ASSESSMENT OF THE THREE-PHASE

ASSESSMENT OF THE THREE-PHASE INDUCTION MOTORS CHARACTERISTCS
AIMING TO SAVE ENERGY: AN ERROR ANALYSIS
Antonio Tade u Lyrio de Almeida ( 1,2 )
Afonso Henriques Moreira Santos (1)
( 1) EFEI - Escola Federal de Engenharia de Itajuba
Av. BPS, 1303 - Pinheirinho
37500 - Itajuba - M.G.
ABSTRACT
This work aims to carry out a critical
analysis of the errors resulting from the
application of several methodologies employed
to evaluate the characteristcs of induction
motors in laboratory and in field.
to -
INTRODUCTION
The electrical sector has a dopted in the
last years an attitude of energy saving. The
identification of saving potentials and the
technical and economical evaluation of the
replacement of certain equipaments, as well
as the consumer faculty in adopting other
attitudes are largely fruits of optimization
studies and of energetic diagnostics.
In such activities , many attention has
been paid to the
three-phase
induction
motors, seeking to obtain their performance
characteristics; this f act rises from their
massive presence in industrial processes in
which they are often inadequate for the l oad
they drive (mainly while moving fluids in
pumps and fans, which respond for the most
part of the demand) and they also often
operate with low efficiencies.
Many procedures have been employed to
determine
such
characteristcs .
These
procedures, however, are grouped in two basic
levels, namely: the procedures perfo rmed in
laboratory and the ones perfome d in the very
work site.
The laboratory tests are based upon
several standards [1,2,3,4,5] and they employ
equipments hardly applicable in field; thus,
the so-called "type c haracteristics "
are
obtained, which, in principle, are considered
identical for the many units manufac tured
based upon a design, even though there is a
diversity in the quality of the material used
and in the workmanship.
On the oher hand, the motors evaluation
in site has been the object of several
studies (12-17] with many methodologies and
formulations
resulting
from
them;
the
emphasis given is justified or the necessity
to examine the real operational situations of
the motor-load group, preventing from only
theoretical simualations (closed upon the
type characteristics like the statement) wich
may disguise results.
In both situations, uncertainties in the
methodologies employed are found out, in
tests and in measurements and extrapolations
as well. The literature has many examples of
Jo~o
Lopes Ferreira Neto (1)
Edson da Costa Bortoni (1)
(2) UNITAU - Universidade de Taubate
R. Daniel Daneli, s i n. - Jd. Morumbi
'Taubate . - S.P.
these features, mainly on the achievement of
the motors efficiency. As the electrical
sector is acknowledge of, the some motor
tested with different standards
presents
efficiency values strongly divergent to each
other [6-8]. It is worth mentioning that even
methods considered accurate (as in the case
of the dynamometer method) present several
soucers of errors.
Thus, it is evident that the economics
of an eventual replacement can be seriously
impaired, because the results obtained in
laboratories and in field are not thoroughly
sure.
With regards the previously mentioned
and aiming to allow for to decrease the
uncertainty rate in using the test data, a
critical analisys of the errors inherent in
the various standardized and
expeditious
methods is carried out.
2.0 - DISCUSSION ABOUT STANDARDS AND
PROCEDURES
TEST
The internationally accepted standards
with respect to the efficiency tests of
induction motors are the IEEE std 112 [1],
IEC Pub. 34-2 [2], JEC std 37 [3] and NEMA
std MG 1 [4], the Brazilian standard ~ the
NBR 5383 [5].
A$
The IEEE
std
112
has
two
test
categories : the tests with straight forward
measurement and
the
ones
with
losses
addition. In the first category are the A
( brake), B (dynamometer) and C (back-to-back)
methods; in the second are the E (losses
segregation) and F (equivalent circuit).
The IEC-34-2 methods are basically the
mentioned ones; the preferred one, however,
is the losses segregation one, making it
different in the way
it
corrects
the
temperature and evaluates the stray load
losses. The JEC-std-37 methods are similar to
the IEEE-std 112 ones, and they are note
applicable to the method C. The method that
the standard prefers is the circle diagram
one, requiring tests with frequencies lowers
than nominal. NEMA adopts as standardized
procedure the IEEE std 112's method
E,
including a specific treatment of the stray
load l o sses.
