U.S. patent application number 12/208272 was filed with the patent office on 2010-03-11 for motor protection using accurate slip calculations.
Invention is credited to Subhash C. Patel, Stanley E. Zocholl.
Application Number | 20100060227 12/208272 |
Document ID | / |
Family ID | 41785037 |
Filed Date | 2010-03-11 |
United States Patent
Application |
20100060227 |
Kind Code |
A1 |
Zocholl; Stanley E. ; et
al. |
March 11, 2010 |
MOTOR PROTECTION USING ACCURATE SLIP CALCULATIONS
Abstract
An accurate slip calculation for providing monitoring and
protection to an electric motor. The slip calculation is made using
a minimum value of stator resistance as the initial stator
resistance, where the minimum value of stator resistance is the
minimum value of stator resistance calculated during an initiation
period of the motor. The initiation period may be a predetermined
time period or a predetermined number of cycles during the motor
startup. The initiation period may start after a predetermined
settling time or after a predetermined condition is met.
Inventors: |
Zocholl; Stanley E.;
(Holland, PA) ; Patel; Subhash C.; (West Chester,
PA) |
Correspondence
Address: |
Schweitzer Engineering Laboratories, Inc.;Richard Edge
2350 NE HOPKINS COURT
PULLMAN
WA
99163-5603
US
|
Family ID: |
41785037 |
Appl. No.: |
12/208272 |
Filed: |
September 10, 2008 |
Current U.S.
Class: |
318/778 |
Current CPC
Class: |
H02H 6/005 20130101;
H02H 7/0816 20130101 |
Class at
Publication: |
318/778 |
International
Class: |
H02P 1/26 20060101
H02P001/26 |
Claims
1. A method for providing thermal protection and monitoring to a
three-phase electric motor during startup of the motor, comprising:
sampling an electrical signal to the motor; calculating
positive-sequence voltage values from the sampled electrical signal
during a startup period; calculating positive-sequence current
values from the sampled electrical signal during the startup
period; calculating resistance values from the calculated
positive-sequence voltage values and the calculated
positive-sequence current values during the startup period;
determining an initial resistance value equal to a minimum of the
resistance values calculated during a predetermined initiation
period; calculating slip values from the initial resistance value;
calculating rotor resistance values from the slip values; and,
calculating rotor temperature from the rotor resistance values and
current values sampled from the power signal to the motor.
2. The method of claim 1, wherein the step of calculating the
resistance values comprises calculating motor resistance
values.
3. The method of claim 2, wherein the step of determining an
initial resistance value comprises determining an initial motor
resistance value equal to a minimum of the motor resistance values
calculated during the predetermined initiation period.
4. The method of claim 2, further comprising the step of
calculating stator resistance values from the motor resistance
values.
5. The method of claim 4, wherein the step of determining an
initial resistance value comprises determining an initial stator
resistance value equal to a minimum of the stator resistance values
calculated during the predetermined initiation period.
6. The method of claim 1, wherein the startup period comprises the
predetermined initiation period.
7. The method of claim 6, wherein the predetermined initiation
period comprises a period including a number of power system
cycles.
8. The method of claim 6, wherein the predetermined initiation
period comprises a period including the first ten power system
cycles during the startup period.
9. The method of claim 6, wherein the predetermined initiation
period comprises a period including the first four power system
cycles during the startup period.
10. The method of claim 6, wherein the predetermined initiation
period begins after the first power system cycle.
11. The method of claim 10, wherein the predetermined initiation
period begins after a predetermined condition is satisfied.
12. The method of claim 1, wherein the step of calculating rotor
temperature comprises: calculating positive-sequence rotor
resistance values from the slip values; calculating
negative-sequence rotor resistance values from the slip values;
and, calculating rotor temperature values from the calculated
positive-sequence rotor resistance values, the negative-sequence
rotor resistance values, and the current values sampled from the
power signal to the motor.
