BACKGROUND OF THE INVENTION
The invention relates to inductor arrangements comprising
an input inductor coupled to the input side of an electrical apparatus and an output
inductor coupled to the output side of the apparatus.
Input and output inductors are used to reduce interference
that an electrical apparatus causes to the input and output side networks. Input
and output inductors are used for instance in frequency converter configurations.
Examples of known inductor assemblies are described in publications US 2002/125854
A1, PATENT ABSTRACTS OF JAPAN vol. 1996, no. 02, 29 February 1996 (1996-02-29) -&
JP 07 263262 A (SONY CORP), 13 October 1995 (1995-10-13), DE 19933811 A and EP-A-0012629.
In frequency converters, rectification is typically implemented
by means of a six-pulse diode bridge, which is known to use line current only at
the surroundings of the peak voltage of a sequence, thus causing extensive current
pulses that stress the network. In order to reduce the amplitude of these current
pulses it is known in the art to use series inductors, i.e. input inductors, placed
in the feeding phases.
Power inversion and pulse-width modulation used to control
the output voltage level of the fundamental wave cause extremely rapidly ascending
and descending edges, a kind of surge waves, to the output voltage. These surge
waves may create two types of problems in the motor to be fed: high turn voltages
of the winding including the risk of discharge and bearing currents. In order to
attenuate each of the mentioned phenomena it is known in the art to employ phase-specific
series inductors, i.e. output inductors, to be placed at the beginning of a motor
cable at the output side of the frequency converter that allow smoothing the voltage
edges observable in the terminals of the motor.
An input inductor is generally a three-columned and two-windowed
three-phase inductor assembled of columns and yokes composed of armature sheets
and copper or aluminium windings. The magnetic path is provided with one or more
air gaps that prevent the magnetic core from being saturated. Such a component intended
for a network frequency is typically the largest and heaviest part of the entire
The output inductor that smoothes the surge waves observable
in the terminals of the motor could electrically be most optimal when it would only
affect with frequency components of such a magnitude that only the edges of the
surge voltages were smoothed.
The structure of an output inductor according to the prior
art is similar to the input inductor. However, such an output inductor also attenuates
a component of base frequency, whereby the terminal voltage of the motor is reduced.
Such an inductor is also so massive that it cannot be placed into the specific frequency
converter as an optional component, instead it is separately mounted.
Output inductor structures are also known which are effective
only in high frequency components. What are used are for instance rings made of
a material provided with an extremely high specific permeability that positioned
around output busbars attenuate the voltage transients. A drawback with these components
is that they are very expensive. Consequently they are generally used only as a
"common mode" inductor, which is common for all phases, whereby the effect is restricted
merely to prevent bearing currents. Another problem with such rings is the relatively
large size thereof.
Another structure in use, which is only effective in large
frequency components, comprises an inductor bar provided with an open magnetic path
placed in each output phase, the structure of such an inductor bar resembles a winding
around a pile of armature sheets. The problems associated with this structure include
high costs and a fairly extensive need for space.
BRIEF DESCRIPTION OF THE INVENTION
It is an object of the invention to provide an inductor
arrangement comprising input and output inductors for a frequency converter or a
corresponding electrical apparatus so as to solve the problems mentioned above.
The object of the invention is achieved with an inductor arrangement, characterized
in what is stated in independent claim 1. The preferred embodiments of the inductor
arrangement are disclosed in the dependent claims.
The invention is based on the idea that a core of an input
inductor is also utilized in the structure of an output inductor. In the inductor
arrangement according to the invention, the output inductor is provided by placing
a certain portion of a conductor in each phase of the output adjacent to the core
of the output inductor so that at least a part of the magnetic flux formed around
the output conductor may penetrate into the core. In the inductor arrangement according
to the invention the input and output inductor are in a sense combined.
The advantages of the inductor arrangement according to
the invention in comparison with the prior art solutions are a less significant
need for space and weight and more advantageous manufacturing costs.
BRIEF DESCRIPTION OF THE INVENTION
In the following the invention will be explained in greater
detail by means of the preferred embodiments with reference to the accompanying
drawings, in which:
DETAILED DESCRIPTION OF THE INVENTION
- Figure 1 shows a side view of a prior art input conductor seen obliquely from
the top; and
- Figure 2 shows an inductor arrangement according to an embodiment of the invention
seen from the end of the inductor structure.
Figure 1 shows a typical three-phase input inductor of
a frequency converter, in which the routes along which magnetic fluxes 5 travel
and close are also indicated. A core is composed of thin armature sheets in such
a manner that the magnetic flux 5 formed around winding turns 3 of the input current
of the frequency converter travels along the armature sheet everywhere except in
air gaps 6 created on purpose. The magnetic flux 5 is most dense in the corners
of the windows and most sparse in the outer corners and back parts of yokes 7 and
Figure 2 shows an inductor arrangement according to an
embodiment of the invention seen from the end of the inductor structure. The inductor
arrangement in Figure 2 is formed of the input inductor of Figure 1 by placing a
predetermined length of insulated current conductors 4 of the output of the frequency
converter adjacent to the yoke 7 of the input inductor, and by adding an additional
yoke 9 made of armature sheet adjacent to the upper yoke 7 so that each conductor
4 remains between the upper yoke 7 and the additional yoke 9.
