SYNCHRONOUS MACHINES:NO-LOAD AND SHORT-CIRCUIT VALUES

NO-LOAD AND SHORT-CIRCUIT VALUES

The no-load or open-circuit voltage is generated in the armature windings of a synchronous machine when the armature terminals are open-circuited and the rotor is rotating at synchronous speed while the field winding is energized. The induced open-circuit voltages in the three phases are given by

Figure 11.5 illustrates a typical open-circuit saturation curve of a synchronous machine. It represents the no-load characteristics of the machine. The air gap line represents the extension of the straight-line portion of the saturation curve. Figure 11.5 also illustrates a typical short-circuit saturation characteristic of a synchronous machine. It is obtained by shorting the terminals of the armature together while the rotor is rotating at synchronous speed and the field current is increased from zero to a small value. The short-circuit saturation curve is a plot of the variations of the current in the shorted armature windings ver- sus the field current.

The zero power factor saturation characteristic is obtained by overexciting the machine while it is connected to a highly inductive load or an idle running synchronous motor. The terminal voltage of the machine being tested is varied while its armature current is held con- stant at the rated value by adjusting the excitation of the load and the machine being tested. This characteristic is important for the analysis of a synchronous motor.