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LIBRARY “7
Michigan St
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A STUDY OF RF-CONNECTING
INDUCTION MOTORS ,,°

A Thesis Submitted to

The Faculty of
MICHIGAN AGRICULTURAL OOLLEGE

/
By

1 3

ER: G: Carrow W: A. Siefert
— -
Candidates for the Degree of

Bachelor of Science

June, 1919.
TRESIS
Bibliograwhy

Electrical Review Vole LXVI ppe 165 Jane fu, Vi.
Electric Journal Vole XIII pPpe El, SHO, iv7e
Proceelings of Ae Ie Ee Ee Avste '07 ppe Lele

May rere EDe G7
Electric Journal Mure ‘17.

94606
PREFACE

The advance of electricity in the commercial field in
replacing steam, water, or gas accentuated prime movers has
been extremely rapid in the past half century. It was soon
found that alternating current was the most feasible form of
power tranamission so that apparatus was designed to use this.

In trying to reproduce the good characteristics of the
direct current machines, the induction motor was the first
to be developed and up to the present time it has not been
surpassed although it is by no means perfect. The large in-
stallations and then the necessity of a change meant that the
machines must be changed to meet these new conditions or else
they must be scrapped and new machines purchased. The reasons
for these changes are:

(a) Mowing the site of the factory.

(bd) Changing over to central station service.

(c) Raising of the line voltage because of over—loaded

ey sten.

If the industry was to prosper some remedy for this fre-
quent necessity of scrapping machines must be found. Inves—
tigation and research discovered as one means of solution the
possibilities of re-connecting the motors to meet the new con
ditions.

This phase of the several solutions has been chosen as
the field of our investization as applied to the machine in
the department which is a rebuilt 220 volt, 4 ampere, 2 phase,
4 pole, 60 cycle, 2 horsepower, series connected induction
motor manufactured by the General Electric Company, Inc.
We wieh to express our thanks to the iepirtment for the

assistance which they vave us in the prepuration of this thesia,
BRT

I
In all problems involving a change in motors there are
at least two fundamental conceptions that must be unierstood,
The first is that

Horse~pover = Torgue fveed

and conversely,

Torque = Horse-power 52
Speed

Torque is the rotative effort produced by current flowing in
conductors in the presence of a magnetic field. It is pro-
portional then, to the current in the conductors and the flux
in the iron. In changing a motor the amperes per square inch
in the copper and the flux density per square inch in the iron
should be kept constant or as near the value which the design-
er intended for the muchine as is possible. This would always
be true were it not for the fact that with these quantities
-gonstant the motor would run hotter at slow speeds due to ‘he
reduction of. cooling air which the machine circulates through
iteelf by centrifugal action. For this reason the capacity of
a motor may increase in proportion with the speed when increas-
ing the speed above normal but when decreasing the speed below
normal there isa more rapid decrease in the capacity of the
motor than the proportional speed decfease would lead one to
assume.. So returning to our equation, keeping the ijiensities
constant means that our torque remains constant and so the
horse—-power will vary with the speed.

It ie essential then that we do not confuse the terms

horse~power and torque. It is the function of a motor to
produce torque but it is not till this torque is allowed to
cause rotation that we have horse-power ;roduced. In other
words althovzh a motor may be drawing a current equal to that
which it draws at full load yet our horse~power is zero until
the motor begins to rotate.

The second conception is that all motors act as genere-
tors in addition to the motor action. If we, for the moment,
forget the torque action of a conductor carrying current ina
magnetic field ani think of that coniuctor as passing through
ea magnetic field thus cutting lines of force, this produces
an electro-motive force which is callei counter electro-motive
force. This counter electro-motive force is nearly equal to
the impressei electro—-motive force or the line voltage so that
the design of a motor is really the design of a generator for
the line voltage. This gives us fundamental equations to work
from in the changing of any of our factors.

