page

13

SI

5Z

Plate I. Experiment 1. 4 ?

Direct -current generator regulation charac-

teristics.

Plate I I. Experiment 2. 4 6

Alternating-current generator. One and three

phase regulation characteristics,

Plate III. Experiment 3.

Donble-cnrrent generator internal charac-

teristics.

Plate IV.Exp3ri nent 3.

Dynamic characteristics, one-phase alternating.

Plate V. Experiment 2.

Short-circuit characteristic, one phase alter-

nate ng

.

Plate VI. Experiment 4

Direct-current motor ; performance and effic-

iency.

Plate VII. Experiment 5. S3Direct rotary. Single phase.

Phase characteristics.

Plate VIII. Experiment 5

Direct rotary. Single phase. Performance

and efficiency.

Plate IX. Experiment 6.

Inverted rotary. Single-. -hase. Speed and

phase characteristics.

Plate X. Experiment 8.

gerformanoe and efficiency. Constant

H.P.M.

Plate XI. Experiment 6. 57Performance and efficiency. Constant

excitation.

Plat- XIIExperiment 7. 5®Synchronous motor. Single phase; Phase

Characteristics.

Plate XIII. Experiment. 7 SSSynchronous motor . Perf or nance and effic-

iency.

SI

ss

5£>

Plate XIY.Experiment 5 P.i^s

Direct rotary. Singie phase.. No-load charao- (p

teristics

Plate XV. Experiment 5. ^ /

Direct rotary. No-load regulation obaracterist-

ics

/

INDEX 0:n OIV^ .S.

Plats XVI. f»ag(

Diagrams of connections for Experiments 1,3 and 3. 6^Plate XVII. 63

Diagrams of connections for Experiment 4.

Plate XVIII. 6^Diagrams of connections for Experiments 5 and 7.

Plate XIX. (p5

Diagrams of connections for Experiment 6.

Plate XX. M (o(o

Diagrams of transformer connections for.phase

working.

Plate I (p 7

Diagrams of collector ring connections.

INDEX OP PHOTOGRAPHS.

Plate XXII- Photographs of parts, armature,. interior £qview of magnetic circuit,and .ensemble view

Plate XXIII-Views of coraraeriial types £,9

Plais XXIV-Photogcaphs of switches and plugs. yo

CLASSIFICATION AND IHDEX.

Synchronous Converters.

537. Electricity and Electrical Engineering .

537. S Applications of Electricity.

.83 ElectrorDynam ic Machinery.

.835 Synchronous Commutating Machines

.8353 Synchronous Converters.

.83509 Historical Development.

001 Statistical information.

002 Bcononio considerations in use of.

0023 Costs and Comparisons cf costs.

003 Specifications (A. I.E. B. Standardization

0037 Gurantees of Performance.

C04 Designs and Calculations.

0041 General, Considerations of design.

0042 Designing formulae, pr inciples,.etc.

0043 Calculations of Rotary designs.

0045 Electrical design.

0046 Magnetic design

0047 Mechanical design.

0C49 3rav;ings, designs, vievjs, etc.

00492 Curves, charts, viiring diagrams.

0050 Construction.

0055 Inspection of Rotaries.

00551 General inspection.

CC552 Mechanical inspection.

00553 Electrical inspection.

00554 Magnetic inspection.

C056 Construction details.

C0561 General features of construction.

00562 Electrical details.

00563 Magnetic details.

0C564 Mechanical details.

00c 7 Manufacturing and production.

0058 .Installation of rotaries.

006 Working, maintenance and operation.

0061 General . performance.

0082 Manipulation, handling, starting, etc.

0063 Functional working.

00631 As D. 0. Mot or; A. 0. Dynamo

C0632 As A.C.L'otor; D.O. Dynamo

00633 As A. C. Motor.

00634 As D.O. Motor.

00635 As Double-current generator

00636 Voltage Ratios.

00637 Limits imposed by construction.

006371 Limit of frequency.

006373 Limit of poles on account of commutator.

0064 Operation, running conditions, paralleling.

0065 Regulation and regulative features.

00651 Speed regulation of inverted rotaries.

00652 Variation of power-factor -with excitation.

00653 Regulation of D . C. E. M.F. output

.

00654 Regulation of A. O.E. M.F. output.

0066 Faulty working and remedies.

0C661 Effect of variation of impressed E.M.F.

00662 Frequency variations.

00663 Pumping effects.

00664 Effect of variable loads, regulation

006641 Rotaries for Stree-railaray use.

00665 Wave-form influences.

0067 Protection from accidents.

0068 Maintenance of operation of rotaries.

0069 Repairs and renewals.

007 hfeasurements and tests.

0071 General instruct ions, formulae.

0072 Single and polyphase power measurements, watt-

meters.

0073 Errors, orecautions^ T.recision of measurements.

0074 Methods of testing.

0075 Determination of machine .constants.

0076 Determination of working .characteristics

.

00761 General chara3terist ics.

007611 Power factor and phase characteristics

00762 Distribution characteristics.

.007624 Wave forms of rotf-aries.

00763 Economic characteristics.

.00764 External characteristics.

00765 Internal characteristics.

00766 Mechanical charactarist ics.

. C0767 Pates characteristics.

0076S Regulation characteristics.

00769 Resistance characteristics.

0077 General loss measurements.

00772 Stray power measurements.

007722 Elliotion and mechanical losses.

007723 Iron and magnetic losses^hystersis.

007724 Eddy current losses.

007725 Oorper looses.

00773 Efficiency determination of the .conversion of

electrical energy.

007731 General .considerations

.

007732 Electrical efficiency of conversion.

0078 Results and conclusions of tests.

0079 Reports of tests and experiments.

.008 .Installation, accessories, etc.

.0081 General considerations.

.0032 Starting and starting devices.

0085 Protective and safety devices.

.0086 Regulating, controlling and .synchronizing devices.

.0088 Distributing arrangements, switchboards.

83531 General

83532 Shunt and self -excited rotaries.

83533 Compound excited rotaries.

83534 Separately excited.

83535 Single-phase rctaries.

33538 Polyphase Rotaries.

61 I'wophase

62 fhreephase.

