AC Machinery Laboratory EEC631L
Experiment No.9 The Synchronous Motor
EXE!"MENT N#. 9
The Synchronous Motor #$%ECT"&ES 1. 2.
To examine the the constructi construction on of the threethree-phase phase synchro synchronous nous motor motor To obtain the startin starting g characteris characteristics tics of the three-p three-phase hase synchron synchronous ous motor motor
'"SC(SS"#N The synchronous motor gets its name from the term synchronous speed, which is the natural speed of the rotating magnetic field of the stator. As you have learned, this natural speed of rotation is controlled strictly by the number of pole pai rs and the frequency of the applied power. i!e the induction motor, the synchronous motor ma!es use of the rotating magnetic field. "nli!e the induction motor, however, the torque develop does not depend on the induction currents in the rotor. #riefly, the principle of operation of the synchronous motor is as follows$ A multiphase source of A% is applied to the stator windings and a rotating magnetic field is produced. A direct current is applied to the rotor windings and a fixed magnetic field is produced. The motor is so constructed that these two magnetic magnetic fields react upon each other causing causing the rotor to rotate at the same speed as the rotating rotating magnetic field. &f a load is applied to the rotor shaft, the rotor will momentarily fall behind the rotating field but will continue to rotate at the same synchronous speed. The falling behind is analogous to the rotor being tied to the rotating field with a rubber band. 'eavier loads will cause stretching of the band so the rotor position lags the stator field but the rotor continues at the same speed. &f the load is made too large, the rotor will pull out of synchronism with the rotating field and, as a result, will no longer rotate at the same speed. The motor is then said to be overloaded. The synchronous motor is not a self-starting motor. The rotor is heavy and, from a dead stop, it is not possible to bring the rotor into magnetic loc! with the rotating magnetic field. (or this reason, all synchronous motors have some !ind of starting device. A simple starter is another motor which brings the rotor up to approximately approximately )*+ of its synchronous synchronous speed. Then a direct direct current is applied applied to the rotor windings. After which, the starting motor is then disconnected and the rotor loc!s in step with the rotating rotating field. The more commonly used starting starting method is to have the rotor rotor include a squirrel squirrel cage induct induction ion windin winding. g. This This induct induction ion windin winding g brings brings the rotor rotor almost almost to synchr synchrono onous us speed speed as an induction motor. The squirrel cage is also useful even after the motor has attained synchronous speed, because it tends to dampen the rotor oscillations caused by sudden changes in loading. our synchronous motorgenerator module contains a squirrel cage type rotor.
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AC Machinery Laboratory EEC631L
Experiment No.9 The Synchronous Motor
AA!AT(S !E*("!E' 1 unit 1 unit 1 unit 1 unit 1 unit 1 unit 1 unit 2< pcs. 1 unit 1 unit
quipment /201$ ynchronous otor3enerator odule quipment /)11$ lectrodynamometer odule quipment //21$ 4ower upply odule 5* 6 12* 2*/ 7, 8-phase9 quipment /:21$ ynchroni;ing witch odule quipment /02:$ A% 7oltmeter odule 52<* 79 quipment /02<$ A% Ammeter odule 5/ A9 quipment /)2*$ 'and =igital Tachometer /)<1$ %onnecting >ires /)02$ %onnection #elt lectronic ulti-tester
C+EC,"N- ACT"&"T"ES CA(T"#N
+i)h /o0ta)es are present in this experiment 'o not mae any connections 2ith the po2er on Mae sure that the 1. main s2itch o the o2er Supp0y mo4u0e is in the 5 position 7. its /ariab0e supp0y nob is set to M"N"M(M an4 3. the set is unp0u). lectronic ulti-tester 1. %hec! battery level by turning the device on 2. %hec! calibration for voltage A% reading by observing a ;ero reading. &f not, inform the instructor /)01$ %onnecting >ires 1. 2.
%hec! each wire for continuity using the continuity test function of the electronic multi-tester %hec! for visible sign of insulation tear or damage
quipment /201$ ynchronous otor3enerator odule 1. 2.
