Electricity: motive power systems – Motor load – armature current or force control during... – Initial – 'cracking' or 'starting from rest' torque control
Reexamination Certificate
1999-12-06
2001-12-11
Ro, Bentsu (Department: 2838)
Electricity: motive power systems
Motor load, armature current or force control during...
Initial, 'cracking' or 'starting from rest' torque control
C318S500000, C318S705000
Reexamination Certificate
active
06329776
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to controlling transfer of an AC machine between power from a power supply and power from a variable frequency drive. More specifically, the invention is a method and apparatus for transferring an AC machine between power from a power supply and power from a drive that does not use a volts per hertz control scheme.
In many applications, it is desirable to transfer an AC machine receiving power a power supply, such as supply mains, to receive power from a variable frequency motor drive, or vice versa. The phrase “power supply”, as used herein, refers to any source of AC electric power such as supply mains, a generator, an uninterruptable power supply, or the like. The phrase “AC machine”, as used herein, refers to any AC rotating machine, such as AC synchronous motors, AC reluctance motors, generators, dynamos, or the like.
For example, in gas turbine static starters, a variable frequency motor drive is used in place of a diesel engine to bring a generator up to a self-sustaining speed at which the generator can subsequently be switched to supply mains. In other applications, such as variable speed fans and pumps, it is desirable to achieve variable speed with a motor drive and subsequently switch the motor to supply mains for sustained operation at a constant high speed. Also, in the event of failure of the motor drive, it is desirable to transfer the motor to the supply mains to continue operation. Similarly, it is often desirable to switch a motor from supply mains back to a motor drive for slowing the motor down or otherwise varying the speed or torque of the motor. Switching of an AC machine from a drive to a power supply is referred to as “transfer” herein and switching of an AC machine from a power supply to a drive is referred to as a “capture” herein.
Of course, AC machine speed varies with frequency of the input signal driving the motor. However, the inductive reactance of the motor drops at low frequencies, resulting in excess current in the motor if voltage is constant. Therefore, conventional V/F (“volts per frequency” or “volts per hertz”) drive control, utilizes a fixed ratio between the drive voltage and frequency and the frequency is controlled under this assumption, up to a nominal operating frequency, to control the speed of a motor driven by the drive output. In V/F control drives, it is relatively simple to synchronize the frequency of the drive output and a power supply to accomplish transfer and capture. However, V/F control is open loop in nature and thus has inherent stability problems. For example, since the frequency is controlled, the machine speed is not controlled directly. Machine slip occurs depending on the load and thus motor speed is not directly proportional to frequency of the input current. Complex slip frequency algorithms have been developed to overcome this problem. However, in addition to the added complexity of slip frequency control, V/F control does not separately control the field producing and the torque producing components of the input current and thus instantaneous torque control cannot be achieved.
“Vector control” drives have been developed to overcome limitations of V/F control drives. The phrase “vector control”, as used herein, refers to any type of AC machine torque control such as field orientation, natural field orientation control, and direct torque control. In vector control drives, torque in the motor is controlled directly or indirectly. Accordingly, vector control allows an AC machine to be controlled in a manner similar to a DC machine. Vector control algorithms are well known and vector control drives are used in various DC machine control applications. However, since vector control inherently does not provide a reference voltage, it is difficult to synchronize the output of a vector control drive with that of supply mains and thus it is difficult to implement line transfer or capture procedures in systems having vector control drives.
BRIEF SUMMARY OF THE INVENTION
A first aspect of the invention is a method for switching an AC machine between power from a power supply and power from a drive comprising the steps of, coupling the output of a drive to an AC machine, controlling the speed of the AC machine with the output of the drive utilizing a control procedure other than a volts per hertz control procedure, freezing voltage and frequency output by the drive at a transfer voltage value and transfer frequency value, controlling the output of the drive in accordance with a quasi-voltage mode using the transfer voltage value and the transfer frequency value as a reference, synchronizing the output of the drive and the output of a power supply after said controlling step, coupling the power supply to the AC machine after said synchronizing step, and removing the output of the drive from the AC machine.
A second aspect of the invention is a method for switching an AC machine between power from a power supply and power from a drive comprising coupling an output of a power supply to an AC machine to drive the AC machine with the output of the power supply, coupling an output of a drive to the AC machine with the drive in an idle state, uncoupling the output of the power supply from the AC machine, detecting operating states of the AC machine, preconditioning the output of the drive to correspond to the operating states detected in said detecting step, and controlling the output of the drive based on said preconditioning step.
A third aspect of the invention is an apparatus for switching an AC machine between power from a power supply and power from a motor drive comprising means for coupling the output of a drive to an AC machine, means for controlling the speed of the AC machine with the output of the drive utilizing a control procedure other than a volts per hertz control procedure, means for freezing voltage and frequency output by the drive at a transfer voltage value and transfer frequency value, means for controlling the output of the drive in accordance with a quasi-voltage mode using the transfer voltage value and the transfer frequency values as a reference, means for synchronizing the output of the drive and the output of a power supply, means for coupling the power supply to the AC machine after said means for synchronizing has synchronized the output of the drive and the output of the power supply, and means for removing the output of the drive from the AC machine after said means for coupling has coupled the power supply to the AC machine.
A fourth aspect of the invention is an apparatus for switching an AC machine between power from a power supply and power from a drive comprising means for coupling an output of a power supply to an AC machine to drive the AC machine with the output of the power supply, means for coupling an output of a drive to the AC machine with the drive in an idle state, means for uncoupling the output of the power supply from the AC machine, means for detecting operating states of the AC machine, means for preconditioning the output of the drive to correspond to the operating states detected by said means for detecting, and means for controlling the output of the drive based on an output of said means for preconditioning.
A fifth aspect of the invention is a method for detecting opening of a switch mechanism in a drive systems having a power supply coupled to a drive to provide power to the drive, a first switch mechanism coupled to an output section of the drive, a second switch mechanism coupled to the power supply, an AC machine coupled to the first switch mechanism and the second switch mechanism, and a controller operatively coupled to the first switch mechanism and the second switch mechanism to control to the first switch mechanism and the second switch mechanism to selectively couple the AC machine to the utility and the output section of the drive. The method comprises detecting frequency at terminals of the AC machine, and generating a signal indicating the second switch mechanism is open based on the frequency at terminals
Bixel Paul S.
Ho Eddy Y. Y.
Duncan, Esq. Kevin T.
General Electric Company
Horton Esq. Carl B.
Hunton & Williams
Ro Bentsu
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