Environmental Engineering Reference
In-Depth Information
Table 4.1
Relationship between pole pairs and synchronous speed
in rev/min
p
Rev/min (50 Hz)
Rev/min (60 Hz)
1
3000
3600
2
1500
1800
3
1000
1200
4
750
900
200
15
18
Figure 4.6
Rotor confi gurations for a one- and two-pole pair wound stators
Table 4.1 shows the relationship between the synchronous speed in rev/min and the number
of pole pairs
p
for 50 and 60 Hz systems.
4.2.3 Synchronous Generator Operation when Grid - Connected
Having established the presence of a rotating magnetic fi eld within the cylindrical interior of
the stator, the rotor is now ready to be incorporated. Figure 4.6 shows the salient rotor con-
fi gurations for a one- and two-pole pair wound stators corresponding to the fl ux patterns in
Figure 4.5 .
A one-pole pair rotor will be considered. The rotor fi eld current is switched on so that a
north-south pole pair is created. With the rotor stationary, the interaction between the rotating
magnetic fi eld (RMF) and the rotor fi eld is to say the least unproductive. As the RMF sweeps
by at 50 rev/s or 20 ms per revolution, the only effect experienced by the rotor body is a pul-
sating torque as the RMF pole pair approaches and then overtakes the rotor pole pair. Due
to the rotor inertia it is just not possible for the rotor to accelerate and lock on to the RMF
within the required milliseconds.
Consider now that some external torque is applied to drive the rotor at increasing speed
until the synchronous speed is very nearly reached. If the speed difference is close enough
and decreasing, at some point the rotor N-S pair will lock on to the RMF S-N pair and the
two magnet systems will rotate in synchronism. With the external torque now removed and
