Title: Device, system and method for on-line monitoring of flow quantities
Abstract: Measures of a torque and rotational speed of a pump or fan are obtained, from which a mechanical input power can be calculated. By further using predetermined relations between mechanical input power and flow rate through the pump or fan, a measure of the flow rate can be derived without use of in-line flow or pressure sensors. In preferred embodiments the torque and/or rotational speed is obtained by measuring the input current and voltage of a motor driving the pump or fan and using predetermined relations between on the one hand current and voltage and on the other hand torque and rotational speed. Furthermore, also other flow properties, such as pump head, pump efficiency and total efficiency can be determined in this manner.
Patent Number: 6,918,307 Issued on 07/19/2005 to Ohlsson, et al.
| Inventors:
|
Ohlsson; Bertil (Västerås, SE);
Windecker; Ulrike (Malsfed, DE);
Zahrai; Said (Västerås, SE)
|
| Assignee:
|
ABB AB (Västerås, SE)
|
| Appl. No.:
|
820149 |
| Filed:
|
April 8, 2004 |
Foreign Application Priority Data
| Oct 09, 2001[SE] | 0103371 |
| Oct 09, 2002[SE] | PCT/SE02/01844 |
| Current U.S. Class: |
73/861.353; 415/30; 415/17 |
| Intern'l Class: |
G01F 001/78 |
| Field of Search: |
73/861353,861.36
417/53
415/30,17
|
References Cited [Referenced By]
U.S. Patent Documents
| 3538767 | Nov., 1970 | Pfuntner et al.
| |
| 4700578 | Oct., 1987 | Fassbinder.
| |
| 4767280 | Aug., 1988 | Markuson et al.
| |
| 4781525 | Nov., 1988 | Hubbard et al.
| |
| 4821581 | Apr., 1989 | Jost.
| |
| 5318409 | Jun., 1994 | London et al.
| |
| 6014006 | Jan., 2000 | Stuntz et al.
| |
| Foreign Patent Documents |
| 199527168 | Feb., 1996 | AU.
| |
| 19851795 | May., 2000 | DE.
| |
| 2313197 | Nov., 1997 | GB.
| |
| WO 9530130 | Nov., 1995 | WO.
| |
Primary Examiner: Lefkowitz; Edward
Assistant Examiner: Thompson; Jewel V.
Attorney, Agent or Firm: Venable LLP, Franklin; Eric J.
Claims
1. A device for on-line monitoring of flow quantities in a system comprising
a flow-creating device such as a pump or a fan, comprising:
means for obtaining a measure of a torque of said flow-creating device;
means for obtaining a measure of a rotational speed of said flow-creating device;
and
means for determining a value of a flow rate through said flow-creating device,
means for calculating a value of a mechanical input power of said flow-creating
device, connected to said means for obtaining a measure of a torque and said means
for obtaining a measure of a rotational speed;
storage means for storage of data representing a predetermined relation between
mechanical input power and flow rate through said flow-creating device; and
whereby said means for determining a value of a flow rate is connected to said
means for calculating a value of a mechanical input power and to said storage means;
said means for obtaining a measure of a rotational speed in turn comprising:
at least one current sensor for measuring an input current of an electric motor,
said electric motor giving said torque to said flow-creating device
at least one voltage sensor for measuring an input voltage over said electric
motor; and
means for determining a value of said rotational speed, connected to said current
sensor, said voltage sensor and said storage means;
said storage means being further arranged for storage of data representing a
predetermined relation between input current and input voltage of said electric
motor and rotational speed of said electric motor.
2. The device according to claim 1, wherein said means for obtaining a measure
of a torque in turn comprises:
at least one current sensor for measuring an input current of said electric motor;
at least one voltage sensor for measuring an input voltage over said electric
motor; and
means for determining a value of said torque, connected to said current sensor,
said voltage sensor and said storage means;
said storage means being further arranged for storage of data representing a
predetermined relation between input current and input voltage of said electric
motor and output torque of said electric motor.
3. The device according to claim 1, further comprising:
means for determining a value of a flow-creating device efficiency, connected
to said means for determining a value of said flow rate and said storage means;
said storage means being further arranged for storage of data representing a
predetermined relation between flow rate and flow-creating device efficiency for
said flow-creating device.
4. The device according to claim 3, further comprising means for determining
a value of an electric motor efficiency connected to said current sensor, said
voltage sensor and said means for calculating a value of a mechanical input power.
5. The device according to claim 4, further comprising means for determining
a value of a total efficiency, connected to said means for determining a value
of a flow-creating device efficiency and said means for determining a value of
an electric motor efficiency.
6. The device according to claim 1, further comprising a diagnosing means, connected
to said determining or calculating means, for evaluation of time dependencies of
mechanical input power, flow or efficiency quantities.
7. The device according to claim 6, wherein said diagnosing means comprises a
processor and is connected to said storage means, said storage means being arranged
for storing data representing said time dependencies.
8. The device according to claim 7, wherein said storage means is arranged for
storing data representing comparison curves of earlier registered time dependencies.
9. The device according to claim 1, further comprising a monitor, connected to
said determining means, for monitoring flow or efficiency quantities.
10. A soft starter device comprising a device for monitoring flow quantities
according to claim 1.
