The A.C. armature winding of an alternator is
A) Delta Star connected Generally
B) Delta Connected
C) Star Delta Connected
D) Always Star connected
Correct answer is D) Always Star connected
Explanation
Alternators are connected primarily in the star to achieve the following motives:
More economical:- The phase voltages in star connection are 57.7 % of the line voltages, i.e. the armature winding in star connection is less exposed to voltage as compared to the delta connection which in turn prove more economic if we consider insulation, breakdown strength, the requirement of conductor material
Availability of neutral:- It is very important for an alternator to have a neutral point. This neutral point is to be grounded through a resistor, for stability purposes. The neutral allows a path for circulating currents under unbalanced loaded conditions, and also during faults. If there was no path for the flow of fault current, and a line to ground fault occurs in one of the three phases, there would be a rise in voltages in the other two healthy phases and eventually, it would cause insulation failure in the other two phases and the line to ground fault would lead to a 3 phase fault. A neutral point would avoid all this and limit the fault condition to one phase only. Insulation would be protected, and the lines can be operational after fault isolation. For all this to happen, the star connection in the stator is absolutely necessary.
Zero sequences current Path:- In star connection, if the neutral is grounded then it also provides a path for the Zero-Sequence currents during faults, whereas in the delta connection the zero sequence currents flow within the delta circuit and hence increasing the load on the winding.
Easy protection:- Neutral grounding is necessary to allow zero sequence currents to flow to the ground in case of a fault.
Elimination of harmonics:- Star connection facilitates a neutral connection which is instrumental in eliminating triple harmonics.
No circulating currents:- In star connection, we don’t have circulating parasitic currents like in Delta which lead to heating losses.