Synchronous Motor:Motor on Load with Constant Excitation and Power Flow within a Synchronous Motor.

Motor on Load with Constant Excitation

Before considering as to what goes on inside a synchronous motor, it is worthwhile to refer briefly to the d.c. motors. We have seen (Art. 29.3) that when a d.c. motor is running on a supply of, say, V volts then, on rotating, a back e.m.f. Eb is set up in its armature conductors. The resultant voltage across the armature is (V – Eb) and it causes an armature current Ia = (V – Eb)/ Ra to flow where Ra is armature circuit resistance. The value of Eb depends, among other factors, on the speed of the rotating armature. The mechanical power developed in armature depends on Eb Ia (Eb and Ia being in opposition to each other).

Similarly, in a synchronous machine, a back e.m.f. Eb is set up in the armature (stator) by the rotor flux which opposes the applied voltage V . This back e.m.f. depends on rotor excitation only (and not on speed, as in d.c. motors). The net voltage in armature (stator) is the vector difference (not arithmetical, as in d.c. motors) of V and Eb. Armature current is obtained by dividing this vector difference of voltages by armature impedance (not resistance as in d.c. machines).

Fig. 38.6 shows the condition when the motor (properly synchronized to the supply) is running on no-load and has no losses.* and is having field excitation which makes Eb = V.

It is seen that vector difference of Eb and V is zero and so is the armature current. Motor in- take is zero, as there is neither load nor losses to be met by it. In other words, the motor just floats.

If motor is on no-load, but it has losses, then the vector for Eb falls back (vectors are rotating anti-clockwise) by a certain small angle a (Fig. 38.7), so that a resultant voltage

If, now, the motor is loaded, then its rotor will further fall back in phase by a greater value of angle a – called the load angle or coupling angle (corresponding to the twist in the shaft of the pulleys). The resultant voltage ER is increased and motor draws an increased armature current (Fig. 38.8), though at a slightly decreased power factor.