The capacity of our machine will depend upon the capacity
of the copper for currying the current ani the capacity of the
iron for carrying the flux. The heating of the machine will
depend upon the anount of current flowing in the coniuctors
and the hysteresis loss in the iron. But if we change the
voltage, frequency, or piase of the supply we must still keep
these densities the same unless we also chinge the speed of
the motor, The result is that the capacity of the machine
must remain constant ani we therefore, sacrifice the possible
horse~power obtainable in altering our machine in any way um

lees we divert from our rule for econstint densities,
To better illustrate this point the following are given
as an examples If a notor was originally connected in series
for 440 volts and we chinged the connections to parallel etar
the woltaze should be cut in half iri the latter connection
draws twice as nucl current e3 the original machine, If we
change frem series st:r to series delta the nex voltage
ehouli be 0.5775 cof the origiral or the reciprocal of 1.73
giving ue the nev vo)ts7e as 254 volts.

In changing fron. the star to the delta ccnnection we
now only have 75% of the original torque. The characteris-
tics of our machine, however, are not chanced aprreciably in

changing from a series toa parallel connection. Another
“change which is by no neuns ecrmon or suvrzesteli as cood prac
tice is changin: frum three -hase to tao phase or vice verea,
The resulting perforrance with regard to heating, efficiency,
power factor, uni insulation is the same ae if the machine
was operated on three ;*ase at 1°5% normal voltaze. That is,
the heating or rise in temperature is greater ani the power
factor is reduced. Also when such. changes are made the position
of the phase coils -ust be ch2ingel in order to bring them at
the point of raximum iifference of potential which is between
phases,

The more cornon practice in re-connecting motors of three
phase work is to make them two speed machines, In such a con
nection the distribution factor is only 86.6% ae gzooi on the
low speed as it isa on the high because the coils for one pole

of the former are spreai over an equal part of the periphery
ase for the machine with the lesser number of poles, The
operation on such a connection is only possible by the pro-
duction of coneequent poles in that all the coils of any one
phase are so connected as to give the sine polarity. It be-
ing impossible to establish a field with only one kind of
pole we have produced immediately cr equal number of con-
sequent poles of the opposite polarity. Another point to
consider in the two speed consequent pole connections is
the resultant torque and horse~power of the motor, To
change from a yarallel stur to a series star on such a con
nection ani applyinz the proper voltsse for such a connec
tion the resuit is to leave the torque constant and there
ie twice the hLorse-po-ver on tie series connection, To
change from a parallel stur to a series delta with the
corresponding voltsaze ch:inge the horse-pover remains con=
stant but the torque is cut in half for the series connection.

The things which are possible of chin:e to the supply
of any motor are vwoltsse, phase, ini frequency. That which
may be changed on the motor iteelf is the number of poles
and consequently the sccei of tre nmuctine. The two factore
which deterrnine the possibility of re-connecting, by their
effect on the operating cnaracteristics arez

(a) The throw o7 the coils.

(b) The phase insulation, it's location uni number,
The span of the coils shouli be the quotient of the peri-
phery dividei by the number of poles, It is a mechanical
possibility to spring forr youni coils one or two slote if
necessary to do so. Such a motor with symretricall: placed
phase windings gives full pitch and is the ideal motor. The
flux density, primary current, ani secondary current in such
a motor are sinusoidal. But there ure times vhen attempting
to obtain the proper number of effective turns, or desiring
to use form wound coils in a motor of small bore and few
poles, or in the desire to reduce the space required for end
eonnections, or wishing to secure better ventilation of the
motor, or desiring to save in the amount of copper ani in
sulation that we cannot use full pitch. Te can then resort
to what is known as fractional pitch or the space covered
by the coil is not equ»l to the quotient of the periphery
divided by the number of poles, The effect of using this
fractional pitch is the same as wouli be proiucei on full
pitch when the number of coniuctors are reluced with the
added reduction of course in the eni~wise length of the coils.
In an induction motor of the lowresistance squirrel care style
supplied with a simple hurnonic electro—-motive force the evap
flux is approximately sinusoidally periphkcrically distributed.
But to use the fractional pitch means a chanze in the chord
factor which causes a loss in the effectiveness of the electro-
motive force generatea because different conluctors of the
same phase are under the influence of electro—motive forces
of opposite siren at the sume instant.

S80, in considerinz the various forns of useful re-com
nection a factor must be taken into ennsiieration hict hae

a great ileal to do with the operating characteristics due to
this differental effect of the motor after the change is
made. Thies factor is the throw of tre iniividual coils or
the number of slots spannei by the two sides of a coil.

