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THE JEFFECT OF VOLTAaE UNBALANCEON THE OUTPUT OF A TWO-PHA8E
INDUCT^^N MOTOR
BY
Ross Elmer CullingsAND
Charles Eugene McCormack
THESIS
FOR THK
DEGREE OE BACHELOR OE SCIENCE
IN
ELECTRICAL ENGINEERING
COLLEGE OF ENGINEERING
UNrVERSITV OF TI.r^rNOTS
PRESENTED JUNE. 1910
V
UNIVERSITY OF ILLINOIS
June 1
THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY
ROSS. ELIJEE CULL.III.G.S _..an.a CEAEL'^IS EU.GElffi liCCOHLIACZ..
ENTITLED TM l!:J^5^CT..._Q£...VQLTAG^ UMALAIICE QM. TEZ QUWl OF A..
_T..7Q.rPHA3,K lilDIJCTI.QIJ IffiT.QE...
IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE
DEGREE OF Bachelor oi ijcience in j^lectrical jiiiiglneering
APPROVED:
HEAD OF DEPARTMENT OF ii.LECTRIGAL EKGIIIEEE.IKG..
167155
TABLE 0? CONTENTS
Pa^e
Introduction ^— /
Theory of Hotcr Operation j
Description of Apparatus : 3
Tests
Rating of D.C. Generator ^
i^ethod of obtaining Unbalance G
Discus sion 7
Conclusions d
Connections
Data i3
Curve s zs
IKTRODDCTION
Polyphase induction iriotors have rr.ar.y advantages over motors of
other types and so have coire to be v:idely used as a source of mechan-
ical pov;er. They are very simple in construction as they have no slid-
ing contacts and no v/earing parts except the bearings. As there is no
commutator or brushes, there is absolutely no trouble from sparking.
These motors can therefore be used amongst inflammable material, in
dirty places etc., vfith perfect safety and Viithout any trouble of
operation. These motors are almost indestructible. They are simple of
construction and of operation and cost very little for attendence and
maint ai nance. They run at nearly constant speed for all loads, have a
high efficiency and a good pov;er factor. Thus it can be seen that in-
duction motors are suitable for practically universal use.
These motors are mostly polyphase, and so take poner from polyuhase
circuits. The different phases of these circuits may vary in term;inal
volt8.ge by a quite considerable percentage due to the fact that one
phase may be m.ore heavily loaded than the others and therefore have a
greater line drop. The effect of this difference in phase voltage on
the operation of tvvo phase motors, especially as to heating effect,
is the subject of this thesis investigation.
THEORY OF MOTOR OPERATION
An induction motor consists of a primary, or stator winding and
a secondary, or rotor, winding. Polyjihase alternating currents are im-
pressed upon the primary Vfiinding, producing a rotating state of magnet-
ism; in the lam.inated iron of the stator. This rotating wheel acts upon
the short circuited rotor conductors, producing currents in then;. The
action of this field upon the currents in the rotor produces a torque
which tends to speed up the rotor to synchronous speed. The rcotor
never quite reaches synchronous speed, however, for at thaf speed ther
would be no lines of force cut by the rotor, no currents produce in
the secondary, and therefore no torque to keep the r.achine running.
Therefore the rotor lags behind the rotating field, or slips an air.ount
varying vvith the load on the motor. The synchronous speed of a motor
in revolutions per second is n = 2f/v, v.'here f equals the frequency
in cycles per second and p equals the number of poles. In the rotor
under consideration p equals 6, f equals CO, therefore n equals 20
R. P. S. , or 1200 B'. P. U. The full load rated speed is 1120 E. P. M.
or a slip of 6.66 percent.
At starting, the currents induced in the rotor cf an induction
motor are very large due to the larje induced 5. I«. on account of
the rapid cutting of flux by the conductors. Therafore, for starting,
the urirrary 3. U. ?. should be cut doicn co a value belov; normal, say
60 percent, in order to keep the c' rrent 'caben by the motor from being
excessive. There are several methods of doing this, such as auto start
ers, starting compensators, etc, but any means of lovjering the supply
voltage will do. For instance, in operating this particular motor the
voltage of the alternator supplying the pcver was cut down by means
of the field rheostat. This was an exceptional case, however, and it
cannot ordinarily be done.
DESCRIPTIOK OF .i^PPARATUS.
The motor tested in this work ttss a Kestinghouse 7. p E. P. , two
phase induction motor, type CCL, taking 10 aznperes per phase at 60
cycle, 400 volts, and running 1120 R. P. Iv!. It is characterized by
strength of parts, high starting torque, large overload capacity, low
operating temperature, nearly constant speed, high efficiency and good
power factor. The rotor was of the squirrel cage type.