%hec! the fiber glass protection panel for any brea! or damage ?pen the fiber glass protection panel and chec! smooth rotation of roller bearing assembly mounted at the side of the module 8. 7isually chec! the visible part of the stator and rotor copper windings for evidence of burn or insulation brea!down. &f there is, inform the instructor. &f none, proceed. 0. %hec! for smooth rotation of the rotor <. %hec! for a good connection of the wires coming from inside the machine and into the connector leads mounted on the fiber glass protection panel :. %lose the fiber glass protection panel and perform continuity test for the three windings 5terminal pairs 1-0, 2-<, and 8-:9 of the ynchronous otor @. et rheostat !nob to minimum setting. %lose switch 1 and connect the multi-meter tester at terminal pair @-/. et instrument to measure resistance. otate rheostat !nob to increase resistance. ?bserve multi-tester resistance reading if it increases. ?pen switch 1. a)e 7
AC Machinery Laboratory EEC631L
Experiment No.9 The Synchronous Motor
quipment /)11$ lectrodynamometer 1. 2. 8. 0. <. :. @.
%hec! the fiber glass protection panel for any brea! or damage ?pen the fiber glass cover and chec! brush and its good contact against the copper rheostat element %hec! smooth rotation of roller bearing assembly mounted at the side of the module %hec! the good connection of the wires coming from inside the lectrodynamometer and into the connector leads mounted on the fiber glass protection panel lide the equipment at the middle part of the wor!station, between the 4ower upply module and the quirrel-%age &nduction otor module %hec! if its shaft pulley groove aligns with the shaft pulley groove of the quirrel-%age &nduction otor module. &f not, inform the instructor et the control !nob to ;ero 5full counter-cloc!wise9
quipment /02:$ A% 7oltmeter and quipment /02<$ A% Ammeter 1. 2. 8.
%hec! the good connection of the wires inside the meter modules and into the connector leads mounted on the panel lide the A% 7oltmeter at the top of the 4ower upply module and the A% Ammeter module at the top of the quirrel-%age &nduction otor module et all meter needles to ;ero. AdBust plastic screw on the front panel below the meter view screen for each meter instrument using a screw driver if necessary
quipment //21$ 4ower upply module 1.
et the variable control !nob to ;ero 5minimum or full counter-cloc!wise9, the 207 A% red translucent switch to off 5* position9, the main 4?> switch to off 5* position9, and selector switch to 0-< 2. %onnect each of the A% 7oltmeter across terminals 1-C, 2-C, and 8-C 8. %hec! the firm connection of the main power cord at the bac! of the module 0. 4lug the power cord of the 4ower upply to an outlet and turn on the 4?> switch 5 " position9. ?bserve uniformity of the lights of all three amber pilot lamps on the front panel labelled, 1, 2 and 8. &f not, inform the instructor. <. ?bserve the three A% volt meters if each will indicate a value of 12*7 A% thereabouts. &f not, inform the instructor. :. otate the control !nob to 1** 5maximum9 @. ?bserve if meter on the 4ower supply module will indicate a value of 2*/7 A% thereabouts. &f not, inform the instructor. epeat for terminals <-: and :-0. eturn the control !nob to ;ero 5minimum9 /. Turn off the 4ower upply odule but do not unplug yet the module ). =isconnect the voltmeters from the 4ower supply module quipment /:21$ ynchroni;ing witch odule 1. 2.
%hec! for a good connection of the wires inside the switch module and into the connector leads mounted on the panel %hec! the toggle switch for any loose part when you toggle the switch
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AC Machinery Laboratory EEC631L
Experiment No.9 The Synchronous Motor
quipment /)2*$ 'and Tachometer 1. %hec! the rubber cone at the tip of the apparatus 2. lide the switch to %ontact position 8. 4ress the blac! button on the right side and chec! if a number ;ero is indicated in the view screen. /)02$ %onnection #elt 1. %hec! for visible signs of wear and tear
M#'(LES SET ( ACT"&"T"ES 1. 2. 8. 0. <.
