Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS OF IMPROVING THE EFFICIENCY OF AN
INDUCTION MOTOR
BACKGROUND
This invention relates to induction motors. More particularly, the invention
relates
to a method and apparatus to maximize the efficiency of an induction motor.
As ail AC induction inotor rotates, the magnetic fields of the rotor and the
stator
interact. The stator windings are typically connected to a supply in three-
phase form or
single phase form. By applying a voltage across the windings, a radial,
rotating magnetic
field is fonned. The rotor has solid aluminum bars cast in a "squirrel-cage"
configuration.
The rotating magnetic fields produced by the stator produce a current in the
aluminum bars
of the rotor. This produces a magnetic field in the aluminum bars which
interacts with the
rotating magnetic field of the stator to generate torque on the rotor. The
rotor reacts to the
magnetic field, but does not travel at the same speed. The rotor actually lags
behind the
speed of the rotating magnetic field. This lag is called slip, aiid is
essentially a comparison
of the speed of the rotor aild the speed of the magnetic field. The slip
typically increases
proportionately with increases in load.
hiduction motors run less efficiently when lightly loaded. In order to
increase
efficiency of the motor, the flux of the motor may be reduced by utilizing the
flexibility
built into most variable speed drives. However, determining the ideal flux for
maximum
efficiency often requires the use of expensive sensors.
A common approach used to increase the efficiency of the induction motor is to
sense the difference between the respective phases of the energizing voltage
and current at
the motor terminals. This requires identifying the zero crossings of the
voltage and current
wavefonns. However, when the motor voltage is pulse-width modulated at a low
freqt"iency, phase detection is difficult because the current has some of the
same
components as the waveform of the carrier frequency. Thus, identifying the
zero crossings
of the phase current may be difficult.
Another approach used to increase the efficiency of an induction motor is to
control the slip of the motor for maximuzn efficiency. To measure the slip,
the actual rotor
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speed must either be measured or estimated. However,
measuring or estimating rotor speed is very expensive.
SUMMARY OF THE INVENTION
Accordingly, the apparatus and method of the
invention provide an accurate estimate of the slip of the
motor using only one phase current sensor combined with
knowledge of the voltage and current at the motor. In the
apparatus of the invention, the motor includes a monitoring
circuit to monitor an analog DC bus voltage and an analog DC
bus current. A circuit connected to the monitoring circuit
estimates the predetermined slip of the motor. A
compensating circuit connected to the circuit adjusts the
voltage applied to the motor such that the motor operates at
the predetermined slip.
In the method of the invention, three phase AC
power is supplied to energize the motor. A DC bus voltage
and a DC bus current are measured. An actual torque
producing current value is calculated based on the DC bus
voltage and the DC bus current along with an estimated phase
voltage. The actual torque producing current value and the
estimated torque producing current value are compared. If
the actual and estimated torque producing current values are
different, an error term is produced representing that
difference. The estimated phase voltage value is then
changed based upon the previous estimated phase voltage
value and the error term. The three phase AC power supplied
to the motor is adjusted based on the estimated phase
voltage.
According to one aspect of the present invention,
there is provided a method of maximizing the efficiency of
an induction motor, the method comprising the acts of:
supplying three phase alternating-current (AC) power to the
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motor to energize the motor; measuring a direct-current (DC)
bus voltage and a DC bus current; determining an estimated
phase voltage; determining an actual torque producing
current value based on the DC bus voltage, the DC bus
current and the estimated phase voltage; determining an
estimated torque producing current value; comparing the
actual torque producing current value with the estimated
torque producing current value; determining an error term
representing the difference between the actual and estimated
torque producing current values; changing the estimated
phase voltage value based on the error term; and adjusting
the three phase AC power supplied to the motor based on the
estimated phase voltage.
According to another aspect of the present
invention, there is provided an induction motor comprising:
a rotor; a stator; a monitoring circuit to monitor a direct-
current (DC) bus voltage and a DC bus current; a circuit to
estimate a one of the slip, power factor and torque
producing current of the motor based on the DC bus voltage
and the DC bus current; and a compensating circuit to change
the flux in the motor based on the one of the estimated
slip, power factor and torque producing current.
According to still another aspect of the present
invention, there is provided an efficiency optimization
circuit comprising: a monitoring circuit to monitor a
direct-current (DC) bus voltage and a DC bus current; a
circuit to estimate a slip of the motor based on the DC bus
voltage and the DC bus current; and a compensating circuit
to adjust a voltage applied to the motor based on the
estimated slip.
According to yet another aspect of the present
invention, there is provided an efficiency optimization
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circuit for an induction motor, the circuit comprising:
measuring means to measure an analog DC bus voltage and an
analog DC bus current; converting means to convert the
analog DC bus voltage and the analog DC bus current to a
digital DC bus voltage and a digital DC bus current;
estimating means to estimate a phase voltage; first
calculating means to calculate an actual torque producing
current value based on the digital DC bus voltage, the
digital DC bus current and the estimated phase voltage;
comparing means to compare the actual torque producing
current value with the estimated torque producing current
value; second calculating means to calculate an error term
representing the difference between the actual and estimated
torque producing current values; and adjusting means to
change the estimated phase voltage value based on the error
term.
