Citation: Bixby, Horace E. and Long, Carl C. Operation of a 3-phase, 15 K.W. alternator as a synchronous motor. Senior thesis, Kansas State Agricultural College, 1908.Introduction: The theory of the synchronous motor and its operating characteristics can be shown very clearly by the use of diagrams. In this way, the variation of the power-factor with the load and with the exciting current is shown

Bixby, Horace E.

Long, Carl C.

English

K-State Research Exchange

"Operation of' a 3-Phase, 15 K.W. Alternator as a Synchronous Mot or." " By Horace E. Bixby. Carl c. Long . 1. Diagram of Connections. 2. Theory. 3. Starting. 4. "Vee" Curve. 5. Power-Factor Curves. 6. Efficiency Curves. To I S'tlorage lattery A. C. Generator. Diagram of co~nections . 20: 1 \ 0 0 0 lsyn. Lamps . Theory of the Synchronous Motor . The theory of the synchronous motor and its operating character-istics can be shown very clearly by the use of diagrams. In this way, the variation of the power-factor with the load and with the exciting current is shown. Figure 1 s r ows the relation between the supply E.M.F.,(A ), t he motor counter E . l.~ . F . ' (B), t he resultant E.U.F.,(E) and the current (I). The angle 9 which the current makes with the resultant E .M. F. is constant for any one circuit of armature winding and connections, and is equal to tan-lX/R, where R is the r es i stance in the circuit and X the inductance. Fig. 1. / / / /e / / / / / / o~/=-----+1---.--:::::-:? -------Fig . 3. ---"1), Figure 2 is the diagram which shows the value of the power in-take of the motor, with the increase of the l oad , the e m fs being kept constant, in other words the field excitation of the motor is kept constant. The supply voltage is supposed to be kept constant throughout this discussion. In t hi s fig., OC represents the motor emf, (B), CP the supply voltage, (A) , OI the current in the circuit. Then OP will represent the resultant e ~ f (E) . If we draw through 0 the line EF, making the angle 9 with the line oc, the projection of (E) upon EF, or OQ will be proportional to the power in-take of the motor. This is proved in t he following manner:-The out-put of power of (B) = B·I·Cos (COI) (POQ) = (COI)-180°, Cos (POQ) =-Cos (OOI) OQ = E·Cos(POQ) = -E·Cos(COI) =-/R2 ~ X2 ·I·Cos(COI) -OQ//R2,.. x2 = I•Cos(COI) , Power out-put of (B) - - ~B:;·:::OQ:::::;;::=_ fR2+ x2 • . . . or power in-take of (B) = B•OQ//R2$ x2 , ( B ) , ( R ) , and (X ) being constants, the power in-take will be proportional to OQ. Figure 2. E \ \ \ \ \ F The supply e rn f and the motor e m f being kept cons tant , the anglo between the e ~ f must change as the load changes, in order that the resultant voltage may b ecome l arger, resulting in an increase of current and power in the circuit of the machine. Suppose. the load to so vary . Ia circle. Then (A) must r evolve about (C), the point (P) describing The line s~ being dra~~ perpendicular to EF , t he scope of operation of (B) as a motor will be points on •be arc sms • At tho points sands', OQ is equal to zero and (B) r ecieven no power frorr the circuit. In order that the e m f (A) , may pass beyond s and s' i t will be necessary for (B) to run as a generator, in which case it will give out power to the circuit . If the line m~~ is drawn parallel to EF, then the limit of steady operation of the motor, the e m f s being constant , is between the po i nts s and m. I n this region, any increase in load, resulting in an increase of phase difference between (B) and (A) will be met by an increase of OQ and the power in-take . Between t he points m and a', an increase in load results i n a decrease of power in-take . Ther efor e 'I if the motor is l oaded until the point (P) passes by m, the motor wi ll s top or "break dmm" • The angl e between the current and the supply voltage will change with the load. The current will be in phase wi th the supply vo l tage when the angle CPO i s equal to 9. At this point we will have unity power-factor and consequently maximum efficiency. If we keep the angle between the two e m f s comstant and vary the motor e m f by varying the field current of the motor, we will have the conditions shown in figure 3. Here CP, CP' and CP"represent three differ ent values of fie l d current. OC r epresents the suprLY em f and the lines on, OD ' and OD " the current which l ags behind t he resultant e m f an angl e equal to 9 . As this angle i s constant, the angle between the cur·rent and the supply e m f will change with the field , the load being kept constant.When t he motor e m f is equal to OP, the current is in phase with the supply emf and the power-factor is unity. If the field current is increased, the current will lead the e m f and the ~otor will have a condenser effect on the circuit . If