63 Sixphase.

AUTHOR

George Vf.Colles, A.B.M.E

Louis Bell Ph.D.

.Frank Perkins

.tf.E.Goldsborough

Maurice F.OUdin

Parshall and Hobart

Hans Bigismund Ueyer

F. iff. Springer

E^W.Rioe Jr.

A.G.Grier, B.Sc. and

J.G.Hyde, 3. Sc.

E.C.Eborall

BIBLIOGRAPHY.

PUB DIG AT ION

, Journal of the Franklin

Institute.

Street Railway Journal,

Western Electrician

Western Electrician

Standard Polyphase

Apparatus and Systems

Engineering (L

)

El ekt r ot e ch a i sche

Zei'Gschrif t

Electrical World and

Engineer.

The Sibley Journal of

Engineering

Canadian .Electrical .News

London Slactrician

DATE

Inarch and ,April.l9Cl

Sept.lS98.

Dec. 22,1900

Feb. 9, 1901

Sept. 29, 1399

April 4,1901.

June 23,1900

June 1899.

Sept. 1900

Mar. 89, 1^8.

MAPTS R I.

DEVELOPMENT 07 TH3 OON'VESTSR.

1. The Rectifier type*, which dates back as far as 1328, vzas the

first form. It consisted merely of a revolving commutator operated

synchronously by an auxiliary motor. It was not of much commercial

use, because of its excessive sparking. This was due to the fact that

the circuit must be periodically broken at each reversal to avoid the

alternative of short-circuiting the primary mains.

2. The Inductcrium type was for direct current only, and was char-

acterized by a series of induction coils, which might or might not be

stationary , arranged in a circle and had their primary and secondary

coils, one or both, connected to the respective segments of a commutat-

or. This machine was at: o externally rotated. This type was brought

out as earln as 1875,being improved and new patents taken out in 1882,

1883 and 1857.

3. The Motor -Generator was the first real form of rotary converter;

being simply an independent generator coupled to a motor, on the same

shaft or otherwise mechanically connected to it. Edison patented this

device in 1881. Van Depoele patented another form or device in 1889,

Henry in 18B4. Their patents had to do with the number of couplings

and modes of excitation.

4. The Dynamotor consisted of two machines merged into one-usually

a single armature, or sometimes two armatures, but always one field, and

always with independent primary and secondary coils. Its use is pract-

ically confined to direct current only, with a commutator,! or each coil.

The first machine of this type was patented by Van Depoele in 1882.

The common direct-current dynamotor has followed the same lines of im-

provement as other closed coil eleetrordynamie machinery, the improve-

ment being in the design, rather than principle.

5. 'The Botary Converter consists of one machine, one armature, and

one coil. It is, in short, a dynamo of ordinary drum or ring-wound arm-

ature type. It is provided with two sets of terminals to its armature

coils, one of which is a set of slip rings for an alternating cY poly-

*notary Transformers; George V. Colles, A.B. , If. B; The Journal of the

Franklin Institute, Vol. 0L1, p., 2C7; March 1901.

chase current, and the other a simple commutating device. This treat-

ment puts the rotary converter last, is the more improved form. It is

rather doubtful whether it is better in many ases than the motor-gen-

erator. The latter has a controllable pressure regulation, as oppos:ed

to the fixed voltage-ratio of the former, which a>Jso possesses com-

pactness, higher eff iciency, and power-factor regulation. In many cases

the fixed voltage ratio has caused the rotary to be taken out and a

motor-generator set installed in its place. The first appearance i

of the rotary converter was not as a transformer, or converter , but as

a double-current generator, used to generate alternating and direct

current according to the needs ,of the owner. Such a machine has been

on exhibition at the Pinsbury technical College since 1885. It was

not patented in this .country .until 1888. when a. patent was issued to

Bradley. Its true value was not recognized, at least practically, until

two German firms exhibited, at the Frankfort Exposition, several large

threerphase rotaries in actual operation. Since that time the rotary-

transformer has come into general use.

6. When the transmission* circuit is vary long and must be kept

clear of .complicated line reactions or when the frequency used ;is . in-

conveniently high or low, it is best to fall back on the motor-generat-

or. Rotary converters .can be built for any f requency ,but a large mach

ine for high frequency forces an extreme multipolar construction to

keep down the peripheral speed, and this means a very .complicated .com-

mutator. On the other hand, a very low frequency .would compel a bipol-

ar construction when a multipolar machine would be cheapar and better.

Under ordinary conditions the rotary converter is at its best at from

twenty to forty cycles per second. A motor-generator set is a few. per

cent., less efficient than a rotary converter and somewhat more costly,

but counting in th. cost and loss of efficiency in the transformers

needed to supply the low voltage to the latter, the difference is less

than woulc. at first be supposed.

7. In general, the use of rotary converters is for cne of two pur-

poses- to utilize water-power situated at a point distant from t.e

railway line, or to feed a long line from a single steam-driven station

The first involves the comparative cost of water-power and steam pow-

er delivered at a given point, the second the relative cost of steam

power in stations of different sizes and with different load factors.

Under present conditions the robary converter finds its most important

*Tfce Rotary Converter in Street .Railway IRailway Journal; Vol. XIV, r.o.9, Seot.lr98.

ork; Louis Bell Ph. D. ; Street

use in the utilization of -rater rpcrer for railway purposes and :in this

field its importance is making .itself felt.

8. It is generally conceded that the motor-generator* is better

than the rotary converter for arc-lighting circuits. The reason is

thav special arc-lighting machines,-such as the Thomson-Houston, Brush,

and Wood types may be operated by alternating .current motors without

difficulty in many cases. From the ^boint of eff iciency , the rotary con-

verter usually has the advantage of any motor-generator set. If the

motor and generator of ^a large and well supported motor-generator set

each has sin efficinecy of 95$, the total efficiency of the apparatus

would not exceed 90S, and in smaller equipments the efficiency would

run as low as 80 cr 85$. A rotary converter may be constructed at even

a higher efficiency than the generator alone,since the weight of the

armature may be less, thus resulting iin a lower hysteresis loss. Even

including the loss of the static transformer used with the rotary, the

efficinecy is usually higher than that of a motor-generator set.

