Secondary voltage may deviate due to incorrect turns ratio, excessive winding resistance, leakage flux, or core saturation. Manufacturing defects like uneven winding tension or poor insulation can exacerbate this. For audio and lighting OEMs, unstable secondary voltage causes flicker, hum, or dimming issues. Ensuring accurate turns ratio, uniform winding, and proper core material reduces these deviations and maintains output stability.

    A secondary output voltage that deviates significantly from the calculated or specified no-load value points to errors in the turns ratio. The handbook identifies the following root causes:

  1. Incorrect Number of Turns (The Most Direct Cause):

    • Primary Winding: Having more turns than designed will reduce the primary voltage per turn, leading to a lower secondary output voltage, and vice versa.

    • Secondary Winding: Having fewer turns than designed will directly produce a lower output voltage, and vice versa. Counting errors or winding machine faults are common culprits.

  2. Incorrect Core Cross-Sectional Area ($A_e$): Using a core with an effective magnetic area different from the design assumption directly impacts the fundamental voltage equation: $V = 4.44 f N B_m A_e$.

    • A smaller $A_e$ than specified forces operation at a higher flux density ($B_m$) to maintain voltage, which may approach saturation. However, if the primary turns are correct for the actual smaller core, the secondary voltage will still be wrong.

  3. Operation at Incorrect Frequency ($f$): The transformer voltage is directly proportional to frequency. Testing a 60Hz-designed transformer at 50Hz will yield a lower output voltage (by about 16.7%) under the same input voltage, as the flux density increases beyond design intent.

  4. Excessive Primary Winding Resistance or Connection Impedance: While this has a minor effect on true no-load voltage (since no-load current is very small), if the voltage measurement point is far from the transformer primary terminals and the connecting wires have high resistance/impedance, the actual voltage at the primary winding will be lower than the source voltage, causing a lower secondary output.

  5. Measurement Errors:

    • Using a low-accuracy or non-true-RMS meter for non-sinusoidal voltages.

    • Incorrect meter calibration.

Troubleshooting Flowchart Suggested by the Handbook:

  1. Verify Input: Confirm rated primary voltage and frequency are applied.

  2. Measure Turns Ratio: Use a simple turns ratio test with a low voltage AC source. This isolates the winding issue from core effects.

  3. If Ratio is Correct but Voltage is Wrong Under Rated Voltage: Suspect core saturation due to wrong core $A_e$ or material property (low permeability). Check no-load current for excessive values.

  4. If Ratio is Incorrect: The problem is definitively in the winding turns. Physically verify or re-count turns.