Communications: electrical – Systems – Selsyn type
Reexamination Certificate
2001-10-22
2003-12-23
Wu, Daniel J. (Department: 2632)
Communications: electrical
Systems
Selsyn type
C340S315000, C340S315000
Reexamination Certificate
active
06667685
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power line noise filter that reduces noise in electric power lines.
2. Description of the Related Art
Telecommunications using electric power lines, in which high frequency signals are superposed on electric power lines, is known as one of the telecommunications technologies used at home and office. During the telecommunications using electric power lines, sporadic noises of uncertain frequency bands are generated in electric power lines (hereinafter referred to as the noise) when various electric and electronic devices that are connected to the electric power lines are operated, causing higher error rates and other defects in telecommunications qualities.
A noise generated in an electric power line by operation of a device connected to the electric power line sometimes adversely affects other devices connected to the same line, even when the line is not used for telecommunications.
There are two types of noises generated in electric power lines, namely common mode noises, which propagate through two conductive lines in the same phase, and normal mode noises generated between two conductive lines. Further, noises generated in electric power lines include a noise that causes a change in electric current (hereinafter referred to as a current-related noise), and a noise that causes a change in voltage (hereinafter referred to as a voltage-related noise).
One of the measures against noise problems described above is use of a filter against electromagnetic interference (EMI) (hereinafter referred to as an EMI filter). An EMI filter is generally formed as an LC filter (a filter comprising inductors and capacitors) in which discrete elements such as a common mode choke coil, a normal mode choke coil, an X capacitor and a Y capacitor are used in combination.
Published Unexamined Japanese Patent Application (KOKAI) Heisei 7-115339 discloses a line filter that absorbs noise currents. The line filter has a first transformer including a primary coil and a secondary coil, a second transformer including a primary coil and a secondary coil, and an amplifier that amplifies a noise current that is electromagnetically induced to the secondary coil of the first transformer when the noise current flows into the primary coil of the first transformer. The noise current amplified by the amplifier is allowed to flow into the secondary coil of the second transformer to cause a change in the impedance of the primary coil of the second transformer. According to this line filter, an attenuation effect on noise is increased through adjustment of the impedance of the primary coil of the second transformer.
Published Unexamined Japanese Patent Application (KOKAI) Heisei 10-303674 discloses an AC line filter that reduces noise in an AC power source line. The AC line filter comprises: a common mode choke coil to which a third winding has been added; a noise extraction circuit that extracts common mode noise in the AC power source line; a noise amplifying circuit that amplifies the extracted common mode noise; and an electric current supplying circuit that supplies a current for providing an electromotive force of inverted phase to the third winding of the common mode choke coil in response to outputs of the noise amplifying circuit. According to this AC line filter, the noise extraction circuit extracts common mode noise in the AC power source line, the noise amplifying circuit amplifies the extracted common mode noise, and, in response to the outputs of the noise amplifying circuit, the electric current supplying circuit supplies a current for providing an electromotive force of inverted phase to the third winding of the common mode choke coil. Common mode noise in the AC power source line are thereby reduced.
Conventional EMI filters formed as an LC filter are advantageous in that the circuits can be easily configured, but on the other hand they have drawbacks such as (1) to (3) described below.
(1) Conventional EMI filters can provide a desired attenuation amount only within a narrow frequency band, because those filters have specific resonance frequencies that depend on inductance and capacitance.
(2) EMI filters need to be optimized according to devices that generate noises, because frequency bands, intensity and characteristics of noises generated differ among electric/electronic devices. Thus, a process of trial and error is required each time a device is designed, to optimize EMI filters so that they are compatible with the standards related to noises, which in turn necessitates a long time for measurements and evaluations, and makes standardization of EMI filters difficult.
(3) Since conventional EMI filters can provide a desired attenuation amount only within a narrow frequency band, their noise reduction effect fluctuates due to variations in noise frequencies among different noise sources and differences in attenuation characteristics among EMI filters.
According to the line filter disclosed in Published Unexamined Japanese Patent Application (KOKAI) Heisei 7-115339, the impedance of the primary coil of the second transformer is adjusted by allowing currents which synchronize after a lapse of one cycle with the noise currents detected by the first transformer to flow into the secondary coil of the second transformer. Therefore, this line filter may be effective for reducing continuous noise with a constant frequency, but it is not capable of canceling sporadic noise. FIG. 4 in this reference shows an example of the line filter configuration in which a line is passed around two cores together, with the secondary coil of the first transformer and the secondary coil of the second transformer being wound around the cores respectively. In this configuration, however, the two cores can easily become misaligned, and cabling would be difficult.
According to the AC line filter disclosed in Published Unexamined Japanese Patent Application (KOKAI) Heisei 10-303674, as shown in FIGS. 1 and 2 thereof, common mode noise is detected by detecting variations in voltage in a neutral line through the use of a high path filter (HPF), and the detected common mode noise is amplified by the noise amplifying circuit. Then, in response to outputs of the noise amplifying circuit, the electric current supplying circuit generates a current for providing the third winding of the common mode choke coil with an electromotive force of inverted phase, and supplies the current to the third winding of the common mode choke coil.
Thus, in the above AC line filter, the voltage of the common mode noise (hereinafter referred to as the noise voltage) is detected, and the noise voltage is amplified and then converted into a current having a phase inverted to that of the common mode noise (hereinafter referred to as a phase-inverted current), so as to cancel the common mode noise using the phase-inverted current.
In the above AC line filter, however, a delay of the phase-inverted current against the noise voltage occurs during the processes of amplifying the noise voltage and converting the noise voltage into the phase-inverted current. Further, waveform of the noise voltage and that of the phase-inverted current do not coincide with each other completely. For these reasons, it is difficult to accurately cancel common mode noise in AC power source lines using the above AC line filter.
Basically, the above AC line filter reduces common mode noise using the common mode choke coil, and, enhances the effect of reducing common mode noise by supplying the phase-inverted current to the third winding of the common mode choke coil. Therefore, it is difficult for this AC line filter to reduce noise over a wide frequency band, because its attenuation characteristics depend on the properties of the common mode choke coil.
Further, in the above AC line filter, the HPF for extracting common mode noise is provided between the neutral line and the frame ground, and the third winding for canceling the common mode noise is connected between the frame ground and the el
Saitoh Yoshihiro
Wasaki Masaru
Nguyen Phung
TDK Corporation
Wu Daniel J.
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