A study of voltage sags in electric motors
Development of a sag generator
John J. Pérez
Grupo de investigación
CALPOSALLE
Universidad de La Salle
Bogotá. Colombia
jhperez@lasalle.edu.co
Camilo A. Cortés
Grupo de investigación
CALPOSALLE
Universidad de La Salle
Bogotá. Colombia
camilocortes@lasalle.edu.co
Abstract - One of the biggest problems in Power Quality is
the disconnection of electric motors due to the presence of
sags. In order to study this phenomenon it became
necessary to build a sag generator to, for example, develop
immunity curves for induction motors as well as for
synchronous motors. In this way it is possible to evaluate
the quantitative impact of sags in these machines. This
paper also analyzes the effects in current, torque and speed
caused by sags, in particular A, B and C types.
addition, this paper presents the methodology used for the
assembly of the laboratory tests.
II.
Sag
100
Voltage Sags are one of the biggest problems in Power
Quality [1], having a particular influence in electric motors.
The impact of PQ in electric motors is an important topic; for
example, more than 50% of the electricity consumption in the
developed countries and approximately 65% of the electricity
used in industry is consumed by electric motors [2]. Thus, it
became necessary to develop a research instrument, in this case
a sag generator for sags type A, B and C, to analyze their
effects in different equipments.
The main effects produced by voltage sags in electric
motors are motor speed loss, peak current and torques that
appear with the voltage recovery [3].
In this work, immunity curves were developed for an
induction motor and a synchronous motor. The most important
results are also depicted for three types of sags: A, B and C. In
50
V o ltage (V )
INTRODUCTION
CLASSIFICATION OF VOLTAGE SAGS
A voltage sag, according to IEEE 1159 – 1995 standard, is
a momentary decrease (10% - 90%) in the RMS voltage
magnitude for a duration from 0.5 cycles to 1 minute, see Fig. 1
[4].
Keywords – sag, immunity curve, DAQ, induction motor,
synchronous motor
I.
Álvaro Gómez
Grupo de investigación
CALPOSALLE
Universidad de La Salle
Bogotá. Colombia
labielec@jupiter.lasalle.edu.co
0
-50
-100
10
20
30
40
Cicles (Sec)
50
60
70
Figure 1. Voltage Sag.
The most common voltage sags are types A, B, C and D.
The following is the characterization of A, B and C sags [5].
Type A is produced because of three phase faults, which
produce a voltage reduction in the three phases of the electrical
system, see Fig. 2. Type B is due to single phase faults and it
produces a voltage reduction in one phase of the electrical
system, see Fig. 3. Type C is due to phase to phase faults and it
produces a voltage reduction in two phases of the electrical
system together with phase-angle jumps, as it is shown in Fig.
4 [5].
phase, electric power, and mechanical parameters such
as torque, speed and mechanical power.
•
Industrial computer PXI. Pentium IV 1.7 GHz, bus 533
MHz, 1 GB Ram.
•
Acquisition card for the PC. 16 single-ended or 8
differentials (software-selectable per channel).
Resolution of 12 bits, 1 in 4,096. Maximum sampling
rate of 1.25 MS/s. Analog input 16 channels, Digital
Input/Output 16 channels and 2 analog outputs.
•
Module of signal acquisition: SCC Analog Input
(AI01, AI02), 2 channels per phase. Digital Output
(DO01) and analog output (AO10).
•
Network analyzer.
The source used for the laboratory tests is controlled by
means of an algorithm, in order to produce sags. Software was
also implemented to conduct the following operations
according to the flow chart showed in Fig. 6.
Figure 2. Sag type A
1) Start of the process: laboratory test. (Definition of the
total duration of the test).
2) Definition of parameters: definition of sag duration,
selection of sag type (A, B or C).
3) Operation of sag generator: According to the time
duration and the sag type.
4) Application of sags to the motor under test.
5) Reading the acquired data (voltages, currents, torque,
speed) and sending them to Excel (Data base)
6) Ending the execution of the test in a safe way.
Figure 3. Sag type B
Fig. 7 illustrates the visualization of mechanical parameters
such as speed and torque as well as electrical parameters such
as power and current. This corresponds to additional software
that was developed using Labview.
.
Figure 4. Sag type C
III.
LABORATORY IMPLEMENTATION OF A SAG
GENERATOR (TYPES A, B AND C)
A sag generator was developed in the atmosphere of virtual
instrumentation of Labview (VI). The equipment used for the
generator development was:
•
A three-phase variable voltage source of 220/380, 10 A
and 15 A with digital and analog input and output.
•
Modules of measurement. Measurement of electrical
and mechanical parameters: voltage and current per
Figure 5. Developed Sag generator in Laboratory
IV.