13. A system for monitoring a rotor temperature of a rotor of a
three-phase electric motor during a startup period, comprising; a
three-phase electric motor comprising a rotor, a stator, and an
electric power input for providing electric power to the electric
motor; a current transformer in communication with the electric
power input for providing a current signal; a potential transformer
in communication with the electric power input for providing a
voltage signal; a first sampler in communication with the current
transformer for sampling the current signal to provide current
samples; a second sampler in communication with the potential
transformer for sampling the voltage signal and to provide voltage
samples; a positive-sequence current calculator in communication
with the first sampler for calculating positive-sequence current
values from the current samples; a positive-sequence voltage
calculator in communication with the second sampler for calculating
positive-sequence voltage values from the voltage samples; a
resistance calculator in communication with the positive-sequence
current calculator and the positive-sequence voltage calculator for
calculating resistance values from the positive-sequence current
values and the positive-sequence voltage values; an initial
resistance calculator in communication with the resistance
calculator for determining an initial resistance value equal to a
minimum of the resistance values during a predetermined initiation
period; a slip calculator in communication with the initial
resistance calculator and the resistance calculator for determining
slip values from the initial resistance value and the resistance
values; a rotor resistance calculator in communication with the
slip calculator for calculating slip-dependent rotor resistance
values from the slip values; and a rotor temperature calculator in
communication with the rotor resistance calculator and the first
sampler, for calculating rotor temperature values from the current
values and the slip-dependent rotor resistance values.
14. The system of claim 13, wherein the resistance calculator
comprises a motor resistance calculator and the resistance values
comprises motor resistance values.
15. The system of claim 14, wherein the initial resistance value
comprises an initial motor resistance value equal to a minimum of
the motor resistance values calculated during the predetermined
initiation period.
16. The system of claim 14, further comprising a stator resistance
calculator in communication with the motor resistance calculator
for calculating stator resistance values from the motor resistance
values.
17. The system of claim 16, wherein the initial resistance value
comprises an initial stator resistance value equal to a minimum of
the stator resistance values calculated during the predetermined
initiation period.
18. The system of claim 13, wherein the startup period comprises
the predetermined initiation period.
19. The system of claim 18, wherein the predetermined initiation
period comprises a period including a number of power system
cycles.
20. The system of claim 18, wherein the predetermined initiation
period comprises a period including the first ten power system
cycles during the startup period.
21. The system of claim 18, wherein the predetermined initiation
period comprises a period including the first four power system
cycles during the startup period.
22. The system of claim 18, wherein the predetermined initiation
period comprises a period beginning after the first power system
cycle.
23. The system of claim 22, wherein the predetermined initiation
period begins after a predetermined condition is satisfied.
24. The system of claim 13, wherein: the rotor resistance
calculator is further configured to: calculate positive-sequence
rotor resistance values from the slip values; and, calculate
negative-sequence rotor resistance values from the slip values;
and, the rotor temperature calculator is further configured to
calculate rotor temperature values from the calculated
positive-sequence rotor resistance values, the negative-sequence
rotor resistance values, and the current values sampled from the
power signal to the motor.
25. An apparatus for monitoring a rotor temperature of a rotor of a
three-phase electric motor during a startup period, comprising; a
first sampler in communication with an electric power input to the
electric motor for sampling a current signal to provide current
samples; a second sampler in communication with the electric power
input for sampling a voltage signal and to provide voltage samples;
a positive-sequence current calculator in communication with the
first sampler for calculating positive-sequence current values from
the current samples; a positive-sequence voltage calculator in
communication with the second sampler for calculating
positive-sequence voltage values from the voltage samples; a
resistance calculator in communication with the positive-sequence
current calculator and the positive-sequence voltage calculator for
calculating resistance values from the positive-sequence current
values and the positive-sequence voltage values; an initial
resistance calculator in communication with the resistance
calculator for determining an initial resistance value equal to a
minimum of the resistance values during a predetermined initiation
period; a slip calculator in communication with the resistance
calculator and the initial resistance calculator for determining
slip values from the resistance values and the initial resistance
value; a rotor resistance calculator in communication with the slip
calculator for calculating slip-dependent rotor resistance values
from the slip values; and a rotor temperature calculator in
communication with the rotor resistance calculator and the first
sampler, for calculating rotor temperature values from the current
values and the slip-dependent rotor resistance values.
26. The apparatus of claim 25, wherein the resistance calculator
comprises a motor resistance calculator and the resistance values
comprises motor resistance values.
27. The apparatus of claim 26, wherein the initial resistance value
comprises an initial motor resistance value equal to a minimum of
the motor resistance values calculated during the predetermined
initiation period.
28. The apparatus of claim 26, further comprising a stator
resistance calculator in communication with the motor resistance
calculator for calculating stator resistance values from the motor
resistance values.
