A good procedure gone astray? BY CHUCK YUNG & TRAVIS GRIFFITH HIS ARTICLE DISCUSSES THE BASIC Stator Cores components of core loss (hysteresis and Although IEEE Standard 432 Guide for Insulation Mainte- eddy-current losses), the effect of frequency nance for Rotating Electric Machinery [1] was withdrawn in and lamination thickness, and other 2004, the loop test described in Appendix A4 has long contributing factors It also explains the effect of operating been used to evaluate the suitability of stator cores for frequency of squirrel cage type-rotors and armatures and rewinding End users and repair facilities generally agree provides correction factors that this test provides a valid data for evaluating polyphase T Digital Object Identifier 10.1109/MIAS.2010.939431 Date of publication: 12 November 2010 IEEE INDUSTRY APPLICATIONS MAGAZINE JAN j FEB 2011 WWW.IEEE.ORG/IAS © FOTOSEARCH stator cores designed for 50- or 60-Hz power systems Specifically, it is used to measure damage or other 1077-2618/11/$26.00©2011 IEEE 57 IEEE INDUSTRY APPLICATIONS MAGAZINE JAN j FEB 2011 WWW.IEEE.ORG/IAS observed overall core performance is a deficiencies in the lamination stack summation of the individual laminaand thus determine the suitability for tion behavior rewind The loop test was narrow in THINNER The familiar right-hand rule [Figure scope and intended to evaluate the LAMINATIONS 2(a)] describing the voltage and current core losses of a three-phase stator produced in a conductor, when there core In addition, the magnetic flux HAVE A LOWER is relative movement between that density with which the core was to be conductor and a magnetic field, also excited (105% of the operating magSTACKING applies to the core The rotating netic flux density) was determined magnetic field passing through the based on an assumed 50- or 60-Hz FACTOR BECAUSE squirrel cage bars induces voltage operating frequency and current into the laminations Unfortunately, the use of the core OF THE HIGHER (which is the origin of the term intest has been expanded to include the PROPORTION OF duction motor) examination of squirrel cage and wound While the voltage between adjacent rotor induction motor (SCIM and INSULATION TO laminations is small (approximately WRIM) rotors, as well as dc arma0.02 V/lamination), the cumulative tures, but without the rigorous scrutiny STEEL effect of hundreds of stacked laminathat the IEEE Standard 432 contained tions is appreciable Therefore, the Because of the lack of understanding LAMINATION laminations must be electrically insuof the underlying principles, some lated from each other When the interrotor and armature cores are conlaminar insulation is compromised, the demned or unnecessarily rebuilt and therefore a loop test fails to identify shorted dc armature laminations short together, the voltage virtually drops to zero, and the resulting power (kVA) is predominantly the cores sometimes 58 What Are Core Losses? When a ferromagnetic material is magnetized, some residual magnetism (remanence) is present after the flux source is removed While the cores under consideration are compressed stacks of punched steel laminations, each lamination exhibits the same property Thus, energy is required to return the flux to a zero or neutral state Consider that in an ac machine, the ac remagnetizes the core continuously, changing the polarity with each half cycle The amount of energy required changes in a nonlinear fashion, resulting in a lag between the application and removal of force and its subsequent effects (Figure 1) This phenomenon, known as hysteresis, might be described as magnetic kinetic energy In a good core, typical losses attributable to the continuous remagnetization are approximately two thirds of the total core losses The other components of core loss are eddy-current and interlaminar losses The Motion or Force Magnetic Field (β ) Induced Current (I ) (a) Flux Density B a Saturation Retentivity b Coercivity –H Magnetizing Force in Opposite Direction c f H Magnetizing Force e d Saturation in Opposite Direction –B (b) Flux Density in Opposite Direction Hysteresis loop (B-H curve) (a) The right-hand rule (b) Flux travels circumferentially around the core, inducing voltage between the laminations (Photo courtesy of EASA.) Losses ¼ heat; more heat ¼ further damage: Eddy-current losses within the laminations produce resistive heating These losses are proportional to the square of the lamination thickness (Figure 3) and the square of the frequency If the two laminations short together, the eddy-current losses increase by a factor of four in that location The larger the hot spot, the more is the generated heat, increasing exponentially Not only