the applications where high starting torque and constant speed are
desired then synchronous induction motors can be used. It has the
advantages of both synchronous and induction motors. The synchronous
motor gives constant speed whereas induction motors can be started
against full load torque.
the motor through slip rings. One phase carries full d.c. current while
the other two carries half of the full d.c. current as they are in
parallel. Due to this d.c. excitation, permanent poles (N and S) are
formed on the rotor.
of starting resistances. When the resistance is cut out the motor runs
with a slip. Now the connections are changed and the exciter is
connected in series with the rotor windings which will remain in the
torque (upto twice full load value) can be developed. When d.c.
excitation is provided it is pulled into synchronism and starts running
at constant speed. The synchronous induction motor provides constant
speed, large starting torque, low starting current and power factor
the d.c. excitation is provided on the stator. This simplifies control
gear. It also gives better facilities for insulation which permits
higher voltages and lower d.c. excitations.
m.m.f. with moderate d.c. excitation power. The excitation loss must be
distributed evenly over the winding. The mmf distribution should be
nearly sinusoidal. It should also provide damping against hunting and it
should be satisfactorily started as induction motor.
induced alternating currents in the rotor and it runs below synchronous
speed. When the rotor carries d.c. currents in the rotor and it runs
below synchronous speed. When the rotor carries d.c. currents the rotor
field and hence the rotor must run at synchronous speed. This means that
slip must be reduced to zero. But if there is any departure from this
speed during normal operation then again induced currents are there in
the rotor. The rotor is of low resistance so its windings act as damping
winding. Hence no separate damping windings are required.
where ? is the angle between stator and rotor field. In addition to
this induction motor torque is also present which is proportional to the
slip (d?/dt), so long as slip is small. There may also be constant load
torque if it is started on load and finally it requires torque J(d2?/d2t) to accelerate the rotor.
acts in opposite direction to that of load torque which tends to reduce
the angular velocity d?/dt of the slip motion. when ?<?<2? then
synchronizing torque acts in conjuction with load torque to increase the
slip i.e. nothing but angular velocity d?/dt and the motor fails to
mechanical strains. Also there may be oscillations in current and power
factor. Hence it is desired that the motor should synchronize as quickly
as possible after switching d.c. excitation. It requires that
synchronizing torque should be sufficiently larger than load torque and
it should be opposite of load torque. The angle obtained at the instant
of switching d.c. excitation also affects pulling into step. Following
figures shows oscillograms of rotor current on application of excitation
for various values of ?. When the excitation is delayed beyond 60o
it is seen that the rotor fails to synchronize as the induction motor
torque and the synchronizing torque work in conjuction and the torque
will have pulsating value.
is applied at a position that the rotor will occupy when both stator and
rotor fields are synchronized.
induction motor, three different types of torques are to be considered.
These are viz the starting torque which indicates capacity of motor to
start against load, pull in torque which indicates the ability of the
motor to maintain operation during change over from induction motor to
synchronous motor, pull out torque which represents the running of motor
synchronously at peak load. The first two torques are closely related
with each other and are the characteristics of the machine running as
induction motor. The pull out torque is characteristics when it is
running running synchronously. The characteristics curves for
synchronous induction motor operating at full load unity p.f. and at 0.8
p.f. leading is shown in the Fig. 3.
machine looses synchronism and runs as an induction motor with
fluctuation in torque and slip due to d.c. excitation. With reduction in
load torque the motor is automatically resynchronized.
Following are the advantages of synchronous induction motor over salient pole synchronous motor.
more than full load torque which is not possible with salient pole
synchronous motor which must be started against light load.
capacity as the gap is not long as compared to normal salient pole
providing excitation and required damping. So no separate damper winding
iv) No separate starting and control equipments are required.
ii) The variation of power factor is large as compared to normal synchronous motor.
iii) The speed variation is not possible for synchronous induction motor as it runs at constant motor.
phase advancing properties of synchronous motors are to be used then use
of synchronous induction motor is better option. Also the applications
where in load torque is remaining nearly constant, this motor can be