*Rotary Converters and Motor-Generators; Frank O.Perkins;. Western

Electrician; Vol. XXVII, No. 25, p. , 396, Dec. 22, 1900.

CHAPTER II.

THEORY OF THE ROTARY CONVERTER.

9. If an ordinary closed-coil armature* were connected at two dia-

mebr ieally opposite points bo collector -rings and rotated in an alter-

nator field of the .usual multipolar type, no current of any significance

could be obtained, because the positive and negative electromotive forc-

es in each half of the armature would at all times be practically equal.

10. !'oreo\/er,a common • rlosedrco.il, direct-current armature does not

oroduce an alternating current, in the propdr sense of the .word, nor does

such a current exist at any time in any individual .coil; for though the

electro notice force developed in each .coil does follow an approximate

aine function of the time, the current in the coil remains .constant

during one-hale a revolution, and .is then abruptly reversed in direction.

This does .not happen to all the coils at once, but to each . one success-

ively at equal intervals of time; so that, if an alternating current is

to be got from such an ar nature, it muv.t be a new and different current,

whether superposed upon it or otherwise independent of :it. The prin-

ciple u^bon which a true sinusoidal alternating cur-rent is obtained from

such an armature depends upon 'the mathematical theorem that the ..inte-

gral of a sinusoidal. quantity will give us a sinusoidal quant ity; hence,

integrating the sinusoidal . electrom ctaive forces developed, in the indi-

vidual coils of the armature over any .given angle of the same, we obtain

a sinusoidal E.M.E.as a resultant. Hence, a sinusoidal form of the cur-

rent curve reauires a sinusoidal space-distribution of the magnetic

flux around the armature, while in direct-current toi&ing it is only

the total-flux that affects the E.M.F. at the motor terminals; this

latter fact affects the ratio of the pressure in the two external cir-

cuits.

11. Having obtained a machine which will yield two species of current,

we may put in one species to run it as a m£tor,aiid take out the other

as from a generator. The theory of the machine is not , however, the

same in both cases. As a dynamo, the two currents are merely added to-

gether or superposed in the armature, whose reactions, resistance losses,

etc., are thus merely due to their joint effects. But when the machine

is used as a converter, the two currents run, in general, in opposite di-

rections, and must be subtracted; so that the resistance losses in the

The Journal of the

armature are greatly diminished, or, looking ati it in another 'ray,

a

larne Dart of the current converged either does not cass through the

armatureat all, or only through a few of its coils. -

12. If the continuous current be on the primary, that is, if the con-

verter is run inverted, the speed of the machine, and hence the frequency

of the alternating current produced, naturally depend on the field-ex-

citation afcd the pressure in the primary circuit, and both will rise anc

fall together with the latter, as well as with variations: in the field-

s'trength. The difference between the primary voltage and the counter

S.M..F of the machine as a generator , necessary to compensate armature

losses and keep the machine running, is accounted for in the secondary

Qicxui t,alternating by .just sufficient phase-lag in the current, caused

by increased self -induction, to bring the ohmic M. F1

. down to the prop-

er level.

13. In the case< she. re the direct current ife the secondary , and the

primary alternating or. polyphase, it is somewhat' nore complex, as .it is

also.much r.ore frequent in practice. This is due to the fact that,

whatever the counter-electromotive ,'orce produced- by the machine and

generated in the secondary circuit, the machine must run in synchronism

with the primary eircuit , or. not at al X If then,.so running, this count-j,

•

.er-E.M.?.is greater or less than bhafc" of the primary mains, the current

will adjust itself automatically to the proper lead or lag necessary

to secure equality, of the two, while at the same time, the armature cur-

rents react upon the field to assist in securing such equality.. If

the count er-E. M. B« of the machine at synchronism .is greater than that

of the primary circuit, the current will lead, and the reactions.mil!

oppose the field; if less, the current will lag, and such reactions will

act to increase the field-strength. So far will thisactiou go, that if

the load be sufficinetly light, the machine will run without field-ex-

citation, by. the mere reaction of its armature coils. Thus the machine

is to a certain extent self -regulat ing;but if the lead or lag of the

current is unduly increased, the wattless current and armature losses

become very large and the effective volts so small that the machine

will fall out of step and stop.

14. It is an interesting fact that rotary converters whose fields

are excited from the direct current winding are self -start ing on open

secondary circuit. In the case of a polyphase primary, this is due to

the rotary field; bat in one-phase primaries, to the fact that when

starting they act like a direct-current motor in the same case; the

current in fields and commutator being simultaneously reversed, the

torque remaining .constant in .one direct ion; and this condition is main-

>

tained until the armature has reached synchronism.

15. .In the case of the machine in hand, the resistance of the copper

pole-tips was so small tint the armature took something like 500 per

cent., more than the full load current, acting like a transformer .wit ha

short circuited secondary, so tint it could not be kept cn the armature

long enough to bring the machine .up to synchronism, on account of the

excessive heating. ,.the.

16. If *an alternating-current voltage is impressed atAcollector

rings of a converter, we do not get the same voltage between adjacent

sets of brushes, that «te do between adjacent collector, rings. The

direct -current voltage w i 11 always be greater than the alternating-

current voltage. This is because the crest of the wave of alternat-

ing E. \L, ?, cannot have a greater value than the maximum E.M.F1

. generat-

ed in the armature conductors that act in series between phase con-

nection taps. Moreover , since the effective value of the alternating

tf. wave is onlyl/2~times the maximum value, it is not possible for

the effective alternating Rl.M.F.ever to equal the direct-current S.M..F

the direct-current S.M.F. being equal to the maximum E.M.F. which can

possibly be generated in the maximum number of armature conductors

which it is possible to have acting in series at any one time. The

table given below shows, first , what the theoretical voltage and current

eatios are in different types of converters. They are the results de-

rived mathematically . by Mr St e i nmet z , and illustrate with accuracy the

conditions that obtain in practice. The constants given assume barman

ic voltage and harmonic current waves, and no allowance is made in the

voltage ratios for the copper losses in the windings. In practice,

therefore, a variation of as much as 15$ may be found from these values

although the departure is not often more than 5%.

17. Comparative Capacity Ratios of Different Types of Rotary Con-

verters.

