How to Test Core Loss in AC Motors

How to Test Core Loss in AC Motors

Core loss, also known as iron loss, is a critical performance indicator of AC motors. It directly affects motor efficiency, temperature rise, and long-term reliability. Accurately testing core loss is essential for motor manufacturers, quality inspectors, and engineers involved in motor design and performance evaluation.

What Is Core Loss in AC Motors?

Core loss occurs in the stator and rotor laminations when they are subjected to an alternating magnetic field. It mainly consists of two components:

1)Hysteresis loss, caused by repeated magnetization and demagnetization of the core material

2)Eddy current loss, generated by circulating currents induced in the iron core

Both losses depend on magnetic flux density, frequency, and the quality of the silicon steel laminations.

Common Methods to Test Core Loss

1. No-Load Test (Indirect Method)

The no-load test is the most commonly used method for estimating core loss in AC motors.

Test procedure:

●Run the motor at rated voltage and frequency with no mechanical load

●Measure input voltage, current, and power

●Subtract mechanical losses (friction and windage) from the total no-load power

The remaining power is considered the core loss plus a small stator copper loss.

Advantages:

●Simple and practical

●Suitable for routine factory testing

Limitations:

●Results are influenced by mechanical losses

●Less accurate for small or high-speed motors

2. Epstein Frame Test (Material-Level Test)

The Epstein frame test is used to measure the core loss of electrical steel before motor assembly.

Test procedure:

●Assemble standardized strips of silicon steel into an Epstein frame

●Apply alternating magnetic flux at specified frequency and flux density

●Measure the input power to calculate core loss per kilogram

Advantages:

●High accuracy

●Ideal for evaluating lamination material quality

Limitations:

●Does not represent actual motor operating conditions

●Not suitable for finished motors

3. Ring Core Test

The ring core test is another material-level method, often used for research and sample evaluation.

Test procedure:

●Wind excitation and sensing coils on a ring-shaped laminated core

●Apply sinusoidal voltage at a defined frequency

●Measure magnetic flux and power loss

This method offers better magnetic uniformity than the Epstein frame but requires precise sample preparation.

4. Back-to-Back Test (Comparative Method)

In some cases, two identical motors are coupled mechanically and tested together to evaluate losses more accurately.

Advantages:

●Reduces the influence of mechanical losses

●Suitable for efficiency verification

Limitations:

●Complex setup

●Higher testing cost

Key Factors Affecting Core Loss Test Results

●Supply voltage waveform (harmonics increase iron loss)

●Frequency accuracy

●Temperature of the core

●Lamination thickness and insulation quality

For accurate results, testing should be performed under stable voltage, frequency, and temperature conditions.

Conclusion

Testing core loss in AC motors can be performed at both material and motor levels, depending on the purpose of the evaluation. While material tests such as the Epstein frame and ring core methods focus on silicon steel quality, no-load tests remain the most practical approach for finished motors. A proper understanding of each method helps engineers choose the most suitable test technique and improve motor efficiency and reliability.