The
determination
of
the
motor
efficiency by the avaiable methods is a
problem in itself, because all of them are
faulty and present
divergences
in
the
results. With this respect, the references
[6-8] gives many examples of the differences
"
} ~
')
which exist in the efficiency values when one
motor
is
tested
by
using
different
procedures.
As
claimed
by
Andreas
[6],
the
disagreements in the results are due to the
stray load losses calculations; consequently,
NEMA has adopted in its standards [4J,
the
recommendation that the polyphase induction
motors be specified with the rated efficiency
NEMA (or NEMA NOMEFF) when tested according
to the IEEE-std 112 [lJ, dynamometer method,
treating the stray load losses and admits a
range of efficiency values for a given motor,
based upon
a
statistical
distribution,
resulting in differences of up to 4.5 per
cent points.
With respect to JEC std 37, Ishizaki and
Hiragama [9J propose changes in some of its
procedures, aiming to achieve a
greater
accurateness in the calculated
characteristics.
It must be observed that, even in tests
using the dynamometer (IEEE 112, method B)
there are several sources of imprecision,
such as the instruments, the dynamometer and
instruments calibration [6J.
3.0 - METHODOLOGIES TO ASSESS THE MOTORS
CHARGING
The standardized procedures are more
applicable to laboratories than in field, due
to the necessity of suitable equipments and
facilities.
With this regard, many methodologies and
formulations have appeared which seek to
determine the charging of the motors in their
own site. Usually,
they
are
based
on
measurements of easy accomplishment and on
nameplate data or on manufacturer's data
sheets.
Some of these methods wich will be
expeditous
assigned
by
generically
methodologies in this work, are analysed as
follows:
3.1 - Kloss Formula:
The Kloss formula permits to obtain the
relationship between the torque
(M)
for a
slip (s) and the maximum torque
(Mk),
that
is:
2
M
~=
(1+ ~/R2
Sk)
~+~+2~slc:
Sk
s
n
TJIoI
- n"
ns
-
(2)
. MN
In wicp: ns is the synchronous speed; and n
is the speed corresponding to the motor load
(t1l.
The speed can be obtained directly or
through measurement of the current absorved
into the grid [11].
3.3 -
Inverse Circle Diagram
The mounting of the Circle Diagram is
possible through data obtained by performing
the tests with free rotor and locked rotor
under reduced
voltage
[10];
with
the
mentioned diagram and the plate-given rated
values in hands,
all the
characteristic
curves are obtained.
On the other hand, not always the tests
mentioned in field are possible to be carried
out; so,
the
authors have developed a
computer program wich,
inputed from simple
measurements of
active
electric
power,
voltage and current taken "in loco" plots the
circle diagram, in a form inverse to what is
usually done. It is necessary to know the
rated values.
3.4 - Methodology Developed by
sa:
[12]
Dr. sa has developed in his Phd thesis
a methodology, also presents in the reference
[12J, based on the solution of the equivalent
circuit in "T" of the induction
motor,
calculating the parameters from manufacturers
data sheets. The rotor reactance and the
resistance are considered varying with the
slip (or with the rotor frequency) between
the start-up and Sk. So, the emplo yment
of
the skin effect in the mentioned range is
incorporated,
but
not
the
saturation
influence. In the so-called operating region
the parameters are kept constant. As the
torque and start-up current values are used,
it is hardly applicable to motors with wound
rotors without adaptations.
3.5 - Other Methodologies:
According to Kostenko [10J, when
the
relationship
between
the
stator
resistances (~) and the rotor
resistence
refered to the stator
(Rz)
cannot
be
determined with more
precision,
it
is
admitted that Rl=R2.
In the equation (1),
" Sk"
is the slip corresponding
to
the
pull-out torque "M)c", wich is obtained by
taking the values given in catalogues for the
rated
conditions
(MN,
TJIoI)
and
the
relationship Mk/MN.
the
M
(1 )
R2
3.2 - Linearization of
ristic M=f(n):
The basic principle of this method is
the linearization of
the
characteristic
"torque x rotor speed",
in the so-called
operating region, that is, between slip zero
and "sic" . The pair MN and TJIoI is considered as
a point of the curve, wich given and taken as
true. So, the load on the motor shaft is
given by:
characte-
There are others methodologies, as the
one mentioned in [13], in wich the power is
obtained from three other values taken from
manufacturer data sheets, corresponding to
50%, 75% and 100% of full load.