Volts bstween adjacent

collector rings

Amperes per line...'....

Ratio of copper loss in

rotary to copper loss of

same machine operated as a

direct -current machine

%t same output

D.C. Singlephase, Threephase, Twophase.

1

1

0.707

1.414

0.682

0.944

0.5

0.707

1 0.555 0.37

Western ftlectricina; Vol.

/

r

D.C. Single phase, Threephase, Twophase.

Relative capacity of

rotary converge?, and

same machine operat-

ed as a direct-cur-

rent generator at the

same temperature 1 C.85 1.34 1.34

IS. One of the most important characteristics of the rotary convert-

er, is the ease with which.it lends itself to the regulation of the pow-

er-factor of the .circuit from which it is operated. If the rotary is

anderexcited^it will act like an impedance coil connected .in the line,

and produce a I'V-'-U 1"

1--?, current. As its excitation is increased, if the

load is kept constant, the current comes more nearly in jfnase with the

.impressed B. MJ?. ,and is at the same time reduced in value. Finally,

a

point is reached at which the armature cuttrent is in phase with the im-

pressed 3. M. P., and the armature is then a minimum for the given load.

Any further increase in the field excitation causes the converter to act

as a condenser in the line. "6ver-exci tat ion, therefore, causes the arm-

ature current to increase again, as well as to attain an angular advance

overthe 5.M..E.. The machine can, therefore, be adjusted to counteract the

inductive effect of induction rotor and transformer loads, and when once

set cor a given power-factor by a given adjustment of the field-current,

.it will maintain this power-factor practically constant for all loads.

'Then operated from generator mains supplying . power to well-loaded trans ••

formers, a rotary converter should be adjusted to a power-factor of 100

per cent., as the power-fact or of the transformers will closely approx-

imate this value, ifhen so adjusted, a gqod rotary will maintain this

condition, practically, at all Ipads. This statement refers primarily to

uncompounded machine, or to the simplest form of rotary converter con-

nected up and operated in the simplest manner!..

CHAPTER III.

CLASSIFICATION OF ROTARY CONVERTERS..

19. Rotary* converters may. be either separately excited or have both

series and shunt field excitation. A third type is sometimes construe!:

ed which has neith>cr separately nor series .excited field,but in which

the magnetic field is induced by the armature current. This type is

known as the "Induction" converter, and has the characteristic of an in-

duction motor, of a lagging current at all leads. It runs, however, at a

synchronous speed. This latter type is not used commercially to any

great extent.

20. The machine in hand is of the second type, for it has both series

and shunt windings* and is also provided with copper grids at the pole

tips to lessen the hunting by the damping effect of the induced eddy-

currents.

21. A further classification includes the"inverted rotary" in which

direct -oar rent is put in at the commutator and alternating taken out

at the collector sings; This type will be discussed in full later.

Standard Polyphase Apparatus and Systems, Oudin, pp., 111-112.

CHAPTER 17.

ELEMENTS 0? THE DESIGN OP ROTARY CONVERTERS.

3*3. A rotary* converter is structurally in many respects similar to

a continuous-current generator, the chief outward difference consisting

in the addition of a number of collector rings, and in the .commutator

being very much larger, in comparison wit h the dimensions of the rest

of the machine, than in an ordinary continuous-current dynamo.

33.. The most interesting . point in the design of rotary converters

is the overlapping of the motor and generator currents in the armature

conductors, in virtue of which, not .only may the conductors be of very

small cross-section for a given output, from the thermal stand-point,

but, the armature reactions also being largely neutralized, large .numbers:

of conductors may.be placed on the armature, which permits of a very

small flux per. pole, and a correspondingly small erossvsection of mag-

netic circuit. But the commutator must be as large as for a continuous

current generator of the same output, hence a we 11 -designed rotary con-

verter should be characterized by a relatively large commutator and

small magnetic system. This is best achieved by an armature of fairly

large diameter and small axial length, and this, furthermore, gives room

for the many small armature conductors, and for the many poles required

for obtaining reasonable speeds at economical periodicities. The me-

chanical limit of centrifugal f orce due to high speed is an important

factor in the design of the armature and commutator of a rotary convert-

er as compared with continuous current gdnerat,rs.

24. Th3 extent to which the motor and generator currents neutralise

one another, and permit of small armature conductors to Gaflfry the re-

sultant current , varies with the number of ph°.3e<s. Table , I, gives the

output of a rotary converter for a given I*B loss in the armature con-

ductors, in the terms cf the output of the same armature when used as a

continue is generator, the latter being taken at iJQD Table, II,

shows the effect of pow?r-f actor.

*T"ne Design of Hotary Converters; Marshall and Hobart, Engineering (L)

Sept . 29, 1899*, Oct . 13, 1899.

Table I.

Output in terms of output of con bin. -cur. Generator for equal I*R loss

in conductors for power-!: act or=l and conversion efficiency of 100 per

q e n t

'1?ype"of Rotary

Converter.

1-phase

3 -phase

4 -phase

6-phase

13-phase

Number of Uniform dist.of

slip-rings. flux over pole-

face spanning

.entire pole pitch.

3 .85

3 1.34

4 1.64

6 1.96

13 3.34

67 per cent., of en-

tire pole pitch.

.88

1.38

1.67

1.98

8.36

Table II.

Output in Terms of output of direct -current Generator for equal S*R.

loss in armature conductors for 100 per cent ., efficiency, and for uni-

form §ap distribution of flux, of 67 per cent. , polar pitch.

Type

1-phase

3-phase

4-phi:se

6-phase

Slip rings Power Factor.

i..ao 0.90 0.80

3 . .88 .81 .73

3 1.38 1..33 1.17

4 1.67 1.60 1.44

6 1.98 1.93 1.77

Table .III.

armature or I*R loss is of that of same

direct-current generator for same output , assuming IDC percent. , convers-

ion efficiency.

Power Factor Per cent., loss.

1.00

.87

.50

58

85

375

infihi by

Gain in I*R loss only g.qod for power factor of about unity.

The winding is connected up to the commutator segments exactly as

for an ordinary direct current dynamo. The slip-rings are connected to

armature at points on the commutator.