the motor e m f is less than OP, the current will lag behind the supply voltaee, and the motor will have an inductive effect on the circuit. As the angle of lead or lag increases, the current in-creases . The increase of the current over that w~ere the power-factor is unity , is a wattless current , and causes a great I2R loss in the circuit. I I f the field uurrent was -either increased enough or decreased enough, a point would be reached where the current was at right angles to the supply voltage . In that case the current vould all be wattless , the motor would recieve no power, and it would stop . The machine used in this test was a General Electric type ATB one to six phase, 15 kilo-watt alternator, ~ith rotating fields and laminated pole faces. Power was supplied by a ~estinghouse, 2300 volt three phase a lternator, and stepped do~ to 110 vo_ts b ' trar~rorners . Care was taken that this voltage as well as the frequency ~as zept constant . For a load, the I!l.otor was belted to a. 1~ · :..:c--a-:.t Westinghouse, compound wound, 110 volt D .C . ge~~ra~r: :cad&d ~:..th a bank of incandescent laffips . This gave us a. -e~ s-~b:e a~d ea.si:J measured load. Before the test was run ~ au e~f:c:..2Lcy c .. xve of the D.o. generator was obtained by ruru1ing it ~~h a ea_:~rated wotor . In this way the out-put of the motor was detel.'"i:::ti.Led. When the machine was first started, t~e~e was trouble due to hunting. This was to be expected as the poles ~ere laminated, and the machine was not designed for ~ operat ion a s a synchronous motor. The difficulty was finally located in the be lting which was too l oose . After that the belts wore kept tight, both on the D. C. generator and the motor . YJe then found no difficulty with hunting during the r emRindor of the test . Due to the inadequacy of the power supply we were unable to load the motor heavily enough to find the break down points of t he different field excitations used. We did get full load on the motor, r ated as a generator at which load the operation was entirely satisfactory. Starting the Synchronou s ~otor . Alternators are convertible into motors; and one alternator will run in synchron~sm with another similar machine after it is broueht to the same speed, or, if of unlike number of poles, to the sa~e multiple of the speed of the driven dynamo , provided the nu~ber of pairs of poles on the Motor is divisible into the multiple. Such rro tors will run as if geared to a driven alternator even up to two or three t imes its normal full torque or capacity . Single- phase synchronous motors hav P- no starting-torque , but synchronous motors of multiphase circuit s wi ll come up to synchronism without much load giving about 25% starting-torque, starting as induction motors, with the d .c. field open . Since the synchronous motor has very little starting- torque, it has to be started at practica lly no-load and the load thrown on gradually after it has reached full speed . One method of starting the synchronous motor is by a separate prime mover, which for s implicity tal{e a d.c . motor . ·rn this ~ethod the alternator which is to b e used as the syncJrronou: motor is brought up to synchronous speed and having the same voltage as the alternator which is to drive the synchronous motor and exactly opposite in phase . One way of knowing when the two machines are in phase is by use of synchronizing lamps. These lamps, one in each phase, Rre placed over the three-pole swi tch as shown in diagram of connections. The lamps will first be bright and then dark and as the two machines are nearing synchron-ism the pulsations of the lamps will be slower and there will be several seconds of darkness each time until they are exactly in phase . When this occurs the switch in the rnain circuit should be closed and the prime mover of the synchronous ootor is cut off. This method is used in paralleling of alternators but not so much in starting of synchronous motors. Another method is the partial voltage method. In this the d.c. field of' the synchronous motor is opened and the voltage of the driving alternator is lowered to about one-half normal and when the switch in the main circuit is closed, the machine will start as an induction motor, and when it has reached full speed the field circuit is closed. The voltage is brought up to normal gradually after starting. In this method as in the fi rst, tee motor is started at practically no-load, e . i . only such load as is necessary in startin~ as a generator on the same shaft or one directly belted. A third oethod is to close switch in ~ain circuit, open the fie ld on the rrotor and as the alternator is started the motor will pick up as an induction motor, and when the alternator has reached full speed, so has the motor and the