29. The apparatus of claim 28, wherein the initial resistance value
comprises an initial stator resistance value equal to a minimum of
the stator resistance values calculated during the predetermined
initiation period.
30. The apparatus of claim 26, wherein the startup period comprises
the predetermined initiation period.
31. The apparatus of claim 30, wherein the predetermined initiation
period comprises a period including a number of power system
cycles.
32. The apparatus of claim 30, wherein the predetermined initiation
period comprises a period including the first ten power system
cycles during the startup period.
33. The apparatus of claim 30, wherein the predetermined initiation
period comprises a period including the first four power system
cycles during the startup period.
34. The apparatus of claim 30, wherein the predetermined initiation
period comprises a period beginning after the first power system
cycle.
35. The apparatus of claim 34, wherein the predetermined initiation
period begins after a predetermined condition is satisfied.
36. The apparatus of claim 25, wherein: the rotor resistance
calculator is further configured to: calculate positive-sequence
rotor resistance values from the slip values; and, calculate
negative-sequence rotor resistance values from the slip values;
and, the rotor temperature calculator is further configured to
calculate rotor temperature values from the calculated
positive-sequence rotor resistance values, the negative-sequence
rotor resistance values, and the current values sampled from the
power signal to the motor.
Description
RELATED APPLICATION
[0001] None.
TECHNICAL FIELD
[0002] This disclosure relates to thermal monitoring and protection
of electric motors. More particularly, this disclosure relates to
monitoring and protecting an electric motor using a resistance
calculated from an accurate determination of slip.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] Non-limiting and non-exhaustive embodiments of the
disclosure are described, including various embodiments of the
disclosure with reference to the figures, in which:
[0004] FIG. 1a is a block diagram of a three-phase motor and an
intelligent electronic device (IED);
[0005] FIG. 1b is a circuit diagram of a rotor thermal model for an
electric motor;
[0006] FIG. 2 is a diagram showing rotor temperature, current, and
slip during motor startup;
[0007] FIG. 3 is a diagram showing per unit motor resistance during
motor startup;
[0008] FIG. 4 is a diagram showing rotor temperature, current, and
slip during motor startup;
[0009] FIG. 5 is a diagram showing per unit motor resistance during
motor startup;
[0010] FIG. 6 is a diagram showing per unit slip during motor
startup;
[0011] FIG. 7 is a diagram showing per unit slip during motor
startup;
[0012] FIG. 8 is a flowchart illustrating a method of monitoring
and protecting an electric motor; and
[0013] FIG. 9 is a block diagram illustrating an apparatus and
system for providing protection to an electric motor.
DETAILED DESCRIPTION
[0014] Three-phase motors are widely used throughout industry to
transform electrical energy to mechanical energy which may be used
to perform work. Motors are often necessary pieces of equipment for
performing numerous industrial tasks from pumping water to
processing materials. Because motors are such an integral part of
many industries, loss of use of a motor can cause great delays and
loss of income while the motor is off line. Therefore, motors are
monitored and protected against overheating, which is one incident
that can shorten a motor's productive lifetime, requiring
replacement of or maintenance to the motor. Further, the monitoring
and protection of motors is carefully controlled such that a motor
is not taken off line unless it is indeed experiencing a condition
that would warrant such action. For example, motor protection
schemes meant to interrupt a motor startup due to overheating
attempt to determine as accurately as possible the temperature
conditions of the rotor. If the schemes are overly conservative,
the motor startup would be prematurely stopped, resulting in
unnecessary and disruptive downtime. However, if the schemes
underestimate the actual temperature conditions, the motor may
experience unnecessary and premature harm due to the elevated
temperature conditions.
[0015] Intelligent electronic devices (IEDs) are often used to
monitor various aspects of electric motors and provide protection
thereto. Using certain values provided by the motor manufacturer as
well as the currents and voltages supplied to the motor, IEDs can
be programmed to determine various conditions of the motor and
provide protection to the motor by taking the motor off line when
certain conditions are determined to be present in the system. For
example, the IED may be programmed to determine the temperature of
the rotor and take the motor off line if the temperature exceeds a
certain value.