Goldenberg and Lobosco [14] use these
data with the power factor and the speed for
the same conditions; besides, the equationing
include other data as full load and locked
current, full load and locked rotor torque
and the maximum torque. Fourteen equations
are obtained wich only six unknowns, with an
analytical solution resulting. Additionally,
an adjustment is done through
numerical
processes to minimize the deviation between
the values claimed by the manufacturer and
the values calculated through Form F.3 from
IEEE std 112 [1] .
The utilization of an accelerometer [15]
allows for the achievement of the M=f(n)
curves;
but
it
requires
the
motor
disconnection and it is more applicable to
laboratories. With this respect, Szabados et
alIi [16] develops improvements
in
the
traditional accelerometers [15] and obtains
the motor ' s characteristics based upon the
measurement of the stator currents and speeds
during the start-up,
employing
a
data
acquisition
board
together
with
the
device. These techniques have their attraction
in the possibility of performing, later, an
on-line monitoring by using a microcomputer
or through
measurement
of
conventional
currents. Artime and Sanz [16] presented
another methodology wich employs the electric
quantities measurement in
two
operating
points, but it needs a value of the torque
developed in the shaft for one of these
points.
4.0 - STATISTICAL MODELS
The methods presented in the previous
section for calculation o f the developed
torque and, consequently, of the power given
in the motor shaft, have a deterministic
character. It is known, however, that such
procedures are subject to errors incurred
from the achievement of the measurements to
the practice of the mathematic models, these
err o rs accumulating during the applications.
One of the main restricti ons presented
to them is the utilization of data sheets or
nameplate data; as a matter of fact, the
diversity of the quality of the material used
and of the workmanship leads to distinct
performances for motors with the same design
and rated characteristics.
With respect to these data, it
is
necessary to verify if they are typical, mean
or guaranteed, if the stray load losses and
the bearing ones are included in
their
determination, what is the test method used
to obtain them and what is the trust level
the motor ' s user wishes. This way, naturaly,
there is a great uncertainty in the results
obtained with the methodologies employing
data sheets or nameplate data.
However, even when employing established
methods
in
the
sundry
outstanding
international standards, concerning questions
arise, that is: What is the
efficiency
correct value? What results must be adopted
to evaluate technically and economically the
fewibility of replacing a motor? Is there any
practical fewibility in the
standardized
methodologies?
The uncertainties are present in the
achievement and in the use of the test
results as well, and not only
in
the
expeditious methodologies. The fact that the
questions are the same done for these last
ones must be enhanced .
Upon the exposed, there is a necessity
of statistical treatment of any
results
obtained, so as to check out the inherent (or
systematic) errors to the test or expeditous
methods,
to
the
measurements
and
extrapolations. It must be observed that the
NEMA recommendations [4] follow this findi ng.
The data consistency can be achie ve d
through adjustments to statistical models
[18] , which must reflect,
however,
the
physical behavior of the motor to make them
valids ; among them, the one presenting the
least standard deviation must be used.
Based upon the theory of the three-phase
induction motors and on the equationing of
their ' equivalent
circuit
[10],
three
statistical models applicable to the region
of steady operati on of the
motor
were
developed which are outlined.
a) Modell:
In this model, as simplification,
parameters are considered not to vary in
motor ' s operating region; so,
1
the
the
(3)
t1
in wich: C1 and Cz are constants
which
aggregate the motor's parameters, applied
voltage and synchronous speed, quantities
considered invarying in the analysis.
By means of a linear regression of
multiple variables, along the
origin
a
straight line best adjusted to the test
points is obtained.
b) Model 2:
In this case, the interception
was
introduced in the linear regression by adding
a new constant (~) to the expression (3), so
as to consider the systematic errors.
The representative expression of the
model is:
(4)
c) Model 3:
As, a matter of fact, some of the
machine parameters vary with the speed along
the operation [10], a new term was added to
the expression (4) trying to represent this
feature, that is:
1
t1 =
C1
+Czs+OI+
S
c;..
2
(5)
S
5,0 - TEST OAT A AND ADJUSTMENT OF
THE MODELS
To illustrate the adjustments procedures
with the statistic models test results of
motors performed by different methodologies
and obtained with various manufacturers are
presented in the tables 1, 2 and 3.
Among more than two dozens of test
sheets, the study of three motors, whose
basic characteristic and standards employed
in their tests are given in the Appendix.
