Introduction And Working Of Synchronous Motor
1. Introduction
If a three phase supply is given to the stator of a three phase
alternator, it can work as a motor. As is is driven at synchronous
speed, it is called synchronous generator. So if alternator is run as a
motor. It will rotate at a synchronous speed. Such a device which
converts an electrical energy into a mechanical energy running at
synchronous speed is called synchronous motor. Synchronous motor works
only at synchronous speed and can not work at a speed other than the
synchronous speed. Its speed is constant irrespective of load, no doubt,
its speed changes for an instant at the time of loading.
The three phase synchronous motor works on the concept of rotating
magnetic field. The field produced by stationary three phase winding,
which rotates in space is called rotating magnetic field. Its speed is
always synchronous and given by,
Ns = 120f/P
The rotating magnetic field can be defined as the field or flux having
constant amplitude but whose axis rotates in a plane at a certain
speed.e.g. permanent magnet rotating in a space produces a rotating
magnetic field. Similarly if an arrangement is made to rotate the poles,
with constant excitation supplied, the resulting field is rotating
magnetic field. So a field produced in an air gap of a rotating field
type alternator is of rotating type. But this is all about production of
R.M.F. by physically rotating poles or magnet. In practice such a
rotating magnetic field can be produced by exciting a set of stationary
coils or wi9nding with the help of polyphase a.c. supply. The resultant
flux produced in such a case has constant magnitude and its axis rotates
in space without physically rotating the winding. Let us study how it
happens.
?1 = ?m sin(?t) = ?m sin ? ………..(1)
. The waveform of three fluxes are shown in the Fig.1(a) while the
assumed positive directions of these fluxes in space are shown in the
Fig.1(b). Assumed positive direction means whenever the instantaneous
value of flux is positive, in vector diagram it must be represented
along its assumed positive direction. And if flux has negative
instantaneous value then must be represented in opposite direction to
assumed positive direction, in the vector diagram.
Case i) ? = 0o
Substituting in equations (1), (2) and (3) we get,
?1 = ?m sin 0o = 0
?2 = ?m sin(-120o ) = -0.866 ?m
?3 = ?m sin (-240o) = + 0.866 ?m
Hence vector diagram looks like as shown in The Fig. 2(a).
BD is perpendicular drawn from B on ‘ ?T ‘.
 |
| Fig. 2 a and b |
... OD = DA = ?T/2
So magnitude of resultant flux is 1.5 times the maximum value of an individual flux.
Case ii) ? = 60o
Substituting in equations (1), (2) and (3) we get,
?1 = ?m sin 60o = +0.866 ?m
So ?1 is positive and ?2 is negative so vector diagram looks like as shown in the Fig. 2(b).
It can be seen that from the Fig. 2(b), that,
?T = 1.5 ?m
Case iii) ? = 120o
Substituting in equations (1), (2) and (3), we get,
?1 = ?m sin 120o = +0.866 ?m
 |
| Fig. 2 c and d |
Case iv) ? = 180o
Substituting in equations (1), (2) and (3), we get,
?1 = ?m sin (180o) = 0
So ?1 = 0, ?2 is positive and ?3 is negative. The vector diagram is as shown in the Fig. 2(d).
From the vector diagram, it can be proved that,
?T = 1.5 ?m
This is nothing but satisfying the definition of a rotating magnetic
field. Hence we can conclude that the three phase stationary winding
when connected to a three phase a.c. supply produces a rotating magnetic
field.
of the fluxes. The relation is exactly similar to what we have
discussed earlier in case of alternator. So resultant flux bears a fixed
relation between speed of rotation, supply frequency and number of
poles for which winding is wound. The relation is derived while studying
an alternator. So for a standard supply frequency of f Hz of a three
phase a.c. supply and ‘P’ poles of the three windings, the speed of the
rotating magnetic field is Ns r.p.m.
The direction of the rotating magnetic field is always from the axis of
the leading phase of the three phase winding towards the lagging phase
of the winding. In the example above the phase sequence is 1-2-3 i.e.
phase 1 leads 2 by 120o and phase 2 leads 3 by 120o.
So rotating magnetic field rotates from axis of 1 to axis of 2 and then
to axis of e i.e. in the clockwise direction as seen above. This
direction can be reversed by changing any two terminals of three phase
winding while connecting them to the three phase supply. So in practice
for a phase sequence of R-Y-B, the rotating magnetic field is rotating
in clockwise direction, then by changing any two terminals of the
winding it can be changed to anticlockwise, as shown in the Fig. 3(a)
and (b).