Chuck Yung
Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO

The paper "Core Testing: A Good Procedure Gone Astray?" by Chuck Yung, presented at the EASA Convention 2009, explores the loop test used to evaluate the suitability of stator cores for rewinding. Originally developed for three-phase stator cores, the loop test has been expanded to test squirrel cage induction motor rotors, wound rotor cores, and DC armatures. However, misinterpretation of the test results often occurs due to a lack of understanding of the principles behind the test.

Core losses consist of hysteresis and eddy current losses. Hysteresis losses occur due to the residual magnetism present after the flux source is removed, requiring energy to return the flux to a neutral state. Eddy current losses are caused by the rotating magnetic field passing through the stator core, inducing voltage and current into the laminations. If laminations short together, high current causes localized heating, leading to further damage and eventual failure of the winding insulation, laminated core, or rotor cage.

The loop test energizes the stator core with an external source of magnetic flux densities to simulate operating conditions. The test can be performed using multiple loop turns or a single loop turn with a high current power supply. Shorted laminations produce higher eddy current losses, which increase dramatically in shorted areas, leading to increased heat and further damage.

Factors affecting core losses include lamination thickness, frequency, permeability of the steel, and stack tightness. Eddy current losses are proportional to the square of the lamination thickness and frequency. Thinner laminations are used to control losses and improve efficiency. The frame material and construction also influence core test results, with steel frames carrying magnetic flux well and affecting ampere-turns required to energize the core.

Rotor and armature operating frequency is determined by the operating speed and synchronous speed of the machine. High-slip designs result in higher operating frequency for rotor cores. Eddy current losses in rotors are minimal due to low operating frequency, making the core test less useful for most squirrel cage rotors. DC armatures are subject to alternating current as each coil reverses polarity while rotating, with operating frequency affecting eddy current losses.

The paper emphasizes the importance of considering frequency when interpreting core test results. Eddy current losses vary as the square of the frequency, making core loss testing more critical for machines operating at higher frequencies. The material of the frame and its construction can skew core test results, and the core should ideally be removed from the frame for accurate testing.

In conclusion, core testing is a valuable procedure for evaluating stator cores, but its application to rotors and armatures requires careful consideration of operating frequency and other factors. Misinterpretation of test results can lead to unnecessary repairs or failure to identify damaged cores.

Key Points Covered:

• Overview of core losses (hysteresis and eddy current losses)
• Principles and methodology of the loop test
• Factors affecting core losses (lamination thickness, frequency, permeability, stack tightness)
• Influence of frame material and construction on core test results
• Operating frequency of rotors and armatures
• Importance of considering frequency in core test results

Key Takeaways:

• Core testing is essential for evaluating stator cores but requires careful interpretation for rotors and armatures.
• Eddy current losses are influenced by lamination thickness and frequency.
• Frame material and construction can affect core test results.
• Operating frequency is crucial for understanding core losses in rotors and armatures.
• Misinterpretation of core test results can lead to unnecessary repairs or failure to identify damaged cores.

Categories: Technical topics, AC motors, Stators/windings, Repair procedures & tips, Testing, Motor testing

Tags: Core loss testing

Rate this article:

No rating

Print