Electrical Engineering ⇒ Topic : Correct and incorrect delta connections of alternator

Correct and Incorrect A Connections of Alternator

When the three phase windings of an alternator are connected into a delta, they must be properly phased" i.e. finishing end of one phase winding should be connected to the starting end of the other phase winding and so on. Any wrong connection may develop high voltage in the windings. Let us illustrate this point.

(1) Correct A connections. We first consider the case when the phase windings of a Δ-connected alternator are connected correctly. Suppose the magnitude of each phase voltage is E_ph. Then,

Thus the phasor sum of three phase voltages is zero and there is no circulating current in the mesh. Therefore, line voltage is equal to the phase voltage.

(2) Incorrect A connections. If the phase windings of a A-connected alternator are not connected properly, there will be circulating current in the mesh. This can be easily proved. Suppose connections to one of the phase windings, say blue phase winding are reversed. Taking E_RY as reference phasor, we have,

Thus the resultant loop voltage has a magnitude twice the value of phase voltage. This high voltage may cause large current to flow through the windings even without external load and may damage the windings. Therefore, when the coils of an alternator or secondary of a transformer bank are connected into a delta connection, a check must be made with a voltmeter before closing the delta.