A good procedure BY CHUCK YUNG

& TRAVIS GRIFFITH

gone astray?

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© FOTOSEARCH

HIS ARTICLE DISCUSSES THE BASIC Stator Cores

T components of core loss (hysteresis and

eddy-current losses), the effect of frequency
and lamination thickness, and other
contributing factors. It also explains the effect of operating
Although IEEE Standard 432 Guide for Insulation Mainte-
nance for Rotating Electric Machinery [1] was withdrawn in
2004, the loop test described in Appendix A4 has long
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
stator cores designed for 50- or 60-Hz power systems.
Digital Object Identifier 10.1109/MIAS.2010.939431
57
Date of publication: 12 November 2010 Specifically, it is used to measure damage or other
1077-2618/11/$26.00©2011 IEEE
 deficiencies in the lamination stack observed overall core performance is a
and thus determine the suitability for summation of the individual lamina-
rewind. The loop test was narrow in THINNER tion behavior.
scope and intended to evaluate the The familiar right-hand rule [Figure
core losses of a three-phase stator LAMINATIONS 2(a)] describing the voltage and current
core. In addition, the magnetic flux produced in a conductor, when there
density with which the core was to be HAVE A LOWER is relative movement between that
excited (105% of the operating mag- STACKING conductor and a magnetic field, also
netic flux density) was determined applies to the core. The rotating
based on an assumed 50- or 60-Hz FACTOR BECAUSE magnetic field passing through the
operating frequency. squirrel cage bars induces voltage
Unfortunately, the use of the core OF THE HIGHER and current into the laminations
test has been expanded to include the (which is the origin of the term in-
examination of squirrel cage and wound PROPORTION OF duction motor).
rotor induction motor (SCIM and While the voltage between adjacent
WRIM) rotors, as well as dc arma- INSULATION TO laminations is small (approximately
tures, but without the rigorous scrutiny 0.02 V/lamination), the cumulative
that the IEEE Standard 432 contained.
STEEL effect of hundreds of stacked lamina-
Because of the lack of understanding LAMINATION. tions is appreciable. Therefore, the
of the underlying principles, some laminations must be electrically insu-
rotor and armature cores are con- lated from each other. When the inter-
demned or unnecessarily rebuilt and laminar insulation is compromised, the
therefore a loop test fails to identify shorted dc armature laminations short together, the voltage virtually drops to
cores sometimes. zero, and the resulting power (kVA) is predominantly the

What Are Core Losses?

When a ferromagnetic material is magnetized, some resid-
ual magnetism (remanence) is present after the flux source
is removed. While the cores under consideration are com-
pressed stacks of punched steel laminations, each lamina- Motion or Force
tion exhibits the same property. Thus, energy is required
to return the flux to a zero or neutral state. Consider that
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in an ac machine, the ac remagnetizes the core continu-

ously, changing the polarity with each half cycle. The Magnetic Field (β)
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 phenom- Induced
enon, known as hysteresis, might be described as magnetic Current (I )
kinetic energy.
In a good core, typical losses attributable to the
continuous remagnetization are approximately two (a)
thirds of the total core losses. The other components of
core loss are eddy-current and interlaminar losses. The