does the additional heat expand the damaged region by destroying the interlaminar coating, but it also deterio- Eddy-current losses increase as the square of lamination rates the various winding insulation materials The use of thickness thinner laminations was a first-line method of controlling losses (heat) in an electric motor This has the effect of keeping the core temperaTABLE TYPICAL LAMINATION THICKNESSES tures lower and within the temperature Frequency 25 Hz 40 Hz 60 Hz 400 Hz limits of the insulation class, thereby improving efficiency Lamination 1.2 mm 0.8 mm 0.61 mm 0.36 mm Motors operating on the 25- and 40-Hz thickness (0.046 in) (0.030 in) (0.024 in) (0.014 in) electrical systems of the recent past were IEEE INDUSTRY APPLICATIONS MAGAZINE JAN j FEB 2011 WWW.IEEE.ORG/IAS much less sensitive to eddy-current amperage component This high curlosses Table illustrates that the rent causes localized heating of the lamination thickness decreased as shorted laminations, thus extending THE ONLY BENEFIT the operating frequencies increased, damage to the interlaminar insulation and the manufacturers strove to maxThis cycle of damage continues, evenOF USING A CORE imize the effective use of active tually leading to a failure of the windTESTER TO CHECK materials and further improve the ing insulation, laminated core, or the efficiency of electrical equipment rotor cage SCIM ROTORS IS This is applicable to motors, generaThe loop test is a procedure for tors, and transformers energizing the stator core with an TO DETECT ROTOR The well-known 10° rule [5] posexternal source of magnetic flux dentulates that the expected insulation sities to simulate operating condiCAGE life is halved for every 10 °C tions The test models a transformer increase in temperature A 50 °C turn ratio on a single phase–single ANOMALIES increase in temperature reduces the coil basis for convenience Depending expected insulation life from 30 on the available power supply (espeyears to less than a year A hot spot cially current limitations), the loop turns and voltage are changed in direct proportion, and does not have to be very large to wreak havoc with the current varies inversely, thus providing the same kil- insulation life ovoltamperes The test can be performed using multiple loop turns, as described in IEEE Standard 432, or with a single loop turn with a power supply capable of supply- Factors Affecting Core Losses ing the high current required for the low excitation volt- Core losses are comprised of hysteresis and eddy-current age [2], [3] For example, a 100-turn, 250-V test is losses at approximately 2:1 ratio in a new core Hysteresis equivalent to a one-turn, 2.5-V test However, the one- losses occur within the steel and are influenced by the type turn test would require 100 times the current as the 100- of steel (carbon steel versus silicon steel) used and are proportional to the lamination thickness, grain size, and operturn loop test Whether caused by a rotor–stator rub from bearing ating system frequency Eddy-current loss is given by failure or core plate deterioration due to excessive burnout temperatures, shorting of the laminations produces higher eddy-current losses [4] The stator core test Pe ¼ 7:47 10À14 (B2 f t2 )(qd), (1) should be performed before and after the burnout process, using the same measurements and test set for both tests When shorted regions of the core are noticea- where Pe ¼ W=lb, B ¼ flux density, q ¼ electrical ble, eddy-current losses increase markedly and represent resistivity ðohms=cmÞ, f ¼ frequency, d ¼ density of core a disproportionate amount of total core losses For exam- material ðg=cm3 Þ, and t ¼ lamination thicknessðcmÞ ple, normal eddy-current losses of a 100-kg core are typiHysteresis losses are higher for carbon steel than silicon cally 2–9 W/kg, whereas a shorted area of 10 cm2 can steel, a fact that must be balanced against the better perincrease these losses to 22–30 W/kg (10–15 W/lb) or meability of carbon steel In addition, grain orientation more Higher losses result in increased heat and higher also affects the loss magnitude Therefore, electrical steel magnetizing current 59 IEEE INDUSTRY APPLICATIONS MAGAZINE JAN j FEB 2011 WWW.IEEE.ORG/IAS 60 Larger regions of shorted laminamanufacturers produce a wide variety tions are more readily detected than of low-loss silicon steels with high perIEEE STANDARD the smaller ones A two-pole core, havmeability to be used in the cores of ing a larger back iron area, is more difelectric motors and generators 432 ficult to evaluate than a core designed The magnetic flux path induced by for a winding with more poles (i.e., a the loop test follows a circumferential