Electricity: motive power systems – Motor-reversing – Armature or primary circuit control
Reexamination Certificate
1997-06-26
2001-05-15
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Motor-reversing
Armature or primary circuit control
C318S432000, C318S496000, C388S907200
Reexamination Certificate
active
06232731
ABSTRACT:
This invention relates to a multi-channel motor winding arrangement which produces an alternating current waveform with low harmonic distortion. The alternating current waveform may be used in a single phase or polyphase synchronous or induction type alternating current motor. This invention also relates to a controller for producing an alternating current waveform exhibiting low harmonic distortion for use in motors or other apparatus constituting inductive loads.
BACKGROUND OF THE INVENTION
A number of pulse width modulation (PWM) inverters have been developed for converting direct current (DC) power or fixed frequency alternating current (AC) power to variable frequency AC power for the purpose of driving AC synchronous or induction motors at variable speeds. Also, a large number of permanent magnet (PM) motors have been developed which resemble AC synchronous motors in construction but are typically driven by a controller that only provides an electronically commutated waveform to the stator windings, similar to that generated by the mechanical commutator in a DC motor. However, full-fledged variable frequency inverters are also being used more commonly in recent years to drive these PM motors, particularly in the higher rating devices. This is primarily because the high frequency components generated by hard-switching electronic commutation results in unacceptable dielectric cycling of insulation as well as hysteresis and eddy current heating in larger motors unless expensive insulation, lamination and conductor types are used in the construction of the motor.
The use of PWM inverters allows effective operation of higher rating motors incorporating conventional lamination and conductor types. However, these motor controllers are still typically limited to ratings of several hundred kilowatts. This rating limitation is primarily due to the ratings of power switching devices currently available and the circuit configurations in which they are typically used. Currently available power switching devices for high voltage (e.g., 500-4500v), high speed switching (e.g., 10-40 KHZ), and high current (e.g., 50-400 A) applications are limited to insulated-gate bipolar transistors (IGBT's). Certain other devices can be used, such as thyristors, plain bipolar junction transistors (BJT's), Darlington BJT's and metal oxide semiconductor (MOS) controlled BJT's with some associated compromise in performance or cost. Other devices under development, such as MOS controlled thyristors (MCT's), promise higher current capacity with comparable voltage and switching speed ratings, which would make current PWM inverter configurations practical at power levels in the several megawatt range or higher. Unfortunately, these devices will require special protection features to avoid the hazards of high-current explosive faults and may require higher switching speeds than devices of lower ratings to avoid parasitic losses in the stator windings and laminations due to harmonic distortion of the inverted waveform. This latter problem could lead to expensive modifications in the motor construction (e.g., thinner laminations and smaller wire gauges), particularly in large motors, to avoid damage to insulation from excessive heating and unacceptable performance losses.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved multi-channel electromagnetic winding arrangement for high rating devices.
Another object of the invention is to provide an improved method for providing current waveforms to multi-channel electromagnetic devices.
It is further object of the subject invention to provide a PWM inverter suitable for driving various classes of AC synchronous and induction motors as well as PM motors, that provides low harmonic distortion of the inverted waveform, can be designed for virtually unlimited motor ratings and uses currently available power switching devices, the only restrictions being that the motor windings carrying the inverted waveform(s) are a group of individual conductors, insulated from one another and that each conductor is connected to its own dedicated power switching devices in an H-bridge or other equivalent configuration.
These and other objects of the invention are attained by providing a multi-channel electromagnetic winding arrangement comprising an electromagnetic winding including devices for generating at least one current waveform comprising at least two channels. At least one switching device controls each channel and is dedicated thereto. The arrangement also includes a control unit for controlling the switching means so as to produce a composite current waveform from the current waveforms generated by each channel.
The invention also provides a controller for producing waveforms in a multi-channel electromagnetic winding arrangement having at least one switch for controlling current through each channel in the arrangement. The controller turns each switch on and off in a predetermined sequence at a rate which is varied in response to the magnitude of the difference between the composite current waveform produced from current waveforms generated by each channel and a desired current waveform. In an alternate controller arrangement, the switches associated with each channel turned on at a fixed interval, each channel being slightly offset from the other, and the duration that each switch remains on is varied by a control signal that represents the voltage waveform to be generated by the control means, or a control signal that varies in response to deviations of the measured current waveform and the target current waveform.
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Baker & Botts
Electric Boat Corporation
Ro Bentsu
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