4lace the ynchronous otor3enerator odule at the lower center slot 5beside the 4ower upply odule9 of the obile >or!station 4lace the lectrodynamometer odule at the lower right slot of the obile >or!station 4lace the ynchroni;ing witch odule on top of the 4ower upply odule 4lace the A% Ammeter odule on top of the ynchronous otor3enerator odule 4lace the A% 7oltmeter odule on top of the lectrodynamometer odule
!#CE'(!E 1.
xamine the construction of the Three-4hase ynchronous otor3enerator, paying particular attention to motor, slip rings, rheostat, connection terminals and the wiring.
2. 7iewing the motor from the rear of the module$ a. &dentify the two slip rings and brushes. b. %an the brushes be movedD No c. Cote that the two rotor windings are brought out to the two slip rings through a slot in the rotor shaft. 4. &dentify the =% damper windings on the rotor. 5Although there are only two windings, they are connected so that their magnetomotive forces act in opposition, thus creating four poles. e. &dentify the four salient poles Bust beneath the damper windings. . &dentify the stator winding and note that it is identical to that of the three-phase squirrel cage and wound rotor motors. 8. 7iewing the front face of the module$ a.
The three separate stator windings are connected to terminals 1 and 8E 7 and , 3 and 6. :1 point;
b. >hat is the rate voltage of the stator windingsD :1 point; 175 7 A% c.
>hat is the rated current of the stator windingsD :1 point; 1 A A%
4. The rotor winding is connected through the 1<* F rheostat and a toggle switch 1 to terminals < and =. :1 point; e.
>hat is the rated voltage of the rotor windingD :1 point; 175 7 =% a)e 8
AC Machinery Laboratory EEC631L
.
Experiment No.9 The Synchronous Motor
>hat is the rated speed and output power of motorD :1 point; peed G 1=55 rpm
4AT&C3 G 1< >
Startin) Characteristics 0.
"sing your Three-4hase ynchronous otor3enerator, 4ower upply, A% Ammeter, connect the circuit shown in (igure ).1. Cote that the three stator windings are wye-connected to the fixed 2*/ 7 8-phase output of the po wer supply, terminals 1, 2, and 8.
>i)ure 9.1. <.
a. Turn on the power supply. Cote That the motor starts smoothly and continues to run as an ordinary induction motor. b. Cote the direction of rotation. :1 point; otation G Counter?c0oc2ise
&1 G 1.7 A A%
c. Turn off the power supply and interchange any two of the leads from the power supply. d. Turn on the power supply and note the direction of rotation. :1 point; otation G C0oc2ise
&1 G 1.7< A A%
e. Turn off the power supply. :.
"sing your lectrodynamometer and ynchroni;ing odule, connect the circuit shown in (igure ).2. %ouple the motor to the electrodynamometer with the connection belt.
@.
a. The synchroni;ing switch module will be used as an on-off switch for the three-phase power to the stator windings. et the switch in its off position. b. The electrodynamometer is connected to the fixed 12* 7 A% output of the power supply module terminals 1 and C. et the dynamometer control !nob for approximately 0*+ excitation.
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AC Machinery Laboratory EEC631L
Experiment No.9 The Synchronous Motor
>i)ure 9.7
/.
c.
The rotor of the synchronous motor is connected to the fixed 12* 7 =% output of the power supply module terminals / and C. et the field rheostat for ;ero resistance 5full counter cloc!wise9.
d.
%lose the switch 1 in the synchronous motor module.
a. Turn on the power supply. Then apply three-phase power by closing the synchroni;ing switch and observe what happens. ?bserve the average reading in the ammeter. '# N#T LEA&E T+E #@E! #N >#! L#N-E! T+AN 15 SEC#N'S Turn off the power supply. b.
=escribe what happened. :7 points; ANS@E!
The current in the ammeter remains at
>hat did the ammeter indicatesD :7 points; ANS@E!
The ammeter indicates
hould a synchronous motor, under load, be started with =% excitation on its fieldD No
a. %onnect the rotor of the synchronous motor to the variable *-12* 7 =% output of the power supply module terminals @ and C. elect @-C i n the meter selector switch on the power supply module. =o not disturb any of the other connections or change any control settings.
