Page 774 - Basic Electrical Engineering

Figure 10.9 Open-circuit characteristic: (a) field system is rotated by prime mover; (b) field current
versus EMF-induced characteristic

10.10 SYNCHRONOUS GENERATOR ON LOAD

When a synchronous generator is loaded, current will flow through its
winding as well as through the load. Since three-phase currents will flow

through the three-phase windings, these currents will develop a resultant
rotating magnetic field. This rotating field is due to the currents flowing

through the armature windings, i.e., the stator windings. This stator field will
rotate at synchronous speed, N . The field magnets are also rotating at speed,
s
N . Thus, these two fields, i.e., the rotor field and the field produced by the
s
stator which is also called the armature will rotate at the same speed, i.e.,

these two fields are stationary with respect to each other. The armature field
flux and the main field flux produced by the field windings will rotate at the

same speed, called the synchronous speed. The air-gap flux will be the
resultant of these two fluxes. The effect of the armature field flux on the main

field flux is called armature reaction. Depending on the power factor of the
load, the armature flux will oppose, aid, or distort the main field flux.
If the load is purely inductive, the armature flux will be opposing the main

field flux. If the load is purely capacitive, the armature flux will aid the main
field flux.

For resistive loads, the armature flux will distort the distribution of the
main field flux.