RECOMMENDS lower synchronous speed), particularly path through the back iron [Figure when damage occurs deep within the 2(b)] Magnetization of the teeth is CORE LOSS back iron also accomplished via a combination High-permeability steel requires eleof sufficient current to force the flux TESTING AT 5% vated magnetic flux densities to detect across the slots and fringing of the flux OVER THE shorted laminations Cores constructed at the upper periphery of the back from thicker laminations are more reiron The flux path of the loop test folMAGNETIC FLUX sponsive to lower magnetic flux density, lows a circular path through the cirwhereas a core of the same physical size cumference of the back iron, as DENSITY OF THE constructed from a larger number of opposed to the normal operating flux thinner laminations may appear to be in path (Figure 4) WINDING DESIGN good condition when evaluated at a low The ampere turns required to raise flux density the flux density to a level capable of Stack tightness is another variable, revealing hot spots is affected by the conditions of the core (i.e., shorted laminations cause an with typical stacking pressures of 5–8.8 kg/cm2 (75–125 increase in magnetizing current), the permeability of the psi) Insufficient stacking pressure translates into greater steel, thickness of the lamination, and other factors movement of the laminations, abrading away the interlaminar coating It also implies less steel per unit length The coating thickness on each lamination has a similar effect as stacking pressure; a thicker coating reduces the ratio of steel to the overall core length A typical core is comprised of a stack of punched laminations, with each lamination coated to reduce eddy-current losses The term stacking factor is used to describe the ratio of actual lamination steel to overall stack length Bur height must also be controlled by the manufacturers by constantly monitoring the condition of the punches used to stamp lamination profiles Laser-cut laminations can achieve a 98% stacking factor, compared with a 95% stacking factor for a core constructed of punched laminations Further complicating interpretation is the influence of the frame containing the core Cast iron does not pass flux well, so a cast iron frame has no discernable influence on the loop test Rolled steel or welded (fabricated) steel frames carry magnetic flux very well, so they significantly influence the loop test If the construction of the frame is steel and forms an uninterrupted circular path Flux path of a four-pole induction motor for the magnetic flux to travel, error is introduced into the results of a loop test The operator must be aware of this influence on the results of the core test Anecdotal evidence supports the belief that when a core test determines low core losses but with higher-than-expected ampere turns, it is an indication of lower permeability steel or a steel frame The closer the contact between the core and frame the greater the effect will be on the ampere turns required to energize the core to the required magnetic flux density In an International Protection code (IP)54 or IP55 [totally enclosed fan cooled (TEFC)] design, there is an interference fit between the core and frame, with a full contact between the two (Figure 5) The core of an IP22 [open drip proof (ODP)] or IP23 [weatherproof (WP)] enclosure by design has limited contact with the frame (Figure 6) This permits more airflow across the core to promote cooling A motor conTEFC stator core and frame fit (Photo courtesy of EASA.) structed with a steel frame, in close contact with the with a design with little or no slip For example, an SCIM rotor with 2% THE USE OF slip has an operating frequency of only 1.2 Hz on a 60-Hz system (1.0 Hz THINNER at 50 Hz) Consequently, because eddycurrent losses are proportional to the Frequency and Core Losses LAMINATIONS square of the frequency, the eddyThe importance of operating freWAS A FIRST-LINE current losses in operation are only quency when interpreting core test 0.04% of the losses determined by results should be understood by exMETHOD OF the 60-Hz core test amining (1) The adaptation of core testing to Eddy-current losses vary with the CONTROLLING SCIM rotors is misunderstood by many square of the frequency and laminaend users and by others who should tion thickness Eddy currents, thereLOSSES (HEAT) IN know better First, the induction rotor fore, have a disproportional impact is subject to line frequency power at on motor efficiency These losses in AN ELECTRIC the time of starting As the rotor accelmachines operating at higher freMOTOR erates, rotor frequency drops quickly quencies are controlled by using pro(in the