Begin
Definition
Sag duration
Select Type of Sag
Sag Generator
Motor
DEVELOPMENT OF IMMUNITY CURVES
The Cbema immunity curve was originally developed by
Cbema (Technology Industry Council), to describe tolerance of
main frame computer equipment to the magnitude and duration
of voltage variations in power systems in conditions of
undervoltages and overvoltages [6].
In 1996 the Cbema curve [6] was substituted by the
Information Technology Industry Council (ITIC) curve [7].
This curve describes an AC input voltage envelope that can be
typically tolerated by most IT equipment without any loss of
function.
Failure
Daq
Excel
End
Figure 6. Flow chart of the implemented software for sag tests in electric
motors
Figure 8. Algorithm to generate sags type A.
A. Methodology of Study
A methodology is proposed in order to test different electric
motors, and to develop immunity curves using the proposed
laboratory. The steps of the methodology are:
1) Development of a laboratory to test A, B and C sag
effects in electric motors.
The assembly has an electric source, a sag source, an
electric motor, an electric generator and a load, as Fig. 9
illustrates.
Figure 7. Visualization of electrical and mechanical parameters of the motor
Four algorithms were developed in order to generate the
different types of sags:
1. Type A (Reduction of the three phases)
2. Type B (Reduction of one phase)
3. Type C (Reduction of two phases), 2 algorithms.
In general the sag generator is able to reduce the voltage of
the phases between 0.1 and 0.9 p.u. of the RMS nominal value.
Fig. 8 shows one of the developed algorithms that allows the
elaboration of immunity curves for the selected motors: the
algorithm developed for the type A.
Figure 9. Assembly of the laboratory tests
2)
Test development
a) Voltage sags were applied in magnitude steps of
10%, from 10% to 90% of the RMS nominal value.
b) Each motor was loaded with two types of load
scenes: 50% of load and 100% of load.
c) Voltage sags were applied in time steps of 100 ms,
from 100 ms to 10000 ms of total sag duration.
d) Sensitivity curves were obtained using the laboratory
tests, having these parameters: torque vs. time, speed vs. time
and current vs. time.
3) Motor selection
In the laboratory, it is possible to test electric motors from
0.5 kW up to 5 kW, having nominal voltage from 220 V to 380
V. This paper shows results of tests for the two motors
described in Tables I (squirrel cage induction motor) and II
(synchronous motor).
TABLE I.
Power
Speed
Frequency
Voltage
Poles
TABLE II.
Power
Speed
Frequency
Voltage
Poles
The immunity curve of Fig. 10 shows that the motor failed
after 500 ms when there is a voltage decrease of 30% of
magnitude for the three types of sags.
2) Case 2 Induction motor, 50% of Load
The immunity curve of Fig. 11 shows that the motor,
having a load of 50%, failed after 1500 ms due to a voltage
decrease of 30% in the three types of sags.
PARAMETERS OF THE INDUCTION MOTOR
1.1 kW
3600 r.p.m.
60 Hz
220 V
2
Figure 11. Immunity curve, induction motor with 50% of load
PARAMETERS OF SYNCHRONOUS MOTOR
0.5 kW
1800 r.p.m.
60 Hz
220 V
4
3) Synchronous motor, 100% of Load
Fig. 12 shows the immunity curve for the synchronous
motor of 0.6 kW, having the same conventions of the previous
curves. The test shows that the motor failed after 500 ms due to
a voltage decrease of 25% in the three types of sags.
4) Development of immunity curve:
Data (mechanical and electrical parameters) are acquired in
Excel, where it is possible to develop immunity curves for
different load cases.
B. Immunity Curves
After testing and validating the sag generator, immunity
curves were developed for the induction motor of 1.1 kW and
the synchronous motor of 0.5 kW. The National Electrical
Code 2002 [8] was considered (Numeral and articles 70 - 284,
70-126) in order to determine the moments for the electric
motor disconnection.
1) Case 1: Induction motor, 100% of Load
Fig. 10 shows an immunity curve for the induction motor of
1.1 kW. The blue region presents the case for type C; the
turquoise region presents the case for sags type A. Note that the
blue region covers most of the turquoise region. Finally, the
orange region presents the case for sags type B, which includes
also the regions of types A and C.
Figure 12. Immunity curve, synchronous motor with 100% of load
V.
A.
Figure 10. Immunity Curve, induction motor with 100% of load
SENSITIVITY CURVES
This section presents some sensitivity curves, which were
developed for the parameters torque vs. time, speed vs. time,
and current vs. time. These curves were obtained for the
induction motor.
Tests for sags type A
Fig. 13 shows a sensitivity curve of speed vs. time for sag
duration of 3000 milliseconds and a Sag depth of 90%. The
induction motor decreases its speed from 3450 r.p.m. to 400
r.p.m., resulting in a speed loss of 88.4% with regard to the
nominal speed of the machine. For a sag depth of 70%, the
speed decreases from 3450 r.p.m. to 750 r.p.m., resulting in a
speed loss of 78.2%.