[0016] Protection against overheating of the rotor is especially
important during the startup of the motor. Certain physical
conditions during startup result in the rotor temperature
increasing rapidly. Motors typically include a rotor with windings
through which an alternating current flows, causing a magnetic
field in the rotor windings. The rotor winding may consist of bars
parallel to the motor shaft. End rings connect the bars to form a
short-circuited assembly. Similarly, the motor typically includes a
stator with similar windings and resulting magnetic fields. The
magnetic fields cause the rotor to spin. During startup, the
magnitude of the current through the windings is much higher than
it is at the rated speed of the motor. This increased current
causes the temperature in the windings to increase dramatically.
When the rotor finally reaches its rated speed, the current flowing
through the windings decreases considerably and the temperature
slowly decreases as well.
[0017] Further, when the rotor is not moving (known as "locked
rotor" condition), the frequency of the current, voltage, and
magnetic flux in the windings of the rotor is the rated frequency
of the power system supplying electric power to the motor
(typically near 60 Hz in the US, 50 Hz in Europe). This high
frequency results in the majority of the current flowing through
the "skin" of the rotor (known as the "deep bar effect"). Because
the cross-sectional area of the conductor through which the current
flows is effectively decreased, the resistance is increased,
resulting in a temperature increase that is more rapid than if the
current were flowing through the entire cross-sectional area of the
conductor. As the rotor reaches its rated speed the frequency of
the current, voltage, and magnetic flux in the windings thereof
decreases to the slip at rated speed (relatively low). As the
frequency in the rotor decreases, so does the deep bar effect and
the resistance of the rotor. Thus, the rate at which heat is
produced by the rotor decreases as the rotor approaches its rated
speed.
[0018] Accordingly, for proper protection of the motor, it is
critical to monitor the rotor temperature during startup, when the
temperature is increasing at its highest rate. Accurate
calculations of rotor temperature during startup are helpful in
providing proper protection during startup.
[0019] FIG. 1a illustrates a block diagram of a system 100
including a three-phase motor 140 and a protective IED 120. The
motor includes inputs from each phase of electric power 102A, 102B,
and 102C. The inputs each include a current transformer 110A, 110B,
110C for providing inputs to the IED representing the currents from
each phase to the motor 140. The inputs each also include a
potential transformer 108A, 108B, and 108C for proving inputs to
the IED representing the voltages of each phase to the motor 140.
With the current and voltage signals from each phase, the IED 120
can monitor various conditions of the motor 140 including the rotor
temperature.
[0020] The embodiments of the disclosure will be best understood by
reference to the drawings, wherein like parts are designated by
like numerals throughout. It will be readily understood that the
components of the disclosed embodiments, as generally described and
illustrated in the figures herein, could be arranged and designed
in a wide variety of different configurations. Thus, the following
detailed description of the embodiments of the systems and methods
of the disclosure is not intended to limit the scope of the
disclosure, as claimed, but is merely representative of possible
embodiments of the disclosure. In addition, the steps of a method
do not necessarily need to be executed in any specific order, or
even sequentially, nor need the steps be executed only once, unless
otherwise specified.
[0021] In some cases, well-known features, structures or operations
are not shown or described in detail. Furthermore, the described
features, structures, or operations may be combined in any suitable
manner in one or more embodiments. It will also be readily
understood that the components of the embodiments as generally
described and illustrated in the figures herein could be arranged
and designed in a wide variety of different configurations.
[0022] Several aspects of the embodiments described will be
illustrated as software modules or components. As used herein, a
software module or component may include any type of computer
instruction or computer executable code located within a memory
device and/or transmitted as electronic signals over a system bus
or wired or wireless network. A software module or component may,
for instance, comprise one or more physical or logical blocks of
computer instructions, which may be organized as a routine,
program, object, component, data structure, and the like, that
performs one or more tasks or implements particular abstract data
types.
[0023] In certain embodiments, a particular software module or
component may comprise disparate instructions stored in different
locations of a memory device, which together implement the
described functionality of the module. Indeed, a module or
component may comprise a single instruction or many instructions,
and may be distributed over several different code segments, among
different programs, and across several memory devices. Some
embodiments may be practiced in a distributed computing environment
where tasks are performed by a remote processing device linked
through a communications network. In a distributed computing
environment, software modules or components may be located in local
and/or remote memory storage devices. In addition, data being tied
or rendered together in a database record may be resident in the
same memory device, or across several memory devices, and may be
linked together in fields of a record in a database across a
network.