Theoretically, the span of the cofl must equal the
quotient of the periphery dividei by the number of poles
or the number of slots in the periphery dividei by the
number of poles. It is, however, not renerally understood
that increasing or iecreasing the span of the coil has an
effect similar to increasing or decreasins the effective
number of turns of wire in a coil. The coil proiuces it's
maximum effect upon thre nagnetizing field when it is exactly
from the center of one pole to the center of the next. The
coil is then said to s;an 180 degrees electrically. It is
sometimes necessary an! appropiate to make the span of the
coil less than or greater than full piteh to obtain the de—
sired number of effective turns. This is know aga fractional
piteh or chord winding.

To illustrate this, let us tuke our machine of thirty—six
slots and wind it for four poles. The coils sill be in full
piteh if wouni with a span of 36/4 or 9 slots. This means
that one side of the coil would lie in slot number one and
the other site of the coil yvoul! lie in slot nunber ten thus
giving us a spun or throw of nine. This coil woul! span 180
electrical jegrees. Tlie sine of one-half of 180 ierrees or
90 degrees ig 1. Therefore, the turns in this coil would
have a maximum effect in producing the nagnetic field,

Suppose that the coils sre left in their slots vith a span
of one to ten but that the coils are re-connected for a six
pole machine. Full pitch on the six pole machine would mean
@ span of one to seven or a throw of 6, Therefore, we are
using 150% pitch on the six pole connection. And 180/6
equals 50, therefore, each represents 30 electrical degrees.
Also 50 x 9 equals 270 degrees which represents the actual
epan of the coil in degrees by using a throw of nine instead
of eix. The sine of one-half of 270 decrees or 135 degrees
equals 0.707 which means that when connected for six pole the
coils have an effect of only 0.707 as against 1.0 when con
nected for four pole. The expression "sine of one-half the
angle® spanned by the coil is called chord factor ani must
always be considered when re-connecting motors. In chang
ing from four pole to six pole in the above example the
chord factor changes from 1.0 to 0.707 and the new line volt~
age should be 1/0.707 of the old, neglecting the effect of
other changes. If the motor had a normal vwoltare of 220

on four poles the new voltage on six poles should be 220/0.707
or approximately 310 volts. If it was still operated at 220
volts it would be thouzht of as operating at about 29% under
voltage. This of course is beyond the limitations of good
operating characteristics,

A clearer conception may be obtained by comparing the
windings of an actual machine with those of an ideal motor,
The theoretically ideal motor would have a large number of
equal, symmetrical, simple harmonic electro-motive forces.

In this case the gap flux would be distributed sinusoidally
arouni the gap periphery at any instant and these distri-
butions wouli revolve snoothly aroun! the periphery at syn
chronous speed,

In the actual machine the nunter of phases is small
however ani there must therefore be several adjacent conduct—
ors carrying current of the same phase at any one instant thus
forming what might be called a "belt of conductors” in which
the current increases ani decreases as a unit. On either side
of this belt wouli be another belt the current in which differs
from that of the first by 60 degrees for three phase and 90
degrees for two phase. Thus it is seen that the current varies
from point to point around the periphery by steps rather than
gradually as in an ideal motor,

If now these coils are wound less than full pitch it can
| readily be secn that the current in the two coils of different
phase but in the sane slot will ower—lap and decrease the ef-—
fectiveness of this magnetizing flux. Also different conduct-
ors of the sune phase belt will at certain periods have induced
in them electro-motive forces of opposite sign. This cutting
of the flux by coniuctors of the sue phase belt may be termed
differential action ani the factor by which the total electro-
motive force must be multiplied in order to obtain the actual
resulting electro-motive force is termed the differential fact~
or. Thus the differential factor thouzh used in a different
eense than chord factor has an effect similar to that of chord
factor. The two terms frequently used iniiscriminately inas-

much as the one cannot be changed without changing the other
by a proportionate amount.

In all induction motors we have reactance caused by
slot leakave, tooth-tip leakage, coil eni leakage, and re-
actance component of the excitine current. The effect of
fractional pitch on these factors is to reiuce the leakage
reactance components, the over-all length of the motor, to
make it rore convenient for windinz ania saving of space,
and to decrease the reactance component of the exciting
current.