field circuit of the motor is closed. In starting a synchronous motor as an induction motor, the a.rnature current is large, taking about 200% of no-load current as a synchronous motor . When the d.c . field circuit is closed the arnature current at once comes dovm to a lower value depending upon the strength of field current used in the synchronous motor. Considerable care must be exercised in the use of synchronous I!lotors, and their best condition is where the load is quite steady, otherwise they introduce inductive effects or- the line that are quite troublesome. Th~ field of such a motor can be adjusted for a particul ar load, so there will be neither leading nor lagging current but unity power-factor. If the load changes, then the power-factor aleo changes, until the field is readjusted; i f the load has been lessened the current will lead, and if it increases the current will lag . Data for Vee curve, no load. Amps . Fld. Amps . Arm. . Watts (1) Watts (2) . 1 . 00 29 - 1400 1900 1.5 17 . 5 800 1350 2 . 0 5. 200 700 2 . 5 0 . 0 .. 400 100 3 . 0 13 1100 - 450 3 . 5 24 . 5 1750 1050 4 . 0 34 . 5 2400 l b50 4 . 5 -44 . 5 3000 2100 5 . 0 55 . 3700 2600 5 . 5 64 . 5 4300 3100 6.0 74 . 4900 3500 6 . 5 84. 5600 4000 7.0 93 6200 4400 The "Vee" curve is one showing the relation between the motor armature current and the field current. This curve gives the no-load armatur e current for any field current between the two limits . Starting with one ampere field current and increasing 0 . 5 ampere at each step, the armature value decreades until when the point corres-ponding to 2 . 5 amperes field current was reached , t he arnature currert had decreased to practically zero, this fi ld current, 2 . 5 amperes, producing the best power- factor at no- l oad . Up to this point the armature current l agged with respect to the e m f. As t he field wac over excited the armature current increased which produced a leading current wit~ respect to the line em f. The over excited synchronous motor thus has a condenser action. Data:-- Curve #1. A1 . A2 A3 Volts : K.W:1 . K . TI . 2 Cos ¢ D.C .Volt.s /l..rcperes : . . 24 24 . 5 22 110 2 . 5 -.4 . 45 104 10 28 28 25 II 3 . 2 00 . 60 105 19 32 . 5 33 31 " 4 . •. 4 .70 106 27 . 5 44 46 42 " 4 . 75 . 9 .743 108 34 . 46 48 .5 44 " 5 . 5 1 . 2 . 755 107 . 5 37 . 55 58 .5 54 . 5 II 6 . 9 1.3 .79 106 4L . 5 Synchronouc kotor field excited to 1.5 amperes . Breakdown point at 68 amperes armature current of motor . A1, A2, and A3 corresp nd to the current in the dif~erent phases of motor aroa.ture. Cos ¢ = %power- factor . 3'0 Data:-- Curve #2 . Al A2 A3 Volta K. W.l K. W. 2 Cos ¢ D. C. Volts : An::peres : 45 46 45 110 3 . 6 - 1 . 4 . 251 106 00 48 48 48 " 4 . 1. . 33 106 6 47 47 47 " 4 . 4 . 5 . 438 106 . 5 14 49 50 49 " 5 . 0 .0 . 53 109 22 51 52 51 II 5 . 65 .. 1 . 1 . 685 108 . 5 31 57 57 57 " 6 . 55 2 . 5 . 835 109 54 63 63 63 " 7 . 3 <;;:: 0 U o v . 934 108 . 5 69 .7 65 65 65 II 7 . 6 4 . 5 . 977 108 76 69 69 69 " 7 . 9 5.4 1.000 104 87 Motor field excited to 4 . 5 amperes . Spee d 1 200 r. p . m. Fr equency 60 cycles . Data: - - Curve 4/=3. Al A2 A3 Volts K. W. 1 K.W. 2 Cos ¢ D.C . Volts Amperes 76 76 76 110 5.6 -2.9 .187 75 76 75 II 6 2.25 .262 107 000 76 76 76 It 7 1 . 435 106 9 79 79 79 tl 8 o.o .532 106 29 83 83 83 " 8.9 -t l . 3 . 645 107 42 . 5 84 85 84 " 9 . 3 2 . . 703 106 62 87 87 87 II 9 . 8 2.9 .755 106 69 90 90 90 II 10.2 3 . 6 .805 104 82 Motor field excited to 6 amperes. No breakdown point. Data:-- Curve #4 . A1 Ar.~ A3 Vol ts K. W. 1 K. W. 2 Cos ¢ D. C.Vo1ts Amperes t:J 1 5 15 15 110 1.7 .1 . 631 2 2 22 22 " 2 .4 .9 . 806 104 1 2 . 25 25 26 25 " 2 . 95 1.6 . 93 105 23 31 3 1 30 II 3 .41 2 . 27 .979 106 32 .1 37 .1 36 35 " 3 . 9 3 .15 1.000 108 42.5 44 .5 44 43 " 4 . 5 4 . 08 1. 000 109 53.7 52 50 51 " 4.9 5 . 08 1.001 109 65 58 56 55 " 5 . 05 5 .9 .96~ 109 73 . 8 Motor field excited to 3 amperes . Power-Factor Curves. The power-factor curves are plotted with watts in-put as abscir-.sa and % power-factor as ordinates, one curve being taken for each value of the ~ield current used . Curve #1, corresponding to a va lue of 1.5 amperes field current shows a maximum power-factor of only 79% when the break down point is reached . The armature current for t his power-fE~ctor 68 amperes , 69 amperes is the rated full load current as a generator . The field for this curve was under excited so tha t the current was lagging wi th respect t o the impressed e m f as is shovm by the"Vee" curve . Curve #4 , which has a value of 3 amperes field current, is some-what higher than the previous curve and therefore has hi gher power-fac tor. In t hi s vre get a power-factor of l OO% or unity and from the Vee curve it may be seen that the current wes leading slightly with respect to the impressed em f,tbis indicated an over excited field . This curve gave t he hi8best power-factor although for one value of input of curve #2 a value of 100% W '"' S obtained . The curve corresponding to 6 amperes field current falls below the other three du.e to the decrease in power-factor which was caused by the ~otor field being very much over excited, therefore producing a leading current with respect to the i mpressed em f . This curve is nearly a s traight line showing that the power-factor and watts input vary nearl y in direct _proportion to each other. The power- factor would probably not become unity. The curves taken were for values of 1.5, 3, 4 . 