[0024] Embodiments may be provided as a computer program product
including a machine-readable medium having stored thereon
instructions that may be used to program a computer (or other
electronic device) to perform processes described herein. The
machine-readable medium may include, but is not limited to, hard
drives, floppy diskettes, optical disks, CD-ROMs, DVD-ROMs, ROMs,
RAMs, EPROMs, EEPROMs, magnetic or optical cards, solid-state
memory devices, or other types of media/machine-readable medium
suitable for storing electronic instructions.
[0025] FIG. 1b illustrates a first-order thermal model 150 of the
rotor temperature of the motor. The model 150 illustrates the
heating effect caused by the power supplied to the motor 152. The
heating effect is a sum of the positive-sequence current (I.sub.1)
and the negative-sequence current (I.sub.2) multiplied by their
respective resistances, thus,
R 1 R N I 1 2 + R 2 R N I 2 2 ##EQU00001##
where R.sub.N is the rotor resistance at rated speed (see Equation
1), R.sub.2 is the negative-sequence rotor resistance and R.sub.1
is the positive-sequence rotor resistance (see Equations 2 and
3).
[0026] The rotor resistance at rated speed R.sub.N can be
calculated using:
R N = 1 - FL .omega. Syn .omega. Eq . 1 ##EQU00002##
where FL.omega. is the full load speed and Syn.omega. is the
synchronous speed.
[0027] The positive-sequence rotor resistance R.sub.1 and the
negative-sequence rotor resistance R.sub.2 can be calculated using
Equations 2 and 3, respectively:
R.sub.1=(R.sub.M-R.sub.N)S+R.sub.N Eq. 2
R.sub.2=(R.sub.M-R.sub.N)(2-S)+R.sub.N Eq. 3
where R.sub.M is the locked rotor resistance (see Equation 4), and
S is motor slip.
[0028] The locked rotor resistance R.sub.M can be calculated
using:
R M = LRQ I L 2 Eq . 4 ##EQU00003##
where LRQ is the locked rotor torque in per unit of rated torque
and I.sub.L is the locked rotor current in per unit full load
current.
[0029] Turning again to FIG. 1b, the capacitive element 154
represents the thermal mass of the rotor and is calculated
using
R M R N , ##EQU00004##
with R.sub.M and R.sub.N being described above.
[0030] The resistive element 156 represents the cooling effect
present in the motor, and can be calculated using
I.sub.L.sup.2(T.sub.A-T.sub.0)
where T.sub.A is the locked rotor thermal limit time, starting with
the motor at ambient temperature, and T.sub.0 is the locked rotor
thermal limit time starting when the motor is at a run
temperature.
[0031] During startup, the model can be assumed to be adiabatic,
modeled by eliminating the resistive element 156. This results in
the temperature U being calculated by the combination of elements
152 and 154.
[0032] The combined effect of elements 152, 154, and 156 is
compared in comparator 158 against the threshold of
I.sub.L.sup.2T.sub.A. If it is greater than the threshold, then the
model produces an output 160 resulting in a trip signal, ultimately
causing the motor to trip off.
[0033] As can be seen above, determining whether to trip a motor
during startup depends on a comparison of the calculated thermal
effect U of the motor against a threshold value. The thermal effect
U on the motor is a function of motor resistance, which is, in
turn, a function of slip.
[0034] Rotor resistance R.sub.r is calculated using:
R.sub.r=(R.sub.M-R.sub.N)S+R.sub.N Eq. 5
where it is plainly seen that rotor resistance R.sub.r is a
function of slip S, locked rotor resistance, (R.sub.M, a constant),
and rotor resistance at rated speed, (R.sub.N, also a constant).
Because the thermal effect on the rotor is a function of the rotor
resistance, a more accurate calculation of slip will yield a more
accurate calculation of rotor resistance, in turn leading to
accurate temperature calculations and better motor protection and
monitoring.
[0035] Slip Scan be calculated using:
S = R N A ( R - R S_I ) - ( R M - R N ) Eq . 6 ##EQU00005##
where A is a constant, R is motor resistance (see Equation 7),
R.sub.S.sub.--.sub.I is initial stator resistance (see Equation 8),
and R.sub.M and R.sub.N are described above.
[0036] Motor resistance R can be calculated using:
R = real ( V 1 I 1 ) Eq . 7 ##EQU00006##
where V.sub.1 is the positive-sequence voltage calculated from the
voltage signals from the three phases, and I.sub.1 is the
positive-sequence current calculated from the current signals from
the three phases.