25. As a synchronous* motor, the rotary converter depends on the fre-

quency and this is considerably greater than in a direct -current machin^

hence we may fix a maximum size.

26. The maximum .current allowable from one brush set is 300 amperes.

This gives the number of poles, which, v/i th the frequency gives, R. P.M.

R. P. 1.'.= (Frequency * 60)«-(pairs of poles). For a certain number of pcles

we need a minimum diaimcter to avoid too great axial length of machine

and to allow for. sufficiently large pole-pitch. But the diameter is

limited with the R.F.M.by the permissible peripheral speed. In consid-

eration of this , we may take as a maximum limit for the size of rotary

-converters 1500ICV, for 25 cycles. For higher frequencies, considerably,

less , especially if 500 volts are required. Smaller . E.M.F's allow

larger capacity, for fewer commutator segments per pair of poles are "re-

quired.

•27. Frequency should be chosen :as low as possible to avoid hunting

and sparking. The .best frequency i to use is 25 to 30 cycles per 'second,

30 to 40 is permissible, or with smaller machines .even .-as high .as 50

cycles. .These low frequencies are advantageous in transmission :because

. of : the smaller .self -induction in the leads, bet ter. parallel-operat ion . .

.of genera'cors^ebejbut .-transformers used in combination with the rotaries

.have a low efficiency and poifir factor.

.23. . For . high frequencies, low .voltage is required, but a minimum limit

.should.be about 320 volts, to ayoid too heavy currents and difficult

.commutation.

29. For use with accumulators, about 25 per cent ., variation of .voltage

should be possible,and :an :arrangement • for altering the applied Fj.!,'.F.

on the alternating side should be attacbedo

30. Since 300 amperes is taken as the maximum allowable current per

set of brushes,the number of poles is given by this. For example, if

we take a 500KW.,40 cycle, oOOvolt machine, this gives 1CO0 am. leres direcij

current, 8 poles and 250 amperes.

per set of brushes.. From* this we have

600 R.P.!L Of course -the assumption of 300 amperes maximum is not a

fixed cne,absolutely, that is, at high frequencies a .smaller number of

poles . and greater 3. P. M..is advisable, even if it passes, the limits. All

such rules are merely . mean values

.

31. As to oeripheral speed. of the armature,we should not go beOow

75 feet per second as the ventilation of the machine is bad otherwise.

*0n the Calculation of Rotary Converters. Hans Sigismund l'eyer;31ektro-

technische Zeitschrif t , Vo 1: XXII. p. , 295; April 4, 1901.

As an upper limit bake 1250 feet per second, but preferably 95 to 110

should be used, The diameter of the armature per pole should be .it

lease 1 5/S".

32. ?or the case where .the .current -and :B.m..F. , arc .g in.phase,

thereis .no distortion .of cield nor armature reaction in the real sense.

But a leading .current brought to bhe converter produces .weakening .of

bhe field by armature reaction. A lagging .current produces a strength-

ening. of field. Reversing .this,.we can produce leading or landing .cur-

rents by increasing or decreasing the field strength. We can, theref ore

use bhe rotary converter . as a compensabor in systems loaded .with .watt-

less .currents. To. carry this out between wide limits requires . many

turns per pole in the armature. Other advantages of this are bhat.it

decreases hunting, since the increased impedance of f ers more .'resistance

to the wattless .equalizing .currents, and self -starting from albernabing

side is made easier, as the .starbing E.M. P. becomes .higher and starting

current less. In general the ampere turns in the armature. per pole ab

full load, and only directicurrent considered, should equal or be larger

than the no lead ampere turns per pole in the field.

33. The . permissible .S.M.F'. per .commutator segment depends on currant

to be cpmmutated. From 10 to 16 volts are usual. .From the assumption

of 2 commutator segments.. per slot, we get the number of slots. For g cod

finding the .number .of slots should be divisable by the .product of the

.number . of £oles and the .number.. of .phases.

34. The heating .should .not exceed ,40°0. , above atmospheric. tenperature

This. will allow from. 450. to 600 amperes.per . square centimeter. This

.is much higher than .in direcb-current machines, for only 60 tri 64. per

.cent., of this .current is. effective in heating the machine.

35. For good commutation .we require minimum self -induction, for . the

brushes. are in the . neutral zone and commutation . must take.plp.ee without

the help of the active field. For lev; self -inluction, many slots. are

used, with few turns per slot, and of the open form. But half closed

slots have other ad/antages, for. poles are usually of .wrought iron to

a^/oid hunting and .with open slot* a c ansiderable .core loss .in pole shoes

, is .experienced which is less for closed slots. The latter are chosen

vben freedom from . sparking is provided for by Iqw voltage. per segment

.and moderate peripheral speed, .^.rallel winding should be adopted,.with

as many .circuits as there . are.

pples

.

36. .Although it is necessary .in direct -current machines to choose a

high density of lines .in airgap and teeth, for soarkless commutation,it is not so in rotary converters. Tie can, theref oite,.work on the l«wer

branch of the magnetization .curve, and since the lack of armature re-

action causes the rotary converter to act like a separately excited

. direct-current machine, there is no danger in working on tkis usually

^unstable Aact of the curve. On the ether hand fthe upper part of the

curve is bettec,in consideration of the tendency to hunt, since at high-!

.er saturation there is a tendency to leep the R.rA.F. const ant and to

resist small periodic variations. Hence it must be decided for. each

case which. part of the .curve to work on.

37. The leakage is somewhat larger than in direct -current machines,

because. of more. poles and less space between .them. A mean value for

machines having 70 to 30 per cent ., cf pole pitch as polar arc, is .about

20 . per cent. , leakage

.

38. A gpod average on bend. of the characteristic is as follows;. per

square centimeter , for . the armature core 3000. to. 9000; for teeth 16000

to 20C00; for.airgap 7000 to 9000;.

pole core 12000 to 14000; and in

ragnetic frame, for ..cast -iron. 5000,.wrought .iron 9000 lines. per square

centimeter; values .which differ but little from .those used .in the direct--

current machines.

