The synchronous motor is not a self-starting motor, so there are the
various methods of starting the synchronous motor.
Starting as an alternator: In this case the synchronous motor is started as an alternator by the external source. The e.m.f. is induced in the armature and is synchronised with the bus bar. After synchronising both the voltages the prime mover is taken off. The machine now will run as a synchronous motor. The motor can be started by means of diesel engine or d.c. motor etc.
Starting by means of d.c. source: Generally for this purpose the d.c. compound motor is used, when the d.c. voltage is available easily. The d.c. motor is coupled with the synchronous motor. The speed of d.c. motor is controlled and brought nearly equal to the synchronous speed. Now the stator winding is energised by the three phase supply, and the field switch is also on. There are now two magnetic fields. one of the stator winding rotating at synchronous speed, other because of the rotor field winding which rotating at synchronous speed by the external source. These fields are interlocked and the rotor will continue to run at synchronous speed. The motor prime mover either can be taken off or if the d.c. motor it can be utilized to excite the synchronous motor. Sometimes the synchronous motor can also be started by the exciter mounted on the same shaft. Then after synchronising it will again perform its main function of excitation.
By means of ponny motor. An induction motor called as ponny motor, is coupled with the shaft of the synchronous motor. This motor has less number of poles than the synchronous motor; so that the running speed of the combination can be raised above than the synchronous speed of the motor. The field winding and armature both are energised as a result of which the magnetic interlocking results and the motor continues to run at the synchronous speed. Then the ponny motor is taken off.
Self-starting method
So far it is discussed that extra means are adopted to start the synchronous motor, but this can be made self starting by providing the following means:
By means of damper windings. The damper winding consists of short circuited copper bars embedded in the face of field poles. Whenever three phase supply is given to the stator, a rotating magnetic field is produced in space. This field will link the dampers and will induce some e.m.f. in them. These dampers being short circuited causes a current to circulate through the damper winding. Now the motor will start as a squirrel cage induction motor. The no load speed of the motor is nearly the synchronous speed. The excitor is also rotating at the same speed, producing some voltage. Now when the speed reaches to the synchronous speed the field is excited. Now because both the rotor and stator are energised, so the magnetic inter-locking will result and the motor will be running at the synchronous speed. In this condition there is no linkage, so there will not be any e.m.f. in the damper winding.
Following are the advantages and disadvantages of this method.
Advantages
- No need of any starting means.
- The motor will continue to run even after losing the synchronism. If the speed falls down the damper winding will come into operation and the torque will develop to run the motor.
- It hunts the oscillations which are due to the loading conditions i.e. the damper are used to prevent the hunting effect.
Disadvantages
- The motor starts as the squirrel cage induction motor so the starting current is more.
- To avoid the danger of shock the field winding should be carefully handled, There are more number of turns in the field winding, so the voltage induced due to induction is much larger. It requires special arrangement for insulation and the field winding should be short circuited through a suitable resistance to limit the Current and distribute the induced voltage.
(2) By means of slip ring induction motor action or by external resistance method. The motor if started by using the damper winding procedure, it takes much current, to avoid this large current, the motor is started by using the slipring induction motor starting method as shown Fig. 20.3 and Fig. 20.4. The rotor resistance is introduced at the time of starting and thus current is reduced and limited with a good starting torque. The external resistance is then taken off step by step.
Fig. 20.3. Slipring induction motor action-I.
The no load speed is nearly equal to the synchronous speed. Now both operations, taking out the rotor external and switching on to d.c. source take place simultaneously. Thus the magnetic interlocking lesults and machine continues to run at the synchronous speed.
Fig. 20.4. Slipring induction motor action ll.