[0037] Initial stator resistance R.sub.S.sub.--.sub.I can be
calculated using either:
R S_I = R P - R M A Eq . 8 ##EQU00007##
where R.sub.P is the initial motor resistance.
[0038] The initial motor resistance R.sub.P can be calculated
using:
R P = real ( V 1 ( cyc ) I 1 ( cyc ) ) Eq . 9 ##EQU00008##
where V.sub.1(cyc) is a positive-sequence voltage at a selected
initial cycle during startup and I.sub.1(cyc) is a
positive-sequence current at a selected initial cycle during
startup. As discussed in more detail herein, initial motor
resistance R.sub.P may be calculated by determining the minimum of
the motor resistance R:
R.sub.P=minimum[R] Eq. 10
[0039] FIG. 2 is a diagram 200 showing traces during a typical
startup of an electric motor. As can be seen, the current into the
motor 206 remains at a relatively high level until the rotor
reaches its rated speed, seen at between about 650 and 700 power
system cycles. The current then drops to a relatively constant
value. The rotor temperature 204 is plotted on a per-unit basis. As
can be seen, the temperature 204 increases quickly during startup
until the rotor approaches and reaches its rated speed between
about 650 and 700 power system cycles. The temperature 204 reaches
a maximum value of around 80% of its maximum allowable temperature.
Once the rated speed is reached, the current decreases, and the
rotor temperature slowly decreases as well. The slip 202 is also
plotted on a per-unit basis. As can be seen, slip starts at a value
of 1 at startup, and decreases until the rotor reaches its rated
speed at between about 650 and 700 power system cycles, after which
it remains at a relatively constant and low value.
[0040] FIG. 3 is a diagram 300 illustrating the value of motor
resistance R 302 calculated using Equation 7 through the first 10
cycles during startup. As can be seen, the value of motor
resistance R 302 settles to a constant value shortly after the
first power system cycle. Because of this rapid settling, the
selected cycle for determining the a positive-sequence voltage
V.sub.1(cyc) and is a positive-sequence current I.sub.1(cyc) is not
important, so long as it is after settling (just over one cycle in
this example). The selected cycle for these calculations in FIG. 2
is the eighth power system cycle.
[0041] By comparison, FIG. 4 shows a diagram 400 of the same
calculations as in FIG. 2 during a different startup. As can be
seen, the current 406 values follow much the same trend as in FIG.
2. The calculated slip values 402, however, remain at a value of
one (except for one minor excursion between about 75 and 125 power
system cycles) for an extended period of time--until just after 300
power system cycles. Slip values 402 then decrease to a relatively
low value once the rotor reaches its rated speed. Because the slip
values remain high, the calculated rotor temperature values 404
also remain high, reaching a value of 100% before falling.
[0042] The IED performing the calculations whose results are
illustrated in FIG. 4 was programmed to reflect a slip value of one
if the slip calculation yielded a slip value greater than one. As
can be seen above, the slip and temperature values are dependant on
the initial stator resistance value (and/or the initial rotor
resistance value), which in turn depends on the positive-sequence
voltage and current values at a particular cycle. FIG. 5 is a plot
500 of the motor resistance values 502 calculated during the first
10 cycles of the same motor startup as in FIG. 4. Instead of
settling to a constant value shortly after the first power system
cycle as illustrated in FIG. 3, the motor resistance illustrated in
FIG. 5 varies. The value of initial motor resistance R.sub.P was
calculated using values at the eighth power system cycle 506,
yielding a motor resistance, R.sub.P value of 0.0289.
[0043] As can be seen in the plot 600 of FIG. 6, when the value of
stator resistance, R.sub.S, calculated using the motor resistance
at the eighth power system cycle, the calculated slip yielded
values 602 that exceeded one. Thus the IED reported values of one
for an extended period of time as previously mentioned and as
illustrated in FIG. 4.
[0044] The present disclosure describes a method of more accurately
calculating slip using the minimum value of initial motor
resistance R.sub.P calculated during an initiation period during
startup instead of a value calculated at a particular power system
cycle. Turning again to FIG. 5, illustrated is the minimum value of
motor resistance R 504 at the third power system cycle of 0.023.
