U.S. patent application number 10/413011 was filed with the patent office on 2004-10-14 for vsd control.
Invention is credited to Holden, Steven J..
Application Number | 20040201354 10/413011 |
Document ID | / |
Family ID | 32990300 |
Filed Date | 2004-10-14 |
United States Patent
Application |
20040201354 |
Kind Code |
A1 |
Holden, Steven J. |
October 14, 2004 |
VSD CONTROL
Abstract
A variable speed drive control system operates a variable speed
drive to power a first load from a power source simultaneously
powering a second load. An AC reactive power is monitored which may
comprise or consist of an AC reactive power of the second load.
Responsive to the monitored AC reactive power, the variable speed
drive is controlled to counter the AC reactive power of the second
load.
Inventors: |
Holden, Steven J.; (Manlius,
NY) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510
US
|
Family ID: |
32990300 |
Appl. No.: |
10/413011 |
Filed: |
April 14, 2003 |
Current U.S.
Class: |
318/268 |
Current CPC
Class: |
H02P 23/26 20160201;
H02P 5/74 20130101; F25B 2600/021 20130101; H02P 2201/15
20130101 |
Class at
Publication: |
318/268 |
International
Class: |
H02P 007/00 |
Claims
What is claimed is:
1. A method for operating a variable speed drive to power a first
load from a power source simultaneously powering a second load, the
method comprising: monitoring a monitored AC reactive power
comprising a second AC reactive power of the second load; and
responsive to the monitored AC reactive power, controlling the
variable speed drive to maintain a first AC reactive power of the
variable speed drive and first load opposite the second AC reactive
power.
2. The method of claim 1 wherein said controlling comprises
maintaining a magnitude of the first AC reactive power of the to at
least 20% of a magnitude of the second AC reactive power.
3. The method of claim 1 wherein said controlling comprises
maintaining a magnitude of the first AC reactive power to at least
the lesser of: 50% of a magnitude of the second AC reactive power;
and 20% of a real power of the variable speed drive.
4. A method for operating a variable speed drive to power a
compressor motor from a power source simultaneously powering an
additional load including at least one additional motor, the method
comprising: monitoring a monitored AC reactive power including an
AC reactive power of the additional load; and responsive to the
monitored AC reactive power, controlling the variable speed drive
to maintain an AC reactive power of the variable speed drive and
compressor motor opposite the AC reactive power of the additional
load.
5. The method of claim 4 wherein the controlling maintains a
combination of the AC reactive power of the variable speed drive
and compressor motor and the AC reactive power of the additional
load to no more than 50% of a wattage of the combination.
6. A variable speed drive system for powering a first load from a
source also powering a second load, the system comprising: means
for monitoring a monitored AC reactive power comprising an AC
reactive power of the second load; and means for controlling the
variable speed drive responsive to the monitored AC reactive power
to maintain an AC reactive power of the variable speed drive and
first load opposite the AC reactive power of the second load.
7. The system of claim 6 in combination with the first load, the
first load being a motor of a compressor.
8. The combination of claim 7 in further combination with the
second load, the second load comprising at least one additional
motor not powered by the variable speed drive.
9. The combination of claim 7 in further combination with the
second load, the second load being remaining group of loads of a
building.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] This invention relates to variable speed drives, and more
particularly to variable speed drives controlling compressor
motors.
[0003] (2) Description of the Related Art
[0004] Variable speed drives (VSDs) are commonly used to power
electric motors of compressors. An exemplary application is
refrigeration compressors used to cool water for building climate
control, industrial cooling or the like. The electricity
consumption of such VSDs may represent a substantial portion of the
energy consumption of the building.
[0005] It is known to control the VSD (e.g., a VSD having an active
rectifier front end) to compensate for losses associated with the
reactive power of the motor being powered. The reactive power
(measured in volt ampere reactive or VAR) is energy used by the
motor to build up its magnetic field. VARs are not converted into
work and thus represent losses. The VSD may be controlled to
ideally zero the reactive power of the combined VSD and motor
system. So controlled, the ratio of real power to apparent power of
the system (the "power factor") will be one.
[0006] The apparent power (measured in volt ampere or VA) is the
mathematical product of voltage and current in an AC system.
Because voltage and current may not be in phase in an AC system,
the apparent power may exceed the real power. Reactive loads
(inductance and/or capacitance) in an AC system will cause the
apparent power to exceed the real power. Apparent power may be
graphically represented in vector form as the hypotenuse of a right
triangle whose other sides are real power and reactive power.