The motor was driven by power taken fron; a substation set. This
set consisted of a 49 K. W. , 220 volt D. C. si:u:it iBotor using current
from the po7;er Silant mains and driving a 4p K. 'A. tv7o phase, 440 volt
alternator. By means of the field rheostat the voltage on the alter-
nator could be dropped to a lov; value to be used when starting the
induction raotor in place of an auto' starter.
Load was applied to the induotion :rotor by nie?*ns of 2 belt connect
ion to a 110 volt, ^2 ampere, 1020 R.P.y'i. separately excited direct
current Northern Generator. The generator v;as loaded by ineans of lamp
banks connected in parallel across its terminals.
Two transformers, 10 K. W. , 440 - 220/110 volts were used as auto
transformers in order to obtain unbalanced conditions. Also two ssiall-
er transformers of one Z.Ti. capacity, 440 - 550/47p volts were used
for the same purpose. The method of using these transformers will be
be explained ir.ore in detail later. The instruments used in this test
were of the portable type, Weston make.
Tvio sets of tests v.ere run on the induction rrotor. During one the
tsraperature rise was kept constant at its maxiinuii; point, 50° C. rise
in the windings, and the rraxiraurj load which the acotor could carry was
deteriained. In the second test the output was kept constant and the
temperature rise and operation of the motor noticed. Tn both sets the
unbalance was varied over as large a range as possible.
The connections used in these tests are shown on page |0 . Po'.-rer
was taken fro- the depart nfient substatior and delivered at the irotor
in an unbalanced condition. The rcotor drove a D.C. generator as load.
The generator itself was loaded by aeans of lamp banks. For each run,
readings of current input, voltage and TJatts per phase into the A. G.
niotor Tfere taken, and the load and temperature rise observed. The
limit of temperature rise was found to be in the windings in each
case. The allovfable rise was 59° ^- above a standard 25° 2. root." LeiTir;-
erature. For each degree variation of room teiTiperat ur e froni the stand-
ard 25°, a correction of one-half of one percent per degree was iiiade
in the observed rise. Thus for a 3p° room temperature, the observed
rise was decreased by five percent. Ther moaet er s v;eve used to detern;-
ine the temperature rises, placing one on the fratne, one on the lamin-
ations, and tvjo on the '.bindings. After shutin-| down one was placed on
the rotor and one in the bearings. V;hile running a test, readings were
taken every 15 minites until the t err:per at ui'^e s became constant. The
limiting rise was found to be in the windings, as stated above. The
highest of the tv.'o t her n'Oinet er readings og the winding ten:perature
was taken as the correct one.
For constant temperature rise, the unbalance nas varied fvoz
to 41 percsnt. For constant output the unbalance could only be carried
to 27 percent owing to the limiting temperature rise.
RATIKG 0? D.C. GSITERATOR. i
The D.G. Generator supplying the load for the .-T^otor had to be
rated for different conditions of operation as to speed and excitation,
in order to find the losses in the machine itself. To rate the ^ener-
ator, it i-jas run as a separately excited niotor aith an adjustable I
resistance in both the field and the armature circuits. The ilux vjas|
kept constant by keeping the fi-^ld current constant, and the notor
mas run light at different speeds by shifting, the arir.ature rheostat.^
The input to the motor was taken by Trsans of an acmster and voltmeter
M
in the armature circuit. The product EI ras equal to the losses, i,
y I'
stray poTJer, and a small j. ^; loss. The arr^ature resistance Tfas meas-
ured by the fall of potential method. The stray po'.fer losses, vrhich
varied as the generated E.M.?., 7;ere plotted as shown on curve sheet II.'
Different curves were plotted for different values of field current.
An I-^R loss curve ^vas also plotted as shown on curve sheet I. As the
'i
D.C. machine was rcn as a separately excited generator, no notice 1
v?as taken of the field losses at all. '
To detertnine the output of the induction irotor, then, fet any
load, all that was necessary was to read field and armature currents,
arciature terininal voltage and speed of the D.C. generator. iVror the
curves, for this speed and niagnet izat ion, the stray power losses could
r nbe read directly. The ' "" ''ossss could be read froir I R curve. The
suit: of these losses, plus the output of the generatoif, t'lus three
percent allowed for belt losses, gave the output of the induction irotor.
As an example of this, let us say that the generator was deliver-
ing 49 amperes at 110 volts at a spe3d of 1020 R.?.}.L., T?ith an exoita
tion of 1.75 aicperes field current. Then froE the curves the I'^R loss
are o30 'kvatts, the stray po7>'er losses are 43O ivatts.