“The voltage suppliei to the machine may be changed
from the normal about 10% either way without changing the
end connection or effecting the performance characteristics
to any great extent. Chinzes above normal which ore greater
than this shoul: not be uttemptei because of the possibility
of insulation break-down in ailition to the effect on the
performance curves, ‘Wanufacturer's have two classes of in
sulation, the first of which 1s goo? for voltare up to 500
and the other beings goo? for 2500 volts. The former is used,
of course, on most of the machines because voltazes as sup-—
plied to the building io not exeeed 500 volts as a rule. As
far as insuletion properties are concerned a decrease in volt—
age from normal would not be objectionable but when this de—
crease is creater than 10% the characteristics are changed to
@ great disadvantaze. More specifically stating the effects
of volture chanr-e, we have the followine chanres in perfor—

mance characteristics for 10% decrease from normals
 

(a)

(b)
(c)

(d)
(e)

(f)

(g)
(h)

(1)

Decrease the rugnetie density because there is less
exciting current at the lower voltaze,

Increase tte leakn-e current.

The starting torque reiains tie sume but the pulle
out torque is lesa.

There is un increase in the percent of alip.

The friction un! wviniage renain the same because
there is no ctanse in the speei of the machine.

In decreasin, tl.e magnetic density we decrease the
iron losses in the lavinations of the machine.

Tie pover functor is slightly better,

The prirary unl the secondary losses are slightly
increased, | .
The apparent efficiency of the nachine reraine the
Sune due to the chunze In the various losses but the

actual efficiency is somewhat lower,

To consider tie other chanze which is operating at over volt—

aze our performance ch.racteristics ure chanvei as followas:

(a)

(b)

(c)

(d)

Tre magnetizing currert is increasei thus raising our
magnetic density ani makins our iron losses freater,
Tle speei remaininzs constunt the friction anid winiage
are not chunsal.

The prinary anil seconiary copper losses ure less thus
decreusing tie leakare current. Thue as an agzrerate
the loases ure sualler ant our efficiency is slichtly
increused.

An increase in our marnetizing current causes rreater
il

flux ani therefore the torque is increased in pro-
portion.

(e) Our power factor is lowered ani our slip ig decreased.

(f) The apparent efficiency of the machine remains the

same anid there isa .n increase in the temperature rise
due to rusnetie density being riiaseil.

The charact2ristic chanves as outlinei for both 10% un-
der and over-voltare are present at fractional lcais as well
as at full loai. As 2 result of these points we micht Buy
that any chanze ereciter than 10% from normal wouli give a
wide varistion from the cerformance at ritel voltaze ani is
therefore prohibitive,

Referinz aviin to echunze in the nurbver of poles it night
be said that a chanze of two poles eitie2r way is vermissible
but not recommeniei. Aniif it is done without chanzin<e the
span of the coils then the chord factor must be ietermined
to be positive that it is not too low or too rizh. At the
seme time the Impressei volt272 shouli be altered in pro=
portion with the chain.e in chor? fuctor or elee it shouli te
clearly unierstooi that the notor is operatins above or bee
low normal voltave. Tris vives rise to a chinve in our per
formance curves ari since? the usual changve is creater than
10% then the operation is onlv fair. The exception to this
when chanving the number of poles is when the number is itoub—
lei or consejuent poles are used,

Phase chanztes have been mentione? before so here we will

Only sum up a few points which refer to this chanze when not
 

in conjuction with other chanves, The most usual chance is
re-connecting the motor fron two phase to three phase or

vice wersa, Theoretically for the sane line voltaze there
should be 25% more current in the two phase motor than in

the three phase. To Lave the «.me current density on the three
phase machine as there is on the two phase the horsepower
should be cut 12% fron that of the two phase. But on the whole
it ie bad practice to chince the phase of the machine not

only for the reason of chanving the performance curves but

also the fact that the phuse coils must be chanved which

means that all the coils must ve taken out of the slots and
then put back in arain in order to properly locate the phase
ecoile, This is a mechanicel job that requires skill and in
herently causes nany shorts due to dsemacine the inaulation
when taking out and replacinz the coila,

