Effect of changes in field excitation on synchronous motor performance
Effect of changes in field excitation on synchronous motor performance:
Fig: Phasor diagram showing effect of changes in field excitation on armature current, power angle and power factor of a synchronous motor
Intuitively we can expect that increasing the strength of the magnets will increase the magnetic attraction, and thereby cause the rotor magnets to have a closer alignment with the corresponding opposite poles of the rotating magnetic poles of the stator. This will obviously result in a smaller power angle. This fact can also be seen in equation
When the shaft load is assumed to be constant, the steady-state value of Ef sinδ must also be constant. An increase in Ef will cause a transient increase in Ef sinδ, and the rotor will accelerate. As the rotor changes its angular position, δ decreases until Ef sinδ has the same steady-state value as before, at which time the rotor is again operating at synchronous speed, as it should run only at the synchronous speed.
This change in angular position of the rotor magnets relative to the poles of rotating magnetic field of the stator occurs in a fraction of a second. The effect of changes in field excitation on armature current, power angle, and power factor of a synchronous motor operating with a constant shaft load, from a constant voltage, constant frequency supply, is illustrated in Fig.
we have for a constant shaft load,
This is shown in Fig., where the locus of the tip of the Ef phasor is a straight line parallel to the VT phasor. Similarly, for a constant shaft load,
This is also shown in Fig., where the locus of the tip of the Ia phasor is a line perpendicular to the VT phasor. Note that increasing the excitation from Ef1 to Ef3 in Fig. caused the phase angle of the current phasor with respect to the terminal voltage VT (and hence the power factor) to go from lagging to leading. The value of field excitation that results in unity power factor is called normal excitation.
Excitation greater than normal is called over excitation, and excitation less than normal is called under excitation. As indicated in Fig., when operating in the overexcited mode, |Ef | > |VT |. In fact a synchronous motor operating under over excitation condition is sometimes called a synchronous condenser.