11. A frequency inverter device comprising a device for monitoring flow quantities
according to claim 1.
12. A flow system, having a flow-creating device such as a pump or a fan arranged
for moving a fluid, and means for on-line monitoring of flow quantities, said means
for monitoring flow quantities in turn comprising:
means for obtaining a measure of a torque of said flow-creating device (
12);
means for obtaining a measure of a rotational speed of said flow-creating device;
and
means for determining a value of a flow rate through said flow-creating device,
wherein said means for monitoring flow quantities further comprises:
means for calculating a value of a mechanical input power of said flow-creating
device, connected to said means for obtaining a measure of a torque and said means
for obtaining a measure of a rotational speed;
storage means for storage of data representing a predetermined relation between
mechanical input power and flow rate through said flow-creating device; and
whereby said means for determining a value of a flow rate is connected to said
means for calculating a value of a mechanical input power and to said storage means;
said means for obtaining a measure of a rotational speed in turn comprising:
at least one current sensor for measuring an input current of an electric motor,
said electric motor giving said torque to said flow-creating device;
at least one voltage sensor for measuring an input voltage over said electric
motor; and
means for determining a value of said rotational speed, connected to said current
sensor, said voltage sensor and said storage means;
said storage means being further arranged for storage of data representing a
predetermined relation between input current and input voltage of said electric
motor and rotational speed of said electric motor.
13. The flow system according to claim 12, wherein said means for obtaining a
measure of a torque in turn comprises:
at least one current sensor for measuring an input current of said electric motor;
at least one voltage sensor for measuring an input voltage over said electric
motor; and
means for determining a value of said torque, connected to said current sensor,
said voltage sensor and said storage means;
said storage means being further arranged for storage of data representing a
predetermined relation between input current, input voltage of said electric motor
and output torque of said electric motor.
14. The flow system according to claim 12, wherein said means for monitoring
flow quantities further comprises:
means for determining a value of a flow-creating device efficiency, connected
to said means for determining a value of said flow rate and said storage means;
said storage means being further arranged for storage of data representing a
predetermined relation between flow rate and flow-creating device efficiency for
said flow-creating device.
15. The flow system according to claim 12, further comprising a diagnosing means,
connected to said determining or calculating means, for evaluation of time dependencies
of mechanical input power, flow or efficiency quantities.
16. The flow system according to claim 12, further comprising a monitor, connected
to said determining means, for monitoring flow or efficiency quantities.
17. The flow system according to claim 16, wherein said monitor is positioned
remotely in relation to said flow-creating device.
18. A device for on-line diagnostics of performance in a system comprising a
flow-creating device such as a pump or a fan, comprising:
means for obtaining a measure of a torque of said flow-creating device;
means for obtaining a measure of a rotational speed of said flow-creating device;
and
storage means,
means for calculating a value of a mechanical input power of said flow-creating
device connected to said means for obtaining a measure of a torque and said means
for obtaining a measure of a rotational speed;
diagnosing means, connected to said calculating means, for evaluation of time
dependencies of mechanical input power;
said means for obtaining a measure of a rotational speed in turn comprising:
at least one current sensor for measuring an input current of an electric motor;
said electric motor giving said torque to said flow-creating device;
at least one voltage sensor for measuring an input voltage over said electric
motor; and
means for determining a value of said rotational speed, connected to said current
sensor, said voltage sensor and said storage means;
said storage means being arranged for storage of data representing a predetermined
relation between input current and input voltage of said electric motor and rotational
speed of said electric motor.
19. The device according to claim 18, wherein said storage means is arranged
for storing data representing said time dependencies.
20. The device according to claim 19, wherein said storage means comprises data
is arranged for storing data representing comparison curves of earlier registered
time dependencies.
21. A method of on-line monitoring flow quantities in a system comprising a flow-creating
device such as a pump or a fan, the method comprising:
obtaining a measure of a torque of said flow-creating device;
obtaining a measure of a rotational speed of said flow-creating device, and
determining a value of a flow rate through said flow-creating device, based on
said measure of a torque and said measure of a rotational speed,
calculating a value of a mechanical input power of said flow-creating device
based on said measure of a torque and said measure of a rotational speed;
said determining step using said mechanical input power and a predetermined relation
between mechanical input power and flow rate through said flow-creating device;
said step of obtaining a measure of a rotational speed in turn comprising:
measuring an input current of an electric motor, said electric motor giving said
rotational speed to said flow-creating device;
measuring an input voltage over said electric motor; and
determining a value of said rotational speed, using said input current and input
voltage and a predetermined relation between input current, input voltage and rotational
speed of said electric motor.
22. The method according to claim 21, further comprising:
controlling parameters of said system based on said flow rate.
23. The method according to claim 21, further comprising:
empirically determining said predetermined relation between mechanical input
power and flow rate through said flow-creating device prior to installation of
said flow-creating device.
24. The method according to claim 21, wherein said step of obtaining a measure
of a torque in turn comprises:
measuring an input current of said electric motor;
measuring an input voltage over said electric motor; and
determining a value of said torque, using said input current and input voltage
and a predetermined relation between input current, input voltage and torque of
said electric motor.
