Download Effective Estimation of Induction Motor Field Efficiency Based on On-site Measurements

Effective Estimation of Induction Motor Field
Efficiency Based on On-site Measurements
T. Phumiphak* and C. Chat-uthai**
* Electrical Power Engineering Department, Faculty of Engineering, Mahanakorn University of Technology,
Bangkok 10530 Thailand
** Department of Electrical Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology
Ladkrabang, Bangkok 10520 Thailand
ABSTRACT
This paper proposes the effective technique for
estimating efficiency of existing three-phase induction
motors in the field. This technique focuses on the
operating efficiency of motors without the need for
removing the motors and without the need for measuring
the output power or torque. This paper describes the use
of a few sets of data (voltage, current, power, speed)
measured from the motor (on-site) coupled with the
genetic algorithms for evaluating the motor parameters
instead of using the no-load and blocked rotor test results.
Once these parameters are known it is possible to obtain
the estimated efficiency of the motor. To illustrate how
well the estimated efficiency match that of the calculated
obtained from the standard evaluations, the results of
various induction motors rating 10 up to 100 hp are
presented. Test results indicate that this proposed
technique has a high accuracy, and then it could be
suitable for conducting on-site energy audits of existing
motors in order to support a decision to replace operating
motors with a higher-efficiency model.
Keywords: Field Efficiency, Genetic Algorithms, Threephase Induction Motors, On-site Energy Audits.
1. INTRODUCTION
The majority of motors in the industry are induction
motors. There may be various reasons for the desire of
testing an existing induction motor in the field, such as
consideration of exchanging out of date or worn motors
with new, or checking the efficiency after rewinding.
Particularly the output power of motor is hard to detect.
One of established procedures is therefore to calculate the
efficiency by measuring the losses and subtract them
from the input to find the output.
There are many methods pertinent to field efficiency
evaluation in the literature and the new methods are
appearing every year [1-5]. In the past, many methods
were used to calculate the induction motor efficiency, one
common method is to test the motor under load
conditions and then monitor the input and output at
different load points using a dynamometer and torque
transducer (if possible). This is the most straightforward
method to measure the output power directly from the
shaft without any need to calculate losses. The shaft
torque method offers the most accurate field efficiency
evaluation method, however, it is not suitable for field
evaluation because this process involves the removal of
motor from service to place it on a test stand and couple it
to the dynamometer. It can be seen that this method is
impractical and costly.
In the field, one may estimate the efficiency based on
information from the nameplate and input measurements
in which a few problems may occur. First, the nameplate
efficiencies of a given motor can be evaluated according
to different standards. Second, the nameplate data are
rounded, the error in estimated efficiency can be very
high. Third, the motor may have been rewound.
Comparison of actual motor efficiencies is certainly a
valid tool to justify the use of one motor over another. In
order to conduct energy consumption of motors (on-site
energy audits), the operating efficiency of these motors
must be known. It is essential that the efficiency of motor
should be estimated quickly with minimum disturbance
to the service. The objective of this paper is to propose a
new technique that can help the plants to make the right
decision in replacing the inefficient motors with more
efficient ones using the results of the effective estimation
of induction motor field efficiency.
2. EFFECTIVE ESTIMATION OF INDUCTION
MOTOR FIELD EFFICIENCY
The calculation of the operating efficiency in this
present work is based on the equivalent circuit method.
The motor parameters can be estimated by using a few
sets of data from the field test and nameplate information
coupled with the genetic algorithms [6] instead of using
the no-load and blocked-rotor test results which are
undoubtedly a tedious and time consuming task if the
motor is already coupled to driven equipment in the
process. The field test data can be obtained from the
measured values at any one operating point test (OPT) or
two operating points test (TPT). Note that this technique
is suited to general purpose when the motor cannot
operate at no-load. This technique relies on measuring the
input voltage, current, electrical power, stator resistance
and output speed of the motor in the field test at any load.
The proposed equivalent circuit parameters per phase
of three-phase induction motor is shown in Fig. 1. This
model can be represented the steady state behavior of an
induction motor where
R1
jX1
jX2
Rc
jXm
I1
V1 Ztotal
parameters proceeds as follows: Input the nameplate
information and measure the stator resistance. Then from
the field test of motor (on-site) directly measure one
operating point or two operating points of motor input
voltage, current, power and output speed which need not
be close to no-load or full-load values while the motor is
in service. Select the method of OPT or TPT. As a result,
it is possible to estimate the motor efficiency in the field
in a short time.
