Electricity: motive power systems – Induction motor systems
Reexamination Certificate
2002-09-09
2004-08-31
Duda, Rina (Department: 2837)
Electricity: motive power systems
Induction motor systems
C318S800000, C318S732000, C318S767000, C290S046000, C322S016000, C322S029000
Reexamination Certificate
active
06784634
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to controls for induction machines, and more specifically, to controls for brushless doubly-fed induction machines, including both motors and generators.
BACKGROUND OF THE INVENTION
Doubly-fed induction machines have been used as variable speed electric motors or generators. Generators of this type have been controlled with a power converter that has a lower power rating than the machine electrical power output, while motors of this type have been controlled with a power converter having a lower electrical power output than the motor mechanical power output. The prior art also teaches that wound rotor induction machines having a stator connected to an alternating current (AC) power line can be controlled with a field oriented or flux vector control that is connected to the rotor to provide accurate control of the machine currents and torque when the machine is used as either a generator or a motor.
FIG. 1A
is a power circuit block diagram illustrating this prior art configuration. The stator of a doubly-fed machine
10
is connected through current sensors
20
to an AC power line
14
, which also supplies power to a current regulating motor control
12
. Control of the current in rotor leads
22
controls the torque. The mathematical model and design basis for such a control, including the use of rotor position and stator and rotor currents to determine the position of the flux vector, are disclosed in Chapter 13.1 of the textbook “Control of Electrical Drives,” by Werner Leonhard, Springer-Verlag (1985).
Flux vector control provides substantially independent control of the distribution of excitation current between the rotor and stator, and of the quadrature stator current, which determines torque. The applied AC stator voltage and machine characteristics determine the total excitation current. The control regulates the stator portion of excitation in response to a reactive current reference and commands the necessary rotor excitation current to attain the required total excitation of the machine. This type of control accurately regulates the excitation and quadrature (torque producing) stator currents within preset limits and provides accurate torque control within preset limits, even if external loads exceed the rated machine or control capability.
As taught by the above-noted Leonhard text and other references, wound rotor machines that have a stator connected to the AC power line require power to flow from the rotor connection to the control when motoring at sub-synchronous speeds, which are speeds below the synchronous speed at which the frequency of the power at the control connection to the rotor of the machine is zero Hz. When the machine is operating as a generator, power flows into the rotor at subsynchronous speeds and from the rotor to the control at super-synchronous speeds.
Flux vector control of singly-fed induction machines, i.e., of a conventional AC induction motor
30
, is also taught by the Leonhard textbook and this technique is commonly used in industrial motor and generator controls. Controls
26
all rely on position feedback
32
of rotor position, or electrical measurements of the stator, to provide the information needed to estimate the flux in the rotor.
FIG. 1B
is a power circuit block diagram of this prior art control configuration.
Control of the rotor with an inverter
44
in a doubly-fed wound rotor generator
36
for standalone applications is shown in the power circuit block diagram of FIG.
1
C. Typically a DC bus power source
42
supplies control power to inverter
44
until the power output of generator
36
to inverter inputs
34
is adequate to supply control power. Inverter
44
controls the frequency and voltage of generator
36
rotor inputs
46
. Voltage taps
18
are monitored for control of output voltage. This configuration is also taught by Leonhard and by other prior art references.
The slip rings of wound rotor doubly-fed machines can be eliminated with brushless doubly-fed machines of several types. These include dual rotor-stator induction machines (referred to below as “Type 1”), such as disclosed in U.S. Pat. Nos. 3,183,431; 3,571,693; 4,229,689; 4,246,531; 4,305,001; 4,472,673; 4,701,691; 5,886,445; and 6,278,211. Single rotor-stator induction machines with two sets of stator windings of different pole counts (referred to below as “Type 2”) are disclosed in U.S. Pat. Nos. 3,183,431; 5,028,804; and 5,239,251; and in other references listed therein. Reluctance machines (referred to below as “Type 3”) are disclosed in U.S. Pat. No. 5,359,272 and by Xu et al. in “A Novel Wind-Power Generating System Using Field Orientation Controlled Doubly-Excited Brushless Reluctance Machine,” IEEE, pp. 408-413 (January 1992). Brushless doubly-fed induction machines of Type 1 with reverse phase rotor connections, and of Type 2, have a rotor construction that tightly magnetically links the two stator winding sets through the rotor currents, so that the total number of poles is equal to the sum of the number of poles of the two stator winding sets. When they are synchronously controlled, their speed is proportional to the sum of the two stator frequencies, and the torques on the shaft from the two sets of stator currents are additive.
Brushless doubly-fed induction machines with one stator connected to the AC power line also require power to flow from the other doubly-fed connection, i.e., the other stator, to the control when motoring at sub-synchronous speeds or generating at super-synchronous speeds. The synchronous speed in revolutions per second, at which the frequency of power at the control connection to the stator is zero Hz, is equal to the quotient of twice the AC power line frequency divided by the sum of the number of poles of the two stator windings. Several of the above-noted references also teach that there is a discontinuity in the control of these machines at the speed above synchronous speed where the rotor frequency is equal to zero Hz. No power can be transferred between the stators by the machine when the rotor frequency is zero. The speed, in revolutions per second, at which this discontinuity occurs is equal to twice the AC power line frequency divided by the number of poles in the stator connected to the AC power line. Thus, the speed range over which a brushless doubly-fed induction machine can be smoothly controlled is from zero speed through the synchronous speed, and up to nearly the discontinuity speed, where the rotor frequency is zero.
The flux vector control techniques developed by Leonhard and others for wound rotor machines have been shown to apply also to brushless doubly-fed induction machines. Papers describing these adaptations include: (1) D. Zhou et al., “Field Oriented Control Development for Brushless Doubly-Fed Machines,” Proceedings of IEEE IAS Annual Meeting, San Diego (1996); (2) Xie Lun et al., “The Research of Brushless Doubly-Fed AC Excited Induction Motor Drive,” Proceedings of Fifth International Conference on Electrical Machines and Systems (2001); and (3) B. Hopfensperger et al., “Combined Magnetizing Flux Oriented Control of the Cascaded Doubly-Fed Induction Machine,” IEEE Proceedings on Electric Power Apparatus (July 2001). The foregoing references teach flux vector control of singly fed induction machines, doubly-fed wound rotor induction machines and brushless doubly-fed induction machines and inverter control of standalone generators like that shown in FIG.
1
C. However, none of these prior art references discloses or suggests a method for control of brushless doubly-fed induction machines that achieves specific desirable operating capabilities for such a machine. It would clearly be desirable to eliminate the position sensor typically used in the prior art and determine rotor position from electrical variables. It would also be desirable to develop a method of substantially “bumpless” doubly-fed motor connection of such a machine to an AC line at or near the zero Hz speed.
There are certain advantages to operating
Anderson Ronald M.
Colon Santana Eduardo
Duda Rina
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