25. The method according to claim 24, further comprising:
empirically determining said predetermined relation between input current, input
voltage and torque of said electric motor prior to installation of said flow-creating
device.
26. The method according to claim 21, further comprising:
empirically determining said predetermined relation between input current, input
voltage and rotational speed of said electric motor prior to installation of said
flow-creating device.
27. The method according to claim 21, further comprising:
determining a value of a flow-creating device efficiency, using said value of
said flow rate through said flow-creating device and a predetermined relation between
flow rate and flow-creating device efficiency for said flow-creating device.
28. The method according to claim 21, further comprising:
registering of time dependencies of said mechanical input power, flow or efficiency
quantities;
evaluating said time dependencies for diagnosing of the operation of said flow-creating
device.
29. The method according to claim 28, wherein said step of registering comprises
storing of said time dependencies in a storage means.
30. The method according to claim 29, wherein said step of evaluation comprises
comparing said time dependencies with earlier registered time dependencies.
31. The method according to of claim 27, further comprising:
determining a value of an electric motor efficiency, using said value of said
mechanical input power, said input current and said input voltage.
32. The method according to claim 31, further comprising:
determining a value of a total efficiency, multiplying said flow-creating device
efficiency and said electric motor efficiency.
33. A computer program product comprising computer code means and/or software
code portions for making a processor perform the steps of claim 21.
34. The computer program product according to claim 33 supplied via a network,
such as Internet.
35. A computer readable medium containing a computer program product according
to claim 33.
36. A method of on-line diagnosing of performance of a system comprising a flow-creating
device such as a pump or a fan, the method comprising:
obtaining a measure of a torque of said flow-creating device; and
obtaining a measure of a rotational speed of said flow-creating device,
calculating a value of a mechanical input power of said flow-creating device
based on said measure of a torque and said measure of a rotational speed;
registering of time dependencies of said mechanical input power; and
evaluating said time dependencies for diagnosing of the operation of said flow-creating
device;
said step of obtaining a measure of a rotational speed in turn comprising:
measuring an input current of an electric motor, said electric motor giving said
rotational speed to said flow-creating device;
measuring an input voltage over said electric motor; and
determining a value of said rotational speed, using said input current and input
voltage and a predetermined relation between input current, input voltage and rotational
speed of said electric motor.
37. The method according to claim 36, wherein said step of registering comprises
storing of said time dependencies in a storage means.
38. The method according to claim 37, wherein said step of evaluation comprises
comparing said time dependencies with earlier registered time dependencies.
39. A computer program product comprising computer code means and/or software
code portions for making a processor perform the steps of claim 36.
40. A computer readable medium containing a computer program product according
to claim 39.
Description
TECHNICAL FIELD
The present invention relates to monitoring and control of flow quantities in
association with pumps or fans.
BACKGROUND
Many process systems of today include flows of different fluids, where the flow
is created by a fan or pump. In many cases, the flow rate and other quantities
related to the flow are of crucial importance or at least of interest for controlling
the process. The measured flow and related quantities are typically used as input
parameters in monitoring, measuring and/or controlling processes. There is thus
a general request in many process systems involving fluid flows for measuring flows
and pressures caused by different flow-creating means, i.e. essentially fans and pumps.
The straightforward approach used in prior art employs the use of direct measurements
of flow-related quantities. In DE 198 51 795 A1, a system and a method for textile
treatment are disclosed. A respective pressure sensor is introduced within a flow
line of treatment fluid immediately before and after a pump. The pressure difference
is used to control the operation of the pump in order to provide the right flow
of treatment fluid in the process.
Hence, in such system and systems of a similar design, a number of sensors
have to be introduced into the flow, which always exposes the sensors to wear and
potential damage. Furthermore, sensors introduced in-line also increase the risk
for changing or disturbing the actual flow in an unfavourable manner. Replacement
or repair of such sensors will typically cause an operation stop of the process
system, which often is very costly. Also the sensors themselves are relatively
costly. There are thus a number of disadvantages with flow measurements according
to prior art.
In the patent application AU 27168/95, a pump monitoring apparatus is disclosed.
This apparatus monitors the pump motor in order to achieve a measure of the run
time of the pump. By an a-priori knowledge of the expected pump capacity and pump
efficiency, a throughput can be calculated by multiplying with the actual operation
time ratio. This procedure is only applicable where capacity and efficiency are
known in advance.
In U.S. Pat. No. 4,781,525, a flow measurement system is disclosed, where blood
flow rate through a centrifugal pump is measured. A rotational speed is obtained
by a rotational speed sensor. The sensor is directly measuring the speed of the
motor shaft, mechanically, optically or in any other direct manner, as clearly
indicated by a broken line in the disclosed figures. Furthermore, a parameter representative
of the motor torque is obtained. The blood flow rate is computed as a function
of the measured rotational speed and the obtained motor torque. A problem with
the devices disclosed in U.S. Pat. No. 4,781,525 is that a separate rotational
speed sensor has to be provided, which is both expensive and in many cases also
difficult to provide for mechanical reasons.
SUMMARY
It is thus a general object of the present invention to provide devices and methods
for improving monitoring, control and diagnosing of systems involving a fluid flow.