For any test method, the best adjustment
was up to mode l 3. The expressions for the
motors' models are presents in the next page.
I
TABLE 1: Comparison f o r the
mode ls - Motor 1
Torque in Cpu]
based on the
rated torque
Speed Te st Hode l
Hodel
[rpm] data
1
3
171 4 1.85
1.66
1. 84
1738 1. 39
1.37
1. 41
1746 1. 25
1. 2 4
1. 24
1753 1.09
1.11
1. 09
1760 0.93
0 . 97
0 .93
1767 0.78
0.82
0.77
1778 0.53
0.56
0.53
1790 0.26
0.26
0 .26
STANDARD DEVIATION
Error r elative
to the t est
data [% ]
Hode l
Hodel
1
3
10 . 3
0 .54
1.44
-1.44
0 . 80
0 . 80
1.83
0.00
-4 . 30
0.00
-5.13
1.28
5.66
0.00
0.00
0 .00
0 . 76
4 . 86
TABLE 2: Comparison fo r the
mode l s - Motor 2
To rque in Cpu]
based on the
rated torque
Speed Test Hodel
Hodel
[rpm] data
1
3
3560 1.31 2.169
1. 239
3573 1. 00 2.087
1.030
3581 0.75 1.791
0.802
3589 0 . 51 0.377
0.495
3594 0 . 27 0.206
0 . 273
STANDARD DEVIATION
statistic
Er ror relative
to the test
data [%]
Hodel
Model
1
3
69.90
-5.20
108.1
2.69
6.65
138.2
26 . 8
-3.88
24 . 5
0 . 00
85 . 4
4 . 33
TABLE 3: Comparison for the
models - Motor 3
Torque in Cpu]
based on the
rated torque
Speed Test Model
Model
[rpm] data
1
3
874 1. 29 1.261
1.265
880 1.02 0.995
0.994
885 0.50 0.512
0.512
890 0.50 0.512
0.512
STANDARD DEVIATION
statistic
stat i stic
Error relativ e
to the test
data [%]
Hodel
Mode l
1
3
1.712
2 . 047
2.874
2.718
0.765
0.470
1 . 332 -1.387
0 . 039
0 . 038
~
= 4.277 + 457.2 s _ 23.49 _ 4.68x:0
s
s
b) Hotor 2 :
H
-2
= 2.06x10 + 394 . 6 s _ 10.27 _ 4.97x10
s
S2
-5
(7)
c) Motor 3:
1 = 0 . 352 + 20.0 s _ 2.5x1 0- 5
--H
S
THE
The u se of model 3 was best adjusted to
the test data, making them co nsi stent; now,
any extrapolation almlng t o obtain othe r
points in t he "torque related to speed "
characteristic can be c alculat e d
through
expressions (6) , (7) and (8) for moto rs 1, 2
and 3, r espectively . Again , it is enhanced
that all the analysis of this work hold for
the region between the speed corresponding to
the maximum torque and the synchronous speed.
Only
the
procedures
with
easy
application in field will be considered, that
is, the Kloss formula [10J, the linearizati on
of H=f(n), the inverse circle diagram, and
the Dr. sa ' s [12] . The procedures mentioned
in [13] and [14] will not be assessed because
not always there are avaiable data about the
operating conditions at 50% and 75% of full
load of an operating motor. The Szabados'
digital
a ccelerometer
[ 16]
requires
a
tachogenerator coupled in the axie shaft
what, many times, is
not
possible
in
industrial plants and, therefore, is not
applicable to every case o f evaluation in
field.
Within this scope, an error evaluation
is carried out
between
the
expeditous
proc edures and the statistical model adjusted
for
each
case.
The
c omparisons,
are
illustrated in correspondence to figures 1, 2
and 3, for motors 1, 2 and 3, respectively.
The table
4
presents
theirs
standard
deviations .
Comparasion among
Standard Deviation
methods
-3
(6)
1
6 .0 - THE EXPEDITIOUS PROCEDURES AND
ADJUSTED STATI STICAL MODEL
TABLE 4:
a) Motor 1:
n
decre ases, i n relat ion to
t he
previous
measurement;
speed
constant
with
l oad
i ncrease;
speed
constant
with
brutal
r e duction of load in relati on to the previous
measurement, with the mo tor not ope rating
with l ight load on the shaft; o r, different
speeds for equal l oads on the shaft,
t he
speed contro l bei ng non - existent.
