This application claims the priority of German Patent Application Serial No. 100 49 817.5, filed Oct. 9, 2000, the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to an induction apparatus for inducing a magnetic filed in a magnetizable core, and more particularly to exciter coil windings for electric machines. The present invention further relates to induction arrangements with several induction apparatuses, and to a corresponding method for operating an electric machine.
Electric machines, in particular synchronous motors, are frequently configured to include specially shaped winding wires placed in slots provided in the rotor or stator. The wires are connected at the winding head to form winding strands. A single winding strand can be connected to several coils, with the number of coils depending on the number of poles. With three-phase machines, at least one winding strand is provided per phase, and the winding strands are connected at a star point. Thus, for a three-phase synchronous motor with four poles, for example, three winding strands are connected at the star point, with each winding strand having four coils. The input voltages of the three-phase current are applied to outer terminals of the respective winding strands, with the outer terminals positioned opposite to the star point.
The windings of linear motors and increasingly also of synchronous motors that are required to provide a high torque are fabricated by a coiling technique in which the winding is placed on a coil body that is separate from the body to be magnetized. This means that the magnetic cores, which are made of transformer sheets, are formed with teeth that serve as a pole core. The pre-fabricated coils are placed on the individual pole cores and suitably connected. The pre-fabricated coils include a carrier or coil body made of an electrically insulating material, and insulated copper wires wound on the carrier, typically by using a machine. Optionally, the carrier may be removed after the winding process so long as the winding body itself has sufficient inherent stability.
FIG. 1 shows a circuit diagram of a conventional winding strand of a three-phase machine, including four coils 1 made by using the afore-described coiling technique. The coils 1 which are depicted as hexagons are connected in series. Voltage is applied via input terminal 2 to one end of the winding strand. The other end of the winding strand, distal to the terminal 2, terminates in a star point 3. As mentioned above, the additional winding strands are connected at the star point 3.
FIG. 2 shows the asymmetric equivalent circuit diagram of the electric circuit illustrated in FIG. 1. The inductance of each coil 1 is designated with reference character xe2x80x9cLxe2x80x9d, whereby the individual inductances L are connected in series, as is evident from FIG. 1. Each coil 1 is capacitively coupled to tile pole core 5 (see FIG. 3) on which the coil 1 is mounted. The respective capacitances C are illustrated in FIG. 2 as bypass capacitors C connected to ground, whereby the ground is formed by the magnetic core. Voltage U is applied to the input terminal 2.
The actual configuration of a magnetic pole with coil winding is depicted in FIG. 3. A magnetic core 4 includes a tooth or pole core 5 which is made from transformer sheets. A coil body 6 is mounted on the pole core 5 and includes a winding 7 which is wound onto the coil body 6 in accordance with the aforedescribed coiling technique. The coil body 6 provides insulation between the winding 7 and the pole core 5. The insulation is sized so as to prevent an electric breakdown between the winding 7 and the pole core 5.
Synchronous motors and in particular synchronous linear motors are frequently controlled by using converters. The converters typically generate rectangular control voltages. In particular with large converters, electric breakdown may occur at the star point of the three-phase motors that are made using the aforedescribed coiling technique.
Similar problems are increasingly encountered with electric machines in the event of transient overvoltages. For this reason, overvoltages are limited to prevent breakdowns. German Pat. No. DE-A-38 26 282 describes electric machines having voltage-dependent metal-oxide resistors connected in parallel with a coil to limit transient overvoltages. German Pat. No. DE-B-28 34 378 describes short-circuiting of winding sections for damping transverse fields. Similarly, German Pat. No. DE-A-24 33 618 describes a synchronous machine with rods to dampen transverse fields to thereby protect against transient overvoltages.
European Pat. No. EP-A-0 117 764 describes placing ferroelectric insulators between adjacent coil turns of a winding for suppressing overvoltages that are caused by resonant phenomena. European Pat. No. EP-B-0 681 361 addresses the problem of higher harmonics encountered in converters and rectifiers with power thyristors. The damping winding is herein connected with capacitors to form resonant circuits. The resonant circuits have a resonant frequency which is set 6n times higher than the fundamental frequency of the synchronous machine. In this fashion, higher harmonics of the fundamental wave can be absorbed.
Although these proposals are appealing, they still do not address the problem of electric discharge or breakdown at the star point of a synchronous motor made by the aforedescribed coiling technique.
It would therefore be desirable and advantageous to provide an improved induction apparatus to obviate prior art shortcomings and to protect against a breakdown at the star point. It would further be desirable and advantageous to provide an improved process for operating electric machines by reducing the risk of breakdown at the star point.
According to one aspect of the present invention, an induction apparatus includes a winding arrangement having a winding start and a winding end for inducing a magnetic field in a magnetizable core, a lossy, magnetizable device through which and/or about which the winding start and the winding end of the winding arrangement is so arranged as to induce a magnetic flux in the magnetizable device.
Advantageously, the magnetizable device is provided in the form of a ring-shaped magnetic core or a rod-shaped magnetic core, which may be made of ferrite.
The induction apparatus according to the present invention is especially effective when the winding arrangement is fabricated by a coiling technique in which the windings are wound layer-by-layer on a separate coil body.
According to another aspect of the present invention, an induction apparatus includes a winding arrangement for inducing a magnetic field in a magnetizable core, wherein a shielding device is disposed between the winding arrangement and the magnetizable core, with an electric resistor connecting the shielding device to the magnetizable core.
According to another feature of the present invention, the shielding device includes an electrically conductive shielding foil which may fully, or only partially, cover the inner surface of the winding arrangement, when the winding arrangement is made by using the aforedescribed coiling technique.
According to still another aspect of the present invention, a method of operating an electric machine of a type having at least one winding arrangement and a magnetizable core, includes the steps of applying a control voltage or a control current to the at least one winding arrangement; and using a lossy capacitive and/or inductive element to dampen a capacitive bypass current flowing between the winding arrangement and the magnetizable core.
In general, the invention is based upon the recognition that a coil winding together with the pole core, which is connected to electric ground, forms an L-C oscillating circuit. When several coils of a winding strand are connected in series, a recurrent network circuit of parasitic elements is formed, as shown in FIG. 2. These parasitic elements include inductances and bypass capacitances of the exciter coils. Principally, this network circuit represents an undamped resonant circuit. If this resonant circuit is not damped further in addition to the naturally occurring damping, then overvoltages of, for example, a factor 4 can readily occur at the resonant frequency. This may cause breakdowns at the star point.
Supply of voltages at frequencies in the range of the resonant frequency of the exciter coils may not always be preventable. In particular, when controlling synchronous motors by using converters, frequencies near the specific resonant frequencies of the employed winding strands are generally produced. However, problems normally appear only when the spectral content of the supplied interference voltage in the range of this resonant frequency is fairly high. This is indicated, for example, by a superposition of the interference voltage produced by the converter on the rectangular voltage, whereby a significant ringing at the resonant frequency is observed at the edges of the rectangular voltage. The resonant amplification of the ringing by the recurrent network circuit of the winding strand causes pronounced overvoltages and possibly also breakdowns at the star point 3.
The present invention resolves these problems by damping the resonance amplification, in particular by damping the capacitive bypass current between the exciter coil and the pole core. Alternatively, the capacitive bypass current can produce a magnetomotive force in a lossy magnetic core. The losses in the magnetic core then dampen the capacitive bypass current and attenuate the oscillation in the recurrent network. Alternatively, the capacitive bypass current can be damped by tapping the bypass current via an ohmic resistor.
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