When a d.c. motor or an
induction motor is loaded, the speed of the motors drops. This is
because the load torque demand increases then the torque produced by the
motor. Hence motor draws more current to produce more torque to satisfy
the load but its speed reduces. In case of synchronous motor speed
always remains constant equal to the synchronous speed, irrespective of
load condition. It is interesting to study how synchronous motor reacts
to changes in the load condition.
In a d.c. motor, armature develops an e.m.f.after motoring action
starts, which opposes supply voltage, called back e.m.f. Eb.
In case of synchronous motor also, once rotor starts rotating at
synchronous speed, the stationary stator (armature) conductors cut the
flux produced by rotor. The only difference is conductors are stationary
and flux is rotating. Due to this there is an induced e.m.f. in the
stator which according to Lenz’s law opposes the supply voltage. This
induced e.m.f. is called back e.m.f. in case of synchronous motor. It is
obtained as Ebph i.e. back e.m.f. per phase. This gets
generated as the principle of alternator and hence alternating in nature
and its magnitude can be calculated by the equation,
As speed is always synchronous, the frequency is constant and hence
magnitude of such back e.m.f. can be controlled by changing the flux ?
produced by the rotor.
So back e.m.f. in case of synchronous motor depends on the excitation
given to the field winding and not on the speed, as speed is always
stator construction is similar to the armature of a three phase
alternator, the impedance of the stator is called synchronous impedance
of synchronous motor consisting of Ra as the stator winding resistance and Xs as the synchronous reactance. All the values are generally expressed on per phase basis.
The difference is that this equation is vector equation as each
quantity is alternating and has different phase. So addition is to be
performed vectorially to obtain the result.
Under this condition, the magnetic locking between stator and rotor is
in such a way that the magnetic axes of both, coincide with each other
as shown in the Fig.1. As this is possible only under no losses
condition, is said to be ideal in case of synchronous motor.
|Fig. 1 Magnetic locking under no load condition|
|Fig. 2 Phasor diagram on no load losses|
In practice this is impossible. Motor has to supply mechanical losses
and iron losses alongwith small copper losses. Let us see how it can be
explained in case of synchronous motor.
Now due to the various losses practically present on no load, the
magnetic locking exists between stator and rotor but in such a way that
there exists a small angle difference between the axes of two magnetic
fields as shown in the Fig.3.
|Fig. 3 Magnetic locking under practical condition|
So the rotor axis falls back with respect to stator axis by angle ‘?’
as shown in the Fig.3 This angle decides the amount of current required
to produce the torque to supply various losses.
Hence this angle is called load angle, power angle, coupling angle,
torque angle or angle of retardation and denoted as ? as mentioned
magnetic locking still exists between the two and rotor rotates at
synchronous speed alongwith rotating magnetic field maintaining angle
difference between the axes of two fields, as shown in the Fig. 3(b).
The flux lines between the two get stretched due to such retardation of
rotor axis with respect to stator.
|Fig. 4(a) Phasor diagram for no load condition with losses|
be drawn to produce the torque which meets the various losses present
in the synchronous motor. Under no load condition, ? is very small and
hence ERph is also very small.
As the load on the synchronous motor increases, there is no change in
its speed. But what gets affected is the load angle ‘?’ i.e. the angle
by which rotor axis retards with respect to stator axis.
Hence as load increases, ? increases but speed remains synchronous.
by the motor increases as load increases. This current produces the
necessary torque which satisfied the increased load demand. The magnetic
locking still exists between the rotor and stator.
So from the above discussion it is clear that on no load, current drawn
by the motor is very small. This is because the stator and the rotor
magnetic axes are almost matching transformer each other i.e. load
angle ? is very small. As load increases, rotor magnetic axis starts
retarding with respect to stator axis i.e. load angle ? increases
maintaining the magnetic locking condition. And hence in case of the
synchronous motor load affects the angle ? without affecting the speed.
As ? increases, the magnitude of ERph increases which shows that motor draws more current from the supply. This satisfies the increased load torque demand.
So torque produced in the synchronous motor depends on the load angle
‘?’ for small values of and to be precise depends on ‘sin?’. The load
angle ‘?’ is measured in degrees electrical. As angle ? increases, the
magnetic flux lines producing the force of attraction between the two
get more and more stretched. This weakens the force maintaining the
magnetic locking, though torque produced by the motor increases. As ?
reaches upto 90o electrical i.e. half a pole pitch, the
stretched flux lines get broken and hence magnetic locking between the
stator and rotor no longer exists. The motor comes out of synchronism.
So torque produced at ? equal to 90o electrical is the
maximum torque, a synchronous motor can produce, maintaining magnetic
locking i.e. synchronism. Such s torque is called pull out torque. The
relationship between torque produced and load angle is shown in the Fig
|Fig. 5 Torque angle characteristic|