What causes a transformer to be noisier under no-load conditions?

When a transformer is operating under no-load conditions, it often produces louder noise than under full load. The primary reason is that, with no load on the secondary winding, the primary voltage tends to be slightly higher than nominal. For example, while the rated voltage is typically 10 kV, the actual no-load voltage may reach around 10.5 kV.

This elevated voltage increases the magnetic flux density (B) in the core. According to the formula:

B = 45 × Et / S
(where Et is the designed volts-per-turn, and S is the core cross-sectional area), with a fixed number of turns, a higher no-load voltage raises Et, thereby increasing B beyond its normal design value.

Higher core flux density intensifies magnetostriction and magnetic hysteresis vibrations, which directly results in louder audible noise during no-load operation. This is the main cause of increased sound.

A secondary effect is the rise in no-load current. While increased no-load current itself doesn’t primarily cause the louder noise, it reflects underlying issues such as core material quality and manufacturing precision. High-quality silicon steel sheets exhibit lower specific core loss, leading to smaller no-load currents. Conversely, using more core material or lower-grade steel (with higher core loss and lower saturation flux density) increases no-load current and can also contribute—secondarily—to higher noise levels due to easier saturation.

Other factors influencing overall transformer noise include vibration damping measures, core clamping tightness, and whether the core design induces mechanical resonance. However, these affect the transformer’s general acoustic performance—not specifically the no-load vs. full-load noise difference.

Note: If the transformer emits an unusually harsh or unpleasant sound under no-load conditions, it likely indicates core saturation. In such cases, check whether the voltages of the two 12 V secondary windings are equal. If they are unbalanced, the windings should be removed and rewound to ensure identical turn counts.

Additionally, when measuring the current through resistor Rs, if the waveform shows peak overshoot instead of a smooth sawtooth rise, it suggests that the 12 V winding needs a few additional turns.

If rewinding the transformer is impractical, an alternative is to slightly reduce the resistance of R_L to raise the oscillation frequency to around 5 kHz (note: likely a typo in original—should be kHz, not Hz). This adjustment has minimal impact on most loads but is unsuitable for frequency-sensitive devices (e.g., certain analog clocks).

To simplify the circuit and reduce cost, this power supply design omits a voltage regulator; thus, the output voltage decreases as the battery voltage drops.

Measured performance of the prototype:

• Maximum efficiency: 94%

• Output voltage: slightly lower than the target 230 VAC, but aligns well with China’s standard nominal output of 220 VAC.

To achieve a true 230 VAC output from a 13 VDC input, either:

• Increase the turns ratio (secondary-to-primary) of the transformer, or

• Replace it with a transformer rated for 230 V secondary and 11 V primary.