time it takes to start the motor) portionately thinner laminations From to slip frequency A typical rotor operFigures and 8, the effect of both lamination thickness and frequency can be readily ates at 1–3 Hz during normal service, although this figure is higher for National Electric Manufacturing Association understood The eddy-current losses of a motor operating at (NEMA) design D rotors at 5–8% or 8–13% slip As in 60 Hz are 144% greater [(60/50) ¼ 1.44] than when Table 2, the rotor frequency is still considerably lower than operating at 50 Hz A motor operating at 120 Hz the frequency of the stator core Rotor frequency is given by would experience four times the eddy-current losses in operation as indicated by a 60-Hz test Core loss is far À ððr=minÞ=synchronous r=minÞ line frequency; more critical for a machine operating at 400 Hz, than for a 50- or 60-Hz core By the same token, the core loss ð2Þ for a vintage 25-Hz core is less critical than for a comparable 60-Hz core However, because the loop test or core test is usually Core Loss Breakdown performed using the power supply available to the repair of 0.6-mm (0.24-in) Silicon Steel facility, the core loss is interpreted at a fixed frequency, 90 Eddy-Current Loss regardless of the operating frequency of the core being 81 72 evaluated Rotor and Armature Operating Frequency The operating frequency of a rotating core is directly determined by the operating and synchronous speeds of the machine The high slip designs result in a higher operating frequency for a rotor core, when compared 63 54 45 36 27 18 Hysteresis Loss 30 50 200 60 100 Frequency 400 Effect of frequency on losses, 0.6-mm lamination thickness Core Loss Breakdown of 0.8-mm (0.031-in) Silicon Steel Percentage of Loss 100 90 80 70 60 50 40 30 20 10 ODP type frame with minimal core contact (Photo courtesy of EASA.) Eddy-Current Loss Hysteresis Loss 30 50 60 100 200 Frequency 400 800 1,000 Effect of frequency on losses, 0.8-mm lamination thickness IEEE INDUSTRY APPLICATIONS MAGAZINE JAN j FEB 2011 WWW.IEEE.ORG/IAS Percentage of Loss entire circumference of the core, is likely to require more ampere turns to raise the magnetizing current to the test level 61 IEEE INDUSTRY APPLICATIONS MAGAZINE JAN j FEB 2011 WWW.IEEE.ORG/IAS 13 kilolines/cm (85k/in ) normally effectively, multiply the percent slip times the line frequency to determine used for testing stator cores In addition, HIGHthe rotor frequency Note that even the infrared thermography (Figure 9) can high slip design D rotor is only be useful for detecting partial open cirPERMEABILITY exposed to 9–15 Hz during normal cuits in the rotor cage There are reoperation ported cases of rotor core testing, at the STEEL REQUIRES Because the rotor frequency is so IEEE Standard 432 test levels, at which ELEVATED low, eddy-current losses are not a the heat generated by current passing significant concern for most induction through a shaft-to-core weld actually MAGNETIC FLUX rotors The core test, performed using bent the shaft 50- or 60-Hz power, is not a useful DENSITIES TO test for most squirrel cage rotors A Armature Frequency possible exception is the two-pole Evaluation of dc armature core test DETECT SHORTED rotor, where the test might reveal results must also consider the role of localized hot spots that might contribfrequency DC machines are designed LAMINATIONS ute to thermal bowing and increased to operate at a wide range of speeds, vibration levels with the operating frequency being Thus, the only benefit of using a core tester to check dependent on the revolutions per minute (r/min) and SCIM rotors is to detect rotor cage anomalies An open poles [6] rotor bar can force the current, normally carried by the Knowing that eddy currents are an ac phenomenon, bar, to pass through the laminations in the vicinity of some are surprised to learn that a dc armature is subject the break, generating heat When used in conjunction to ac; each coil reverses polarity as it passes from pole to with magnetic imaging paper or iron filings to check pole (Figure 10) while rotating Unlike ac machines, the the integrity of the rotor cage, the core loss tester can relationship between r/min and number of poles for dc be useful machines is not fixed; therefore, the actual r/min affects To avoid overheating of the rotor core, the magnetic the eddy-current losses in dc armatures The operating flux densities should be 1.8–2.3 kilolines of flux/cm frequency of the armature should be a factor in evaluat(12–15 kilolines of flux/in ) as opposed to the ing the condition of the armature core To calculate the frequency of an armature, use the r/min and number of poles: 62 TABLE ROTOR OPERATING FREQUENCY