According to a further aspect of the present
invention, there is provided a data processing system for
determining the slip of an induction motor, the processing
system comprising: a computer processor for processing data;
a storage medium for storing data; a trigger signal for
initializing the storage medium; a monitoring circuit to
monitor a direct current (DC) bus voltage and a DC bus
current; a circuit to estimate a slip of the motor based on
the DC bus voltage and the DC bus current; and a
compensating circuit to change the flux in the motor based
on the estimated slip.
According to yet a further aspect of the present
invention, there is provided a computer program product for
determining the slip of an induction motor, the computer
program product comprising: a computer usable medium having
computer readable program code embodied therein for
estimating the slip of the motor; computer readable program
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code for causing the computer to determine an estimated
voltage needed to operate the motor at the predetermined
slip; computer readable program code for causing the
computer to compare the actual voltage applied to the motor
with the estimated voltage needed to operate the motor at
the predetermined slip; and computer readable program code
for causing the computer to adjust the voltage applied to
the motor to operate the motor at the predetermined slip.
According to still a further aspect of the present
invention, there is provided a method of maximizing the
efficiency of an induction motor, the method comprising the
acts of: measuring a direct-current (DC) bus voltage, a DC
bus current, and a phase current; determining an initial
value of a phase voltage; determining a calculated power
factor value based on the DC bus voltage, the bus current,
the phase current and the initial value of the phase
voltage; determining a desired power factor value; comparing
the calculated power factor value with the desired power
factor value; determining an error term representing the
difference between the calculated and desired power factor
values; and changing the phase voltage based on the error
term.
According to another aspect of the present
invention, there is provided an efficiency optimization
circuit comprising: a monitoring circuit to monitor a
direct-current (DC) bus voltage and a DC bus current; a
circuit to estimate a torque producing current of the motor
based on the DC bus voltage and the DC bus current; and a
compensating circuit to adjust a voltage applied to the
motor based on the estimated torque producing current.
According to yet another aspect of the present
invention, there is provided an efficiency optimization
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circuit comprising: a monitoring circuit to monitor a
direct-current (DC) bus voltage and a DC bus current; a
circuit to estimate a power factor of the motor based on the
DC bus voltage and the DC bus current; and a compensating
circuit to adjust a voltage applied to the motor based on
the estimated power factor.
According to still another aspect of the present
invention, there is provided a data processing system for
determining the slip of an induction motor, the processing
system comprising: a computer processor for processing data;
a storage medium for storing data; a trigger signal for
initializing the storage medium; a monitoring circuit to
monitor a direct current (DC) bus voltage and a DC bus
current; a circuit to estimate a torque producing current of
the motor based on the DC bus voltage and the DC bus
current; and a compensating circuit to change the flux in
the motor based on the estimated torque producing current.
According to still a further aspect of the present
invention, there is provided a data processing system for
determining the slip of an induction motor, the processing
system comprising: a computer processor for processing data;
a storage medium for storing data; a trigger signal for
initializing the storage medium; a monitoring circuit to
monitor a direct current (DC) bus voltage and a DC bus
current; a circuit to estimate a power factor of the motor
based on the DC bus voltage and the DC bus current; and a
compensating circuit to change the flux in the motor based
on the estimated power factor.
The principal advantage of the invention is to
optimize the running efficiency of the motor by determining
the motor slip.
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Another advantage of the invention is to provide an accurate estimate of the
slip of
the motor using only one phase current sensor.
Other features and advantages of the invention will become apparent to those
skilled in the art upon review of the following detailed description, claims
and drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an induction motor including an efficiency
optiinization circuit for controlling the induction motor.
FIG. 2 is a schematic view of another embodiment of the efficiency
optimization
circuit of the induction motor.
Before one enibodiment of the invention is explained in detail, it is to be
understood that the invention is not limited in its application to the details
of the
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construction and the arrangement of the components set forth in the following
description
or illustrated in the drawings. The invention is capable of other embodiments
and are
carried out in various ways. Also, it is understood that the phraseology and
terminology
used herein is for purpose of description and should not be regarded as
limiting.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Shown in FIG. I of the drawings is a schematic view of an induction motor 10
embodying the invention. The details of the induction motor are commonly known
in the
art and form no part of the invention. Accordingly, the induction motor will
not be
described in detail. The motor 10 includes an efficiency optimization circuit
100 for
coiitr-olling the excitation of the stator 38. The efficiency optimization
circuit 100 includes
a measuring circuit 104 for measuring a DC bus voltage 108 and DC bus current
112. The
DC bus voltage and current are input to analog to digital (A/D) converter 122
which is
electrically connected to a circuit 124. The circuit 124 may be a
microprocessor or may be
comprised of discrete components. In the embodiment of the invention including
a
nlicroprocessor, the method of the invention is preferably implemented using a
computer
software program or programs stored in the memory for the microprocessor. The
circuit
124 includes a torque producing current circuit component 144, a comparator
circuit
component 148, a regulator circuit component 152, and a feed back circuit
component
156. The circuit 100 also includes an inverter 132, and an RMS conversion
circuit 168
connected as shown.