A further object of the present invention is to provide devices and methods, which
allow for on-line measurements of flow quantities. Another further object is to
provide such devices, which are cheaper than the prior art solutions. Another object
is also to provide devices and methods, which are easily integrated into existing
equipment, such as frequency converters or soft starters of pump or fan motors.
The objects above are achieved by devices and methods according to the enclosed
claims. In general words, by obtaining measures of a torque and rotational speed
of a pump or fan, a mechanical input power can be calculated. By further using
predetermined relations between mechanical input power and flow rate through the
pump, a flow rate that corresponds sufficiently accurately to the actual flow rate
under normal circumstances can be derived without use of in-line flow or pressure
sensors. The torque and/or rotational speed is obtained by measuring the input
current and voltage of a motor driving the pump or fan and using predetermined
relations between on one hand current and voltage and on the other hand torque
and rotational speed. Furthermore, also other flow properties, such as pump head,
total efficiency and pump efficiency can be determined in this manner. By evaluating
time dependencies of flow, power or efficiency quantities, diagnosing of the operation
can be obtained. The invention is easily applicable on constant speed pumps and
fans, but can also be used with other types of equipment.
Further advantages are that there is no need for installing any sensors directly
in or on the flow path, which makes installation, substitution or repair possible
without interrupting the flow. However, the measurements and calculations of the
requested quantities are performed in the direct vicinity of the flow path, which
makes the method possible to apply for controlling purposes. No time-consuming
off-line analyses have to be performed.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further objects and advantages thereof, may best
be understood by making reference to the following description taken together with
the accompanying drawings, in which:
FIG. 1 is a flow system according to prior art making use of in-line flow sensors;
FIG. 2 is another flow system according to prior art making use of direct rotational
speed measurements;
FIG. 3 is a flow system, based on current and voltage measurements of a constant
speed pump;
FIG. 4 is a flow system according to an embodiment of the present invention,
based on current and voltage measurements of a variable speed pump;
FIG. 5 is a flow system according to an embodiment of the present invention,
having evaluation means for time dependencies;
FIG. 6 is a flow system according to another embodiment of the present invention,
having evaluation means for time dependencies;
FIG. 7 is an illustration of the processing of information in a flow system
of FIG. 2;
FIG. 8 is an illustration of the processing of information in a flow system
of FIG. 3;
FIG. 9 is an illustration of the processing of information in a flow system
of FIG. 4;
FIGS. 10A-E are diagrams illustrating different pump or motor characteristics
used by the present invention;
FIGS. 11A-D are diagrams illustrating the possibility for diagnosing by the
use of time dependencies;
FIG. 12 is a flow diagram illustrating a preferred method according to one aspect
of the present invention; and
FIG. 13 is a flow diagram illustrating a preferred method according to another
aspect of the present invention.
DETAILED DESCRIPTION
The potential application areas of the present invention are covering a wide
range of technical fields. Non-excluding examples are e.g. water transport, food
industry, pulp manufacturing, slurry pumping, oil and gas transport, chemical industry
and ventilation systems. Anyone skilled in the art easily understands that the
list of possible applications can be made far more extensive.
The present invention can be utilised in systems where a fan, pump or other flow-creating
device causes a flow of fluid or pressure rise. Preferable embodiments involve
such flow-creating devices driven by electric motors.
In order to illustrate the benefits of the present invention, this description
starts with a brief description of a typical system according to prior art, illustrated
in FIG. 1. A flow system
1 comprises a flow path
10 of a fluid, e.g.
a liquid, a gas, a mixture of gases, a mixture of liquids, a mixture of gases,
liquids and solids, an emulsion, a slurry, a suspension etc. A flow-creating device
12, e.g. a pump or a fan, creates a flow (indicated by the arrows
14)
of the fluid within the flow path
10. The fluid is thus transported from
an inlet point
16 to an outlet point
18, being the interface with
the rest of the system, indicated by the broken square
2. The flow-creating
device
12 is driven by an electric motor
20, in this example a three-phase
motor. In the flow path
10, two pressure sensors
22,
24 are
inserted on each side of the flow-creating device
12. Furthermore, a flow-rate
meter
26 is also present in the flow path
10. A valve
28 is
present as an example of a device possible to use for controlling the flow in the
flow path
10. A data acquisition unit
30 is connected to the three
sensors
22,
24 and
26 for receiving signals from sensors representing
pressures and flow-rates. The measurements are evaluated and the result in the
form of a determined flow is presented at a monitor
32. The result may also
be used for controlling purposes as indicated by the broken line
34.
Throughout this description, different sensors are discussed. It is obvious
for anyone skilled in the art that most sensors are accompanied by some kind of
control devices, either provided separately or integrated in any other equipment.
In FIG. 2, another prior-art flow system is illustrated. Identical or similar
parts are given the same reference numbers as in FIG.
1. Here we assume
that the flow-creating device is a pump
12. However, the same principles
are valid also for fans. Anyone skilled in the art immediately realises that all
three in-line sensors are removed. Instead, means for obtaining a measure of a
torque T, in this embodiment a torque sensor
42, is introduced, which obtains
a signal representing a measure of the torque T on the pump
12. The torque
sensor
42 is connected to an evaluation means
40 and the torque T
value is inputted thereto. A signal representing a measure of the rotational speed
n of the pump
12 is in this embodiment obtained by a rotational speed sensor
43, as indicated in the figure. A flow-rate determination means
48
uses the torque T and the rotational speed n to determine an estimated value of
the flow rate.