5 and 6 amperes fie ld current . They all resemble a magnetization curve in form . None start at the origin as the lowest value we obtained for power-factor ~--~----~~==~----------------------------------~--- ' ~) was about 18%. From these curves we would conclude that the best value of field current for operating the machine as a synchronous motor is 3 amperes. This current g ives a value of practically unity power-factor, over the greatest range of load. The curves were obtained by reading the t otal watts input to the synchronous motor and amperes in each phase . The field current was kept constant at the value desired and the i mpressed voltage and speed kept constant . From this equation: - Cos ¢ = p , where P is the sum of the ~ff watt-meter readings , E = voltage, I = average current of the 3-phases, and Cos ¢ = power-factor, the % power- factor was computed . Effi ciency Curves . The curves on the pr eceeding page show the effi ciencies of the synchronous motor at f our different values of field current . They were obtained in the fo l lowing manner. The Westinghouse, 15 K.W.,D . C. generator was operated by 26 horse-power Crocker-Wheeler calibrated motor and an efficiency curve obtained . By reading the out- put of the generator at any load , the out-put of the synchronous motor could be calculated . The in-put to the motor was measured by two watt-meters placed in the supply c i rcui t. The out - put divided by the in-put gave the efficiency at that l oad . B taking readings at several different loads , from no-load up to maximum , the efficiency curves were obtained. It is seen that an efficiency of 92 .8% is obtained with an excitation of 3 amperes . This data was checked at a subsequent time and found to be correct. A·s that is generally cons i dered as too high an efficiency for a synchronous motor of this size, the discrep-ancy may have been in the efficiency curve of t~e calibrated motor which was used . If thi s curve had given, in general, too high an efficiency for the motor, the result would have been an efficiency generally too low for the generator and consequently too high for the synchronous motor. However the shape of the curves and their r elative position with respect to each other would not be affected by this error . A comparison of the four curves shows that for extremely high and extremely low values of exciting current the efficiency is very low, maximum efficiency being obtained with a field current of 3 amps . T · · · wi tl' the power-factor h~s ~s what one would expect from a compar~son 1 \44 curves at the above excitations. Three amperes field current gives a power-factor of nearly unity at full load which we will consider as 10 K.W. This should therefore give us maxi mum efficiency borne out by the efficiency curves. The reason for the decrease of the effic-iency with the power-factor is that on low power-factors a considerabl~ amount of wattless current is circulating in the system causing in-creased I 2R losses in comparison with the energy derived from the watt currents. Because of this variance of power-factor the efficiency of the motor with 1.5 amperes field current is a maximum at 8 K.W., the point of maximum power-factor; while with 6 amperes field current, maximum efficiency i s at 14 K. W. because of the higher power-factor with the greater load at this excitation. We would then conclude that maximum efficiency is obtained when the field i s normally exci ted, a fact which can be proven from theoretical considerations. We also see that an under excited synchronoun motor has maxim~ efficiency at less than full load; and an over excited motor has maximum efficiency above full load. Owing to our lack of power in the supply circuit we could not determine the efficiencies above full load, but we have evidently reached the point of maximum effici ency in most of our curves. * 

Operation of a 3-phase, 15 K.W. alternator as a synchronous motor

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Operation of a 3-phase, 15 K.W. alternator as a synchronous motor

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