In operation, the slip at which a motor operates with maximum efficiency
(Smax.err.)
is constant for a given induction motor. Thus, lowering the operating voltage
Vg until
(Smax.eff.) is attained is one way of operating the niotor at maximum
efficiency. When the
motor is operating at any slip less than the rated slip, the efficiency
optimization circuit
100 of the invention uses the relationship between the torque producing
component Iy and
flux producing component Id of the current Is supplied to the motor.
The measuring circuit 104 measures a DC bus voltage 108 and a DC bus current
112. These voltages are preferably measured via a voltage and current bus. The
DC bus
voltage 108 and the DC bus current 112 are analog signals that are converted
by an
analog-to-digital (A/D) converter 122 to a digital voltage signal 116 (Vds)
and a digital
current signal 120 (Id,).
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The digital voltage signal 116 and the digital current signal 120 are supplied
to the
torquc producing current circuit 144 along with an initial condition value of
the phase
voltage 128 (V,,). Assuming that the inverter 132 lias no losses, the
conservation of power
equation in the inverter is:
P.ry=Vd, =II =lVra l .110 l =cos(B)=Pou,;
wliere 0 is the angle between the phase voltage V0 the phase current I0.
Solving for
the power factor yields:
cos(b) = Vd' Id'
IVol = 110 1
Assuming that 0 and 0 (the angle between the pliase current and the torque
producing current) are approxitnately equal, then the torque producing
coinponent of the
current l,i is approximately deGned by the following equation:
Id COS (&)= Vdc I dc
IvOl
The torque producing current circuit 144 calculates the approximation of the
torque
producing component of the current 158 (Iy) as Ie cos0, and inputs the current
158 to a
comparator circuit 148. The comparator circuit 148 compares the current 158
with a
desired torque producing current value 160. The desired torque producing
current value (IA
cos0,,,,d) 160 is determined by the feedback circuit 156. The pliase current
136
(I9) is measured using a sensor (not shown) at the motor coils. The phase
current 136 (Io)
is input into the RMS circuit 168, where the AC phase current 136 (Ie) is
converted to an
RMS value 172 (I9RMS) of pliase current 136 (le). The RMS value 172 is input
into the
feedback circuit 156, along with the power factor for maximum efficiency
cos0,n,d, which
is a known constant for the motor. The feedback circuit 156 determines the
appropriate
angle, based ou niotor paraineters, between the phase voltage V,, and the
phase current IA
for maximuin efficiency. More specifically, the feedback circuit 156
calculates the cosine
of lhe angle, thus factoring the change of angle froni L to ly, resulting in
the estiniated
torque producing current value I0cos0c,,,d 160.
Coinparator circuit 148 compares the desired torque producing current value
160
(1oCOS0omd) and the current 158 to generate an error tenn 170 representing the
difference
between the calculated and desired torque producing current. The error term
170 is
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supplied to the regulator circuit 152. If the calculated and desired torque
producing
cui-rent values are the same value, the error term 170 is zero.
The regulator 152 increments or decrements the commatided pliase voltage 128
(V,,) by the error term 170, generating a new value for a commanded phase
voltage value
128. If the calculated and desired torque producing current values are the
same, the error
te--m 170 is equal to zero, and accordingly, the commanded phase voltage does
not change.
"The new commanded pliase voltage is then input into the inverter 132, which
converts the
rectiGed DC power to tliree-phase AC power. The three-phase AC power is then
fed to the
tei-minals 38 of motor 10.
The new commanded phase voltage 128 (Ve) is also fed back from the regulator
152 and used as the next conimanded phase voltage (V ) for determining the
actual torque
producing current, and the cycle repeats. By constantly updating the commanded
phase
voltage Va, the motor 10 is able to continually operate at the slip, thereby
increasing the
efficiency of the motor.
t 5 FIG. 2 scliematically illustrates anotlier embodiment 200 of the
efficiency
optimization circuit of the induction niotor. Like parts are identified using
like reference
numerals. As sliown in FIG. 2, the input 160 to computation circuit 148 is
simply the
desired power factor cos0amd, and the input 158 to computation circuit 148
from the torque
producing current circuit 144 is a calculated power factor (cosO), which is
calculated
similar to I cos0, except that the additional division operation is performed
using le. This
is achieved by directly supplying the RMS pliase current 172 (IoRms) to the
torque
producing current circuit 144 tlirougli A/D converter 122, instead of through
a feedback
loop as is sliown in Fig. 1. The overall operation of the efficiency
optimization circuit 200
is simpler than that of the efficiency optiniization circuit 100, and thus
requires less time to
update the cominanded phase voltage (V0).
Various features and advantages of the invention are set forth in the
following
claims.