In FIG. 3, another embodiment of a flow system is illustrated. In this system,
the pump is a constant speed pump, why the value of the rotational speed n is assumed
to be known in advance. No separate input of this value is needed since the value
is incorporated directly in the calculations as a constant. In this embodiment,
the means for obtaining a measure of a torque
42 comprises a current sensor
50 and a voltage sensor
51, measuring the electrical input quantities
of the motor
20. The measured values of input voltage and input current
of the motor
20 are supplied to a torque determining means
52, in
this embodiment comprised in the evaluation means
40. The torque determining
means
52 is further connected to a storage means
46, which is supplied
with relations
54 between input current and voltage, and output torque of
the motor
20 in question. By using these motor characteristics
54,
possibly by use of an appropriate motor model, the measured values of input current
and voltage can be transferred into an output torque of the motor. This calculated
torque is then used in a calculating means
44 to provide the mechanical
input power of the associated pump or fan
12. This is possible to obtain
by the relation:
where n is the rotational speed of the pump
12. Since n is already known,
the mechanical input power is readily calculated based on the determined torque.
If the motor
20 is a three-phase motor, more than one current sensor
50
and/or more than one voltage sensor
51 might be necessary. In cases where
the current and voltage measurement do not involve phase information, the phase
difference between the voltage and current has preferably to be measured.
The storage means
46 of the evaluation means
40 further comprises
data representing a relation
47 between mechanical input power P
m
and flow rate Q for the pump in question. This relation
47 may be stored
as an equation, as a look-up table or in any other suitable form. The flow-rate
determination means
48 uses the calculated mechanical input power P
m
to determine an estimated value of the flow rate Q.
In FIG. 3, the pump or fan was assumed to be a constant speed device. However,
also many varying speed devices are used for creation of flows. In such cases,
the rotational speed has to be measured, directs or indirectly, and entered as
a parameter in the mechanical input power calculations of the calculating means
44. According to the present invention, as illustrated in FIG. 4, the rotational
speed is calculated, based on the already measured input current and voltage to
the motor
20, using a predetermined relation
58 between input current
and voltage and the rotational speed, typically obtainable from motor characteristics
and a motor model, discussed more in detail below. In a frequency converter, the
rotational speed is proportional to the (known) frequency of the supplying electric
network. The means for obtaining a measure of a rotational speed
43 may
in such an embodiment comprise the current and voltage sensors
50,
51,
a rotational speed determining means
57 and the part of the storage means
46 comprising the appropriate motor characteristics
58.
Furthermore, selected values can be displayed at a monitor
32
in a conventional manner, and/or be used for controlling purposes
34. Most
parts of the evaluation means are preferably implemented by software in processors.
In the embodiment of FIG. 4, the storage means
46 further comprises a
relation
56 between flow rate and pump efficiency η
P. This data
is then used to obtain the pump efficiency η
P of the particular
pump in question, in this embodiment within the flow determination means
48
or connected thereto. The pump efficiency η
P may also be displayed
at a monitor
32. In similar fashions, other flow system related quantities
might be obtained, such as pump head or total efficiency η
T, by
using predefined relations between the quantities and e.g. the determined flow rate.
FIG. 5 illustrates a further embodiment of a flow system according to the present
invention. This embodiment is closely related to the embodiment of FIG. 4. A diagnosing
means in the shape of an event registration means
60 is connected to the
calculation means
44 and registers the time evolution or time dependency
of the mechanical input power P
m to the pump or fan
12. This
event registration means
60 preferably comprises a processor for evaluating
the registered behaviours. This time evolution is preferably also stored in the
storage means
46 together with current values of other relevant process
parameters. The time evolution is preferably compared with changes of process parameters
performed during the registered time period, and changes in mechanical input power
P
m can thus be correlated to such parameter changes. Preferably, the
time evolution is also compared with stored time evolutions of earlier operations,
also stored in the storage means
46. In this manner, changes in flow quantities
caused by alterations of process parameters may be separated from changes having
other causes, such as wear, malfunctioning equipment, obstacles in the flow etc.
Examples of such diagnosing procedures are given further below.
FIG. 6 illustrates an alternative embodiment. Here, the event registration means
60 instead registers the time evolution of the calculated flow Q. Since
there is a direct relation between the two parameters, either quantity should be
possible to use as an event indicator. Also other flow parameters, such as pump
head, pump efficiency η
P etc. could be used for diagnosing purposes.
A unit or functionality for determining a value of an electric motor efficiency
η
E is also easily integrated, connected to the current sensor
50 and voltage sensor
51 and the calculation means
44. The
electrical motor efficiency η
E is then easily obtained as the
ratio between the electrical input power to the motor and the mechanical power
transferred to the pump
12. A total efficiency η
T can also
be obtained by e.g. multiplying the electric motor efficiency η
E and
the pump efficiency η
P, preferably in a means connected to relevant
other means in the evaluation means
40.
