What is tested in a turn ratio test for a transformer, and what indicates a correct result?

• A. Compares primary-to-secondary voltage ratio; a result matching nameplate turn ratio indicates correct ratio.
• B. Tests insulation resistance; high value indicates good ratio.
• C. Measures only the phase angle difference.
• D. Checks mechanical alignment.

Answer: (A)

The main idea behind the turn ratio test is to verify that the relationship between the primary and secondary voltages matches the number of turns in those windings. In an ideal transformer, the voltage ratio Vp/Vs equals the turns ratio Np/Ns. So, when you apply a known voltage to the primary and measure the voltage on the secondary, the ratio you observe should align with the nameplate turns ratio within a specified tolerance. If it does, you’ve confirmed the windings are proportional to their turns and properly connected.

This test doesn’t assess insulation resistance, phase angle, or mechanical alignment. Insulation tests check how well the winding insulation holds up, phase-angle measurements relate to the timing between windings rather than their voltage magnitude ratio, and mechanical checks look at physical alignment or mounting. The turn ratio test focuses purely on the electrical ratio to ensure the windings’ turn counts are correct.