FOURPOLE MOTOR, 60 HZ (50 HZ) Armature frequency ¼ poles r=min=120: Full Load (r/min) Synchronous Speed (r/min) % Slip Rotor Frequency (Hz) 1,746 (1,455) 1,800 (1,500) 3% 1.8 (1.5) 1,728 (1,440) 1,800 (1,500) 4% 2.4 (2) 1,692 (1,410) 1,800 (1,500) 6% 3.6 (3) 1,584 (1,320) 1,800 (1,500) 12% 7.2 (6) ð3Þ An armature of a four-pole dc machine, rotating at 1,800 (1,500) r/min, is subject to 60 (50) Hz At 900 r/min, the frequency drops to 30 Hz At higher speeds, the frequency increases, causing eddy-current losses to increase as the square of the speed increases The righthand column of Table summarizes the impact of Pole Iron Field Coil FLIR 87.8 117 74 °F Thermal image of SCIM rotor under test (Photo courtesy of EASA.) Interpole Armature 10 DC armature polarity reverses to ac TABLE ARMATURE r/min AND FREQUENCY Poles Speed (r/min) Armature Frequency (Hz) Relative Core Loss 3,500 116.7 4.0 1,750 58 1.0 1,100 36.7 0.4 350 21.5 0.14 12 Armature undergoing core test (Photo courtesy of EASA.) TABLE ADJUSTMENT FACTORS FOR DIFFERENT OPERATING FREQUENCY AND CORE LOSS Hz 25 50 60 120 240 400 Base 1.02 (1.5) 2.04 (3) 2.45 (3.6) 4.9 (7.2) 9.8 (14.4) 16.3 (24) Watts loss/lb (kg) 1.04 (2.25) 4.17 (9) 6.0 (13) 24.0 (51.8) 96.0 (207) 267 (576) IEEE INDUSTRY APPLICATIONS MAGAZINE JAN j FEB 2011 WWW.IEEE.ORG/IAS Spindle Motors, Frequency, operating frequency on core losses for and Lamination Thickness a four-pole armature at different operWhen high frequencies are discussed, ating speeds EDDY-CURRENT the spindle motor, often operating at When a loop test or traditional 240 Hz or more, is another special tester is used for armature or rotor LOSSES WITHIN case Eddy-current losses vary in cores, several drawbacks exist The proportion to the square of the lamisearch coil must fully encompass the THE LAMINATIONS nation thickness, so thinner laminaback iron If the core has vent opentions (0.25 mm) are used to control ings (Figure 11) in the back iron PRODUCE the eddy-current losses Thinner and the search coil lead is passed RESISTIVE laminations have a lower stacking through a vent opening, only part of factor because of the higher proporthe back iron is an active part of the HEATING tion of insulation to steel lamination circuit Generators and electric motors asThe transformer ratio of the onesociated with the aircraft industry turn current path and the search coil assumes the same active iron for both When the amount of active iron is changed, such as by routing the sensing leads through a vent opening, the sensed voltage will not be the same as when the search coil encompasses the entire back iron Why a one-turn coil? Physical arrangement of the stator core allows full access inside and outside the stack, varying with only the inclusion of the frame material (as noted earlier), whereas with many rotors or armatures, there is no free internal space to pass a cable The only means of passing a current through the center of the rotor or armature assembly is through the shaft; thus, the test leads attach directly to the shaft ends (Note that the only exception to this would be for a vertical hollowshaft-type machine.) When the sensing leads are connected to the shaft, the tester can only read the voltage drop across the shaft This bears emphasis The sensing 11 leads not measure the induced voltage in the core Consequently, the calculations for core losses as calcu- Armature core with axial vent ducts (Photo courtesy of lated by the on-board algorithm are not using the EASA.) induced voltage—the input value for induced voltage is actually the voltage drop across the shaft The results are erroneous and may directly result in failure to reveal a damaged core or cause expensive repairs to a core that does not require it (Figure 12) 63 Common sense should be applied when evaluating a core NEMA T-frame motors generally use lower loss steel HYSTERESIS than older U frame and pre-NEMA Importance of Frequency motors Energy-efficient motors with To factor in the effect of frequency LOSSES ARE conservative densities and better grades on core testing, consider the relative of steel have lower losses than many watts loss/lb for the same core at variHIGHER FOR metric design motors Vintage 25- or ous applied frequencies (Table 4) 40-Hz motors are more likely to have Because eddy-current losses are proporCARBON STEEL thicker laminations, so a core test will tional to the square of the frequency, THAN SILICON report higher eddy-current losses than it is logical to apply the square root of for a more recently manufactured 60the W loss/kg limit (3.6) and change STEEL or 50-Hz core Finally, since eddythat in proportion to other operating current losses are proportional to the