39. The breadth of .machine and parts is decried by the assumption of

the litfe density. Air passages should be about 1 7/8" apart. The in-

sulation of .sheet ir cn amounts to about 10. per .cent. The . latter is us-

ually oxidized and .japanned. The pcles are made a little narrower than

the armature to avoid lines entering the sides of the armature.

CHAPTER V.

lEGHAHXQiHi BLECTRIGAL DAM.

Mechanical Data.

40 Items.

Manufacturer

Banufiacturer *s number

Type of Machine

.No. of Poles

Capac it y : X . R . (Dyn . ) ; H . F . ( Mot . )

Speed 3. P.M.

Frequency, cycles per second

.Volts, normal excitabion

Ourren t , normal load

TCestinghouse

13575

Rotary Converter

4

10 kw.

1800

60

625 D.C. 440 4.

16 D.C. 23 A:C

Mechanical Data.

Shaft; material

" Diim. ,coce portion

" collector portion

" journal bearing

pulley fit

3«arings: kind

" length of,

Pulley rDia a . over crown

" face width

keyway ; width x depth

material

thickness, single

in inches

lineal speed, f t. per ,min.

Toothed .Armature

Di am. over teeth

" at bottom. of slots

Length y over all

" of core, effective

No. of teeth

it

BeltIT

f!

1?

steel

4.85"

4.85"

1.75"

1.62"

Self oiling rin;

5 7/8"

10 13/32"

6 9/16"

1/2" * 1/4"

Leather

Single

1/4"

4900

9 1/2"

7 1/8"

10 3/4"

4.81"

43

findings

Conductors per slob

Size of '\rm. Conductors

Conductors in Multiple

Turns per coil in shunt field

Size of Shunt field conductors

Turns per coil in series field

Size of series field conductors

Commutator.

Di am. external

Length over all1 active

Insulation, thickness btw. seg

Segments, '.;o. , of

Collector Rings

Number of,

Diam. external

Width of,

Distance, centre bo centre

Brushes

Type of brush, tangent , radial

Material of brushes

Number of sets of brushes

Mo. of brushes one set

Length . cf each brushT

.'iidth,of each brush

Thickness of each brush

Area of contact, one brush

" " " set brushes

Pibch mean dis btw.. pie cent.

Pole arc: angle subtended

Diam. of polar bore

Depth of air 5ap, single

Mean length Mag. Giro; of Pole

Magnet Core

Set area magnet core

Yoke

Sketch cross sect ion, and dimension

43

No. 16

2

3490

No. 22

22

No. 11

6.2

§ an

2.15"

.018"

129

15/16"

1 1/8" •

Radial

Carbon

1

2

1 7/16"

1 3/4"

3/3"

.65 sq.ins.

1.3 sq.ins.

6 15/18"

54°

9.71"

.195

4.4"

39 scj . ins

.

fo"

"Aft'S

.41 RESISTANCES.

Resistance of .shunt field (hot)

.Resistance of series field (hot)

Resistance of Armature (hot)

Including brushes

Under brushes

CHAPTER VI.

DESCRIPTION OF R0T4RY CONVERTERS.

42. -.ob-iry converter's ere ir.aite in standaBd generator sizes for fre-

quencies of 25,30 and 60 .cycles.

43. The standard voltages. are 250 and 550 volts. Only two and three

phases rotaries are used. to any great extent , although the six-phase

.connection described elsewhere, is new coning into use. The.sin$le-

. phase .is .not .as .'efficient .and .is .not .extensively .used, because .neirly

all high tension transmission >is .either two or three-phase.

44. 'The machine which .was tested in this thesis .was .not a standard

size, but it .was r aevertheless, a fair example of the modern rotary con-

verter. The following .cuts and photographs show the machine .in hand

and .also .some .modern types in operation today.

45. There are numerous starting and synchronizing devices for rotary

converters, but the .one generally adopted to-day is to have a small in-

duction motor mounted on the rotary shaft which quickly brings the

machine .up to synchronous speed. 'There has been recently patented anthe

ingenious device for synchronizing from„direct-current end*. It con-

sists of an electro-magnet mechanism which bfieafes the direct-current

circuit at the same time that the synchronizing switch is closed, thus

preventing any conflicting between the two sources.

46. Starting from the collector-rings is not advisable because.it

takes. a heavy current. at a very low power-factor and is very detriment-

al to the regulation of the system. Nevertheless, this method , is being

used some to-day, it being a very quick method.

47. Inverted rotaries: are, now, practicably all excited from separate

exciters driven from. the rotary shaft.

48. A rotary . converter is inverted when converting direct to alter-

nating current instead. of vice versa. This requires no change .in its

design, it being merely another use to which the ordinary rotary may be

.put.

49. The inverted rotary is used mainly .in low-tension direct-current

supply systems for supplying suburban, districts with alternating .cur-

rent. It may be used as a connecting link between a station. with low-

*4 New Synchronizing Device for Rotaries;?. I. Springer; Electrical .'florid

.and Engineer;7ol.35 r p. , 944; June 23,1900.

°The Sotary Converter;^ .fl.Bice, Jr . , The Sibley . Journal of Engineering,Vol. XIII.p., 337, June, 1399.

tension .direct -currant .generators,, and .a distant .station .containing .alt-

.ernating-current generators. .It. may be a connecting link between al-ternating and direct-current generators in the same station. .In this

.case the distribution of load . on the generating .station may be changed

.to .suit the demand.

50. When a converter is used direct, the speed of the machine.must.be

synchronous, independent of its. field .excitation; any variation in the

field-excitation simply ".changing the phase -relation .of the .alternating

.current .to .the .impressed ,ELMJ8.

•51. .when .used inverted, however, with direct current as the only . source

ikt .energy, the speed lav; of the converter follows that of a direct-cur-

rent constant-pptential motor , other conditions remaining .constant. .Its

speed depends .upon the f ield .strength, its speed increasing .with a de-

crease of field strength, and iecceasing .with an increase of field

.strength. Under such .circumstances^if the alternating .current supplied

.by the inverted converter is not in .hase with its. E. M.F. , that is, if

the p a/er -f act or is .not unity, but is lagging r which is a usual commerc-

ial condition, variations of the alternating-current load will produce

variations in the field strength. of the inverted converter, through the

effect of armature reaction. Lagging currents demagnetize the field,

which results in increased speed, and a corresponding increase in the

frequency which in turn adds to the lagging of the nascent and .conse-

quent demagnetizing .power .and .even under some conditions the .increase

.in speed may become dangerous. It is , therefore, necessary to. provide

inverted converters with safety devices which can cut them out of cir-

cuit in case tfhe speed exceeds the danger limit.

