TECHNICAL INFORMATION ON INDUCTIVE REACTORS
When an electrical current passes through a conductor, a magnetic field is generated around it..
This phenomenon is called electromagnetism. The magnetic field acts on the orientation of the electrons within the atoms, causing the appearance of a physical force between the atoms. This physical force can act at a distance also through empty space. The magnetic field can be measured in two ways:
- as a force or concentration at a given point (MMF = magnetomotive force, measured in ampere-turns);
- as a magnetic flux, which is the amount of magnetic field in space (Ø symbol and the Weber unit of measure).
The flux value of the magnetic field is directly proportional to the value of the magnetomotive force that generates it.
The flux of the magnetic field relies on a certain inertia value in order to accumulate in the electrons that pass through the conductor that generates the field, and the inductors (inductive reactance coils) are designed to take advantage of this phenomenon.
That is to say that the magnetic field surrounding an inductor increases when the current passing through it increases and it decreases when the current decreases, but by virtue of the kinetic energy of the moving electrons there is an accumulation phenomenon. This phenomenon causes resistance to the passage of increasing current and, inversely, it acts as a generator when the current decreases.
The characteristic of an inductor of storing energy in the form of a magnetic field is referred to as inductance (L), the unit of measure of which is the ‘henry’ (H). Inductors are commonly referred to as inductive reactors or more simply as reactance coils and in high power applications they are sometimes called reactors.
Creating the inductance in the form of a coil, a more intense magnetic field is generated and introducing a solid core made of particular materials the intensity of the magnetic field is further increased. The factors affecting the value of the inductance are the number of turns of the coil and the magnetic permeability of the core.
Permeability (μ symbol, and Tesla units x metres//ampere-turns) is a term used to quantify the capacity of a material to become magnetized. An air core has a permeability value equal to 1 (one), while a soft iron core has a degree of permeability 600 times greater (approximate values).
The intensity of the magnetic field is the measure of the magnet-motive force (MMF) distributed over the length of an electromagnet (symbol H, and unit of measure ampere-turns/metre) and sometimes it is referred to with the name of magnetizing force. The intensity of the magnetic field determines the density of the magnetic flux (induction B and Tesla units).
H is obtained by determining the MMF and dividing it by the length of the material, while B is obtained by dividing the total flux Ø by the cross section of the material. By plotting the trend of B according to H it is possible to obtain a normal magnetization curve (also called a B-H curve) for each specific material. Fig.1 shows two magnetization curves of two different types of silicon steel.
There are two important characteristics that are revealed by the curve when the field intensity H first increases and then decreases (or vice versa); the curve changes shape and shows a hysteresis and when it approaches the maximum field strength the curve tends to flatten off because the more the flux is stacked in the transversal section it is found that fewer electrons are capable to be aligned. This flattening off is referred to as saturation.
The designers of inductors try to minimize this effect by selecting a core that will result in a situation where the flux density B will never approach the levels of saturation and the inductor will work in a linear portion of the BH curve.
Fig. 2 highlights BH cycles measured with a flux density (modulated with a sine wave at 50 Hz) ranging from 0.3 to 1.7 Tesla (Weber/sq.m). The material is conventional silicon steel sheet with oriented granules. Br indicates residual induction. Hc refers to the coercive field. Inductions greater than 1.8 T are not used because the cores would become saturated. To obtain a relatively constant inductance value there is a tendency to work with induction values (flux density) between 0.8 and 1.4 T.
The presence of air gaps in the core increases the value of the resistance to magnetic flux and makes it possible to use the inductors with higher currents without entering a saturation phase. Ideally, these air gaps should be inside the windings to minimize the fields of dispersion. When possible it is thus better to have various small air gaps rather than one large one, so as to reduce the dispersed flux and together with it the further losses that generate heat.
The use of electric and electronic equipment is rapidly growing and this means that in the fields of energy, control and data-processing ever-increasing numbers of such devices are being placed in close proximity to each other, with a reciprocal influence occurring during their use.