Using this value for the initial motor resistance R.sub.P yields
the values for slip 702 plotted in FIG. 7, which illustrates a plot
700 of slip values. As can be seen, in FIG. 7 values for slip 702
remain below one.
[0045] Accordingly, the present disclosure includes accurate
calculations of slip using a minimum value of motor resistance R
calculated during an initiation period during startup, and using
that value as the initial motor resistance R.sub.P to determine
slip and ultimately to provide thermal monitoring and protection
during startup of the motor.
[0046] It should be noted that the stator resistance R.sub.S is a
function of the initial motor resistance R.sub.P and constants.
Thus, the present disclosure also includes accurate calculations of
slip using the initial stator resistance R.sub.S.sub.--.sub.I as
described above. For simplicity, this disclosure may include
descriptions using the initial motor resistance. Further, for
simplicity, this disclosure may refer to "initial resistance",
which includes either the initial motor resistance or the initial
stator resistance.
[0047] The initiation period during startup may include any
predetermined period during startup. For example, the initiation
period may be defined by a particular number of power system cycles
during startup, or a particular period of time during startup.
Further, the initiation period may begin after some predetermined
time during startup, or after some predetermined condition occurs.
This may be beneficial if the calculated motor resistance R
oscillates before settling, as can be seen in FIG. 3. For example,
the IED may include an overcurrent element that does not start the
initiation period counter until after the current exceeds a certain
value, or settles to a particular value. Typically such settling
requires only a few power system cycles.
[0048] The initiation period may be during the first 10 power
system cycles during startup. The initiation period may be during
the first 8 power system cycles during startup. The initiation
period may be during the first 6 power system cycles during
startup. The initiation period may be during the first 4 power
system cycles during startup. The initiation period may be from
about the beginning of the second power system cycle through the
tenth power system cycle during startup. The initiation period may
be from about the end of the second power system cycle through the
fourth power system cycle. The cycle counting may begin after the
first power system cycle after closing in the breakers to the
motor. The cycle counting may begin after a predetermined condition
is met such as a current condition detected by an overcurrent
element (for example, a current of 2.5 per unit), a settling
condition, or the like.
[0049] A method 800 for providing thermal monitoring and protection
to an electric motor is further described as illustrated in FIG. 8.
The method 800 starts 802 with determining whether the motor is
within its startup period 804. If not, the method proceeds to using
running motor monitoring and protection schemes 840 (assuming that
the motor is running). If the motor is within its startup period,
then the method determines whether the motor is within its
initiation period as discussed above 806. If the motor is within
the initiation period, the method starts by calculating the rotor
temperature 807 using rotor resistance R.sub.r equal to R.sub.M.
The method continues to the steps where the initial resistance
value is determined starting with the steps of sampling the
electrical signals into the motor 808. Positive-sequence voltage
V.sub.1 and current I.sub.1 values are then calculated 810. From
those values, the motor resistance R is then determined 812. The
values of the motor resistance R are then used to calculate values
of stator resistance R.sub.S 814. The method then determines if the
calculated value of stator resistance is the minimum value, and
stores it as the initial stator resistance value
R.sub.S.sub.--.sub.I if it is the minimum 816. The rotor
temperature is then compared against a predetermined threshold 828.
If the rotor temperature exceeds the predetermined threshold, then
the method trips the motor 830 and ends 832. Otherwise, the method
returns to the step of determining whether the motor is within the
startup period 804.
[0050] Turning back to the step of determining whether the motor is
within the initiation period 806, if the motor is not within the
initiation period, the method continues to sample the electrical
signals into the motor 818. The method then calculates the motor
resistance R as described above 820. The method proceeds to
calculate slip, S, 822 using the values of motor resistance R
initial stator resistance R.sub.S.sub.--.sub.I locked rotor
resistance R.sub.M rotor resistance at rated speed R.sub.N and
constant A. The method then calculates the rotor resistance R.sub.r
824 using the rotor resistance R.sub.M rotor resistance at rated
speed R.sub.N and slip S values. The method then calculates the
rotor temperature 826 and compares the rotor temperature against a
predetermined threshold 828. If the rotor temperature exceeds the
predetermined value, then the method trips the motor 830 and ends
832. Otherwise, the method returns to step 804 to continue
monitoring and protecting the motor during startup.
