As seen earlier, synchronous
motor is not self starting. It is necessary to rotate the rotor at a
speed very near to synchronous speed. This is possible by various method
in practice. The various methods to start the synchronous motor are,
In this method, the rotor is brought to the synchronous speed with the
help of some external device like small induction motor. Such an
external device is called ‘pony motor’.
Once the rotor attains the synchronous speed, the d.c. excitation to
the rotor is switched on. Once the synchronism is established pony motor
is decoupled. The motor then continues to rotate as synchronous motor.
In a synchronous motor, in addition to the normal field winding, the
additional winding consisting of copper bars placed in the slots in the
pole faces. The bars are short circuited with the help of end rings.
Such an additional winding on the rotor is called damper winding. This
winding as short circuited, acts as a squirrel cage rotor winding of an
induction motor. The schematic representation of such damper winding is
shown in the Fig.1.
|Fig . 1 Starting as a squirrel cage I.M.|
Once the rotor is excited by a three phase supply, the motors starts
rotating as an induction motor at sub synchronous speed. Then d.c.
supply is given to the field winding. At a particular instant motor gets
pulled into synchronism and starts rotating at a synchronous speed. As
rotor rotates at synchronous speed, the relative motion between damper
winding and the rotating magnetic field is zero. Hence when motor is
running as synchronous motor, there can not be any induced e.m.f. in the
damper winding. So damper winding is active only at start, to run the
motor as an induction motor at start. Afterwards it is out of the
circuit. As damper winding is short circuited and motor gets started as
induction motor, it draws high current at start so induction motor
starters like star-delta, autotransformer etc. used to start the
synchronous motor as an induction motor.
The above method of starting synchronous motor as a squirrel cage
induction motor does not provide high starting torque. So to achieve
this, instead of shorting the damper winding, it is designed to a form a
three phase star or delta connected winding. The three ends of this
winding are brought out through slip rings. An external rheostat then
can be introduced in series with the rotor circuit. So when stator is
excited, the motor starts as a slip ring induction motor and due to
resistance added in the rotor provides high starting torque. The
resistance is then gradually cut off, as motor gathers speed. When motor
attains speed near synchronous. d.c. excitation is provided to the
rotor, then motors gets pulled into synchronism ans starts rotating at
synchronous speed. The damper winding is shorted by shorting the slip
rings. The initial resistance added in the rotor not only provides high
starting torque but also limits high inrush of starting current. Hence
it acts as a motor resistance starter.
|Fig. 2 Starting as a slip ring I.M.|
It can be observed from the Fig. 1(b) that the same three phase rotor
winding acts as a normal rotor winding by shorting two of the phases.
From the positive terminal, current ‘I’ flows in one of the phases,
which divides into two other phases at start point as 1/2 through each,
when switch is thrown on d.c. supply side.
Many a times, a large synchronous motor are provided with a coupled
d.c. machine. This machine is used as a d.c. motor to rotate the
synchronous motor at a synchronous speed. Then the excitation to the
rotor is provided. Once motor starts running as a synchronous motor, the
same d.c. machine acts as a d.c. generator called exciter. The field of
the synchronous motor is then excited by this exciter itself.