**Subject :**Electrical Machines II (AC Machines)

**Unit :**Synchronous motor

## Effect of Changing Excitation on Constant Load

**Effect of Changing Excitation on Constant Load:**

As shown in Fig. (a), suppose a synchronous motor is operating with normal excitation (E_{b} = V) at unity p.f. with a given load. If R_{a} is negligible as compared to X_{S}, then I_{a} lags E_{R} by 90º and is in phase with V because p.f. is unity. The armature is drawing a power of V.I_{a} per phase which is enough to meet the mechanical load on the motor. Now, let us discuss the effect of decreasing or increasing the field excitation when the load applied to the motor remains constant.

(a**) Excitation Decreased**

As shown in Fig. (b), suppose due to decrease in excitation, back e.m.f. is reduced to Eb1 at the same load angle α_{1}. The resultant voltage E_{R1} causes a lagging armature current Ia1 to flow. Even though I_{a1} is larger than Ia in magnitude it is incapable of producing necessary power VIa for carrying the constant load because I_{a1} cos φ_{1} component is less than I_{a} so that VI_{a1} cos φ_{1} < VI_{a}. Hence, it becomes necessary for load angle to increase from α_{1} to α_{2}. It increases back e.m.f. from E_{b1} to E_{b2} which, in turn, increases resultant voltage from E_{R1} to E_{R2}. Consequently, armature current increases to I_{a2} whose in-phase component produces enough power (VI_{a2} cos φ_{2}) to meet the constant load on the motor.

(b**) Excitation Increased**

The effect of increasing field excitation is shown in Fig. (c) where increased E_{b1} is shown at the original load angle α_{1}. The resultant voltage E_{R1} causes a leading current I_{a1} whose in-phase component is larger than I_{a}. Hence, armature develops more power than the load on the motor. Accordingly, load angle decreases from α_{1} to α_{2} which decreases resultant voltage from E_{R1} to E_{R2}. Consequently, armature current decreases from I_{a1} to I_{a2} whose in-phase component I_{a2} cos φ_{2} = I_{a}. In that case, armature develops power sufficient to carry the constant load on the motor. Hence, we find that variations in the excitation of a synchronous motor running with a given load produce variations in its load angle only.