Chokes with cores

The core chokes have become an inherent element of the modern power electronics and power engineering systems.
The possible locations in the system where the core chokes can operate are presented on the diagram be-low: 


The chokes with cores made of electrical sheet are applied within the frequency range up to a few hundred Hz. The basic parameters of chokes include the inductance and rated current. In case of chokes of that type produced by ELHAND these parameters are within the range from a few dozen (µH) to a few hundred (mH), and from a few to a few hundred (A). In alternating current circuits the chokes with steel cores are applied in low-pass output voltage filters or in LC filters allowing for the elimination of selected current harmonics. In direct current circuits the chokes are applied usually for smoothing the characteristics of rectifiers’ output voltages and currents and for ensuring the constant run of rectified current within a wide range of converter operation. It is worthy to mention that the induction chokes are also applied for the compensation of reactive capacitive power.

1. Line chokes of ED1N and ED3N type.
Thyristor converters belong to the receivers which require, in most cases, to apply the line chokes in the cir-cuits which connect the converter with supply network. These chokes fulfill a protective functions in relation to the converter itself and the supply network. The thyristors of converting systems need the protection re-straining the forward current increase until the moment when PNPN structure is switched to the state of con-duction. In such systems it is necessary to use the line chokes.
The application of line chokes in thyristor converters system causes also the weakening of mutual influence between the converters supplied from the same transformer during commutation. Using of chokes causes the decrease of backward current pulse amplitude when the thyristor is switched off as well as the decrease of commutation overvoltages and power emitted in the suppressing circuits. Thus the commutation process runs considerably smoother. Moreover the line chokes protect the supply network against unfavourable in-fluence of converters, limiting the propagation of higher harmonics in the network. In case of line chokes the attention should be paid so that the magnetic core characteristics prevents the choke from entering into the saturation state within the entire range of expected receiver’s current.

2. Motor chokes of ED1S and ED3S types.
The motor chokes are applied commonly in the converting power transmission systems, both DC and AC. They are installed in the circuits connecting the converter with motor. Depending on the power transmission system type, with which they cooperate, they perform a lot of tasks, including ensuring the motor current continuity and smoothing its pulsation, minimization of the short-circuit current in converter load system as well as the limitation of commutation overvoltages and compensation of the supply line capacitance.

2.1. Tasks of motor chokes in controlled rectifying systems.
The ripples of rectified current in the circuit of motor supplied by a controlled rectifier causes the sparking under the brushes and it makes the commutation process difficult.
Lack of current run continuity in the motor supply system causes unfavourable changes in the run of motor mechanical characteristics and results in the deterioration of the dynamic properties of drive. Therefore one of the most important tasks of ED1S motor choke is to ensure a maximal possible conduction of continuous current in the converter output circuit. This current takes a non-continuous character the more often the smaller are the values of load current and inductance.

2.2. Role of motor chokes in AC power transmission systems.
Output voltage of converters is a series if rectangular pulses of adjustable width and frequency. The rate of voltage run pulses accretion is very high and endangers to the insulation of supplied machines. Limitation of voltage accretion rate and thus the risk of damage to the motor insulation is achieved by installing the ED3S motor choke between the motor and inverter.
ED3S motor chokes are applied also in order to limit the shot-circuit currents until the protections are trig-gered and the current is switched off in the circuit. In most cases the selection of appropriate inductance of motor choke is the only possibility to protect the thyristors (power transistors) of converting systems. The selection of ED3S motor choke inductance depends on maximal value of short-circuit current in the system. This current cannot exceed the unique peak current value of thyristor (ITSM).
In practice it is often necessary to supply the voltage to drives which are located far away from the supply source. Long supply lines are characterized by great capacitancy, which contributes to a significant increase of power losses in the system. In addition to machine insulation protection the ED3S motor choke compen-sates the capacitance of the supply line and limits the harmonics and commutation overvoltages in the motor circuit.