14
4000
12
Sag 10% Phase A
Sag 10% Phase B
Sag 10% Phase C
Sag 90% Phase A
Sag 90% Phase B
Sag 90% Phase C
3500
10
Current (A)
Speed (r.p.m.)
3000
2500
2000
1500
Sag 90%
Sag 70%
Sag 50%
Sag 30%
Sag 10%
1000
500
8
6
4
2
0
0
1
61
121
181
241
301
361
421
481
1
541
Figure 13. Sensitivity curve Speed vs. Time. Induction motor, sags type A of
3000 ms
B. Tests for sags type B
Fig. 14 shows a significant reduction in the torque of the
induction motor, which is reduced from 2.3 Nm to 0.5 Nm.
This reduction goes with different oscillations. Once the
machine recovers the nominal voltage in the operation an
oscillating torque appears, which raises until a value of 2.8 Nm.
This torque is well-known in the literature as the reacceleration
of the machine [9].
3
2.7
2.4
11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 171
time (cicles)
Figure 15. Sensitivity curve for Current vs. Time. Induction motor, sags type
C of 1000 ms, magnitude of 10% and 90%.
TABLE III.
Time
Sag (ms)
1000
1000
1000
2000
2000
2000
3000
3000
3000
SUMMARY OF TEST, SAG TYPE A, INDUCTION MOTOR
Magnitude of
Sag
10%
50%
90%
10%
50%
90%
10%
50%
90%
% Loss
Speed
0%
4%
5%
1%
28%
37%
1%
43.4%
88%
% Loss
Torque
0%
1%
2%
0%
17%
26%
0%
56.5%
78%
Operation of
machine
Acceptable
Acceptable
Acceptable
Acceptable
Unacceptable
Unacceptable
Acceptable
Unacceptable
Unacceptable
Torque (N*m)
2.1
TABLE IV.
1.8
1.5
1.2
0.9
0.6
0.3
0
1
21 41 61 81 101 121 141 161 181 201 221 241 261 281 301 321 341 361 381
time (Cicles)
Figure 14. Sensitivity curve for Torque vs. Time. Induction motor, sag type B,
3000 ms, magnitude of Sag 90 % of depth.
C. Tests for sags type C
Fig. 15 shows two sensitivity curves for the phase A, B and
C currents of the motors, when it is applied a sag type C having
two depths: 10% and 90% of the RMS nominal voltages. The
duration of the sags was 1000 ms.
The Figure shows how the phase C current increases, in the
case of a 90% sag, from an operation value of 3A up to a value
of 13.5 A, i.e., almost 5 times its nominal value. For a 10% sag
the increase of the current was up to 12A.
D. Other results
Tables III and IV show a summary of other tests for sags
type A and C. Duration and magnitude of the sags and the
percentage speed loss and torque are shown. Finally, the
operation of the induction motor is classified as acceptable or
unacceptable.
Time
Sag (ms)
1000
1000
1000
2000
2000
2000
3000
3000
3000
SUMMARY OF TEST, SAG TYPE C, INDUCTION MOTOR
Magnitude of
Sag
10%
50%
90%
10%
50%
90%
10%
50%
90%
% Loss
Speed
0%
4%
8%
1%
8%
39%
1%
49%
95%
% Loss
Torque
0%
1%
2%
0%
3%
24%
0%
64%
83%
Operation of
machine
Acceptable
Acceptable
Unacceptable
Acceptable
Acceptable
Unacceptable
Acceptable
Unacceptable
Unacceptable
Tables III and IV show that for the cases of sag durations
larger than 1000 ms, the loss in speed and torque can be
superior to 50%. Moreover, in the cases of sag durations
smaller than 1000 ms, these oscillations can affect processes
that require constant speed or torque, as it is the case of several
industrial processes.
VI.
CONCLUSION
This paper shows the construction of a sag generator for
studies of Power Quality, which allows the development of
immunity and sensitivity curves to determine the quantitative
effects in the operation of electric motors. In addition, a
discussion of a group of immunity and sensitivity curves was
presented.
Measured and observed effects in the tests can cause:
•
Possible damages in the mechanical structure of the
machine, such as damages in bearings, in the shaft, etc.
•
Possible damages in the isolation due to the increase of
current in the windings, which produces heating.
•
Loss of useful life, as resulting from the mentioned
effects.
•
In the experimentation mechanical damages were
noticed after performing a large amount of tests. These
damages were evidenced as, for example, audible
vibrations in the machine.
ACKNOWLEDGMENT
The authors express their gratefulness to Universidad de La
Salle, Bogotá – Colombia, which financed the project number
34-342-06-2-01. The authors express a special gratefulness to
Professor Estrella Parra of Universidad Nacional de Colombia
for her contributions in the development of this paper.
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