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AC Machinery Laboratory EEC631L
Experiment No.9 The Synchronous Motor
b.
>ith the variable output voltage control at ;ero, turn on the power supply. Apply three-phase power by closing the synchroni;ing switch and observe what happens.
c.
=escribe what happened to the rotation and the starting and running currents. :7 points; ANS@E!
otation of the motor $ %loc!wise tarting %urrent $ pi!es at 0A unning %urrent $ 1.< A d.
&s your motor operating as an induction motorD Bes
e.
%arefully adBust the power supply output to 12* 7 =% as indicated on the power supply meter.
f.
=escribed the changes in the values of the stator current. :7 points; ANS@E!
>hen the rotor voltage is set to 12* 7 and the exciter switch is ?C, the stator current reading decrease to *.8 A. ). &s your motor operating as a synchronous motorD Bes h. eturn the voltage to ;ero and turn off the power supply. ?pen switch 1 in the synchronous motor. 1*. a. %onnect the circuit shown in (igure ).8. Cote that the synchronous motor is wired in its normal starting configuration 5as a three-phase squirrel-cage induction motor9. b. et the dynamometer control !nob at its full cloc!wise position 5to provide a maximum starting load for the synchronous motor9 c. %lose the witch 1 in the synchronous motor.
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AC Machinery Laboratory EEC631L
Experiment No.9 The Synchronous Motor
>i)ure 9.3 11. a.
Turn on the power supply and quic!ly measure 1, 2, &1 and the developed starting torque. Turn off the power supply. :8 points; 1 G 75= 7 A%
2 G 779 7 A%
&1 G 8 A A%
tarting torque G 71 lbf-in b.
%alculate the apparent power to the motor at starting torque. :7 points; G 8 52*/9509 G 1,001.*@ 7A
c.
TAT&C3 G 1881.5< 7A
%alculate the full load torque corresponding to 1@< > at 1/** rpm. :7 points; 51@<@0:9 G 51.<) H51/**95T991**,***
T (" ?A= G =.17 lbf-in
T G /.12
d.
%alculate the ratio of starting torque to full load torque. :7 points; 21 /.12 G 2.:1/
T AT&? G 7.=6
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AC Machinery Laboratory EEC631L
e.
Experiment No.9 The Synchronous Motor
xplain why a large A% voltage 2 was induced in the rotor windings. :7 points; ANS@E!
The large A% voltage 2 was induced in the rotor windings because the rotor has squirrel cage induction windings which produce a magnetic field that is pushed and pulled by the stator magnetic field in order to start the motor. Additionally, there is a greater magnetic reaction between the stator and rotor fields which causes a stronger or higher torque.
12. a.
b.
>ith your circuit unchanged, turn on the power supply and slowly turn the dynamometer control !now counter cloc!wise to reduce the loading. The motor will come up to full speed and run as a squirrel cage induction motor. Cote the effect upon the induced voltage 2. >hy does 2 decrease as the motor speed increasesD :7 points; ANS@E!
2 decreases as the motor speed increases because the torque is decreasing as well as the current. &n this situation, there is a less load on the motor so, the squirrel cage induction motor requires less voltage to get it up the speed.
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AC Machinery Laboratory EEC631L
Experiment No.9 The Synchronous Motor
!E&"E@ *(EST"#NS 1.
>hat precaution should be ta!en during start-up period of a synchronous motorD :3 points; ANS@E!
The stator must be first before you switched ?C provided the rotor so that it would be rotate closing to the synchroni;e speed. 7.
&f the squirrel cage winding were removed from a synchronous motor, could it start by itselfD 5 1 point; No
3.
tate two reasons why the rotor winding of a synchronous motor usually connected to an external resistance during start-up. :3 points; ANS@E!
a. To reduce current so that 2 will decrease b. To increase the torque 8.
%ompare the starting characteristics of the synchronous motor with those of the three-phase squirrel cage induction motor. :3 points; ANS@E!
&n previous experiment which is squirrel cage induction motor, the amount of starting torque is 1).2 lbf-in and starting current is
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