Motor on Load with Constant Excitation
Motor on Load with Constant Excitation of synchronous motor:
- 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 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).
Fig (a) Fig (b) Fig (c)
- Fig. (a) 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 intake 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 α (Fig. (b)), so that a resultant voltage ER and hence current Ia is brought into existence, which supplies losses.
- If, now, the motor is loaded, then its rotor will further fall back in phase by a greater value of angle α − 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. (c)), though at a slightly decreased power factor.