As anyone skilled in the art realises after reading the present disclosure, the
features of the present invention do not directly influence the flow in the flow
path. There is no need for installing any sensors directly in or on the flow path,
which makes installation, substitution or repair possible without interrupting
the flow. However, the measurements and calculations of the requested quantities
are performed in the direct vicinity of the flow path, which makes the method possible
to apply for controlling purposes. No time-consuming off-line analyses have to
be performed.
Due to this modular design, the different means of the present invention are
easily incorporated in or together with other equipment of the system. A frequency
converter, which often is applied for controlling the motor operation, has already
the ability for obtaining a measure of the input voltage, input current, rotational
speed and sometimes even torque. These values can therefore easily be forwarded
to be used according to the present invention. The different means of the present
invention are therefore easily integrated in frequency converters, or supplied
as add-on modules.
Similarly, many motors driving a fan or pump are controlled by soft starter
equipment. Such equipment is mainly operable at starting and stopping the motors
in appropriate manners, but the use may also be extended to incorporate also operation
control. Thus, by introducing a device according to the present invention in a
soft starter, the utilisation of the soft starter equipment can be extended also
to cover steady-state operation. A soft starter typically measures the input current
and/or the input voltage of the motor, and an integration of the characterising
means of the present invention into the soft starter is therefore easily performed.
In the different embodiments illustrated above, the most parts directly associated
with the present invention are collected in the evaluation means
40. Anyone
skilled in the art understands that the evaluation means
40 can be supplied
as one unit as well as distributed units, being parts of other system means. Similarly,
the storage means
46 is illustrated as a single means, but may also be divided
into smaller units.
In many applications, the position of an operator is often remote from the actual
equipment. It is therefore in many cases preferable to have the monitor positioned
remotely in relation to the pump or fan. This monitor may then also be a part of
an overall control system. In particular, results of the evaluation may advantageously
be communicated by use of Internet, for the purpose of remote monitoring and control.
An important feature of the present invention is the use of predetermined relations
between mechanical input power and flow rate in a pump or fan. In preferred embodiments
further predetermined relations are used. This processing of different quantities,
not directly associated with flow quantities, and predetermined relations characterises
the present invention. In FIG. 7, a simple sketch illustrating the processing of
information is shown, which corresponds to the embodiment of FIG.
2. The
quantities T and n, i.e. the torque and rotational speed, are determined in one
or another way. Together, they give an opportunity to estimate the flow rate Q.
In FIG. 8, a similar sketch illustrates the conditions of FIG.
3. Here,
the input voltage U and current I of a motor driving the pump or fan are processed
into a torque T. Since this particular embodiment concerns a constant speed machine,
the rotational speed n is known, and the flow rate Q is then calculated.
Finally, in FIG. 9, the system of FIG. 4 is illustrated in an Information
flow sketch according to the present invention. Here, in addition to the flows
of FIG. 8, the input current and voltage are used together with predetermined motor
characteristics to compute the torque T as well as the actual rotational speed
n of the motor. These quantities are subsequently used to calculate the mechanical
input power P
m. This quantity in turn can with the support of predetermined
pump characteristics, in particular the relation between input power and flow rate,
be used to estimate the flow rate Q.
Furthermore, the pump or fan efficiency η
P is here
also deduced using the mechanical input power of the pump or fan and other pump
characteristics, namely a relation between mechanical input power and efficiency
η
P. Alternatively, the efficiency η
P can be calculated
starting from the flow rate and using a predetermined relation between flow rate
and efficiency η
P instead.
The predetermined pump or fan characteristics can be obtained in different ways.
Manufacturers often supply performance data together with a pump or fan. A typical
such curve for a constant speed centrifugal pump is illustrated in FIG.
10A.
The curve is only an illustrative example, and other flow-generating devices, e.g.
displacement pumps or fans, have other characteristic features in their relations.
The method of the present invention is, however, applicable to all different equipment
as long as there exist a monotonic relation between P
m and Q. In the
present example, the input pump power P
m is shown as a function
100
of flow rate Q. The function is monotonic and can thus be expressed also as an
inverse function. By inputting a certain input pump power into such an inverse
function, the corresponding flow rate will result. By storing this relation between
input pump power and flow rate in a storage means in the control means of the flow
system, a conversion between input pump power and flow rate is readily available.
The storage can be performed as a mathematical function, as a look-up table or
similar conventional ways of representing a relation.
If the manufacturer does not supply a curve, such as the one shown in FIG. 10A,
the relation can be easily determined in a test set-up, prior to the installation
of the pump. In a typical case, only a few points in the diagram have to be determined,
and equations or models are then used for interpolation and extrapolation of these
measured points.
Another pump property that often is supplied by the manufacturers is the
pump head as a function of the flow rate. An example is illustrated in FIG.
10B.
The pump head or the pressure difference over the pump is in a typical case a smooth
function of the flow rate. Since the input pump power is a monotonically increasing
function of flow rate, there is a defined relation also between input pump power
and pump head. In a similar manner as for the flow rate, the pump head can be calculated
only from the input pump power, or via the already calculated flow rate.
When knowing the pump head and flow rate, the efficiency η
P
of the pump can be calculated as:
##EQU1##
where H is the pump head, ρ is the fluid density and g is the gravity constant.