[0051] An apparatus and system for monitoring and protecting
electric motors using an accurate slip calculation are also
disclosed in the present specification. FIG. 9 illustrates one
particular example of an apparatus and system 900 according to the
present disclosure. As with FIG. 1, a three-phase electric motor
140 is protected by an IED 120 which receives power system signals
in the form of signals representative of the currents and voltages
from all three phases to the motor. The IED may include an
intelligent electronics device capable of monitoring and protecting
the motor using the methods described herein. Some examples of IEDs
that may be used includes protective relays, motor protective
relays, and the like. These signals may be provided to a processor
906 via various filters (such as low-pass filters, not separately
illustrated), an analog-to-digital converter (A/D) 902, and a
multiplexor 904. Various other pre-processing devices and steps may
be incorporated as needed. These various pre-processing devices and
steps may be performed on a processor or the like. Further, the
pre-processing devices and steps may include a sampler (the A/D may
function as a sampler) for sampling the signals. The signals
representing the currents and voltages from the three phases are
ultimately provided to a processor (such as a microprocessor,
microcontroller, application specific integrated circuit (ASIC),
field programmable gate array (FPGA), and the like), where the
methods described herein are performed. The IED may include data
storage 908 where the particular modules, calculators, and/or
computer instructions for operating the present methods may be
stored. Further, values calculated by the processor 906 may be
stored using the data storage 908 as is needed. The IED thus
contains various calculators and/or modules for making the various
calculations described herein, though the various calculators are
not separately illustrated.
[0052] The various modules, calculators and/or computer
instructions may include a positive-sequence current calculator for
calculating positive-sequence current values from the current
samples. A positive-sequence voltage calculator may also be
included for calculating positive-sequence voltage values from the
voltage samples. A resistance calculator may be included for
calculating a resistance from the positive-sequence current values
and the positive-sequence voltage values as described above. An
initial resistance calculator may be included for determining an
initial resistance value that is equal to a minimum of the
resistance values calculated by the resistance calculator during
the predetermined initiation period. The initial resistance
calculator may be an initial rotor resistance calculator or an
initial stator resistance calculator, as described above. A slip
calculator may be included for determining slip values using the
initial resistance value and the resistance values, as described
above. A rotor resistance calculator may be included for
calculating a slip-dependent rotor resistance from the slip values
using the methods described herein. The rotor resistance calculator
may further calculate positive-sequence rotor resistance values
from the slip values and calculate negative-sequence rotor
resistance values, as described above. Further, a rotor temperature
calculator may be included for calculating a rotor temperature from
the current values and the slip-dependent rotor resistance values,
using the methods described herein. The rotor temperature
calculator may be configured to calculate rotor temperature values
from the calculated positive-sequence rotor resistance values, the
negative-sequence rotor resistance values, and the current values,
using the equations and methods described above. Thus, the IED is
capable of monitoring the motor using an accurate value of slip,
calculated using a minimum of the resistance values during an
initiation period.
[0053] The IED 120 further includes a communication device 916 that
is capable of receiving commands from the processor 906 and
transmitting them to receiving devices such as circuit breakers
910A, 910B, and 910C. If the protection modules operating on the
processor 906 call for the motor to be tripped, the processor 906
can send a signal to the communications device 916 which signals
circuit breakers 910A, 910B, and 910C to open, thus tripping off
the motor.
[0054] Further, the communications device 916 may include a
transceiver for communicating with a human-machine interface (HMI)
918 such as a computer, a laptop computer, a computer accessed via
a network, or the like. Certain inputs such as motor parameters
(e.g. full load current (FLA in amps), locked rotor current (LRA in
per unit of FLA), locked rotor time (LRTHOT in seconds), locked
rotor torque (LRQ in per unit of rated torque), full load slip (FLS
in per unit)) and the like may be entered using the HMI and
communicated to the processor 906, which may then store the values
using the data storage 908.
[0055] As described above, with the necessary information entered,
the processor may execute modules and/or computer instructions to
accurately calculate slip and perform the necessary motor
monitoring and protection functions.
[0056] While specific embodiments and applications of the
disclosure have been illustrated and described, it is to be
understood that the disclosure is not limited to the precise
configuration and components disclosed herein. Various
modifications, changes, and variations apparent to those of skill
in the art may be made in the arrangement, operation, and details
of the methods and systems of the disclosure without departing from
the spirit and scope of the disclosure.
* * * * *