frequencies squared frequency, the core losses For example, to determine the equivaduring a test applied at 60-Hz lent losses for 120 Hz, double the 3.6 value, and square it If a 60-Hz core loss test of a 120-Hz should be carefully evaluated for motors that operate at core results in a good value of 13 W/kg (6 W/lb), the higher frequencies The material of the frame itself (steel, aluminum, cast expected eddy-current losses operating at 120 Hz would be 51.8 W/kg (24 W/lb) See (4) and (5) Reasonable limits for iron) can influence the core test results Although the frequencies other than 60 Hz should be determined by col- frame does not affect the running core losses, this can greatly skew the interpretation of the core test Frame lecting actual data construction has a lesser effect, as it affects stack presExpected loss is estimated by sure If the core is in full circumferential contact with p W=lb ¼ ½ðf =60Þ 6 ð4Þ the core (as in a TEFC machine), the effect is much greater than if the core is in intermittent contact (as in an ODP design) Ideally, if the core test results are in question, the stator should be removed from the frame or to avoid conflicting results p Rotor cores function at comparatively minimal frequenð5Þ W=kg ¼ ½ðf =60Þ 132 ; cies Application of the stator core loss testing method is generally inappropriate and except for minor, coincidental rotor bar appearance, yields misleading information where f is the frequency applied Note: Although IEEE Standard 432 was administratively withdrawn in 2004, work in the IEEE Power & Conclusions For core loss testing of stators operating on 60-Hz (or Energy Society continues toward merging 432 and IEEE 50 Hz) sinusoidal power, IEEE Standard 432-1992 is the Standard 56, both of which pertain to insulation maintesource document Reference [4] builds upon the document nance When this article was submitted, the work was in to establish procedures based on the core dimensions, draft 18 weights, and standardized magnetic flux densities IEEE Standard 432 recommends core loss testing at References 5% over the magnetic flux density of the winding [1] Guide for Insulation Maintenance for Rotating Electrical Machinery (5 hp to than 10 000 hp), IEEE Standard 432, 1992 design Industry practice adopted a value of 1.3 T, or [2] less J A Britton, Recommendations for Core Loss Testing on Cage Type Induction 85 kilolines/in2, corroborated by major manufacturers of Motors Accident, MD: Phenix Technologies commercial core testers Some evidence suggests using [3] Core Loss Testing in the Practical Motor Repair Environment Louisville, KY: Lexseco, 1989 T (64.5 kilolines) as the target saturation level might result in greater repeatability of results The higher the [4] Stator Core Testing EASA, Tech Note 17, 1992 [5] Standard Test Procedure for Evaluation of Systems of Insulating Materials for magnetic flux density, above the knee of the saturation Random-Wound AC Electric Machinery IEEE Standard 117 curve, the greater the margin for error [6] Core Loss Testing: Tips for Special Cases EASA Currents, Feb.2002 As measured using a static core test, the losses of a [7] Recommended Practice for the Repair of Rotating Electric Machinery, ANSI/ EASA AR1002006 good 50- or 60-Hz stator range between and W/kg (1 and W/lb) depends on steel grade; higher losses usually indicate a defective core and require corrective measures As eddy-current losses vary as the square of Chuck Yung (cyung@easa.com) is with EASA in St Louis, the applied frequency, the importance of core loss test- Missouri Travis Griffith is with GE Oil & Gas in Houston, ing increases with the frequency of the equipment Texas Yung and Griffith are Senior Members of the IEEE under consideration Spindle motors and armatures that This article first appeared as “Core Loss Testing: A Good operate at higher frequencies are far more critical than Procedure Gone Astray?” at the 2009 Petroleum and Chemical induction rotors Industry Conference IEEE INDUSTRY APPLICATIONS MAGAZINE JAN j FEB 2011 WWW.IEEE.ORG/IAS commonly operate at 400 Hz, requiring thinner laminations 64 ... or 60-Hz core By the same token, the core loss ð2Þ for a vintage 25-Hz core is less critical than for a comparable 60-Hz core However, because the loop test or core test is usually Core Loss Breakdown... core, typical losses attributable to the continuous remagnetization are approximately two thirds of the total core losses The other components of core loss are eddy-current and interlaminar losses... interpreting core test 0.04% of the losses determined by results should be understood by exMETHOD OF the 60-Hz core test amining (1) The adaptation of core testing to Eddy-current losses vary