Method
kloss
formula
linear .
inverse
c ircle
diagram
Hotor1
26.40
26.60
18 . 40
---- -
Hotor2
15 . 50
18.50
-- - --
15.90
Hotor3
0.262
2. 367
0.724
4 . 86
sa
-5
-
8 . 46x10
S2
( 8)
Model 2 was neglected because in more
than twenty motors evaluated it did not have
physical significance . Model 1 p resented a
standard deviation superior to Hodel 3 in
every case; this way, expressions (6),
( 7)
and (8) are employed as the best approach to
the motor
performance,
preventing
from
introducing
systematic
errors
(mainly
resulting from the use of the dynamometer
method) as, for example; decrease of the
speed when the load on the motor shaft
7.0 - CONCLUSIONS
It has been found that the most suitable
statistical adjustment was the one performed
through model 3. It is observed that i n the
motors tested through JEC std
37 ,
the
standard devi at ion is minimal, indicating
that, even though using the ci r c le diagram
and extrapol ations to obtain
the
rotor
parameters at low fr e quenc y (12 [HzJ ) , the
data present statistical cons istency. The
application of the IEEE-B/ NEMA method, also
has resulted in go od conformity of the data .
TABLE 1: Comparison for the
models - Motor 1
Torque in Cpu]
based o n the
rated torque
Speed Te st Hodel
Hodel
[rpm] data
1
3
1714 1.85
1.66
1.84
1738 1. 39
1.37
1. 41
1746 1. 25
1. 2 4
1.24
1753 1. 09
1.11
1. 09
1760 0.93
0 . 97
0 . 93
1767 0 .78
0.82
0 . 77
0.56
1778 0.53
0.53
1790 0.26
0.26
0 . 26
STANDARD DEVIATION
Error r elative
to the test
data [% ]
Model
Model
1
3
10.3
0 .54
1.44
-1. 44
0 .80
0 . 80
0.00
1. 83
-4.30
0 .00
1.28
-5.13
5.66
0.00
0.00
0 .00
0.76
4 . 86
TABLE 2 : Comparison fo r the
models - Motor 2
To rque in [pu]
based on the
rated torque
Speed Test Hodel
Hodel
[rpm] data
1
3
3560 1. 31 2.169
1. 239
3573 1.00 2 . 087
1.030
3581 0.75 1.791
0.802
3589 0.51 0.377
0 . 495
3594 0.27 0 .206
0 . 273
STANDARD DEVIATION
statistic
Error relative
to the test
data [%]
Hodel
Hodel
1
3
69.90
5.20
108.1
2.69
138.2
6.65
26 . 8
-3 . 88
24 . 5
0 . 00
85.4
4 . 33
TABLE 3: Comparison fo r the
models - Hotor 3
Torque in [pu]
based on the
rated torque
Speed Test Hode l
Model
[ rpm] data
1
3
874 1. 29 1.261
1.265
880 1.02 0.995
0.994
885 0.50 0.512
0.512
890 0.50 0.512
0.512
STANDARD DEVIATION
statistic
statistic
Error relative
to the test
data [%]
Hode l
Model
1
3
2.047
1.712
2.874
2.718
0.765
0.470
-1 . 332 -1.387
0 . 038
0 . 039
1
6 .0 - THE EXPEDITIOUS PROCEDURES AND
ADJUSTED STATI STICAL MODEL
= 4.277 + 457 . 2 s _ 23.49 _ 4.68x10
s
Within this scope, an error evaluation
is carried out
between
the
expeditous
procedures and the statistical model adjusted
for
each
case.
The
comparisons,
are
illustrated in correspondence to figures 1, 2
and 3, for motors 1, 2 and 3, respectively.
The table
4
presents
theirs
standard
deviations.
S2
b) Hotor 2:
-2
= 2.06x10 + 394 . 6 s _ 10 . 27 _ 4 . 97x10
t1"
s
52
-5
(7 )
c) Motor 3 :
~
= 0 . 352
S
+ 20 . 0 s _ 2 .
