Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1999-10-11
2002-02-26
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S807000
Reexamination Certificate
active
06351095
ABSTRACT:
FIELD OF INVENTION
This invention is related to the field of electrical rotating machinery for the conversion between electrical energy and mechanical energy.
BACKGROUND OF THE INVENTION
The basic principal of nearly all electrical rotating machinery is that a current of electrical charge located within a magnetic field will experience a force perpendicular both to the flow of charge and the lines of force of the magnetic field. Most electrical rotating machines make use of this principal by generating a magnetic field directed radially about a cylinder, causing current to flow axially along the cylinder, thus developing a tangential force which causes the cylinder to turn. Other geometries are possible, for example, so called ‘axial flux’ machines make use of a magnetic field generally parallel to the axis of rotation, and a generally radial current flow, again causing tangential force and thus causing rotation. If a conductor is forced through a magnetic field by some sort of external prime mover, then an electrical current can be caused to flow; this is the principal of the generator.
In the method of the AC induction motor, a rotating magnetic field is produced by the stator or stationary portion of the machine. This rotating magnetic field has two functions. First, it interacts with current carried by conductors of the rotor, causing the rotor to turn. Second, it produces the rotor currents by means of transformer action. Thus the rotor needs no connections to means of electrical supply, and is simply supported by bearings which allow free rotation. Such design simplifies motor construction, and greatly enhances motor reliability. The essence of the AC induction motor, and by extension the AC induction generator, is the production of a smoothly rotating magnetic field in the stator.
The rotating magnetic field is produced by coils of wire or windings, suitably placed in the stator. Each winding, when energized with a direct current, would produce a fixed magnetic field. By energizing a winding with a sinusoidal alternating current, a smoothly varying magnetic field of fixed orientation may be produced. Finally, by placing several windings of differing orientation within the same stator, and energizing the windings with alternating currents of differing phase, a rotating magnetic field may be produced which is the sum of the time varying fixed orientation magnetic fields generated by each winding/phase.
The difficulty with this approach is that production of a smoothly rotating magnetic field depends upon two factors. First, the fixed magnetic field generated by each winding must have a generally sinusoidal distribution of intensity. Second, the alternating currents used to energize the winding must also be sinusoidal in nature. Any deviation from the ideal sinusoidal relations will produce harmonic rotating magnetic fields, that is magnetic fields which rotate at a different rate and/or direction from the fundamental field. These rotating fields are superimposed and added to the fundamental rotating magnetic field. Each of these harmonic fields exerts its own pull upon the stator, reducing power output, and each results in its own electrical losses, again making the motor less efficient.
Harmonic fields generated by the non-sinusoidal nature of the field generated by each winding are termed spatial harmonics or air-gap harmonics. Harmonic fields generated by non-sinusoidal drive wave-forms are termed temporal harmonics.
Methods for the analysis of harmonic rotating magnetic fields in three phase induction machines are known, and may be found in textbooks on rotating machinery.
Spatial harmonics are mitigated in three phase machines through the use of distributed windings and chorded windings. These are winding techniques which result in a decrease in the fundamental efficiency of the machine, increasing resistance losses in the windings by up to fifteen percent or more. However these winding techniques disproportionately reduce the strength of harmonic fields. The net result is that both machine operation and total efficiency is improved.
Temporal harmonics are only considered a problem with the advent of inverter based variable frequency motor control systems. These systems produce wave-forms rich in harmonic content. Mitigation of these harmonics has been limited to improving the characteristics of the inverter drive systems, reducing the harmonic content of the output wave-forms through pulse shaping and higher switching frequencies.
Temporal harmonics also become a problem when high magnetic saturation levels are used. Ferromagnetic materials are used in motor construction because of the much higher magnetic fields which are developed for a given current flow. However, as the magnetic field strength is increased, the relationship between current flow and generated magnetic field becomes nonlinear. Even if a perfectly sinusoidal alternating current is applied to a winding, temporal harmonics in the resulting magnetic field will be generated. The intensity of these harmonics increases with increasing saturation, thus setting a limit on the saturation levels which may be used. Winding techniques cannot effectively reduce the strength of harmonic fields generated by high saturation in three phase machines.
SUMMARY OF THE INVENTION
From the foregoing, it may be appreciated that a need has arisen for a polyphase induction electrical rotating machine that overcomes the disadvantages of the prior art. In the present invention, an AC induction machine is operated by an inverter drive system. The stator is wound with little or no chording, meaning a chording factor of 1, and with little or no winding distribution, meaning a winding distribution factor of 1, both allowing windings with fewer turns to be used. Thus, resistance losses owing in the stator windings are reduced. Large machines with low pole counts are facilitated by the reduced winding distribution, again enhancing efficiency because low pole count machines are more efficient. Great control of stator magnetic field structure is possible, to the point that motor pole configuration may be changed purely electronically.
The present invention reduces substantially the problems associated with harmonic rotating magnetic fields. Specifically, the use of many phases causes harmonic fields up to a number equal to the number of phases to rotate in synchronism with the fundamental rotating magnetic field. Both spatial harmonic rotating magnetic fields and temporal harmonic rotating magnetic fields are still developed, but such rotating fields add beneficially to the fundamental rotating magnetic field of the machine. Harmonics of higher order than the number of phases still excite non-synchronous rotating fields; however such high order harmonics are in general very weak. Thus motor efficiency losses associated with harmonic rotating magnetic fields are reduced.
The present invention allows for the use of drive wave-form with high harmonic content, and in an embodiment of the present invention, square wave inverters are used in place of the more complex and expensive sine wave inverters to drive the induction rotating machine. The present invention allows for the use of high saturation levels, and in an embodiment of the present invention high voltage is used to produce high flux densities, thus increasing the overload output capabilities of the induction rotating machine. Further, the present invention may be driven by alternating current having different wave-forms, for example a square wave-form or a sinusoidal wave-form.
Accordingly, besides the objects and advantages of the methods of operating an AC induction machine described above, several objects and advantages of the present invention are the following:
It is a technical advantage of the present invention to provide a method by which rotating machinery of few poles can be constructed which demonstrate good chord factor and winding distribution factors. Another technical advantage of the present invention is that rotating machinery with low pole counts, and thus grea
Borealis Technical Limited
Ro Bentsu
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