RSLL
R2
s
Fig.1: Proposed Equivalent Circuit of Induction Motor
R1 : stator resistance,
General Data :
Rated Output Power (kW)
Rated Voltage (V)
Rated Current (A)
Rate Speed (rpm)
Measure Stator Resistance
X 1 : stator leakage reactance,
Rc : core loss resistance, X m : magnetizing reactance,
R2 : rotor resistance referred to stator,
X 2 : rotor leakage reactance referred to stator, s : slip
OPT
TPT
Select Method
Most models used in the past eliminated the stray
load loss. Neglecting this loss introduces a significant
error in estimating the efficiency. The stray load loss will
decrease with decreasing output power proportional to the
square of the torque [7]. It is worth mentioning that this
proposed model includes the stray load loss parameter
RSLL . The parameter RSLL can be expressed as
RSLL =
m (1 − s f ) R2
(1)
sf
where m denotes the per unit of the full-load power
which is set to be the variable ( 0.002 < m < 0.018 ) [7]
and s f is the full-load slip. From the equivalent circuit,
Point 1 :
- Input Voltage (V)
- Input Power (W)
- Input Current (A)
- Speed (rpm)
Point 1 :
- Input Voltage (V)
- Input Power (W)
- Input Current (A)
- Speed (rpm)
Point 2 :
- Input Voltage (V)
- Input Power (W)
- Input Current (A)
- Speed (rpm)
Process :
Motor Parameters
Evaluation Using
Genetic Algorithms
Process :
Motor Parameters
Evaluation Using
Genetic Algorithms
Output :
Performances of Motor
at Point 1 Load
Output :
Performances of Motor
at Any Load
the equations of stator phase current I1 and total input
power Pinput can be expressed as
V1
I1 =
,
Ztotal
3. EXPERIMENTAL RESULTS AND DISCUSSION
Pinput = 3 V1 I1 ( PF )
(2)
where V1 and I1 are rms values of the input phase
voltage and current respectively, and PF is power factor.
The equivalent circuit parameters can be estimated
by using the data from field test coupled with genetic
algorithms. The aim of genetic algorithms is to minimize
the error of (3), or to maximize the fitness of (4).
n
Fobjective = ∑
I1i ,cal
i =1 I1i , mea
Fitness =
2
n
−1 + ∑
Pinput i ,cal
i =1 Pinput i , mea
100
100 + Fobjective
Fig.2: Flow Chart of Proposed Technique
The proposed technique described in the preceding
section is applied to three different size three-phase
induction motors as shown in Table 1. These motors were
tested using standard performance evaluations, i.e. IEEE
Std 112 Method B [7] (for 100, 15, 10 hp) and IEC 34-2
[8] (for 100 hp). The motor efficiencies were calculated
using the analysis procedure described in those standard
performance evaluations.
Table 1: Nameplate Information of Motors
2
−1
(3)
(4)
Hz
V
A
r/min
Pole
100 hp [9]
(75 kW)
50
400/690
131/76
1488
4
15 hp [10]
(11.2 kW)
50
380
21.6
1450
4
10 hp [10]
(7.5 kW)
50
380
15.2
1445
4
where I1i ,cal and Pinput i ,cal are the calculated values
using (2). I1i ,mea and Pinput i ,mea are the measured
values obtained from the field test when n = 1 for the
OPT and n = 2 for the TPT methods.
The flow chart of the proposed technique is shown in
Fig. 2. The technique for estimating the equivalent circuit
Table 2 shows the field test data of 100 hp motor at
various loads (25, 50, 75, 100%). The measured data in
Table 2 can be used to estimate the motor parameters and
to calculate the motor efficiency which can select the
method of using the OPT or TPT. The data of the OPT
(one operating point) can be at any 25% or 50% or 75%
Measured
Input Voltage (V)
Input Current (A)
Input Power (kW)
Speed (r/min)
25%
400
56.8
20.51
1497.5
Motor Load
50%
75%
400
400
77.8
103.8
39.68
59.07
1495
1492
100%
400
132.5
78.53
1489
The results of estimated efficiency of the 100 hp
motor using the proposed technique are summarized in
Table 3 and Fig. 3. At one-quarter load, the measured
efficiencies obtained from IEEE 112 Method B and IEC
34-2 are 93.2 % and 92.9 % respectively, while the
estimated efficiency obtained from the TPT is 91.4 % and
the OPT is 91.9 %. At full-load, the measured efficiencies
are 95.8%, while the estimated efficiencies are 95.2 %
and 95.9 %. It can be observed that the accuracy of
estimation is higher at full-load than at one-quarter load.
