Every armature winding has its own resistance. The effective resistance of an armature winding per phase is denoted as Raph Ω/ph or Ra Ω/ph.
Generally the armature resistance is measured by applying the
known d.c. voltage and measuring the d.c. current through it. The ratio
of applied voltage and measured current is the armature resistance. But
due to the skin effect, the effective resistance under a.c. conditions
is more than the d.c. resistance. Generally the effective armature
resistance under a.c. conditions is taken 1.25 to 1.75 times the d.c.
resistance.
While measuring the armature resistance, it is necessary to
consider how the armature winding is connected whether in star or delta.
Consider a star connected armature winding as shown in the Fig. 1.
 |
| Fig. 1 Star connected alternator |
When the voltage is applied across any two terminals of an
armature winding, then the equivalent resistance is the series
combination of the two resistance of two different phase windings,
... RRY = Resistance between R-Y terminals
= Ra + Ra = 2Ra
where Ra = armature resistance per phase
... Ra = RRY/2 Ω/ph
Thus in star connected alternator, the armature resistance per
phase is half of the resistance observed across any two line terminals.
Consider the delta connected alternator as shown in the Fig. 2.
 |
| Fig. 2 Delta connected alternator |
When voltage is applied across any two terminals, then one phase
winding appears in parallel with series combination of other two.
Hence the equivalent resistance across the terminals is parallel combination of the resistance Ra and 2Ra.
... RRY Ra | | Ra Ω/ph
Thus in delta connected alternator, the armature resistance per
phase is to be calculated from the equivalent resistance observed across
any two line terminals.
