Suction circuit
A suction circuit is a series connection of a capacitor and an inductance (choke or coil) with the lowest possible ohmic resistance. This circuit has a particularly low total resistance in the region of the resonance frequency because the reactance disappears at the resonance frequency. The main application is the short circuit at a certain frequency.
Physical Properties
The resonance frequency of a suction circuit is calculated as with any electrical resonance circuit according to Thomson's vibration equation
Here f is the blocking frequency, L is the inductance of the coil and C is the capacitance of the capacitor.
Applications
The aim is to absorb a certain narrow-band frequency component of the current or a magnetic field in the suction circuit as completely as possible and - in the case of use as a filter - to divert the AC voltage component at this frequency via the suction circuit and ideally bring it to zero.
Suction circuits are used in electrical engineering, electronics, radio technology as filters or blocking circuits, as well as for the wireless transmission of small electrical power over short distances.
Filtering in power engineering
One application is in the intermediate circuit of traction converters . In single-phase traction current networks , the intermediate circuit is not only equipped with an intermediate circuit capacitor battery, but also with a suction circuit. Here the resonance frequency is matched to twice the line frequency (e.g. 33.33 Hz for 16.67 Hz systems, 100 Hz for 50 Hz systems). This optimizes the size and the costs of the entire intermediate circuit. However, the construction volume, costs and space for the suction circuit throttle must be planned. The network frequency components are filtered optimally, as twice the network frequency component in the DC link ripple is almost completely eliminated. The high-frequency and broadband switching frequency components are filtered through the intermediate circuit capacitors. With conventional industrial converters on the three-phase network , such a suction circuit is usually not required, since the decisive frequency component is already 6 times the network frequency and the filtering is therefore done sufficiently well via the intermediate circuit capacitors. The resonance frequency of the suction circuit is usually tuned using several capacitors that can be switched in parallel and switched on or off.
In energy technology , suction circuits are operated directly on the grid. The aim is to filter individual low-frequency harmonics from the 50 Hz network. This may be necessary, for example, if converters (e.g. thyristor controller) cause high network perturbations (see also reactive power , distortion reactive power , reactive power compensation ). Due to the high costs of the passive components, there is an increasing trend towards filtering with active power electronic devices (converters).
Generation of high voltage
When choosing the components of a suction circuit, it must be noted that both components often have to be dimensioned for surprisingly high voltages. Example: In the case of resonance, both reactances are the same size and are 1200 Ω. If a current of 0.5 A flows, 600 V is measured on each component, although the total voltage - depending on the loss resistance of the coil - is only a few volts. This effect is used in resonance converters to generate high voltages from low voltages. This is required to operate cold cathode tubes (CCFL) for backlighting flat screens or energy-saving lamps .
Wireless energy transfer
The inductive coupling of a suction circuit (in this case a parallel resonant circuit) allows the selective transfer of energy to it from an alternating magnetic field of a frequency that is equal to its natural frequency. This principle, on which Nikola Tesla had already carried out tests, has recently been brought up again by American researchers; it allows small consumers in the milliwatt to watt range to be supplied with power over distances of a few meters. The feeding magnetic field is generated with the largest possible air-core coil, although questions about electromagnetic environmental compatibility are unanswered. The principle is similar to older systems for audio signal transmission to headphones with a magnetic loop in the room, but uses the resonance case for additional selection.
Radio and measurement technology
In radio technology , suction filters or notch filters are used to suppress unwanted signals either at receiver inputs or transmitter outputs . Here, too, it is a resonant circuit with a series connection of a capacitor and an inductance (coil) or an inductively coupled parallel resonance circuit. The bandwidth of the suction filter can be influenced depending on the choice of the C / L ratio and the quality of the coil .
Undesired suction circuit effect is observed with all throttles if their series resonance is close to the operating frequency. Then the choke acts almost like a short circuit, although a high AC resistance is expected. At other frequencies, however, the opposite is also measured , namely parallel resonance with particularly high impedances. The frequencies depend on the wire length and the self-capacitance of the coil. They can only be calculated imprecisely, but measured very precisely with a dipmeter . If the choke is short-circuited, series resonance is measured; if the ends are open, parallel resonance. As can be seen in the pictures, there are several points of resonance.
In modern measuring devices with digital signal processing no resonant circuits are used, but rather unwanted frequencies are suppressed by comb filters .
See also
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- ↑ Broadcast “Research Current” on Deutschlandfunk on June 8, 2007