Note that from Table 2 (100 hp), the synchronous speed
is 1500 r/min while the speed at one-quarter load is
1497.5 r/min and at full-load is 1489 r/min. This can be
attributed to the fact that at lower load the slip becomes
smaller. Therefore, any small error in measuring speed
will result in a significant error in slip.
88
2/4
95.4
94.6
94.6
94.5
94.6
94.5
95.5
95.5
94.3
-
1/4
4/4
95.6
95.2
95.2
95.1
95.2
95.2
95.8
95.8
95.9
95
Efficiency
5/4
95
94.7
94.7
94.6
94.7
94.7
95.5
95.4
25&50
25&75
25&100
89
50&75
50&100
75&100
IEEE112
87
IEC34-2
93
91
85
2/4
3/4
4/4
2/4
3/4
4/4
5/4
6/4
Motor Load
Fig. 4: Estimated Efficiency Using TPT and Standard
Evaluations Versus Load for 15 hp Motor
25&50
25&75
25&100
90
88
86
84
82
80
78
76
74
50&75
50&100
75&100
IEEE112
1/4
2/4
3/4
4/4
5/4
6/4
Motor Load
97
1/4
82
76
5/4
Motor Load
Fig. 3: Estimated Efficiency Using TPT and Standard
Evaluations Versus Load for 100 hp Motor
Fig. 5: Estimated Efficiency Using TPT and Standard
Evaluations Versus Load for 10 hp Motor
OPT
95
IEEE112
IEC34-2
90
Efficiency
1/4
93.0
91.4
91.4
91.4
91.4
91.4
93.2
92.9
91.9
-
50&75
50&100
75&100
IEEE112
84
78
85
80
75
70
10 hp
15 hp
100 hp
Fig. 6: Estimated Efficiency Using OPT at 25 % load and
Standard Evaluations Versus Various Motors at 1/4 load
OPT
98
96
94
Efficiency
Method
25%&50%
25%&75%
25%&100%
50%&75%
50%&100%
75%&100%
IEEE 112 B
IEC 34-2
25%
50%
75%
100%
86
80
Table 3: Summary Results of Estimated Efficiency for
100 hp Motor at Various Loads Using TPT and OPT
Motor Load
3/4
95.8
95.3
95.3
95.2
95.3
95.3
95.9
95.9
96.2
-
25&50
25&75
25&100
90
Efficiency
Table 2: Field Test Data of 100 hp Induction Motor
The estimated efficiencies using the TPT at various
loads of the 15 and 10 hp motors can be illustrated in
Figs. 4 and 5 respectively. For the OPT, the estimated
efficiencies of various motors at 1/4, 2/4, 3/4 and 4/4 load
can also be illustrated in Figs. 6 to 9 respectively.
Efficiency
or 100%. The data of the TPT (two operating points) can
be at any 25 and 50%, or 25 and 75%, or 25 and 100%, or
50 and 75%, or 50 and 100%, or 75 and 100%.
IEEE112
IEC34-2
92
90
88
86
84
82
80
10 hp
15 hp
100 hp
Fig. 7: Estimated Efficiency Using OPT at 50 % load and
Standard Evaluations Versus Various Motors at 2/4 load
OPT
98
IEEE112
96
IEC34-2
Efficiency
94
92
90
88
86
84
82
10 hp
15 hp
100 hp
Fig. 8: Estimated Efficiency Using OPT at 75 % load and
Standard Evaluations Versus Various Motors at 3/4 load
OPT
IEEE112
IEC34-2
98
96
Efficiency
94
92
90
88
86
84
82
10 hp
15 hp
100 hp
Fig. 9: Estimated Efficiency Using OPT at 100 % load and
Standard Evaluations Versus Various Motors at 4/4 load
Figs. 3 to 9 show that the agreement between the
estimated and measured efficiencies of the actual motors
is good (error less than 2%). As mentioned in Section 2,
the stray load loss was not ignored in this analysis; as a
result the accurate value of motor efficiency can be
obtained. The effect of neglecting the stray load loss can
lead to dramatic error in the estimation. It is clear that the
motor replacement decisions can not be made using such
large errors in estimation since these errors can wipe out
the potential savings, thus leading to a wrong decision.
