Magnetic performance is characterized by the B-H curve and core loss measurements. Standard tests include: single-sheet testing for core loss and permeability, Epstein frame for small transformers, and hysteresis loop measurement under rated voltage and frequency. These tests ensure that silicon steel meets design specifications, minimizes no-load current, and supports stable operation in OEM audio and lighting transformers. Proper measurement prevents overfluxing and overheating.

    Accurate measurement of core material properties is crucial for design verification and quality control. The handbook describes both standardized material tests and practical transformer-based tests.

I. Standardized Material Testing (On Epstein Frame or Single Sheet Testers):
These tests are performed on sample strips of the core material.

  • Specific Core Loss ($P_c$ or $W/kg$): This is the most critical parameter. It is measured using an Epstein frame or a Single Sheet Tester (SST) under standardized sinusoidal flux conditions (e.g., at 1.5 T / 1.7 T and 50/60 Hz). The power loss in watts divided by the sample’s mass gives the specific loss. Lower values indicate higher quality material.

  • Peak Magnetizing Force ($H_m$) or Apparent Power ($VA/kg$): Measured simultaneously with core loss. It indicates the magnetizing current required and is related to the material’s permeability.

II. Practical Transformer-Based Testing (After Core Construction):
For a finished transformer or core assembly, the handbook recommends:

  1. No-Load Test (Open-Circuit Test):

    • Procedure: Apply rated sinusoidal voltage at rated frequency to the primary winding, with the secondary open.

    • Measurements: Input voltage ($V_1$), no-load current ($I_0$), and input power ($P_0$).

    • Analysis:

      • The measured $P_0$ is approximately equal to the total core loss (hysteresis + eddy current) at rated voltage.

      • The shape and value of $I_0$ reflect the core’s magnetization characteristics. A symmetrical, sinusoidal $I_0$ indicates good material; a distorted or high $I_0$ suggests saturation, poor material, or air gaps.

      • The no-load power factor is low (typically 0.1-0.3).

  2. Build a Dedicated Test Winding (for Core Evaluation):
    For consistent core quality checks, the handbook suggests winding a standard test coil (e.g., a fixed number of turns) on sample cores from a batch.

    • Measure the inductance ($L$) or magnetizing current of this test coil under a standard AC voltage.

    • Measure the $Q$ factor of the test coil, which is related to the core loss.

    • Compare results against a known-good reference core. Significant deviations indicate inconsistencies in material properties or core assembly.

Key Takeaway: While specific loss data from material sheets is essential for design, the no-load test on the finished transformer is the ultimate performance check, integrating the effects of material quality, core geometry, and assembly工艺.