In the structure shown in Figure 2, the additional yoke
9 is made of a similar armature sheet as the core, and the armature sheets of the
additional yoke 9 are placed in parallel with the armature sheets of the core.
In the arrangement shown in Figure 2, the conductors 4
of the output of the frequency converter are placed along the back surface of the
upper yoke 7 so that the conductors 4 are substantially parallel with the main direction
of travel of the magnetic flux caused by the input current of the frequency converter
and passing through the upper yoke 7 adjacent to the conductors. Then, the magnetic
flux formed around each conductor by impact of the output current of the frequency
converter penetrates into the upper yoke 7 in such a manner that the travel route
thereof is substantially perpendicular in relation to the main direction of travel
of the magnetic flux caused by the input current of the frequency converter, in
which case the effect of the magnetic flux of the output inductor 2 on the magnetic
flux of the input inductor is practically non-existent.
In the inductor arrangement illustrated in Figure 2, the
additional yoke 9 is provided with grooves 11 for the conductors 4. In the arrangement
of Figure 2 the grooves 11 are almost as deep as the conductors 4 so that the distance
between the additional yoke 9 and the upper yoke 7 equals the size of the air gap
6. The magnetic flux 5 of each conductor 4 is thus closed through the yoke 7, the
additional yoke 9 and two air gaps 6.
In the inductor arrangement according to a preferred embodiment
of the invention the grooves 11 of the additional yoke 9 are formed to be as deep
as the diameter of the conductor 4, whereby the magnetic flux formed by the current
moving in the conductor 4 does not pass through a single actual air gap 6, but through
several small air gaps formed of the surface insulator in the armature sheets. The
division of an air gap into several parts along the route of the magnetic flux is
preferable in view of the saturation and loss of the core and the additional yoke
9. If the small air gaps formed of the surface insulator of the armature sheets
do not provide a sufficiently large air gap for the magnetic path, then an "actual"
air gap 6 can be formed between the additional yoke 9 and the upper yoke 7 in accordance
with Figure 2.
The inductor arrangement according to the invention can
also be implemented also without the additional yoke 9 placed adjacent to the core,
whereby the magnetic flux of each conductor 4 is closed mainly through the air.
Thus the inductance of the output inductor 2 is substantially smaller than when
the structure illustrated in Figure 2 is used.
The additional yoke 9 is therefore used for increasing
the inductance of the output inductor 2. Shaping the additional yoke 9 appropriately
enables to dimension the inductance of the output inductor as desired. The more
armature sheets on the magnetic path, the greater the inductance.
In the solution shown in Figure 2 the additional yoke 9
is provided with three grooves 11, in other words one groove 11 for each phase.
Each groove 11 is of the same length as the additional yoke 9. Each groove 11 is
provided with one conductor 4 of the output of the frequency converter. Each conductor
4 thus proceeds alongside the core a distance that substantially equals the size
of the upper yoke 7.
The distance that each conductor 4 of the output of the
frequency converter moves alongside the core may be shorter or longer than in the
solution shown in Figure 2. Placing the conductors 4 over a longer distance adjacent
to the core allows increasing the inductance of the output inductor 2, and vice
The same groove 11 may be provided with several portions
of the same conductor 4. The additional yoke 9 may also comprise more than one groove
11 for one phase, in which case each groove 11 is provided with one or more portions
of the same conductor 4.
The inductor arrangement according to the invention may
comprise more than one additional yokes. In addition to an additional yoke 9 placed
adjacent to the upper yoke 7, another additional yoke may be provided that is placed
adjacent to the lower yoke 8. The additional yoke placed adjacent to the lower yoke
8 may be similar to the additional yoke 9 placed adjacent to the upper yoke 7. It
is obvious that all additional yokes are placed adjacent to the conductors 4 of
the output of the frequency converter. If an additional yoke is thus placed adjacent
to the lower yoke 8, then a portion of the conductors 4 is placed between the lower
yoke 8 and the additional yoke.
The grooves 11, in which the conductors 4 of the output
side are placed, can be formed in the inductor arrangement according to the invention
in the additional yoke or in the yoke of the core of the input inductor. It is also
possible to provide an inductor arrangement, in which both the additional yoke and
the yoke of the input inductor comprise grooves 11 ranged to receive the conductors
The inductor arrangement in which the grooves of the conductors
4 are placed in the yoke of the input inductor can be implemented without the additional
yoke 9 or with the additional yoke 9.
The inductor arrangement according to the invention is
applicable to be used with such electrical apparatuses that provide interference
of the above-mentioned type typical for the frequency converters to the input and
output inductors thereof. The inductor arrangement according to the invention can
be implemented as a single or multiple phase inductor arrangement.
It has been noted in the above specification that armature
sheet can be used for manufacturing the core and the additional yoke 9. Here, armature
sheet refers to a thin sheet made of steel provided with an insulated surface. The
armature sheet is employed in magnetic circuits to reduce eddy-current losses. Especially
when transformers are concerned the same thin sheet provided with an insulated surface
is referred to as the transformer sheet.
It is apparent for those skilled in the art that the basic
idea of the invention can be implemented in various ways. The invention and the
embodiments thereof are therefore not restricted to the above examples but may vary
within the scope of the claims.