This means that the two diagrams of FIGS. 10A and 10B can be combined to a relation
between input pump power P
m and the pump efficiency η
P,
as illustrated by FIG.
10C.
For motors, where the rotational speed is not constant during operation, e.g.
operation with frequency converters, or pumps operated at a different rotational
speed than the nominal one for which characteristics are provided, the quantities
of interest can easily be derived by affinity laws:
##EQU2##
where n
1 and n
2 are two different rotational speeds.
In the embodiments using calculations of the torque of the pump, motor characteristics
and possibly also motor models are used. FIG. 10D illustrates an example of a typical
predetermined relation between current and rotational speed for an asynchronous
motor. The relation is monotonic, which means that there is a one-to-one relation
between each value of rotational speed and current for a specified voltage. One
such relation exists for every value of supplied voltage. For each set of current
and voltage, a corresponding rotational speed is derivable.
Furthermore, FIG. 10E illustrates other motor characteristics, a relation
between rotational speed and torque for a given input voltage of an asynchronous
machine. This implies that also here, there exists one such relation for each input
voltage. By combining the curves of FIGS. 10D and 10E, associated with a certain
input voltage, a current value corresponds to one unique torque value. In this
manner, both torque and rotational speed can be derived starting from input current
and input voltage, by means of predetermined motor characteristics. A relation
can also be obtained by using different models, as discussed more in detail below.
Such motor characteristics are sometimes available from e.g. the manufacturers,
as examples of the motor properties. If there are not sufficient data, the relations
may be directly determined either in connection with the manufacturing of the motor
or prior to installation. Such relations are then obviously reliable, but the amount
of efforts to obtain it is large. Another possibility is to measure just a few
critical points at the curves and estimate the rest of the relation based on these
few values. These values should preferably be standard data almost always supplied
by a motor manufacturer. The behaviour from one motor to another or from one type
to another is believed to be rather similar, and the variations are smooth, which
would lead to rather reliable relations.
When obtaining the different motor relations, one may preferably make use of
a model of the motor. In one possible motor model, the motor is approximated by
an equivalent circuit. In this circuit, the rotor and stator windings are expressed
as a circuit of ideal inductances and resistances. By measuring a few points experimentally,
values of these inductances and resistances can be obtained, and the circuit can
then be used to determine points in between the measured ones. The detailed design
of the equivalent circuit depends on the required accuracy as well as on the type
of motor.
For an electric circuit model, the following relation can be used:
where P
m is the mechanical power, P
e is the electrical
power supplied to the motor and P
loss is the power losses in the motor.
P
e is easily determined by measurements of supplied current and voltage,
and P
loss can be obtained by use of e.g. an electric circuit model of
the motor.
The electric circuit model of the motor is one suitable model. However, there
are also other different models, which may serve as models for calculating e.g.
the output torque of a motor.
It is obvious that the present invention relies on that the used predetermined
relations are sufficient reliable. The reliability of the motor characteristics
has been tested experimentally in connection with soft starters, and has been found
to be sufficient, at least for soft starter applications. Such reliability will
be sufficient also for the applications suggested in the present invention. The
reliability of the pump or fan characteristics have been experimentally evaluated
in a test equipment, measuring the pump torque as well as pump head and flow rate.
The actually measured relations between on the one hand the torque and on the other
hand flow rate and pump head have been compared with the characteristics supplied
by the manufacturers. The tests verified that most curves supplied by the manufacturers
were sufficiently reliable for the present application. Furthermore, the measured
relations were reproducible within surprisingly small margins. These results thus
point to the possibility to use predetermined pump characteristics not only as
a guide for selecting the appropriate pump for the task in question, but also for
controlling purposes.
The above discussions are mainly valid for processes involving steady-state operation
or quasi-steady-state operation of the fan or pump. For rapid courses of events,
one may not generally rely on the steady-state relations. However, for rapid operation
changes, which are occurring frequently, special relations may be utilised. The
present invention is thereby possible to use also on non-steady-state operations,
provided that the proper relations are available.
In embodiments registering the time evolution of input pump power or flow quantities,
a further advantage is available. By just measuring e.g. the flow, the system can
be controlled by utilising this value. However, if other system parameters are
changed, and one wants to verify that the purpose of the change was achieved, the
time factor has to be taken into account. A change in system parameters influencing
the flow leads to a certain time evolution of the derived flow parameters. By storing
earlier events, when the changes had the requested results, and comparing the actual
situation with such stored operation situations, it is possible to determine if
the change was successful. FIG. 11A illustrates a possible measured time evolution
of the input pump power. The surrounding events were as follows. The system was
turned on at t
0. Between t
1 and t
2, the surrounding
temperature slowly increased. (In this example, the fluid is most probable a gas,
having a large temperature dependency of the density.) At t
3 a certain
system parameter was changed. FIG. 11B-D illustrates stored time evolutions of
the input pump power at certain earlier occasions of the same system. FIG. 11B
illustrates a successful system start half a year earlier. FIG. 11C illustrates
the conditions during a controlled cooling of surrounding temperature. FIG. 11D
illustrates the input pump power at an earlier change of the same system parameter
at time t
4.