Flux Density B
a Saturation
Retentivity
b

Coercivity
–H c f H
Magnetizing Force Magnetizing Force
in Opposite Direction
e

d (b)
Flux Density
Saturation in –B 2
in Opposite Direction
Opposite Direction (a) The right-hand rule. (b) Flux travels circumferentially
1 around the core, inducing voltage between the
58
Hysteresis loop (B-H curve). laminations. (Photo courtesy of EASA.)
amperage component. This high cur- much less sensitive to eddy-current
rent causes localized heating of the losses. Table 1 illustrates that the
shorted laminations, thus extending THE ONLY BENEFIT lamination thickness decreased as
damage to the interlaminar insulation. the operating frequencies increased,
This cycle of damage continues, even- OF USING A CORE and the manufacturers strove to max-
tually leading to a failure of the wind- imize the effective use of active
ing insulation, laminated core, or the TESTER TO CHECK materials and further improve the
rotor cage. efficiency of electrical equipment.
The loop test is a procedure for SCIM ROTORS IS This is applicable to motors, genera-
energizing the stator core with an TO DETECT ROTOR tors, and transformers.
external source of magnetic flux den- The well-known 10° rule [5] pos-
sities to simulate operating condi- CAGE tulates that the expected insulation
tions. The test models a transformer life is halved for every 10 °C
turn ratio on a single phase–single ANOMALIES. increase in temperature. A 50 °C
coil basis for convenience. Depending increase in temperature reduces the
on the available power supply (espe- expected insulation life from 30
cially current limitations), the loop years to less than a year. A hot spot
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 pro-
turn loop test. portional to the lamination thickness, grain size, and oper-
Whether caused by a rotor–stator rub from bearing ating system frequency.
failure or core plate deterioration due to excessive burn- Eddy-current loss is given by
out temperatures, shorting of the laminations produces
higher eddy-current losses [4]. The stator core test
should be performed before and after the burnout Pe ¼ 7:47 3 1014 (B2 f 2 t2 )(qd), (1)

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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 typi- Hysteresis 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 per-
increase 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.

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 3
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 tempera- TABLE 1. TYPICAL LAMINATION THICKNESSES.
tures lower and within the temperature
limits of the insulation class, thereby Frequency 25 Hz 40 Hz 60 Hz 400 Hz
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)
59
electrical systems of the recent past were
 manufacturers produce a wide variety Larger regions of shorted lamina-
of low-loss silicon steels with high per- tions are more readily detected than
meability to be used in the cores of IEEE STANDARD the smaller ones. A two-pole core, hav-
electric motors and generators. ing a larger back iron area, is more dif-
The magnetic flux path induced by 432 ficult to evaluate than a core designed
the loop test follows a circumferential for a winding with more poles (i.e., a
path through the back iron [Figure RECOMMENDS lower synchronous speed), particularly
2(b)]. Magnetization of the teeth is CORE LOSS when damage occurs deep within the
also accomplished via a combination back iron.
of sufficient current to force the flux TESTING AT 5% High-permeability steel requires ele-
across the slots and fringing of the flux vated magnetic flux densities to detect
at the upper periphery of the back OVER THE shorted laminations. Cores constructed
iron. The flux path of the loop test fol- from thicker laminations are more re-
lows a circular path through the cir- MAGNETIC FLUX sponsive to lower magnetic flux density,
cumference of the back iron, as whereas a core of the same physical size
opposed to the normal operating flux DENSITY OF THE constructed from a larger number of
path (Figure 4). WINDING DESIGN. thinner laminations may appear to be in
The ampere turns required to raise good condition when evaluated at a low
the flux density to a level capable of flux density.
revealing hot spots is affected by the Stack tightness is another variable,
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 inter-
laminar 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 lami-
nation steel to overall stack length. Bur height must also
be controlled by the manufacturers by constantly monitor-
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ing 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 con-
structed 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 signifi-
4 cantly influence the loop test. If the construction of the
Flux path of a four-pole induction motor. frame is steel and forms an uninterrupted circular path
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 deter-
mines 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
5 airflow across the core to promote cooling. A motor con-
60
TEFC stator core and frame fit. (Photo courtesy of EASA.) structed with a steel frame, in close contact with the
entire circumference of the core, is with a design with little or no slip.
likely to require more ampere turns THE USE OF For example, an SCIM rotor with 2%
to raise the magnetizing current to slip has an operating frequency of
the test level. THINNER only 1.2 Hz on a 60-Hz system (1.0 Hz
at 50 Hz). Consequently, because eddy-
Frequency and Core Losses LAMINATIONS current losses are proportional to the
The importance of operating fre- square of the frequency, the eddy-
quency when interpreting core test WAS A FIRST-LINE current losses in operation are only
results should be understood by ex- 0.04% of the losses determined by
amining (1). METHOD OF the 60-Hz core test.
Eddy-current losses vary with the CONTROLLING The adaptation of core testing to
square of the frequency and lamina- SCIM rotors is misunderstood by many
tion thickness. Eddy currents, there- LOSSES (HEAT) IN end users and by others who should
fore, have a disproportional impact know better. First, the induction rotor
on motor efficiency. These losses in AN ELECTRIC is subject to line frequency power at
machines operating at higher fre- the time of starting. As the rotor accel-
quencies are controlled by using pro- MOTOR. erates, rotor frequency drops quickly
portionately thinner laminations. From (in the time it takes to start the motor)
Figures 7 and 8, the effect of both to slip frequency. A typical rotor oper-
lamination thickness and frequency can be readily ates at 1–3 Hz during normal service, although this figure
understood. is higher for National Electric Manufacturing Association
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) 2 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.
would experience four times the eddy-current losses in Rotor frequency is given by
operation as indicated by a 60-Hz test. Core loss is far
more critical for a machine operating at 400 Hz, than 1  ððr=minÞ=synchronous r=minÞ 3 line frequency;
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

