E9 the Synchronous Motor (1)

AC Machinery Laboratory EEC631L

Experiment No.9 The Synchronous Motor

EXE!"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 motorgenerator module contains a squirrel cage type rotor.

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AC Machinery Laboratory EEC631L

Experiment No.9 The Synchronous Motor

AA!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 otor3enerator 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 52ires 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 otor3enerator 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  #! 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 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.

TAT&C3 G 1881.5 at 1/** rpm. :7 points; 51@