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### Why a Synchronous Motor does not have Starting Torque?

Theory of Operation:

The operation of the dc separately excited synchronous motor can be explained in terms of the air-gap magnetic-field model, the circuit model, or the phasor diagram model of Fig. 5.1. In the magnetic-field model of Fig. 5.1a, the stator windings are assumed to be connected to a polyphase source, so that the winding currents produce a rotating wave of current density Ja and radial armature reaction field Ba. The rotor carrying the main field poles is rotating in synchronism with these waves. The excited field poles produce a rotating wave of field Bd. The net magnetic field Bt is the spatial sum of Ba and Bd; it induces an air-gap voltage Vag in the stator windings, nearly equal to the source voltage Vt. The current-density
Distribution Ja is shown for the current Ia in phase with the voltage Vt , and (in this case) pf=1. The electromagnetic torque acting between the rotor and the stator is produced by the interaction of the main field Bd and the stator current density Ja, as a J×B force on each unit volume of stator conductor. The force on the conductors is to the left (_ф) the reaction force on the rotor is to the right (+ф) and in the direction of rotation

The operation of the synchronous motor can be represented by the circuit model of Fig. 5.1b. The motor is characterized by its synchronous reactance xd and the excitation voltage Ed behind xd. The model neglects saliency (poles), saturation, and armature resistance, and is suitable for first-order analysis, but not for calculation of specific operating points, losses, field current, and starting.

The phasor diagram of 5.1c is drawn for the field model and circuit model previously described. The phasor diagram neglects saliency and armature resistance. The phasors correspond to the waves in the field model. The terminal voltage Vt is generated by the field Bt ; the excitation voltage Ed is generated by the main field Bd; the voltage drop jIaxd is generated by the armature reaction field Ba; and the current Ia is the aggregate of the current-density wave Ja. The power angle d is that between Vt and Ed, or between Bi and Bd. The excitation voltage Ed, in pu, is equal to the field current Ifd , in pu, on the air-gap line of the noload (open-circuit) saturation curve of the machine.