No load and load Operation, Effect of Excitation, V and Inverted V curves,Hunting
No load and loaded operation:
The engagement between stator and rotor pole is not absolutely rigid one as the load on the motor increases.At no load, if there is no power loss in the motor, the stator poles, and rotor poles will be along the same axis and phase difference between the applied voltage V and the back emf Eb developed in the armature winding will be exactly 180°. But this is not possible in practice because some power loss takes place due to iron loss and friction loss. Hence, the rotor pole lags by some angle with the stator pole.
When the load on the synchronous motor increases, rotor poles lags the stator poles by larger angle and
the phase angle between V and Eb will increases.
Effect of excitation and power factor control, V, and inverted V curves:
DC current supplied to the rotor field winding is known as excitation in a synchronous motor. As the speed of the synchronous motor is constant, the magnitude of back emf remains constant provided the flux per pole is constant. The magnitude of back emf can be changed by field excitation. If the excitation is changed at a constant load, the magnitude of armature current and power factor will change. By changing the excitation, the motor can be operated at both lagging and leading power factor.
If the excitation is more than 100%, then the motor is said to be over excited and if the excitation is less than 100%, then the motor is said to be under excited.
Consider a synchronous motor operating with a constant load. when Eb = V (in magnitude). The armature current Ia lags behind V by a small angle ø. θ is the phase angle between Ia and ER, whose magnitude is given by θ = tan1 (XS/Ra). Since XS and Ra are constant, angle also remains constant.
If the motor is under excited, the magnitude of Eb will be less than V. Therefore, the resultant of Eb & V i.e. ER will shift upward by some angle, then the direction of Ia will also shift by the same angle so that angle again remains constant as shown in phasor diagram.Here, the magnitude of Ia has increased and Ia lags V by greater angle so that power factor is decreased, but the active component Ia cosø remains same, so the output power also remains constant.
In the case of overexcited motor i.e. when Eb>V, therefore, the resultant voltage vector ER is pulled in the anticlockwise and Ia is also shifted in anticlockwise direction. It is seen that now the motor is drawing a lagging current.
The magnitude of armature current varies with excitation. The current has large values at both low and high values of excitation. In between, it has a minimum value corresponding to a certain excitation for which power factor is unity. The variation of Ia with excitation are shown in graphs.which are known as V curves. For the same input armature, current varies between a wide range and power factor also vary accordingly with excitation. When over excited, motor runs with leading power factor and the motor runs with lagging power factor when under excited. The variations of power factor with excitation are known as inverted V curve. It should be noted that minimum armature current corresponds to unity power factor.
In Conclusion ,we can say that An overexcited synchronous motor operates at leading power factor, underexcited synchronous motor operate at lagging power factor and normal excited synchronous motor operate at unity power factor.
Hunting:
On increasing the shaft load gradually load angle will increase. Consider that load P_{1} is applied suddenly to unloaded machine shaft so the machine will slow down momentarily. Also, load angle (δ) increases from zero degrees and becomes δ_{1}. During the first swing, electrical power developed is equal to mechanical load P_{1}. Equilibrium is not established so rotor swings further. Load angle exceeds δ_{1} and becomes δ_{2}. Now electrical power generated is greater than the previous one. Rotor attains synchronous speed. But it does not stay in synchronous speed and it will continue to increase beyond synchronous speed. As a result of rotor acceleration above synchronous speed the load angle decreases. So once again no equilibrium is attained. Thus, rotor swings or oscillates about a new equilibrium position. This phenomenon is known as hunting or phase swinging.
Hunting is suppressed by providing damper winding also known as squirrel cage winding or damper grids.The damper winding consists of shortcircuited copper/bronze bars embedded in faces of field poles of synchronous motors.
 V curve is the graph of rotor current versus stator current & inverted v curve is a graph of power factor versus rotor current.

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