We now come to the iliscussion of a chanre which though
rare is not nevertheless without the scope of possibility
and which when it is encountered touches upon several points
all of which must be carefully considered in raking the de-~
eired re-connection. If no alterations vere maie on the mae
chine then a chanrze in frequency would be accomranied by a
change in speed of the motor. The first ani most important
question then isa "Will the mechanical deeieon stan the new
speed?* If so then the loai must be adapte? to this new
speadd by some means such as a change in the ratio of the
pulleys if it is a belt drive. If not possible to rake the

load operate under the new coniitions then the motor should
be changed to a new load where it will meet tre requirenents
of the drive under it's nev performince characteristics, If
the frequency of the supply is to be chunsei ani it ls desir-
ed to keep the speed of the motor the same then the number of
poles in the machine must be changeil in the same ratio as the
frequency was cliuncei. Also to keep the sare tensity in the
motor the woltaze should be variei in the eame proportion ae
the frequency. In chanrinz the number of poles in the motor
to keep the aarne speei we encounter the iifficulties of tak-
ing care of the chan;es in the mignetie circuit. Because we
must. remember that the total magnetic flux is divided into

as many equal purts as there afe poles. Ani the iron below
the teeth has to carry this flux for each cirevit. The great-
er the number of poles which we have the less amount of flux
there will be for the iron below the teeth to carry and com
sequently the crosssectionnl] srea nay be reivced. We can
thus see that it is an eas’ ratter to raise the frequency
because therew ould te less material reguired ani conse-
quently a motor has a surplus of iron for the rarpnetie cir-
cuite. This reaiilv e plains the reason that the rotor di-
aceter is iifferent In machines of the same horse~power and
speed operated on lifferent frequencies shen the outside

size of the motor is the sare for both conditions. Ani, too,
we must remerber that the rot@ short circvitine rinesa have
their current paths divided in propéttions to the number of
poles, the total fotor current being constant. This would

require rings in motors for low frequencies.
 

14

The question now arises what effect will the chanze of

frequency have upon performance characteristics? To lower

the frequency we have the following changes:

(a)

(b)

(c)
(a)

(e)
(f)
(g)

(h)

The iron loss is higher iue to the greater density
of the iron.

The friction ani winiage are less due to a decrease
in speed.

The copper losses remain constant.

Magnetization current is increased because vorking
higher on the B-H,.

The power factor is decreased.

The slip is the same.

The torque is greater because of increase in map
netic density.

The resulting efficiency is the same,

As a total result we have the same efficiency, a lower powef

factor, reduced speed, ani an incfeased torque.

On increasing the frequency the changes are almost di-

ametrically opposite to the preceeling discussion. To enum

erate them they are as followas

(a)
(b)
(co)
(a)
(e)
(f)
(g)

Better power factor,

Decreased torque due to decrease in magnetic density.
Friction and windage increased,

Iron loases are lower,

Copper losses remain the sure.

Decrease in magnetizing current.

Higher leakage factor.
(h) The slip is the sane.

(i) Efficiency not changed.

As a general rule then we nay 8a; that a change in frequency
ehouli be accompanied by a corresponding change in volta ze to
correct the changes in performance curves,

In groupins the slots to determine the number per phase
per hole it is not always necessary that the total number of
slots shall equil the préduct of the phase ani the number of
poles. In other xcrisa the groupin= neei not bé equal and
those groups which bFuve a different number of coils in the
group from the requirei number can be placed synmetrically
around the machine ani th total effect will not distort the
performance chiracteristics., Mr. FE. M. Tingley has an article
in the Electricul Review, January 1915, on this problem which
he calls the Least Common Multiple method of winting induction
motors of odd number slots.

As a general conclusion to our liscussion on possibili-
ties of re-connection we may say that those chanres which
give satisfactory performance are:

(a) Voltaze chan-es,.