At a comparison between the registered time evolution and the stored previous
behaviours, it is easily observable that the registered curve first presents a
normal starting behaviour. The exact reasons for the details in the curve have
not to be known in detail. The general shape and the size are normal and the start
seems to be successful. At time t
2, an increase in surrounding temperature
occurs, which causes a change in the measured parameter, which is in accordance
with the behaviour of FIG.
11C. When the system parameter change occurs
at t
3, the resulting curve also presents the assumed behaviour. The
system behaves as expected.
However, at time t
4, the time evolution in FIG. 11A shows some
features, which can not be correlated to any intentional or unintentional parameter
changes. The system acts strange and this calls for a more thorough investigation
of the system. The causes can be of vastly different origins. There might be malfunctions
or damages in the pump or motor, which means that the used relations are not longer
valid. Other possibilities could be that the flow path is affected, e.g. a leak
or an obstructing object. Dryrunning of the pump, cavitation at the pump blades
or uncontrolled changes in the fluid properties might also be the cause. Even if
the actual reason for the strange behaviour can not be obtained directly, the behaviour
calls for a further investigation of the system.
At a closer evaluation, it is possible to see that the detailed shape and size
of the starting portion of the curve in FIG. 11A differs a small amount compared
to the curve of FIG.
11B. By registering also such small changes, time trends
might be found. In this case, the wear of the pump during the last year has changed
the pump characteristics in such a way that a somewhat differing input pump power
is required. By analysing the time scale of changes, different types of changes
may thus be distinguished.
Although one advantage of the present invention is the exclusion of in-line
sensors, it might be of advantage in certain applications to combine these two
approaches. By introducing in-line sensors together with calculated ones, the calculated
ones could be treated as target values and a discrepancy to the actually measured
ones could indicate an error in the operation.
The different system embodiments above are mainly described as autonomous or
separate systems. However, anyone skilled in the art recognize that the described
systems in many applications are integrated as parts of a larger system.
FIG. 12 illustrates a preferred embodiment of the method according to one aspect
of the present invention. The procedure starts in step
200. In step
202,
the input current of motor driving a pump or fan is measured. In step
204,
the input voltage of the motor is measured. An output torque of the motor is estimated
in step
206 using predetermined relations between on one hand the input
current and voltage and on the other hand the output torque. The rotational speed
of the motor is estimated in step
208 using predetermined relations between
on one hand the input current and voltage and on the other hand the rotational
speed. In step
210, rotational speed and torque of the pump or fan is used
to calculate the mechanical input power to the pump or fan. Finally, a flow rate
of the fluid moved by the pump or fan is estimated in step
212 using predetermined
relations between on one hand the mechanical input power and on the other hand
the output flow rate. The procedure ends in step
214. However, despite the
flow-chart character of FIG. 12, the above procedure should not be considered as
a closed procedure, but rather a continuous one. The obtained output results can
also preferably be used further, e.g. for purposes of monitoring or control, as
described earlier.
FIG. 13 illustrates a preferred embodiment of the method according to another
aspect the present invention. The procedure starts in step
300. In step
302, the input current of motor driving a pump or fan is measured. In step
304, the input voltage of the motor is measured. An output torque of the
motor is estimated in step
306 using predetermined relations between on
one hand the input current and voltage and on the other hand the output torque.
The rotational speed of the motor is estimated in step
308 using predetermined
relations between on one hand the input current and voltage and on the other hand
the rotational speed. In step
310, rotational speed and torque of the pump
or fan is used to calculate the mechanical input power to the pump or fan. The
time dependency of the mechanical input power is registered instep
312,
and in step
314, the time dependency is evaluated for purposes of diagnostics.
The procedure ends in step
316. However, despite the flow-chart character
of FIG. 13, the above procedure should not be considered as a closed procedure,
but rather a continuous one.
The methods according to the present invention may be implemented as software,
hardware, or a combination thereof. A computer program product implementing the
method or a part thereof comprises software or a computer program run on a general
purpose or specially adapted computer, processor or microprocessor. The software
includes computer program code elements or software code portions that make the
computer perform the method using at least one of the steps previously described
in FIG.
12. The program may be stored in whole or part, on, or in, one or
more suitable computer readable media or data storage means such as a magnetic
disk, CD-ROM or DVD disk, hard disk, magneto-optical memory storage means, in RAM
or volatile memory, in ROM or flash memory, as firmware, or on a data server. Such
a computer program product can also be supplied via a network, such as Internet.
The present invention has obviously a number of advantages compared to prior
art. Other less sensitive sensors combined with software processing replace prior
art hardware in-line sensors. This gives a cheaper process monitoring and control.
Since the sensors of the present invention are not inserted in the actual flow
path, sensor maintenance is simplified and cheaper. Since the pump efficiency η
P
easily is monitored instantly, an energy consumption optimisation can be obtained.
On a superior level, one may e.g. decide the optimum trade-off, in terms of total
energy efficiency η
T, between flow-rate and the operational time,
giving a certain transported volume. Furthermore, load diagnostics and process
diagnostics is easily available.
It will be understood by those skilled in the art that various modifications
and
changes may be made to the present invention without departure from the scope thereof,
which is defined by the appended claims.
*
