Subject : Electrical Machines II (AC Machines)
Unit : Synchronous motor
Effect of changes in load on armature current, power angle, and power factor of synchronous motor
Effect of changes in load on armature current, power angle, and power factor of synchronous motor:
Fig: 1 Phasor diagram showing effect of changes in shaft load on armature current, power angle and power factor of a synchronous motor
- The effects of changes in mechanical or shaft load on armature current, power angle, and power factor can be seen from the phasor diagram shown in Fig. 1; As already stated, the applied stator voltage, frequency, and field excitation are assumed, constant. The initial load conditions are represented by the thick lines. The effects of increasing the shaft load to twice its initial value are represented by the light lines indicating the new steady state conditions. These are drawn in accordance with
- When the shaft load is doubled both I_{a}cosΦ_{i} and E_{f} sinδ are doubled. While redrawing the phasor diagrams to show new steady-state conditions, the line of action of the new jI_{a}X_{s} phasor must be perpendicular to the new I_{a} phasor. Furthermore, as shown in Fig. 1, if the excitation is not changed, increasing the shaft load causes the locus of the Ef phasor to follow a circular arc, thereby increasing its phase angle with increasing shaft load. Note also that an increase in shaft load is also accompanied by a decrease in Φ_{i}; resulting in an increase in power factor.
- As additional load is placed on the machine, the rotor continues to increase its angle of lag relative to the rotating magnetic field, thereby increasing both the angle of lag of the counter EMF phasor and the magnitude of the stator current. It is interesting to note that during all this load variation, however, except for the duration of transient conditions whereby the rotor assumes a new position in relation to the rotating magnetic field, the average speed of the machine does not change.
- As the load is being increased, a final point is reached at which a further increase in δ fails to cause a corresponding increase in motor torque, and the rotor pulls out of synchronism. In fact as stated earlier, the rotor poles at this point, will fall behind the stator poles such that they now come under the influence of like poles and the force of attraction no longer exists. Thus, the point of maximum torque occurs at a power angle of approximately 90◦ for a cylindrical-rotor machine, as is indicated by Eqn.
- This maximum value of torque that causes a synchronous motor to pull out of synchronism is called the pull-out torque. In actual practice, the motor will never be operated at power angles close to 90◦ as armature current will be many times its rated value at this load.