Synchronous Motor:Equivalent Circuit of a Synchronous Motor

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Equivalent Circuit of a Synchronous Motor

Fig. 38.9 (a) shows the equivalent circuit model for one armature phase of a cylindrical rotor synchronous motor.

It is seen from Fig. 38.9 (b) that the phase applied voltage V is the vector sum of reversed back e.m.f. i.e., -Eb and the impedance drop Ia ZS. In other words, V = (-Eb + Ia ZS). The angle a* between the phasor for V and Eb is called the load angle or power angle of the synchronous motor.

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Power Developed by a Synchronous Motor

Except for very small machines, the armature resistance of a synchronous motor is negligible as compared to its synchronous reactance. Hence, the equivalent circuit for the motor becomes as shown in Fig. 38.10 (a). From the phasor diagram of Fig. 38.10 (b), it is seen that

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