52. The only means of regulating the speed of inverted rotables is

by changing the .excitation, of the field,.increasing it as the lagging

.power-factor goes down, and decreasing it when the leading power-factor

goes down. This may be done by means of the field rheostat, but re-

quires someone to attend to it all the time, which would be inconvenient

as well as expensive, especially for a small machine, so that an automatic

regulator is required. The .speed may be kept approximately .constant

by. making the .converter drive its own exciter*, either belted, or.

placed

on the .end. of the. shaft. It will be S3en that with such an arrangement

as the speed . of the converter rises, its excitation is increased more

rapidly than the speed, not only as already explained, but also by oppos-

ing the accumulative demagnetizing .effect of the lagging current in the

*3ome Sxperiments with ffotary Converters;A.G.Grier,B.Sc. , and J.C.Hyde,1. Sc. Canadian Electrical Uews and lingineering Journal, Vol, X, p. , 179.Sept, 1900/

**********

V

armature. The exciter should be worked low .on .its magnetization .curve

so that a small variation of speed is accompanied by a large variation

.of .voltage. .With .an .exciter designed .properly, the speed may be kept

very nearly coast int.

53. An inverted rotary running in parallel with an alternate? driven

by another .prime mover does not have its speed changed by a change in

its field-excitation, or by a change in the phase relation of the. alter-

nating .current supplied by it, but behaves in a manner entirely similar

.to the direct converter , the alternator in the parallel circuit hold-

ing it in step after being once synchronized.

54. For large capacity* units separate transformers . are always used

for each phase of transmission; but for three and six-phase rotaries,up

to 100 kilowatts capacity, it is always preferable to use three -phuse

transformers, on account of the common .magnetic circuit, by. means of which

the pressure variation is reduced, and they act as a sort of balancer

to the. system. The best .arrangement of transformers for three-phase

working is to use three transformers, mesh-connected for both high and

low-pre : sure sides. For if an accident happens to one transformer , such

as a blown fuse, the supply is not interrupted, because the remaining

two will supply three-phase current to all three phases of the rotary.

The rotary reactions tend to keep the phases equatLy balanced. The

tetanies .can be kept fully loaded for a short time, two transformers do-

ing the work of three. Star-connected transformers, under the same con-

ditions, would supply only one-phase and the machine w culd have to be

stopped on account of sparking at the commutator. The machine would

not carry anything like full 1, ad, and the system would be .unbalanced.

Mesh-connected transformers require 58 per cent., of copper area in the

secondary as compared to that in the star-connected transformer. Star-

connected transformers have 58 per cent., of line pressure across each

primary winding. This gives a less winding space on account of the

smaller amount of insulation. The safety from break down of the mesh

system makes it by far the better.

55. The same arguments apply to the six-phase system, where the sec-

ondaries are arranged with either double "mesh" or "star" preferably

"mesh". This system has two distinct mesh connections, one superposed

on the other, being electrically connected through the armature of the

rotary. It has three single-^hase transformers, with two secondary

*S.0.?f orall; London Slectrician; Vol, XLVI, Some Notes on Polyphase Sub-

station Machinery; - p., 656, March 89,1901.

~1windings.

,

d,c,and f (FigjJ are connected in the opposite direction to

a,b,and 3. The six-phase system is more complicated, but it pays. Stein-

metz and "iapp have demonstrated, that, for the same .mean heating of the

armature coils, the output of any rotary may be incceassd 40 to 50 per

cent., by its use. It insures more uniform heating, the maixmum temper-

ature being rarely more than 20 per cent., higher than the minimum.

Twelve-phase connections .would increase the output in the . same ratio,

but would be too complicated.

56. Rotaries feeding direct-current three-wire systems cannot be paral-

leled from common bus-fars on the .alternating-current side, because . of

the necessary. short circuit through the armatures, so .that .multiple con-

nection- of .the transformers is necessary. Each secondary has two... f'-jfoo-Zje. bb

windings. (Fig. v

57. Pressure gulation consists of regulating .the direct-current

voltage by regulating the alternating -currant volts at the cpllector-

rings. (l)Direct variation of pressure by changing the transformer .rat

i 3, and by induction regulators . riiieh .should always be ,placed in the .sec-

ondary circuit on account of insulation dif f icult-ies. (2)B;y compound-

ing the rotaries and .introducing .induction .coils between the. secondaries

of the transformers and the collector-rings. The power-factor varies

as the excitation .according .to the .we'll -known V current .curve for each

.load. The rotary excitation.is.arranged.se that at no load, the input

.current .is lagging, being .about 30 to40.per cent of .the .full-load cur-

rent, partly due to the under-excitation and. partly due to the reactance

-in the line. .As the .load .comes on, the field increases, due to the ser-

ies coils, the lagging eurrentdiminishfis, the .impressed .3. 'vi.F. rises .pro-

ducing .a corrpsp aiding .rise in the direct-current voltage. M full load,

the field flux is a maximum, the current is leading, due to over-excitat-

.ion by series .coils, and .the .direct-current voltage is raised to its .pro-

per value. By suitable .proportioning the reactance and series .windings

excellent .pressure regulation .can be .attained .in this ,uay in a.perf ect-

. ly .automatic rannec. The actual direct-current .pressure regulation be-

ing nearly as good. as .in an ordinary over-compounded direct-current

generator.

58. The enly objection to this method is that it affects the regula-

tion at the high-pressure feeding points, due to the variation of the

power -factor

.

59. Induction regulators are better for lighting .work, while: . compound-

ing regulation is best for str set-railway; work, where the load varies

suddenly .and through a wide range, k combination of the two is some-

times used sith good results.

60. The phenomenon known as "hunting" which is prevalent in all rotarj

working is caused by the irregular rotation of the engine fly-wheel, and

the rotary not being able to follow each variation in speed, commences to

surge from one side of synchronism to the other, now above, now below.

