Field Excitation:

For a constant load, the power factor of a synchronous motor could be varied from a leading value to a lagging value through adjusting the DC field excitation show in the figure. Field excitation could be adjusted so in which PF = 1 show in the next figure.  Within a constant load on the motor, while the field excitation is increased, the counter EMF (V_G) increases.  The output is a change in phase among stator current (I) and terminal voltage (V_t), so in which the motor operates at a leading power factor.   V_P in Figure is the voltage drop in the stator winding's due to the impedance of the windings and is 90^o out of phase along with the stator current.  If we decrease field excitation then the motor will operate at a lagging power factor.  Notice that torque angle α, also differ as field excitation is adjusted to modify power factor.

Figure: Synchronous Motor Field Excitation

Synchronous motors are used to accommodate huge loads and to improve the power factor of transformers in huge industrial complexes.