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The Basics of Core Loss Testing

Core loss testing provides a quick and efficient method for determining core losses found in the core steel of stators, rotors, and armatures.  Core Loss testing is one of the most important quality assurance tools in the motor repair industry and is virtually a requirement when rewinding a motor.  This proven technology is even more critical in today's competitive market as it predicts motor reliability, helps maintain motor efficiency and reduces motor repair and warranty costs.

Introduction to Losses

A motor's job is to convert electrical energy to mechanical energy.  The efficiency of a motor is a measure of how effectively the motor does its job via the power output (horsepower, kilowatts, etc.).  By subtracting the output power from the input power, the losses of the motor can be determined.  The efficiency is the ratio of the output power to the input power.  All cores experience some inherent loss.  Increased loss results from physical damages or overheating of the core.

The rate at which heat is produced by a steady current in an part of an electric circuit is proportional to the resistance and the square of current.  In motor stator windings, the stator I2R loss is typically the largest contributing factor to the stator winding temperature rise along with windage, friction, and stray load losses.  Core loss, dissipated as heat, increases the operating temperature of the motor which causes more heat and can shorten the winding life.  A vicious cycle of increasing inefficiency is established leading to higher operational costs and premature motor failure.  In DC armatures, core loss can cause commutator sparking and spotting which impedes motor performance.  A relatively small temperature increase in the motor winding can significantly reduce the thermal life of the insulation.

Ferrous, or steel materials have various electrical, magnetic and mechanical properties that are taken into consideration during the design of an electrical apparatus.  The goal of the design is to maximize output while minimizing the associated electrical and mechanical losses. The grade of the core steel, thickness of the steel, and the processing techniques are chosen in a similar way - minimize the steel losses without incurring a substantial increase in material costs.

There are two types of losses from the steel core

  • Hysteresis losses - the amount of input energy expended to change the magnetic polarity of the steel in conjunction with the changing polarity of the alternating current waveform.
  • Eddy Current losses - circulating currents resulting from the magnetic fields generated in the electric motor.  When an electrical conductor is placed within a magnetic field, a current proportional to the cross-section of the conductor and the strength of the field is known to flow perpendicular to that field.  For this reason, steel lamination thickness is minimized to reduce the amount of eddy current flow.

Examination of the Core Through History

Before the invention of the core loss tester the only way to determine if a motor core had gone bad was the loop / ring test.  For this test, an equation was used to calculate the number of turns and wire size to be wrapped around a core to achieve a known target back iron flux density.  This method proved to be labor intensive, time consuming, and therefore, not practical in the motor repair environment.  Additionally, Hot Spot testing required rewiring of the turns and the test in general only applied to stator cores.  For more information about this test, check out EASA's Tech Note 17 - Stator Core Testing.

Now, commercial core loss testers only require one turn of the wire and the voltage applied to that wire is varied.  This achieves the same goal as the Loop Test while simplifying the test procedures tremendously.  The Core Loss Tester automates the test process and reduces the testing time to 5-10 minutes.  Accuracy and consistency are increased.  One of the greatest advantages of the Core Loss Tester is the fact that wound rotors & armatures can also be tested.

For more details about how Core Loss Testers work and the mathematics behind it, check out Core Loss Testing in the Practical Motor Repair Environment.

The Necessity for a Core Loss Tester in Motor Repair

A significant percentage of motors tested have core loss exceeding statistical acceptability.  Some special types, such as hermetic refrigeration and traction motors, suffer especially high losses.  Moreover, government efficiency mandates making detecting sources of energy loss increasingly important.  Preeminent technical authorities have acknowledged the critical importance of core testing.  EASA's Guidelines for Maintaining Motor Efficiency During Rebuilding indicates core loss testing as a recommended practice.

Studies have shown that core loss is the 1st or 2nd leading causes of energy waste in rewound motors and can account for 25% or more of motor efficiency.  Core testing not only identifies motors that should be replaced instead of repaired, but also reveals repairable problems.  The increasing costs of electrical energy and energy legislation make it more important than ever that rewound motors maintain the optimum level of efficiency and performance.

Learn More About Core Loss Testers!(click here) 

 

 

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