(bo) Pole chanzes which are limited by the mechanical

form of the coils,

(c) Frequency chanzes either alone or in combination

with volta;re or phuse, but at the expense of a
@ehange in the speei of the motor. Also in such
changes the fact must be always in mini that the

peripheral speei of the rotor should never be over
7000 feet per minute without checking the mechan-
fcal design for the strains proiuced.
In determining the effect of changes either phase, poles,
or frequency it is best to hanile the change by reducing
them to an equivalent voltage chanre ani then determine

the factors for pgood performance.
PaRT II
EXPFRIMENTAL DATA

In order to perfor the experincnt of re-connecting
the machine for tke varlous two ani three phise connections
and to determine the relationship of voltaze exsisting with
these various connections it was necessary to remove the form
wound coils which were oricsinall: in the machine. These coile
consisted of nineteen turns of wire per coil. In their place
were inserted six numver fourteen rubber covered wires in
each slot thus mukins three turns per coil. The enis of each
wire vere brousht to s separate terminal on the board which
was fastened 2t each «ni of the rotor and were arranzed in
symmetricul, radial rows on the circurference of six ejually
spaced circles. The back coil und connections were then per
manently made with a throv of on? to ten, This is full piteh
for the machine as crigin.lly vouni.

No connections or tests were run sehich necessitated the
changing of the pitch or the removsl of the coils from their
slots if form wrouni coils were usei or which covld not be run
4f form woun:! ccils were used in place of the single conduct
ors. Number fourteen wire was also used for all @oil ends
and coll group connections. In all the teste however it was
impossible to tuke uny brake tests ani rake comparisons be-
cause of the absence of a pulley to fit the rotor shaft,
Therefore all tests ani duta collected are under no-load con

ditions,
Gennection

required applied

Two phase series

four pole 52
fwo phase two parallel

four cole 26
Two phase three "

four vole | 1?
Three pnase series star
four pole 65
Three phase series ielta
four pole 358
Three phase parallel star
four pole 335
Three phase purallel delta
four pole 19

Three phase series stur
eight pole (cones uent) 65

Three phase series stur

six pole 91
Three phase series delta

six pole 53
Three phase :arsllel star
six pole | 46
Three phase parallel delta
 geix pole 27

Three phase series ielta
twelve pole (consequent) 53

Three phase series star
twelwe pole (conseyuent) 91

56

21

87

86

48

55

Voltuze Voltaze Current R. P. KM.

amps. Gen. Motor

15

58

82

21

55

40

47

46

54

66

65

(Current too

50

85

88

56

1800

1800

1600

1775

1775

1820

1805

1750

1760

1790

1770

1790

1790

1785

1730

1770

1761

1756
1789
1785

\

800

1175

1190

Lis

590

593

Slip
g

0.835

5.88

1.61

0.6

1.07

1.7

lll

10

large to run test)

1.18

0.59
19

The winding diagran shown in this thesis were all em
ployed on the machine tested and are typical of those in
general use by all notor manufacturers. They are by no
means complete in the sense of covering all possible Con
binationsa of phase ani poles but they serve to illustrate
the points brought out in this thesis ani are sufficiently
general also to cover all points which msy arise in con
nection with similar iilazrams for other number of poles and
phases.

Fig. 7 represents a combined conventional and schematic
nethod of representing a three cphase four pole winiing con
nected in series str. Since trere ure three times four or
twelve polar groups ani thirty-six coils there are three
colls connected in series to form what is called a polar
phase group. It should be borne in mini thut there are not
actually twelve narnetic poles in a machine for the reason
that three consecutive polar jphase groups unite to form one
wagnetic pole by wirtue of the pliunse iifference of the cur-
rents in the three phases, Considered magnetically there
are two north and two south poles formed by the winding at
any instant ani these poles are equally spaced around the
air gap just as in the case with four mechanically project-
ing pole pleces excited by four coils carrying direct current,
Something of this conception is gained if one imagines the
armature of a direct current generator held etationary and
the field poles rotate! around it. At any given instant the
magnetic field can be coneeived to be the sure as the field
 

