What is the difference between no-load loss and load loss?

What is the difference between no-load loss and load loss?

Both no-load loss and load loss are important indicators for evaluating transformer performance, but they differ in their causes, testing methods, and influencing factors. Simply put, no-load loss primarily originates from the iron core, while load loss primarily originates from the windings.
1. What is no-load loss?
No-load loss refers to the active power consumed when rated frequency and specified voltage are applied to one side of the transformer while the other side remains open-circuited. In this state, the transformer carries no actual load, but magnetic flux is established within the iron core, resulting in power loss.
No-load loss primarily includes:
① Iron core hysteresis loss;
② Iron core eddy current loss;
③ Minor additional losses.
It is mainly related to the core material, silicon steel sheet thickness, lamination process, core structure, operating voltage, and frequency. Excessive no-load loss may be attributed to poor core material properties, improper core assembly, damaged insulation between silicon steel sheets, multi-point grounding of the core, or abnormal test voltage waveforms.
2. What is load loss?
Load loss refers to the loss caused by winding resistance and leakage magnetic fields when the transformer operates at a specified current.
Load loss primarily includes:
① Copper or aluminum losses caused by winding resistance;
② Losses in leads and connection points;
③ Additional losses caused by leakage magnetic fields;
④ Eddy current losses in structural components.
Load loss is closely related to winding materials, conductor cross-sectional area, winding structure, connection quality, test current, and temperature. Excessive load loss may be attributed to insufficient conductor cross-sectional area, high winding resistance, poor welding or connections, unreasonable winding structure, or excessive additional losses.
3. Key Differences Between the Two

4. Why Perform Both Tests?
No-load loss and load loss reflect the performance of different parts of the transformer.
Measuring only no-load loss makes it impossible to determine if the windings and connections are sound; conversely, measuring only load loss cannot evaluate the core material or core assembly quality. Therefore, factory acceptance tests for transformers typically require both no-load and load tests.
By analyzing data from both tests, one can comprehensively assess:
① The stability of the transformer manufacturing process;
② The soundness of the core and winding design;
③ Whether product energy efficiency meets requirements;
④ Whether actual operational energy losses fall within a reasonable range.
5. What Should Be Noted During Testing?
For no-load loss testing, focus on test voltage, frequency, and waveform quality. Voltage deviations or waveform distortions can affect test results.
For load loss testing, accurately measure the test current and winding temperature. Since winding resistance varies with temperature, the measured load loss usually requires conversion to a specified reference temperature before evaluation.
Conclusion
No-load loss primarily reflects losses generated by the transformer core, while load loss primarily reflects losses generated by windings and related components under current flow. The former focuses on core material and manufacturing processes, whereas the latter focuses on winding resistance, connection quality, and structural design. Only by combining these two tests can one comprehensively evaluate the transformer’s energy efficiency and manufacturing quality.

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