Considering the results obtained, it can be stated with
confidence that this simple fast and low-cost technique
will yield virtually the same results as using the
sophisticated standard tests.
4. CONCLUSION
The proposed technique for estimating efficiency of
induction motor in the field has been described. This
technique relies on the on-site measurement of the input
voltage, current, power and actual motor shaft speed
without conducting no-load and blocked-rotor tests. The
measured data need not be close to no-load or full-load
values and can be only one or two operating points. These
data are used for estimating the parameters of equivalent
circuit which includes the proposed stray load loss
parameter for improving the accuracy. The estimated
efficiency is then determined by using these parameters.
Note that the OPT can accurately estimate the efficiency
only at that test point while the TPT can accurately
estimate at any motor load. The important advantages of
this proposed technique over other methods are that it is a
simple procedure, therefore it is possible to estimate the
efficiency of motor in a short time while the motor is in
service (its shaft is permanently connected to its load)
without the removal of motor from service and that such
procedure is inexpensive. The results indicate that the
acceptable estimated efficiencies compared to those
achieved from standard tests are obtainable (error less
than 2 %). However, it is recommended when using this
technique that the input frequency and revolutions per
minute must be measured as accurately as possible. It is
worth noting that this proposed technique is suitable for
conducting on-site energy audits of actual motors which
are then used to project a cost savings and payback. This
information can then be used to guide the decisions
regarding the investment in higher efficiency motors.
5. REFERENCES
[1] J. S. Hsu, and B. P. Scoggins, “Field Test of Motor
Efficiency and Load Changes Through Air-gap
Torque,” IEEE Trans. Energy Conv., vol. 10, no. 3,
pp. 471-477, Sept. 1995.
[2] J. S. Hsu, J. D. Kueck, M. Olszewski, D. A. Casada,
P. J. Otaduy, and L. M. Tolbert, "Comparison of
Induction Motor Field Efficiency Evaluation
Methods," IEEE Transactions on Industry Appli.,
vol. 34, issue:1, pp. 117-125, Jan./Feb. 1998.
[3] F. Alonge, F D’Ippolito, G. Ferrante and F.M.
Raimondi, “Parameter Identification of Induction
Motor Model Using Genetic Algorithms,” IEE
Proc.,Control Theory Appl., vol. 145, no. 6, pp. 587593, Nov. 1998.
[4] A. Gastli, “Identification of Induction Motor
Equivalent Circuit Parameters Using the Singlephase Test,” IEEE Transactions on Energy Conv.,
vol. 14, no. 1, pp. 51-56, March 1999.
[5] K. S. Huang, Q. H. Wu, and D. R. Turner, “Effective
Identification of Induction Motor Parameters Based
on Fewer Measurements,” IEEE Trans. Energy
Conv., vol. 17, no. 1, pp. 55-60, March 2002.
[6] T. Phumiphak and C. Chat-uthai, “Estimation of
Induction Motor Parameters Based on Field Test
Coupled with Genetic Algorithm,” Proceedings of
2002 International Conference on Power System
Technology, PowerCon 2002, vol. 2, Kunming,
China, pp. 1199–1203, Oct. 2002.
[7] IEEE Standard Test Procedure for Polyphase
Induction Motor and Generators, IEEE Standard
112-1996, New York, May 1997.
[8] Methods for Determining Losses and Efficiency of
Rotating Electrical Machinery from Tests
(Excluding Machines for Traction Vehicles), IEC
34-2, 1972.
[9] Technology Procurement Project IEA Hi-Motor
Competition, Jury Report, International Energy
Agency, Demand Side Management, Dec. 1998.
[10] Motor Efficiency Test : According to IEEE Std 1121996 (Method B), Test Report, Metropolitan
Electricity Authority (MEA), Thailand, March 2002.