 

which is formed by the winding as shown by Fix. 7 where
each polar phase group is numberet 1, 2, 3, etc., uni is
ahown by short urcs on all of tle dfusrionsa., The arrowa

are shown sinply to iniicuate a rethod of checking up to
insure the proper prise relutions, There is considerable
danger with «. three ;hase connection of retting a 60 de-
gree relation between the plases irstesni of 120 decrees

or as it might be expresesei on the diagram there is dan
ger that the wrons eni of the B prave for exarnple ruy te
connected to the star point. Asa creck asrainst thia xhen
the ilacran is cenpletel the current is aseurel as going

in at all three leais tovard the Y :oint. Arrows ure put
on each pole phrase zroup as shown and when all three prus
es are traced throu.r the winiinz is correct if the urross
on consecutive eroucs run alternitel” clockwise and counter
clockwise. It riiy be argued that this is an artificial
assumption ani th.t at no instant is tre current flowing
towar:is the étir in all three phases. It ruy elso be ar
gued thit in a correct winding if tre current be aseunei
as flowing towards the stur in two phases ani always away
from the stur in the third the arrowva will fall in succes
sive flizhts of three in the sare ‘Jrection ani then three
in the reverse direction. These statcrunte are true but a
little experinentine will show that an incorrectly eonnected
or 60 degree winding can in this vay te stovwn to give suc~
ecasive flishts of three arrows ani still te wrong. There

is but one exception to the correctness of the check ue
21

shown in Fig. 7 and succeediine figures where the current
as flowing towaris the stur in all three phases an! the
arrows altern:te in direction. This excevtion to the rule
is the case le¢re the winiin: forms consequent poles or
pases throurh oll the phusge pole creups ina north direc-
tion insteai of alternitelyvy north ani south as in Figures
S53, 11, 17, ani 1&8,

The schematic divvron: placsi inside of the winding
diagrams tells at a vlaunce what the winiins diazroms repre-
sents without the necessity of tracinz it out. The sheen=
atic diacram may be suid to te the envineer's concevtion
of the connection while the windine diagrsm proper rav be
@eallei the winier'gw conception of tre rotor, Faeh, hovever,
serves it's purycse «as cither one of the two may be more
easily vraspei by « riven inlivilual.

Very little nore need be said of these diarrarts as
they are self—ex;lanitorv. However, u little obstruction
may be net with In the berinner's ind hen windine the
machine for two phrase four pole connection. Two tines four
equals eicht pols.r groups an! the rachine has thirvt—eix
slots. If four ecils sre put in series in each polar croup
1t will mean the use of only thirty—-tvo salots. And if five
coils are place! in series it will necessitate forty slots,
The windine diagranus io not show how to over-cor: tnis dif-
ficulty. Thirty-six coils may be useil thus making use of
the thirty-six slots by placing four ani five coils in

series alternately in each polar thuse group of each phase,
Thies distributes tre unequel winilines syreetrieally around
the periphery of the machine un! sives perfectly belunced
VOlte.es in exch phase, This is one of the vernier or Less
Conmon Multiple mett.ois of connection x9 refered to before,

In analyzing the experimental iata for a comparison of
the woltares required on tire rachine and those urplied on the
Various connections as iniic..ted 1t is ssen thut nearly all
of the voltages applied cers within the 10% limit of cood one
eratins coniitions wzith the exception of tne twzo plage .:re
@llel connections. Here it seexs trat it reyulrei twice the
necessary volture to sturt the motor, After the uechine is
in running conilition, however, it was possible to cut this
applied volt:ge in ftalf ani still hive the rotor operate un-
der normal ceniitions., The voltuse reyuirei for tre six pole
connections were corrected from the reyultred four pole voltazce
for chord factor.

The notor vas also conrectei for eizht poles but no data
was Obtuinel as 1t was Ingossible to muke the motor come up
to reguired speed due to great decrease in effective maznetiz—
ing flux causei by the large chord fartor,

In econelusion it may be stutei that any Induction motor
may be connectei for t#o different pole connections uni not
more than two excluling the consejzvent pole connection which
is rarely usel ani then usually on special notors wound for
multi-speeis,.

Note: In reference to Fie. 3 the ei~ht pole connection is as

followe: The ends y andi y’ are connected to-gether
puttin=s half
with half of
Use A, ~— B)
ani bl — be

235

of the coils in one ch:ise Inseries
{re eolle in the other chase.

and Ap —— Bo for lewis 1ni leave ay -—- ap

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