Why Alternator Armature Winding always connected in STAR?

click on the image to enlarge

Alternator armature winding is always connected in STAR, not in DELTA because of some important reasons. It is a very important question which frequently asked in Interviews that Why Alternator Armature Winding always connected in STAR, not in DELTA? Today we discuss all the reasons with an explanation.

Reasons for using Star connected Armature winding in Alternator :

We know that in a three-phase system, there are two types of connections, one is STAR connection and Another is DELTA connection. Both of them have their own advantages and disadvantages but in the case of alternator armature winding, we always prefer the STAR connection rather than the DELTA connection. Actually, if you have the basic knowledge of STAR connection and DELTA connection then you can easily understand it.

Let's know the Reasons,

Reason 1:

The reason that will come first in our mind that is the Star connection provides a Neutral Point. We know that in the Delta connection, there is no neutral point, but in the case of the Star connection, there is a neutral point present.

The neutral point helps to neutral grounding the system for healthy purposes of the system. Generally, we ground the neutral for two reasons that are,

1.      The neutral grounding system helps to balance the flow of current in each phase.

2.      When a fault occurs the excessive fault current goes to the ground through the neutral grounding system. 

So as the STAR connection provides a neutral point we prefer the STAR connection for the alternator armature winding.

A simple diagram of the connection between the Alternator and Transformer  in an Electrical Power generating station is given below,

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As you see in the above figure the neutral of the Alternator and Transformer are grounded. The Primary winding of the Transformer is STAR Connected and the Secondary winding of the Transformer is DELTA Connected. The output of the Transformer is connected to the Grid.

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Reason 2:

We know that in Star Connection,

                                                          or, 

and in Delta connection,

Suppose an alternator has an output rating of 11KV which indicates the Line Voltage. 

So if the armature winding is connected in STAR then the voltage across the winding will be,

but if the armature winding is connected in DELTA then the voltage across the winding will be,

click on the image to enlarge

So from the above explanation, we can say if we connect the armature winding in Star then the voltage stress across the winding will be less, therefore less insulation arrangement is required which reduces the cost.

This is the second important reason for the use of Star Connection.

Reason 3:

As the Star connection has a neutral point and we ground the neutral so there is very little harmonic, but in the case of the delta connection as it does not have a neutral point, the output voltage has triple Harmonics.

So in the Star connection, we get pure sinusoidal voltage but in the Delta connection, we do not get pure sinusoidal voltage.

Reason 4:

 For the same Line Voltage, the star-connected armature winding required less number of turns than the delta-connected winding. So the star-connected winding reduces the cost. As less number of turns are required so the weight also decreases.

Reason 5:

In the case of Star connection, when the alternator is not connected to any load the windings will open the circuit so no circulating current will flow through the windings.

But in the case of Delta connection, the windings of the alternator are always in the closed circuit even when the alternator is not connected to any load. So a circulating current always flows through the winding which causes heat.

These are the important reasons Why Alternator Armature Winding is always connected in STAR.

  

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