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facility, the core loss is interpreted at a fixed frequency, 90
regardless of the operating frequency of the core being 81 Eddy-Current Loss
Percentage of Loss

evaluated. 72
63
54
Rotor and Armature 45
Operating Frequency 36
The operating frequency of a rotating core is directly 27
determined by the operating and synchronous speeds of 18
9 Hysteresis Loss
the machine. The high slip designs result in a higher 0
operating frequency for a rotor core, when compared 30 50 60 100 200 400
Frequency
7
Effect of frequency on losses, 0.6-mm lamination thickness.

Core Loss Breakdown

of 0.8-mm (0.031-in) Silicon Steel
100
90
Percentage of Loss

80
Eddy-Current Loss
70
60
50
40
30
20 Hysteresis Loss
10
0
30 50 60 100 200 400 800 1,000
6 Frequency
ODP type frame with minimal core contact. (Photo courtesy 8
61
of EASA.) Effect of frequency on losses, 0.8-mm lamination thickness.
 effectively, multiply the percent slip 13 kilolines/cm 2 (85k/in 2 ) normally
times the line frequency to determine used for testing stator cores. In addition,
the rotor frequency. Note that even the
HIGH- infrared thermography (Figure 9) can
high slip design D rotor is only PERMEABILITY be useful for detecting partial open cir-
exposed to 9–15 Hz during normal cuits in the rotor cage. There are re-
operation. STEEL REQUIRES ported cases of rotor core testing, at the
Because the rotor frequency is so IEEE Standard 432 test levels, at which
low, eddy-current losses are not a ELEVATED the heat generated by current passing
significant concern for most induction through a shaft-to-core weld actually
rotors. The core test, performed using MAGNETIC FLUX bent the shaft.
50- or 60-Hz power, is not a useful
test for most squirrel cage rotors. A DENSITIES TO Armature Frequency
possible exception is the two-pole DETECT SHORTED Evaluation of dc armature core test
rotor, where the test might reveal results must also consider the role of
localized hot spots that might contrib- LAMINATIONS. frequency. DC machines are designed
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 2 frequency of the armature should be a factor in evaluat-
(12–15 kilolines of flux/in 2 ) 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:
TABLE 2. ROTOR OPERATING FREQUENCY FOUR-
POLE MOTOR, 60 HZ (50 HZ) Armature frequency ¼ poles 3 r=min=120: ð3Þ
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Rotor An armature of a four-pole dc machine, rotating at

Full Load Synchronous Frequency 1,800 (1,500) r/min, is subject to 60 (50) Hz. At 900
(r/min) Speed (r/min) % Slip (Hz) r/min, the frequency drops to 30 Hz. At higher speeds,
1,746 (1,455) 1,800 (1,500) 3% 1.8 (1.5) the frequency increases, causing eddy-current losses to
increase as the square of the speed increases. The right-
1,728 (1,440) 1,800 (1,500) 4% 2.4 (2)
hand column of Table 3 summarizes the impact of
1,692 (1,410) 1,800 (1,500) 6% 3.6 (3)
1,584 (1,320) 1,800 (1,500) 12% 7.2 (6)

