Power factor motor control system
First Claim
1. A method for controlling a doubly fed motor driving a load, the doubly fed motor having stator windings and rotor windings and receiving power from an electric power grid having a system frequency, the method comprising the steps of:
- monitoring the values of the electric power grid input to the doubly fed motor;
setting a reference angular rotor speed;
synthesizing the monitored values of the electric power grid input and the set reference angular rotor speed to produce a polyphase controller signal; and
converting input power from the electric power grid, according to the polyphase controller signal, to produce a polyphase doubly fed motor input for supply to the doubly fed motor so as to control the power factor of the doubly fed motor, irrespective of variations in the load.
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Accused Products
Abstract
A device and associated method to control the electromechanical energy conversion process of a variable speed drive utilizing a doubly fed machine, which provides the mechanical energy as demanded by some mechanical load. Alternatively, a device and associated method is provided for a synchronous motor. The terminals of either the stator or rotor winding of the doubly fed machine are directly connected to a polyphase AC power grid of a certain grid frequency. The device establishes through electronic control a stable operation of the drive at a desired power factor at the terminals of these windings, irrespective of varying load conditions. This capability is provided without the need for computing or sensing this desired power factor and the subsequent feedback of its deviation from the desired value through a properly designed regulator. All signal processing as required can be conveniently and reliably implemented; only the easily accessible quantities of the stator voltage and real power supplied to the motor are sensed and subjected to simple algebraic manipulations. Torque speed characteristics produced by the drive are compatible with any demanded load condition within design limits. These flexibilities are retained along with the capability of electronically adjusting the drive speed without the need for detecting the mechanical load condition.
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Citations
44 Claims
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1. A method for controlling a doubly fed motor driving a load, the doubly fed motor having stator windings and rotor windings and receiving power from an electric power grid having a system frequency, the method comprising the steps of:
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monitoring the values of the electric power grid input to the doubly fed motor; setting a reference angular rotor speed; synthesizing the monitored values of the electric power grid input and the set reference angular rotor speed to produce a polyphase controller signal; and converting input power from the electric power grid, according to the polyphase controller signal, to produce a polyphase doubly fed motor input for supply to the doubly fed motor so as to control the power factor of the doubly fed motor, irrespective of variations in the load. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. An AC drive system which receives electrical power from an electrical power grid at a system frequency, and which delivers mechanical power to a rotating mechanical load, operating substantially at a desired power factor as seen from the electric power grid, the system comprising:
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an AC motor having a stator, a rotor, polyphase power windings wound on one of the stator or rotor, and excitation windings wound on the other of the stator or rotor, and which receives electrical power from the electrical power grid and delivers rotating mechanical power to the load; energy conversion means responsive to a controller signal for converting electrical power from the electrical power grid into excitation power which is applied to the excitation windings of the motor; input sensing means for sensing input values of the electrical power grid to the AC motor; and drive controller means for controlling the energy conversion means by producing the control signal from at least one signal processing means including amplitude signal processing means for producing a maximum amplitude signal from the sensed input values of the electrical power grid. - View Dependent Claims (17, 18, 19, 20, 21, 22, 23, 24)
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25. A method for controlling a synchronous motor under mechanical loads, irrespective of any varying torque or speed requirements of such a load, while maintaining a desired power factor operation, the synchronous motor having DC excitation field windings and receiving power from an electric power grid, the method comprising the steps of:
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monitoring the values of the electric power grid input to the synchronous motor; synthesizing the monitored inputs of the electric power grid with a precalibrated constant to produce an initial maximum amplitude signal; generating a controller signal from the synthesized initial maximum amplitude signal; and converting input power from the electric power grid, according to the controller signal, to produce a DC excitation input for supply to the field windings of the synchronous motor so as to control the power factor of the synchronous motor, irrespective of variation in the load. - View Dependent Claims (26)
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27. A method of providing variable speed control to a variable speed drive having an energy converter supplying a doubly fed motor which has stator windings and rotor windings and which receives power from an electrical power grid having a system frequency, the method comprising the steps of:
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establishing a frequency signal from a reference angular rotor speed and the system frequency; synthesizing the input values of the electric power grid received by the doubly fed motor with a precalibrated constant to produce a maximum amplitude signal; iterating the synthesizing step to produce the maximum amplitude signal corresponding to a desired power factor operation of the doubly fed motor; and generating a polyphase controller output signal from the maximum amplitude signal and the established frequency signal for input to the energy converter of the variable speed drive, so as to control the power factor of the doubly fed motor of the variable speed drive, irrespective of variations in a load driven by the motor. - View Dependent Claims (28, 29, 30, 31, 32)
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33. A variable speed drive controller for controlling a variable speed drive having an energy converter supplying a doubly fed motor which has rotor windings and stator windings, and which receives power from an electric power grid having a system frequency, the controller comprising:
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means for synthesizing the input values of the electric power received by the doubly fed motor from the electric power grid with a precalibrated constant to produce a maximum amplitude signal corresponding to operation of the doubly fed motor at a desired power factor; means for establishing a frequency signal from a reference angular rotor speed and the system frequency; and means for generating a polyphase controller output signal from the maximum amplitude signal and the established frequency signal for input to the energy converter of the variable speed drive which supplies the doubly fed motor. - View Dependent Claims (34, 35, 36, 37, 38, 39, 40)
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41. A drive controller for controlling a synchronous speed drive having an energy converter supplying a synchronous motor which has a rotor and which receives power from an electric power grid having a system frequency, the controller comprising:
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means for synthesizing the input values of the electric power grid received by the synchronous motor with a precalibrated constant to produce a maximum amplitude signal corresponding to a desired power factor operation of the synchronous motor; means for limiting a power angle, dependent upon operating parameters of the synchronous motor at an operating point, to values less than 90°
;means for intermittently calibrating the precalibrated constant of the synthesizing step, to correct for aging and hysteresis effects on the performance of the synchronous motor; means for continually determining a magnetizing current from the precalibrated constant and the input values of the electric power grid; and means for adjusting the precalibrated constant when the determined magnetizing current exceeds a predetermined saturation value.
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42. A power factor synthesizer, for use in a variable speed drive control of a doubly fed motor having excitation windings and receiving power from an electrical power grid having a system frequency, the synthesizer comprising:
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means for establishing a frequency signal from an input reference angular rotor speed and the system frequency; means for synthesizing the input values of the electric power grid received by the doubly fed motor with a precalibrated constant to produce a maximum amplitude signal for operation of the doubly fed motor at a desired power factor; means for intermittently calibrating the precalibrated constant to correct for aging and hysteresis effects on the performance of the doubly fed motor; means for continually determining a magnetizing current from the precalibrated constant and the input values of the electric power grid; means for comparing the determined magnetizing current with a predetermined saturation value; and means for adjusting the precalibrated constant when the magnetizing current exceeds the saturation value. - View Dependent Claims (43)
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44. A power factor synthesizer for controlling a synchronous speed drive comprising an energy converter and a synchronous motor having excitation windings responsive to the energy converter, the synchronous motor receiving power from an electrical power grid having a system frequency, the synthesizer comprising:
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means for synthesizing the input values of the electric power grid received by the synchronous motor with a precalibrated constant to produce a maximum amplitude signal for input to the energy converter; means for intermittently calibrating the precalibrated constant to correct for aging and hysteresis effects on the performance of the synchronous motor; means for continually determining a magnetizing current from the precalibrated constant and the input values of the electric power grid; means for comparing the determined magnetizing current with a predetermined saturation value; and means for adjusting the precalibrated constant when the magnetizing current exceeds the saturation value.
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Specification