One reason is that a star-connected armature allows the alternator to operate at a lower voltage than a delta-connected armature. This is because the voltage between any two points on a star-connected circuit is equal to the phase voltage divided by the square root of three (√3), whereas the voltage between any two points on a delta-connected circuit is equal to the phase voltage. Since the phase voltage of an alternator is typically much higher than the line voltage, connecting the armature in a star configuration allows the alternator to operate at a lower voltage and reduces the risk of electrical accidents or equipment damage.
One more reason why the armature of an alternator might be connected in a star configuration rather than a delta configuration is that it can reduce the amount of insulation required for the armature winding. This is because in a star configuration, the voltage across the armature winding is lower than it would be in a delta configuration. The voltage in a star configuration is about 58% of the line voltage, while in a delta configuration it is the full line voltage. Therefore, less insulation is needed to protect the armature winding from overheating or damage.
Another reason is that a star-connected armature allows the alternator to provide a balanced three-phase output. In a three-phase system, it is important to maintain a balanced load on each phase in order to prevent voltage imbalances and improve power quality. A star-connected armature naturally provides a balanced output, whereas a delta-connected armature does not.
Another reason why the armature of an alternator might be connected in a star configuration is that it allows for the creation of a neutral point. In a star configuration, the neutral point is the point where the winding connections meet. This neutral point can be used to derive a single-phase supply, which can be useful in certain applications. The neutral point can also be used for ground fault protection of the alternator, which can help to prevent damage or malfunction.
Finally, a star-connected armature allows the alternator to be connected to the electrical grid more easily. In many cases, the electrical grid is also connected in a star configuration, so a star-connected alternator can be easily connected to the grid without the need for additional transformer or switching equipment.