Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2002-09-19
2004-02-10
Masih, Karen (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S805000, C318S448000, C318S807000, C318S812000
Reexamination Certificate
active
06690139
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The field of the invention is motor controllers and more specifically a method and apparatus for limiting current in an open loop adjustable frequency motor drive at low operating frequencies.
Induction motors have broad application in industry. An induction motor system typically includes a driver or controller, a power conversion configuration and an induction motor itself. The power conversion configuration generally receives power via supply lines and converts the received power into a form that can be provided to the motor thereby causing a motor rotor to rotate. The conversion configuration typically includes a plurality of semiconductor switching devices that link the supply lines to motor terminals and, based on switch turn on and turn off cycles, provide power to the motor phases linked thereto.
One common type of motor is a three-phase induction motor that includes a stator and a rotor. The stator typically forms a cylindrical stator cavity. One common rotor design includes a “squirrel cage winding” in which axial conductive rotor bars are connected at either end by shorting rings to form a generally cylindrical structure. The rotor is mounted in the stator cavity for rotation about a rotor axis. The stator windings are linked to three separate phases of the converter configuration to receive currents therefrom. The stator currents are controlled so that their combined effect is to generate a magnetic stator field that rotates about the stator cavity. The rotating stator field flux cuts across the conductive rotor bars and induces (hence the label “inductance motor”) cyclic current flows through the bars and across the shorting rings. The cyclic rotor bar current flows in turn produce a rotor field. Interaction (e.g., pulling or pushing action) between the rotor field and the stator field causes the rotor to rotate.
By using induced rotor current to generate the rotor field, the need for slip rings or brushes (i.e., wearable mechanical components) is eliminated which renders induction type motors relatively maintenance-free and reduces overall costs associated with motor design. Among other reasons, relatively limited costs have made inductance motors preferred for many applications throughout industry.
To a first approximation the torque (i.e., rotational force on the rotor) and speed of an induction motor may be controlled by changing the frequency of the driving voltage and thus the angular rate of the rotating stator field. Generally, for a given torque, increasing the stator field rate will increase the rotor speed (which generally follows the stator field). Alternatively, for a given rotor speed, increasing the frequency of the stator field will increase the torque by increasing the slip, that is, the difference in speed between the rotor and the stator field. An increase in slip increases the rate at which flux lines are cut by the rotor bars thereby increasing the rotor-generated field and thus the force or torque between the rotor field and stator field.
Referring to
FIG. 10
, the rotating phasor
13
of the stator magneto motive force (“mmf”) will generally form some angle &agr; with respect to the phasor of rotor flux
19
. The torque generated by the motor is proportional to the magnitudes of these phasors
13
and
19
but also is a function of their angle &agr;. The maximum torque is produced when phasors
14
and
18
are at right angles to each other (e.g., &agr;=90°) whereas zero torque is produced if these phasors are aligned (e.g., &agr;=0°). Phasor
13
may, therefore, be usefully decomposed into a torque producing component
15
perpendicular to the phasor
19
and a flux component
17
parallel to rotor flux phasor
18
.
These two components
15
and
17
of the stator mmf are proportional, respectively, to two stator currents i
qe
, a torque producing current, and i
de
, a flux producing current, which may be represented by orthogonal vectors in a rotating or synchronous reference frame of the stator flux having slowly varying magnitudes. The subscript “e” is used herein to indicate that a particular quantity is in the rotating or synchronous frame of stator flux.
Accordingly, in controlling an induction motor, it is generally desired to control not only the frequency of the applied voltage (hence the speed of the rotation of the stator flux phasor
13
) but also the phase of the applied voltage relative to the current flow and hence the division of the currents through the stator windings into the i
qe
and i
de
components. Control strategies that attempt to independently control currents i
qe
and i
de
are generally termed field oriented control (FOC) strategies.
The production of any given set of currents i
qe
and i
de
requires that the stator be excited with voltages V
qe
and V
de
as follows:
V
qe
=(
R
s
)(
i
qe
)+(2
&pgr;f
e
)(&lgr;
rated
) Eq. 1
V
de
=(
R
s
)(
i
de
) Eq. 2
where
R
s
=stator resistance;
i
qe
, i
de
=synchronous motor currents aligned with the d and q-axis typically reflecting motor load and no load currents, respectively;
f
e
=electrical field frequency in Hertz; and
&lgr;
rated
=stator flux linkage=motor nameplate voltage/motor nameplate frequency (in Hertz).
The first terms on the right hand sides of each of Equations 1 and 2 are referred to as the stator resistive voltage drops. As the labels imply, the resistive voltage drops R
s
i
qe
and R
s
i
de
correspond to components of the voltage provided at a stator winding terminal that are dissipated by the stator winding resistance R
s
. Because the resistive drops are provided to boost the commanded voltages and, in effect, overcome the stator resistance R
s
, the resistive drops are often referred to as “voltage boost” terms. The second term 2&pgr;f
e
&lgr;
rated
on the right hand side of Equation 1 is referred to generally as a reactive voltage drop and, as its label implies, corresponds to the component of the voltage provided at the stator winding terminal that causes inductance or interaction between the stator and the rotor.
Equations 1 and 2 above are the fundamental command equations employed by most voltage/frequency controllers. To implement Equations 1 and 2, the controller has to be provided with several of the terms in each of Equations 1 and 2.
In order to minimize costs, often controller/converter configurations are designed to be useable for many different purposes (i.e., to drive many different load types). For instance, one controller/converter configuration may be capable of driving any of several differently sized three phase motors where the motors have different operating characteristics. Thus, when designing controller/converters, manufacturers typically do not know exact characteristics of loads that will be linked to and driven by the controller/converters and, therefore, some controller operating parameters have to be set by customers after system configuration is completed.
The rated flux &lgr;
rated
can be determined by dividing a name plate motor voltage by a nameplate frequency values while the stator winding resistance R
s
is typically determined by performing a commissioning procedure (e.g., see U.S. Pat. No. 5,502,360 for a commissioning procedure to determine R
s
). The d-axis current i
de
may be determined in any of several different ways including use of a look-up table that correlates d-axis current with various motor parameters or by performing some type of commissioning procedure. Each of the d-axis current i
de
, the stator resistance R
s
and the rated flux &lgr;
rated
are stored in a controller memory for use during motor operation. The d-axis current i
de
typically is not adjusted during motor operation and therefore the d-axis voltage V
de
is set upon commissioning.
In addition to the components described above, most controllers also include some type of feedback m
Masih Karen
Quarles & Brady LLP
Rockwell Automation Technologies Inc.
Walbrun William R.
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