BRIEF SUMMARY OF THE INVENTION
[0007] Accordingly, certain aspects of the invention relate to the
operation of a VSD to power a first load. The VSD is powered by a
power source simultaneously powering a second load. An AC reactive
power is monitored which may comprise a second AC reactive power of
the second load. Responsive to the monitored reactive power, the
VSD is controlled to maintain a first AC reactive power of the VSD
and first load opposite the second AC reactive power.
[0008] In various implementations, the magnitude of the first AC
reactive power may be maintained to at least 20% of a magnitude of
the second AC reactive power. The magnitude of the first AC
reactive power may be maintained to at least the lesser of: 50% of
the magnitude of the second AC reactive power; and 20% of a real
power of the variable speed drive.
[0009] Other aspects of the invention are directed to a VSD system
having means for monitoring the monitored AC reactive power and
means for controlling the VSD responsive to the monitored AC
reactive power.
[0010] The controlling may maintain a combination of the AC
reactive power of the variable speed drive and compressor motor and
the AC reactive power of the additional load to no more than 50% of
a wattage of the combination.
[0011] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic diagram of a building electrical
system including a VSD-driven load and an additional load.
[0013] FIG. 2 is a power triangle of an additional load.
[0014] FIG. 3 is a system power triangle according to the prior
art.
[0015] FIG. 4 is a system power triangle according to principles of
the invention.
[0016] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0017] FIG. 1 shows a building power system 20 including an AC
power source 22 (e.g., a generator output or utility output). The
building has a refrigeration compressor 24 mechanically driven by
an electric motor (e.g., a three-phase AC motor 26). The motor is,
in turn, electrically powered by a VSD 28 drawing input power from
the source 22. Beyond the load of the compressor/motor/VSD, the
source 22 powers an additional load 30 which may include various
individual loads such as industrial motors 32 and 34, lighting 36
and additional building systems and contents (not shown). The VSD
receives input from a controller 38 coupled to the source 22 and
which includes a power factor sensor 40 to measure the reactive
power of the system (e.g., all loads powered by the source 22)
either directly or indirectly. The VSD includes a power module 42
coupled to the source 22 to receive power therefrom and to the
motor 26 to supply power thereto. The power module is also coupled
to the controller 38 so that the controller may control delivery of
power by the power module.
[0018] FIG. 2 shows a power triangle for the additional load 30. An
apparent power 50 is shown as the hypotenuse of a triangle whose
remaining sides are a real power 52 and a reactive power 54. If, as
in the prior art (FIG. 3), the VSD 28 is operated to zero the
reactive power of its combination with the motor 26, a system
apparent power 60 will be the hypotenuse of a triangle whose first
side is the combination of additional load real power 52 and
VSD/motor real power 62 and whose second side is merely the
additional load reactive power 54. According to the present
invention, depending upon the size and other properties of the
loads in question, the VSD may be operated to maintain a reactive
power of the VSD/motor combination to fully or partially cancel the
reactive power of the additional loads. FIG. 4 shows the apparent
power 70 in such a partial cancellation. The first side of the
triangle remains the combination of the real powers 52 and 62. The
second side is the additional load reactive power 54 reduced by the
oppositely-directed reactive power 72 of the VSD/motor combination
or subsystem. Thus the system power factor may be brought closer to
one than in the system of FIG. 3.
[0019] An exemplary power factor sensor does not directly measure
the power factor but, rather, includes components measuring other
parameters from which the power factor is computed. Existing VSD
drives may include such components for measuring such parameters in
order to control the active rectifier power module to minimize the
reflection of harmonic distortion onto the input power.
[0020] In an exemplary implementation, the additional load 30 is
dominated by induction motors powered directly from the source 22.
Such motors have an exemplary power factor of 0.8-0.9 with current
lagging voltage. It would be advantageous to raise the system power
factor to at least 0.9, preferably to at least 0.95. With such
inductive additional load the VSD may be controlled to appear more
capacitive as illustrated. Alternatively, if the additional load is
capacative in nature the VSD may be controlled to appear more
inductive (not illustrated).
[0021] One or more embodiments of the present invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. For example, although a basic system has
been shown, the principles may be applied to more complex systems.
Details of the particular application and underlying VSD technology
may influence details of any associated implementation. Although
monitoring of the system reactive power is identified, monitoring
of just the additional load reactive power is also possible.
Accordingly, other embodiments are within the scope of the
following claims.
* * * * *