Discussion
The d.c. armature resistance of a Y-connected alternator measured across its two terminals is 0.5 Ω. The per phase resistance is _______.
Answer: C
The per phase resistance is 0.25 Ω.
In a star (Y) connection, a measurement between two line terminals passes through two phase windings in series, so the measured value is twice the phase resistance.
\[ R_{ph} = \frac{0.5}{2} = 0.25\ \Omega \]
0.5 Ω forgets that two windings are in the path, and 1 Ω doubles instead of halving. In practice this dc value is then raised by a factor of about 1.2 to 1.6 to allow for skin effect when the ac resistance is wanted.
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