For fault-free operation, however, the conditions of electromagnetic compatibility must be present and, that is, an electromagnetic environment in which a device must have the possibility to operate satisfactorily without causing disturbances that would be unacceptable to others.
Electromagnetic interference can cause various types of malfunctioning, which, above all, will often not be easily diagnosed.
Such disturbances cause crackling and hissing in radio receivers, data errors and the blocking of processors in computer systems and even the perforation of insulation.
Inductive line reactors are used as passive components to reduce harmonics and the load in networks where there are various types of converters, DC power supplies and in the production of alternative energy. Fig 3
An inductive line reactance lightens the power supply network by compensating the reactive power of harmonics. The high-value pulses of current caused by the operation of rectifiers are attenuated by more than 60%. Besides current peaks, inductive reactance systems also limit voltage sagging/dips. These problems are caused by switching processes and discharges to ground.
The switching reactors must be used with SCR drivers (thyristors).
Use of only an EMC filter without a line reactor is not permitted. Inversely, the use of only a reactor, also without an EMC filter, is possible. Practical applications show that, with SRC drivers, the sole use of reactors provides excellent results, especially in industrial environments.
The types of reactors required are indicated in the converter manuals. In the case of applications with networks having differing voltage/frequencies the sizing formulas indicated in the manuals may be applied.
The line inductors must be placed between the EMC filter and the driver. The EMC filter must never be placed between the reactors and the driver.
If there is an adaptation transformer/auto-transformer, the EMC filter is normally placed upstream on the network side. If it is inserted downstream, it must then follow the relative inductance.
Without line reactance
With line reactance 1%
With line reactance 2 %
With line reactance 4 %
The input voltage has superimposed high frequency disturbances reaching the network.
The current has high peaks and brief periods of conduction.
By inserting a reactor with input impedance that creates a voltage drop from 1% to 4% of the nominal line value, the voltage remains unvaried but the maximum value of the current is reduced by 40-60%. The harmonic component is decreased and reactive power is decidedly lower.
Our inductive line reactance coils are designed to block all harmonics and to allow the fundamental frequency to pass. This is achieved because the inductive impedance grows as the frequency increases.
The greater the inductance value, the greater the value of the short circuit voltage (the voltage difference between a loaded and unloaded inductor) will be, and the level of attenuation of the harmonics will also reach a higher point. The input impedance of a reactor is expressed as the percentage of voltage drop and normally an inductor has a value equal to 2% —– 4%, but other values can be used, depending on the circumstances.
Advantages offered by the installation of an inductive line reactor.
From an analysis of the phenomena illustrated up to this point we may affirm that the very high current peak, occurring at the moment of maximum voltage, due to operation of the rectifier circuits, is attenuated up to a value of 40% by the additional line inductance.
The deformation of the mains voltage from sinusoidal to trapezoidal waveforms is thus minimized. A line reactor discharges the network, compensating the harmonic reactive current. There is thus an elevation of the power factor (influenced by the harmonics and not to be confused with the cos-phi, the dephasing ratio between voltage and current) to values not lower than 0.9.
The line reactor also restricts the disturbances that appear in the network, such as abrupt changes of voltage and current surges, which may appear from time to time in the power-supply network. These phenomena are caused by the insertion operations and discharges towards ground, as well as by transient start-up processes in the power-supply network.
Conventional inductive output reactors have an excellent accumulation capacity and extend motor life.
They reduce the slope of the dv/dt rising edge to earth and between the phases, reduce engine noise and level the current.
They act as a typical series inductance and level both the active symmetrical current and the asymmetrical interference current.
This solution attenuates the cable-conducted disturbances very well also in the lower frequency range. Electromagnetic radiation of the power line is attenuated to a significant degree. Losses and the typical noise of the ferromagnetic pack of the motor caused by harmonics are reduced. .
The diagrams below show the voltage jumps due to pulse-width modulation. With the inductive reactance for motors the maximum value of voltage and the voltage rise rate are clearly lower. In this way the isolation of the motor is protected.
For a correct dimensioning of this type of reactor in addition to the rated data it is also important to know the modulation frequency (PWM) and the percentage of ripples with respect to the nominal value.