Pole Iron

Field Coil
FLIR 1 87.8
117

74
°F
9 Interpole Armature
Thermal image of SCIM rotor under test. (Photo courtesy of 10
62
EASA.) DC armature polarity reverses to ac.
operating frequency on core losses for Spindle Motors, Frequency,
a four-pole armature at different oper- and Lamination Thickness
ating speeds. When high frequencies are discussed,
When a loop test or traditional EDDY-CURRENT the spindle motor, often operating at
tester is used for armature or rotor LOSSES WITHIN 240 Hz or more, is another special
cores, several drawbacks exist. The case. Eddy-current losses vary in
search coil must fully encompass the THE LAMINATIONS proportion to the square of the lami-
back iron. If the core has vent open- nation thickness, so thinner lamina-
ings (Figure 11) in the back iron PRODUCE tions (0.25 mm) are used to control
and the search coil lead is passed the eddy-current losses. Thinner
through a vent opening, only part of RESISTIVE laminations have a lower stacking
the back iron is an active part of the factor because of the higher propor-
circuit. HEATING. tion of insulation to steel lamination.
The transformer ratio of the one- Generators and electric motors as-
turn current path and the search coil sociated with the aircraft industry
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 hollow-
shaft-type machine.) When the sensing leads are con-
nected to the shaft, the tester can only read the voltage
drop across the shaft. This bears emphasis. The sensing
11

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leads do 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).

TABLE 3. ARMATURE r/min AND FREQUENCY.

Poles Speed Armature Relative
(r/min) Frequency (Hz) Core Loss
4 3,500 116.7 4.0
4 1,750 58 1.0
4 1,100 36.7 0.4
4 350 21.5 0.14 12
Armature undergoing core test. (Photo courtesy of EASA.)

TABLE 4. 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)
63
 commonly operate at 400 Hz, Common sense should be applied
requiring thinner laminations. when evaluating a core. NEMA T-frame
motors generally use lower loss steel
Importance of Frequency HYSTERESIS than older U frame and pre-NEMA
To factor in the effect of frequency LOSSES ARE motors. Energy-efficient motors with
on core testing, consider the relative conservative densities and better grades
watts loss/lb for the same core at vari- HIGHER FOR of steel have lower losses than many
ous applied frequencies (Table 4). metric design motors. Vintage 25- or
Because eddy-current losses are propor- CARBON STEEL 40-Hz motors are more likely to have
tional to the square of the frequency, thicker laminations, so a core test will
it is logical to apply the square root of THAN SILICON report higher eddy-current losses than
the 6 W loss/kg limit (3.6) and change for a more recently manufactured 60-
that in proportion to other operating STEEL. or 50-Hz core. Finally, since eddy-
frequencies. current losses are proportional to the
For example, to determine the equiva- squared frequency, the core losses
lent losses for 120 Hz, double the 3.6 during a test applied at 60-Hz
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.
expected eddy-current losses operating at 120 Hz would be The material of the frame itself (steel, aluminum, cast
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
lecting actual data. greatly skew the interpretation of the core test. Frame
Expected loss is estimated by construction has a lesser effect, as it affects stack pres-
p 2 sure. If the core is in full circumferential contact with
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
or question, the stator should be removed from the frame
p to avoid conflicting results.
W=kg ¼ ½ðf =60Þ 132 ; ð5Þ Rotor cores function at comparatively minimal frequen-
cies. Application of the stator core loss testing method is
generally inappropriate and except for minor, coincidental
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where f is the frequency applied. rotor bar appearance, yields misleading information.
Note: Although IEEE Standard 432 was administra-
Conclusions tively withdrawn in 2004, work in the IEEE Power &
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 mainte-
source 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,
1 T (64.5 kilolines) as the target saturation level might KY: Lexseco, 1989.
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/
good 50- or 60-Hz stator range between 2 and 9 W/kg EASA AR1002006.
(1 and 4 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.

64