The copper pole-tips on the machine in hand.flampen this effect by the

eddy -currents induced in them, but the only true remedy is a priire mover

whose angular velocity is perfectly uniform. The steam and water tur-

bines have solved the problem.

CHAPTER VII.

PERFORMANCE.

61. This machine may be used as a direct-current generator. In this

test four sets of readings were taken:

-

(1) With separate excitation and with series windings in.

(2) With separate excitation and with series windings in.

(3) Simple shunt

(4) Compound -wound.

For separate excitation the plant 500 volt source was used. A water

rheostat qas used for a load.

The curves plotted from these observations are shown on Plate I,

page,'/? .

It will be readily seen that the inherent regulation of this machine as

a direct -current generator is not very good. The armature reaction is

evidently excessive, and apparently the only way to keep the voltage up

is to separately excite the machine and use the series windings, with

results shown in Curve I. The machine is slightly over-compounded for

atout half -load^but does not hold up well. The heating is excessive

on account of the small magnetic area and small cross-section of arma-

ture conductors. It is not designed to be used in this way, but still

will render fair service if required.

62. As an alternating-current generator this machine gives only part-

ial satisfaction. Three sets of observations were taken:

-

(1) Single-phase with a non-inductive load.

(2) Three-phase with a non-inductive load.

(3) Three-phase with an inductive load.

The machine was excited by the plant SCO volts. The single -phase

non-inductive load was a water rheostat , while the three-phase load was

one of the General Electric rotary converters. The power-factor was

approximately adjusted by means of the excitation cf the latter machine

The power of the three: -phase circuit was measured by the two wattmeter

mwltrod. The curves plotted from these readings are shown in Plate II,

page ^8. There being no compensating turns on the field, the inherent

regulation is not good. The speed was about normal, be cause the gene-

rator pulley wa;: slightly too small.

63. Plate III, page f9, shows the internal characteristics as a direct-

current generator and as a single phase alternator. The field was

*

V

excited by the plant 500 volts and the TJeston plant 125 vclt machine in

series. This was done to get a high excitation for the bend in the curve

For the low values of the excitation, the "eston was reserved, thus cutting

down the 500 volts. The vcltageratio carve shows a slight decrease for

high values cf excitation, but in no case coming down as low as the theo-

retical 0.7C7.

64. Plate IV, page SO, shows the dynamic characteristic as a double-cur-

rent generator. Here the voltage ratio curve shows more of a curved

form than in the open-circuit curves, and does not come down to quite as

low a value.

The loads for both direct and alternating-current sides were water

rheostats.

65. Plate V, page J7, shows the short-circuit characteristic as a single-

phase alternator. The armature was shcrt -circuited through as ammeter,

^

the field being much under-excited bfcr the V'eston 125 volt machine. The

inductance of the armature kept the current down as shown in the curve,

which is a straight line.

66. Plate VI, page shows the wcrking characteristics as a direct-

current motor. The machine was separately excited from the plant 500

volts, and the series coils were not in service. The plant 500 volts

was put in series with the ?/eston to supply the 625 volts necessary to

run the motor. The power v;as absorbed by a rope-brake arranged as

shown on pa<je, 63, Pig. 6. The curves show fair performance. The

slight depression in the efficiency curve was due, probably, to the fact

that the voltage fell off somewhat at that point.v

67. Plates VII and VIII, pages J~3 v S4:tshow the working characteristics

as a single-phase rotary -converter. The connections for1 this test are

shown on paj^e hi. Fig.9d*t/0/?he voltage ratio does not come anywhere near

the theoretical value, and the efficiency is rather poor.

6S. Plates IX, X and XI, pages S^fS^ & b , show the characteristics used

as an inverted rotary. In this test,C25 volts was supplied to the di-

rect-current brushes and one of the General Electric rcbtaries was run

through a nest inghouse O.D. transformer from the single-rhase collector-

rings. The direct-current end of the General Electric rotary carried

a load of lamps. The connections are shown on page 65", Fig./-?.

The input direct -current voltage was kept constant, and four sets

of readings taken:

-

(1) The true-watts output was kept constant and the excitation

constant; speed, volts and amperes were read from which to calculate the

power-f actor.

(2) The speed and true-watts output -were kept constant by varying

the excitation, varying the power-factor by changing the excitation of

the secondary rotary.

(3) An efficiency test, with constant speed and variable excitation.

(4) An efficiency test, with constant excitation and variable speed.

On the last test (4) just as my readings were finished, having a^heavy'"

'|

lagging current in the machine, it began to hunt or race, the speed surg-

ing back and forth. Every connected instrument pulsated with the speed.

If finally blew a fuse in the input circuit. The speed varied from

17C0 to 2300 R. P.M. about once in two seconds. This shows the necessity

of some regulating device or cut out for an inverted rotary.

69. Plates XII and XIII, pages ^"8x53 show the characteristics as a single-

phase synchronous motor. The no-displacement curve of the characterist-

ic form and efficiency is fair.

Diagram of connections on page^^-f^F iss.6 .

70. Plate XIV Ashows the no-load characteristics as a single-phase ro-

tary converter. Curves I and II show the effect of the compounding.

The series coils were separately excited by the Weston, and the shunt

excitation kept constant. In order to make the rotary self -excitat ing,

it was brought up to speed and synchronized with separate excitat ion, then

the field was broken and connected acorss the direct-current brushes,

the proper polarity having been ascertained with a direct -current volt-

meter. The rotary ran with no field excitation without falling out of

step, but it took 48 amperes and pulled the input voltage down about 30

volts. The connections for curves I and II are shown on page iff- , FigS.

//a»d

71. Plate XV, page b I , shows <-he no-load characteristics as a single-

phase rotary, witha series reactince in the line as a means of boosting

the direct -current voltage by raising the excitation*. The secondary

of one of the Westinghouse O.D. transformers was placed in series with

the rotary. The curves show some interesting results. £ onn&cf/ o vi.S-

*Some Experiments with Rotary Converters. AG, Grier, B. Sc. , and J.C.Ryde,

B.Sc. Canadian Electrical News Vol. X, p. , 175, Sept. 1900-

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