Electricity: motive power systems – Induction motor systems
Reexamination Certificate
2001-09-28
2003-10-21
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
C318S490000, C318S727000, C318S728000, C318S729000, C318S807000, C318S808000, C318S809000, C318S810000, C318S811000, C318S823000, C318S824000, C318S825000
Reexamination Certificate
active
06636012
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 AC induction motor drives and more specifically the area of injecting high frequency voltage signals into an AC induction motor and using high frequency feedback signals to identify dynamic motor operating parameters.
Induction motors have broad application in industry, particularly when large horsepower is needed. In a three-phase induction motor, three phase alternating voltages are impressed across three separate motor stator windings and cause three phase currents therein. Because of inductances, the three currents typically lag the voltages by some phase angle. The three currents produce a rotating magnetic stator field. A rotor contained within the stator field experiences an induced current (hence the term “induction”) which generates a rotor field. The rotor field typically lags the stator field by some phase angle. The rotor field is attracted to the rotating stator field and the interaction between the two fields causes the rotor to rotate.
A common rotor design includes a “squirrel cage winding” in which axial conductive bars are connected at either end by shorting rings to form a generally cylindrical structure. The flux of the stator field cutting across the conductive bars induces cyclic current flows through the bars and across the shorting rings. The cyclic current flows in turn produce the rotor field. The use of this induced current to generate the rotor field eliminates the need for slip rings or brushes to provide power to the rotor, making the design relatively maintenance free.
To a first approximation, the torque 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 speed of the rotor (which 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 fields. An increase in slip increases the rate at which flux lines are cut by the rotor, increasing the rotor generated field and thus the force or torque between the rotor and stator fields.
Referring to
FIG. 1
, a rotating phasor
1
corresponding to a stator magneto motive force (“mmf”) will generally have some angle &agr; with respect to the phasor of rotor flux
2
. The torque generated by the motor will be proportional to the magnitudes of these phasors
1
and
2
but also will be a function of their angle &agr;. Maximum torque is produced when phasors
1
and
2
are at right angles to each other whereas zero torque is produced if the phasors are aligned. The stator mmf phasor
1
may therefore be usefully decomposed into a torque producing component
3
perpendicular to rotor flux phasor
2
and a flux component
4
parallel to rotor flux phasor
2
.
These two components
3
and
4
of the stator mmf are proportional, respectively, to two stator current components: iq, a torque producing current, and id, a flux producing current, which may be represented by orthogonal vectors in the rotating frame of reference (synchronous frame of reference) of the stator flux having slowly varying magnitudes.
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
1
), 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 iq and id components. Control strategies that attempt to independently control currents iq and id are generally termed field oriented control strategies (“FOC”).
Generally, it is desirable to design FOCs that are capable of driving motors of many different designs and varying sizes. Such versatility cuts down on research, development, and manufacturing costs and also results in easily serviceable controllers.
Unfortunately, while versatile controllers are cost-effective, FOC controllers cannot control motor operation precisely unless they can adjust the division of d and q-axis currents through the stator windings to account for motor-specific operating parameters. For this reason, in order to increase motor operating precision, various feedback loops are typically employed to monitor stator winding currents and voltages and/or motor speed. A controller uses feedback information to determine how the inverter supplied voltage must be altered to compensate for system disturbances due to system specific and often dynamic operating parameters and then adjusts control signals to supply the desired inverter voltages.
To this end, in an exemplary FOC system two phase d and q-axis command currents are provided that are calculated to control a motor in a desired fashion. The command currents are compared to d and q-axis motor feedback currents to generate error signals (i.e., the differences between the command and feedback currents). The error signals are then used to generate d and q-axis command voltage signals which are in turn transformed into three phase command voltage signals, one voltage signal for each of the three motor phases. The command voltage signals are used to drive a pulse width modulated (PWM) inverter that generates voltages on three motor supply lines. To provide the d and q-axis current feedback signals the system typically includes current sensors to sense the three phase line currents and a coordinate transformation block is used to transform the three phase currents to two phase dq frame of reference feedback currents.
In addition to requiring two phase signals and three phase signals to perform 2-to-3 and 3-to-2 phase transformations, respectively, a flux position angle is also required. One common way to generate a flux angle feedback estimate is to integrate a stator frequency. A stator frequency can be determined by adding a measured rotor frequency (rotor speed) and calculated slip frequency. Slip frequency calculations require motor parameter values such as a rotor resistance, leakage inductance, etc. Therefore, precise parameter values are necessary to eliminate errors in flux angle determinations.
In the case of drives that do not include a speed sensor it is necessary to estimate both the rotor frequency and the slip frequency to determine the flux angle. Thus, these drives also require precise knowledge of motor parameter values. Usually, drives that do not includes speed sensors cannot operate at low rotor frequencies (e.g., below 3 Hz) due to motor parameter estimation errors.
Recently, a method for flux angle feedback determination was developed to overcome the low speed operating problems in drives that do not include speed sensors. This method includes injecting a known high frequency voltage signal into each of the command voltage signals used to drive the PWM inverter and use feedback current (or voltage) signals to determine the flux angle. To this end, an exemplary system converts a high frequency command signal into a high frequency phase angle and then generates a first injection signal that is the product of a scalar and the sine of the high frequency phase angle. Second and third injection signals are also generated, each of the second and third signals phase shifted from the first signal by 120 degrees. A separate one of the first, second and third signals is then added to a separate one of the three voltage command signals.
Algorithms to generate a flux position angle estimate as a function of a negative sequence high frequency current components or zero sequence high frequency current (or voltage) components are well known in the controls art and therefore will not be explained here in detail.
The high frequency voltage injection methods can be used to identify a flux angle position without using m
Harbaugh Mark M.
Kerkman Russel J.
Royak Semyon
Gerasimow Alex
McCloud Renata
Nappi Robert E.
Quarles & Brady LLC
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