3. Passive filter chokes and protective chokes for capacitive batteries of ED3F type.
3.1. Passive filter chokes.
The introduction of thyristor converters for supply and control high-power electric machines caused the
appearance of higher harmonics which get through to the network during the operation of converting driving systems. The systems of higher harmonics filters are used in order to limit an unfavourable impact of the converting systems on the electric network and on the operation of batteries of capacitors which may be connected to that network.
Filters are LC resonance systems in series, switched in parallel into the converter supply circuit. They play a double role: they compensate the reactive power consumed through the power transmission system and prevent the higher harmonic permeation to the power network. Depending on the harmonic number, the filter reactance is as follows:

where: Lf ,Cf – inductance and capacity of the circuit branch being a filter; n – harmonic number; ω – pulsation.

When the induction values and capacities are selected properly for the basic harmonic and for the harmon-ics with numbers lower than nr (resonance frequency), the filter will create a capacitive load and for all the harmonics with higher numbers, it will create the induction load. For the resonance frequency, LC branch will have rather small impendence. The current with the resonance frequency will close between the converter and filter, not permeating to the supply network. In 3-phase systems the filtering circuits for the frequency of 5, 7, 11 and 13 harmonics are applied mostly . For the basic harmonic, the filter branches are always capaci-tive in their character, which means that they reduce the reactive current of the basic frequency. 

3.2. Protective chokes for capacitive batteries.
Capacitive compensating batteries installed a dozen or so years ago were not protected with sup-pressing chokes. At present it causes easy to predict operational problems, ending usually with the damage to the battery capacitors. ED3F chokes protect the capacitive batteries against overload-ing with harmonic currents.
In most of industrial networks the installation of many non-linear receivers caused that the level of higher harmonics of voltage and current is dangerous for transformers, motors and, in particular, for the batteries of capacitors. A correct determination of ED3F chokes parameters decided on the efficiency of compensating battery protection. For that purpose it is almost necessary to make the measurements of higher harmonics contents at the designed place of compensating battery operation. An appropriate resonance frequency of the battery–choke system is selected on the basis of received spectrum of harmonics. In most cases it is a frequency intermediate between those frequencies of voltage and current harmonics, which do not occur in the measuring spectrum or have the smallest share in it. Adjusted battery-choke resonance system will show an insignificant impedance for harmonic frequencies located in the vicinity of resonance frequency. At the same time it will suppress strongly the currents with frequencies distant from resonance frequency.

4. Compensating chokes of ED3K type.
The group of compensating chokes is intended for compensating capacitive reactive power, resulting from synchronous machines operation and widespread low- and medium voltage cable networks with their insuffi-cient load. Very often the chokes are combined in choke batteries cooperating with automatic cos regula-tors. Such batteries allow for group, much more efficient follow-up compensation of reactive power, which prevents a possible overcompensation of the network.
The basic operational parameter of such chokes is the reactive induction power produced by them, which is determined from the following formula: 

where: In – rated effective value of the sinusoidal alternating current ; ω –rated pulsation; L – choke inductance.

5. Smoothing chokes of ED1W type.
In addition to constant component of voltage and current also the undesirable varying components occur usually in the load system at the output of diode and thyristor rectifiers and converters. A smoothing choke connected in series with the load allows to obtain a sufficiently small pulsation of voltage and current at high frequency of varying components. Smoothing of voltage and current in the rectifying systems supplied with voltage of industrial frequency with using of a choke only would require the application of elements with very high inductances. In such a case the choke is connected with the capacitance, forming LC filter. Important parameters of smoothing choke, determining its electromagnetic power, are the inductance, rated current value and pulsation amplitude and frequency.

6. Chokes limiting the voltage rise steepness du/dt, ed3du type.
ED3du chokes are installed at the converter’s output. They limit the voltage steepness du/dt circa 2-2,5 times up to 700V/µs and the overvoltage amplitude up to 1000 V. They protect the windings of the asyn-chronous motor stator against overvoltages and allow to lengthen the supply wire up to 100 mb.

print