5x10- 5 -
Comparasion among
Standard Deviation
methods
-3
Method
kloss
formula
linear.
inverse
circle
. diagram
Motor1
26 . 40
26.60
18.40
- -- - -
Hotor2
15.50
18.50
-----
15 . 90
Motor3
0.262
2.367
0.724
4 . 86
( 6)
1
THE
The use o f mode l 3 was best adjusted to
the test data, making them co ns istent; now,
any extrapolation a1m1ng t o obtain othe r
points in the "torque related to speed "
characteristic can be c alculat ed
through
expressions (6) , (7) and ( 8 ) for moto rs 1, 2
and 3, r espectively . Again, it is enhanced
that all the analysis of this work ho ld for
the regi on between the speed corr espond ing to
the maximum torque and the synchronous speed.
Only
the
proce dures
wi t h
easy
application in field will be considered, that
is, the Kloss formula [10], the linearization
of H=f(n), the inverse circle diagram, and
the Dr. sa "s [12] . The proc edures mentioned
in [13J and [14] will not be assessed because
not always there are avaiable data about the
operating conditions at 50% and 75% of full
load of an operating motor. The Szabados'
digital
a ccelerometer
[ 16J
requires
a
tachogenerator coupled in the axie shaft
what, many times, is
not
possible
in
industrial plants and,
therefore, is not
applicable to every case of evaluation in
field.
TABLE 4:
a) Motor 1:
t1"
decreases,
in relation to
the
previous
measurement;
speed
constant
with
load
increase;
speed
constant
wi th
brutal
reduction of load i n relati on to the previous
measurement, with the mo tor not ope rating
with l ight load on the shaft ; o r, different
speeds fo r equal l oads on the sha ft,
the
speed control being non -ex i stent.
sa
-5
8 . 46x10
82
(8)
Model 2 was neglected because in more
than twenty motors evaluated it did not have
physical significance . Hodel 1 presented a
standard deviation superior to Model 3 in
every case; this way, expressions ( 6),
(7 )
and (8) are employed as the best approach to
the motor
performance,
preventing
from
introducing
systematic
errors
(mainly
r e sulting from the use of the dynamometer
metho d) as, for example; decrease of the
speed when the load on the motor shaft
7.0 - CONCLUSIONS
It has been found that the most suitable
statistical adjustment was the one performed
t hrough model 3. It is observed that in the
motors tested through JEC std
37,
the
standard deviation is minimal, indicating
that, even though using t he ci r cl e diagram
and extrapol ations to obtain
the
rotor
parameters at low fr equenc y (12 [Hz] ) , the
data present statistical consistency. The
application o f the IEEE-B/ NEMA method, also
has resulted in good conformity of the data .
=
(pu)
"EA5~ RE"ENTS
- - "'.~nSilCAl VOD EL
~
-
KLOSS i'C·Ro,cULA
. - - UNEAAlZATION
- - -
SA'
FIGURE 1: M = f(n) characteristic obtained
through different methods, Motor 1_
et=J f.lEASUREMENTS
_
-
-
.- -
STAnSilCAl MODEl
KLOSS fORf.lULA
UNe:-RIZATION
- - - CIRCLE DIAGRAI.I
n (rpm)
'i596' ,
'isba
FIGURE 2: M = f(n) characteristic obtained
through different methods, Motor 2_
1.40
11.1
(pu)
=
MEASUREM~NTS
- - STAnSilc:Al MODEL
- - KLOSS fORf.lULA
1.20
.- -
UNEARIZATION
-
SA'
• - - CIRCLE OIAGRAI.!
,001
0. 90
I
0.60
~
J.4O ]
G.20
1." "';;~o' "
'815"' "
aeo"""
'~e5"" " 'B~b'" ., 'a~5", . n 'g~:m)
FIGURE 3: M = f(n) characteristic obtained
through different methods, Motor 3 .
On the other hand, the IEC-34-2 methodology
employed gave the largest error among all
methods; it can be supposed that such a fact
is because of
the
stray
load
losses
calculation as being 0 _5 %
of the active
power absorbed
from
the
grid
for
a
de terminated load on the shaft, which causes
distortions _
From the expeditious methodologies, only
the Kloss formula and the linearization of
M=f(n) were applic able to all the motors _ The
Dr. sa-s and the Inverse Circle Diagram
method not even presented results in some
use
is
c ases
and,
therefore,
their
restricted_
The two first methods present as a f law
the dependence on the catalogue or name plate
data . It is observed that such data were
supposed as the test ones and, even so,
the
errors were rather significant; it is likely
that the imprecision would be much greater if
this . was not done . Besides, the results
obtained do not incorporate any conditions
adverse to the network, such as voltage
unbalance or presence of harmonics.
