Fig: 2.24 shows the phasor diagrams for different field excitations at constant load. Fig: 2.24 (i) shows the phasor diagram for normal excitation (Eb = V), whereas Fig: 2.24 (ii) shows the phasor diagram for under-excitation. In both cases, the motor has lagging power factor. Fig: 2.24 (iii) shows the phasor diagram when field excitation is adjusted for unity p.f. operation. Under this condition, the resultant voltage Er and, therefore, the stator current Ia are minimum. When the motor is overexcited, it has leading power factor as shown in Fig: 2.24 (iv). The following points may be remembered:
(i) For a given load, the power factor is governed by the field excitation; a weak field produces the lagging armature current and a strong field produces a leading armature current.
(ii) The armature current (Ia) is minimum at unity p.f and increases as the p.f. becomes less either leading or lagging
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