Effect of Increased Load with Constant Excitation
Effect of Increased Load with Constant Excitation:
Fig 1: Normal Excitation
Fig: 2 Phasor Diagram normal excitation & under excitation
With normal excitation, Eb = V, with under excitation, Eb < V and with over-excitation, Eb > V. Whatever the value of excitation, it would be kept constant during our discussion. It would also be assumed that Ra is negligible as compared to XS so that phase angle between ER and Ia i.e., θ = 900.
(i) Normal Excitation
Fig. 1. (a) shows the condition when motor is running with light load so that (i) torque angle α1 is small (ii) so ER1 is small (iii) hence Ia1 is small and (iv) φ1 is small so that cos φ1 is large.
Now, suppose that load on the motor is increased as shown in Fig. 1 (b). For meeting this extra load, motor must develop more torque by drawing more armature current. Unlike a d.c. motor, a synchronous motor cannot increase its Ia by decreasing its speed and hence Eb because both are constant in its case. What actually happens is as under :
1. Rotor falls back in phase i.e., load angle increases to α2 as shown in Fig. 1 (b),
2. The resultant voltage in armature is increased considerably to new value ER2,
3. as a result, Ia1 increases to Ia2, thereby increasing the torque developed by the motor,
4. φ1 increases to φ2, so that power factor decreases from cos φ1 to the new value cos φ2. Since increase in Ia is much greater than the slight decrease in power factor, the torque developed by the motor is increased (on the whole) to a new value sufficient to meet the extra load put on the motor. It will be seen that essentially it is by increasing its Ia that the motor is able to carry the extra load put on it. A phase summary of the effect of increased load on a synchronous motor at normal excitation is shown in Fig. 2 (a) It is seen that there is a comparatively much greater increase in Ia than in φ.