The other methodologies, as shown, a re
not that
applicable
to
evaluation
of
operating motors, since the available data
are not much reliable or non-existent .
The identification of saving potentials
and the technical and economical evaluation
of replacing the motors
requires
swift
methodologies for application in field; yet,
as analysed, these are not enough_ The test
procedures ,
which
might
supply
this
necessity, are of difficult application, they
present divergent results according to the
standard (and many . times, discrepants for
motors of a same design) and, dependins upon
the size, they are more expensive than the
motor itself.
Within this scope, the philosophy of
evaluating the efficiency or
the
motor
charging becomes a dangerous fact, because
there are high mistake risks (what,
of
course, results in considerable financial
damages) .
The term "oversized motor" is
very
relative,
because
it
depends
on
the
requirements imposed by the
load
( work
cycle), on the room conditions and on the
supply network; besides, not always
the
products "efficiency per power factor" are
the largest for full load conditions. The
real situation
of
a
motor,
and
the
convenience of an eventual replacement, must
be
evaluated
from
the
binomial
"heating-energetic
efficiency",
these
features being obtained through the typical
work cycle_
Anyway, it can be concluded that the
expeditious methods to evaluate the load of
motors are subject to the reliability of data
and
characteristics
supplied
by
manufacturers, this one holding for test
results from the manufacturer . With this
respect, it is interesting to notice that the
expeditious methodologies
have
a
great
correspondence with the results
obtained
through application of the JEC-std 37;
in
other words, it can be said that all the
methods are applicable because there are no
certainty and that the validity depends on
the standard through which the motor was
originally tested .
REFERENCES
[1] Test Procedure for Polyphase Induction
Motors and Generators
IEEE Standard
.l.l2, 1984_
[2] "Methods
for
determining
losses
and
,
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
electrical
efficiency
of
rotating
Rotating
machinery
from
tests"
IEEE
Electrical Machines
Part 2
Publication ~. (1972)
Induction Machine, Standard QI Japanese
37,
Electrotechinical Committee, JEC
1961.
American Nacional Standard for Motors
and Generators, NEMA MG ~- 1978.
NBR 5383
HAquinas polifasicas
de
Indu~~o - Metodo de Ensaio - ABNT.
Andreas, J.C.; ~ Efficient Electric
~ Selection And Aplications
Marcel Deckker, Inc, New York, 1982.
Cumings, P.G.; Bowers, W.D.; Martiny,
W.J. - "Induction motor efficiency test
methods" - IEEE T.r.arul Q.Il lA, vol. lA-I?,
no 3, may/june 1981, pp 253-272.
Cummings, P.G. - "Comparison of IEC and
NEHA/IEEE motor standards - part I"
IEEE ~ Q.Il .Irui APP.L., vo 1. IA-18 ,
n 2 5, sep/out - "1982 - pp 471-478.
Ishizaki, A.; Hirayama, K.;
"Determination
of
equivalent
circuit
parameters for performance calculation
of polyphase
induction
machines
Electrical Engineering in ~ - 87 (1)
1967 - pp 71-75.
Kostenko, M.; Piotrovski, I. - Electri
~ Machines. - Mir Publ., Moscou.
Ferreira Neto, J.L.; Santos, A.H.M;
"Metodologia
expedita
de
avalia~~o
tecnica e econ6mica de substitui~~o de
motores em operac;:~o"
Winner
of
Pirelli"s ~ SaYing~, 1988.
Ruppert Fo, E.; Arango, H.; sa; J.S.
"Analysis of Squirrel Cage Induction
Motor Rotor Bars Thermal Behavior "
~ ~ ~ Q.Il Electrical
Machines
(ICEM) - Cambridge; 1990, pp 245-250.
Woodham, J.B.;
"Motor loading for
lowest losses" EQ1 - feb 1979, pp 66-69.
Go ldemberg ,
C.;
Lobosco,
O.S.
"Determination
of
Induction
Motor
Characteristics form manufacturers data
sheets" - £I:o.Q.... In:t.... QQn:L. Q.Il Electrical
Machines (ICEM) - Cambridge, 1990, pp
458-463.
Cristofides, N.; Adkins, B.; - "Determination of load losses and torques in
squirrel-cage induction motors" - ~
lEE, vol 113, no 2, Dec. 1966
pp
1995-2005.
Szabados, B.; Findlay, R.D.; Obermeyer,
G.M.; Drapher, R.E.