Output = 40 X 110 = 4400 ;7att3
Input to generator = 4400 + 450 + 53O = 5380 watts.
Allovjing three percent for belt losses the output of the inducti
motor is
1.05 X 5530 = 5550 viatts or 7.45
If S,;^ = E3 = 400 volts, Ij^ = I3 = 9.5 amperes, Ka^^ = XgFiE = 5.
Total X.?i. =6.2.
CoiriDeroial efficiency 4= 5550/620O = 39- 5;1.
Total volt amperes - 2 x 400 '< 9. = 7600.
Apparent efficiency = 5550/7600 = 75;'i.
2.(ETH0D 0? OBTAINING UNBALANCE.
The obtainin,^ of the unbalanced conditions '.vas one of the prob-
lem-s of this thesis v-ork. The method decided upon vjas to use trans-
formers to step up and dov.'n the voltage on one phase by ceans of an
auto-transformer connection. As can be seen in description of appar-
atus, there vjere two different sized t cansf orniers used, giving diff-
erent ratios of transformation. These transformers were connected up
and combined in different manners as is shovm on connection sheets,
pages I I toJ5, to give the different unbalances. At first sight, it
Tsould look as if the sTnallearft transformer, of one K.Vi. normal caoaci-
ty would not carry such an overload as was put upon it, but this is
explained by the fact that in the auto connection the greater part of
the current only goes bhri^ugh the lov; voltage coil and so does not
cause much heating effect in the transformer.
As an exarafle of the method of connscting of connecting up these
transformers, take the case of 22 percent unbalance, where the voltage
of one phase is reduced from 400 volts to 512 volts. One end of the lovi
tension coil is connected directly to one end of the high tension
coil; the free end of this lovj tension coil and a viire froni the other
end of the hi^h tension coil run directly to the induction motor. This
connection reduces the voltage in this phase as the S.M.P. generated in
the lov? tension coil opposes that of the high tension coil. The other
phase is taken to the i?.otor directly iron the 400 vol I. bus bars. Other
conneotionB r:ere irade as shov.'n on diagram sheet.
DISCU3cICr CF CUFV??.
Plata?) I and II have to deal v?ith the rating of the B.C. generator
vihich supplied the load for the induction ir.otor. Plate i sho'.vs the i R
loss in ??atts for any current in the armature, and Plate II shows the
stray po^fer losses at any speed and magnetization. These curves shov.'
that the stray power increases ?iith an increase of speed, vjith field
current constant.
Plates III to VII show the effect of unbalancing upon the motor
vfhen run at constant full load output. Plate III shows hov; the temp-
erature rises v;ith an increase of unbalancing. It is noticed that for
a 30 percent unbalance the temperature rises about 100 percent above
normal running temperature. This motor operates under low tempera-
ture. In other motors such a rise vfould be exceedingly dangerous.
The curves on Plate IV shoivs hor: the efficiency drops off with
increase of unbalance. There is about a 10 percent drop in both true
and apparent efficiencies for 28 percent unbalance.
Plate V indicates how the T;atts in one phase, the high one, in-
crease, vjhile those taken by the lov; phase decrease as the differer.ce
in voltages of the phases increases.
Plate VI shov,'s that the speed remains nearly constant. Also the
power factor, total ivatts divided by total volt-anperes is nearly cons-
tant, as is indicated din curve sheet 7«
Curves VIII and IX show the operation of the cotor with a constant
limiting temperature rise. They shov; clearly that the output and the
efficiencies of the motor drop off quite considerably v?ith increasing
unbalance. The output varies inversely with percentage unbalance.
COKCLUSIOKS.
The results of this thesis investigation can be sumned up as
f ollo^Ts:
For a constant output of the motor, an increase of unbalance re-
sults in a iTiore than proportional increase in temperature rise, and also
in a decrease of efficiency. It has practically no effect on the speed
however.
For a constant t err^per at ure rise an increase of unbalance raeans a
decrease in output, in a straight line ration, The efficiencies also
fall off in value with increase of unbalance.
Although these tests vrere run only on an induction irotor, it can
be seen that the results vrould be socerrhat the sane for a synchronous
motor also, and for any polyphase machine as ?;ell as a tvfo phase irotor.
In actual practise such a large unbalance is seldom if ever Eet with.
The vailtage variation is generally limited to IC or Ip percent, ^here-
fore for a motor giving constant output, as is generally the case, the
increased tem.perature rise kouIc not be dangerous. The greatest effect
of an unbalance -nould be the loss in efficiency resulting therefrom.
NesflnQhouse Tvvo Fhase Induction Nofor:
Typz CCL
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