Alternator or generator armature winding is always connected in STAR not in DELTA. We know that rotating magnetic filed in the air gap can only be created if the phase winding and phase currents are 120º displaced from each other in the space and time respectively. This can be achieved either in star connection or delta connection armature winding in the alternator. But star connection is always preferred over the delta connection due to its advantages.
Advantage of STAR Connection of Generator Armature Winding:
Generator Armature Winding is connected in STAR to achieve technical and economical advantages. Following are some of the advantages:
1) STAR connection provides a neutral point. This neutral point is very important from the stability point of view of generator. Generally, the neutral point is grounded through NGR (Neutral Grounding Resistor). Neutral grounding provides a path for the flow of circulating current during the unbalanced loading condition of generator. In addition to this, it also provides a path for the flow of zero sequence current during the single line to ground fault / double line to ground fault. Furthermore, grounding of neutral point maintains the voltage of healthy phase to the normal phase voltage.
If there were no neutral, the voltage of the healthy phases would have been increased during the ground fault, which may eventually lead to insulation failure of the healthy phases. Thus a single line to ground fault may result in three phase fault due to insulation failure.
Also, the grounding of neutral through NGR limits the value of fault current and hence damage to the stator armature winding.
2) The insulation requirement in STAR connection is less. Since phase voltage is 1/√3 times i.e. 57.7% (100 / √3 = 57.7%) of the line voltage, therefore the insulation requirement of phase winding reduces. This is a great advantage from economic point of view.
Since the generated emf in phase winding,
Ef = √2πfNØ
where N = Number of turns
3) In STAR connection Phase voltage = Line Voltage / √3, whereas phase voltage = line voltage in DELTA connection. Therefore for generating the same line voltage, the requirement of number of turns in STAR connection of armature winding will be less as compared to DELTA connection.
4) STAR connection eliminates the triplen harmonics in the generated terminal voltage of generator armature winding. Read “How Harmonics Eliminated from Alternator Generated Voltage?” to know the reason.