"Measurement of
the torque-speed
characteristics
of
induction motors using an improved new
digital approach" - IEEE Trans
Q.Il
En....
~,vol
5, no 3, sept. 1990, pp
565-571.
Artime, J.; Sanz, J. - "A new proposed
method for the determination of circuit
parameters in squirrel-cage induction
motors by steady-state tests". £I:o.Q....
In:t.... ~ Q.Il Electrical Machines (ICEM)
- Cambridge, 1990, pp 522-526.
Davies, O.L.; Goldsmith, P.L. - Stat is~
methods
in
research
And
production. Hafner Pub. Co, New York,
1972.
APPENDIX
MOTOR"S TEST REPORT:
A.1 - Motor 1:
Test method: IEEE std 112 / B (dynamome t er) /
NEMA MG 1.
PN = 11 kW; 0-. = 440 V; IN = 22 A; 60 Hz;
=
ON = 1730 rpm& stator resistence (Rt)
0.45945 0 / 20 C;
I [A]
11.46 17.68 19.99 22.36 24.76 22.17
Pel[kW] 7.57 10.46 12.34 14 . 19 15.99 17.75
n [rpm] 1778 1767 1760 1753 1746 1738
M [kgf] 3.22 4.76 5.75 6.72 7.66 8.57
p . [kW]
5.88 8.64 10.40 12.10 13.74 15.30
0.78 0.83 0.84 0.85 0.86 0.86
n
co/W
0 . 69 0.78 0.81 0.83 0.85 0.86
A.2 - Motor 2:
Test method: IEC - 34 - 2
PN = 450 cv; UN = 4000 V; IN = 59. 1 A; 60 Hz;
ON = 3570 rpm; 2 poles; MK/MN
2.63;
HI'/MN = 1.39; Ip/IN = 6.32
100
LOAD %
125
75
50
25
Pel[kW] 455.0 350.0 264.0 183.5 101.5
S [kVA] 516.1 396.3 311.8 228.6 159.3
co/W
0.882 0.883 0.847 0.803 0.637
s%
1.111 0.750 0.528 0.306 0.167
PJ1.[kW]
7.40
4.36
2.69
1.44
0.70
PH [kW]
4.92
2.56
1.36
0.54
0.16
PsL [kW]
1.75
2.28
1.32
0.92
0.51
Total
23.97 18.04 14.74 12.27 10.74
P [kW] 431.03 331.96 249.26 171.23 90.76
0.947 0.948 0.944 0.933 0.894
T/
74.5
57.2
45.0
33.0
23.0
I [A]
A.3 - Motor 3:
Test Method: JEC - std 37;
PN = 55 kW; UN = 440 V; IN = 96.0 A; 60 Hz;
ON=880 rpm; 8 poles;
= 3.10;
HI'/MN=1.007; Ip/IN =
Test
f [Hz]
Free Rotor
60
Locked Rotor
60
Load Test (Circle Diagram Method)
LOAD %
125
100
75
50
co/W
0.844 0.813 0.757 0.643
s%
2.87
2.24
1.07
1.64
0.924 0.927 0.924 0.909
T/
77.4
I [A]
115.6
95.6
61.8
A.4 - NOMENCLATURE
Pel - Active electric power; S - Total electric power; I - L~ne current; P - Mechanical
power; PJ1, PJ2 - I R losses (stator and rotor
); PsL - Stray load losses; s
Slip; n
Efficiency; M - Developed torque.
Antonio T. L. Almeida
Elec. Eng.
(EFEI/
1980); Msc in Elect.
Eng.
(EFEI/1986).
Currently the is working toward his Phd
degree in elec. eng. at Universidade Estadual
de Campinas (UNICAMP). He
is
presently
professor of Electrical Machines at EFEI and
UNITAU.
Afonso H. M. Santos - Elec. Eng. (EFEI/1978);
Msc in Elec. Eng. (EFEI/1980); Phd (UNICAMP/
1987). He works at EF~I, but presently is a
Postdoctoral FellowautClRED, FRANCE.
Jo~o L. F. Neto
Elec. Eng.
(EFEI/1989);
currently he is working toward his Msc degree
at EFEI. He is the 1988 recipient of the
Pirelli"s Energy Saving Award.
Edson C. Bortoni - Elec. Eng.
(EFEI/1990);
Presently he is working toward his Msc degree
at UNICAMP.