Electricity: motive power systems – Synchronous motor systems – Antihunting or damping
Reexamination Certificate
2001-07-31
2003-06-24
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Antihunting or damping
C318S448000, C363S040000
Reexamination Certificate
active
06583598
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for damping resonant peaks at a motor star point in an electric motor which is operated using an intermediate voltage circuit converter with an input-side inductance. In particular, the present invention relates to a mains system input inductor which has a frequency response with at least one resonant frequency with respect to ground potential, and to a corresponding electric motor in which resonant peaks are damped in such a manner.
BACKGROUND OF THE INVENTION
In present-day converter systems having an intermediate voltage circuit, in particular in multi-shaft converter systems, system oscillations can be formed which are virtually undamped. This relates essentially to converters having an intermediate voltage circuit and a regulated supply in the form of a regulated mains-system-side converter, which is also referred to as an input converter.
In principle, converters are used for operating electrical machines with a variable supply frequency. Such an intermediate circuit frequency converter allows an electric motor, e.g. a three-phase machine such as a synchronous machine, to be operated in such a manner where it no longer needs to be linked directly to the mains system and hence has a fixed rotation speed, but such that the fixed mains system can be replaced by an electronically produced, variable-frequency and variable-amplitude mains system for supplying the electrical machine.
The two mains systems, the supplying mains system whose amplitude and frequency are fixed and the mains system which supplies the electrical machine with a variable amplitude and frequency, are decoupled via a DC voltage storage device or a direct current storage device in the form of what is referred to as an intermediate circuit. In this case, such intermediate circuit converters essentially have three central assemblies: (a) a mains-system-side input converter which can be designed to be uncontrolled (for example diode bridges) or to be controlled, in which case energy can be fed back into the mains system only by using a controlled input converter; (b) an energy storage device in the intermediate circuit in the form of a capacitor in the case of an intermediate voltage circuit and an inductor in the case of a current intermediate circuit; and (c) an output-side machine converter or inverter for supplying the machine, which generally uses a three-phase bridge circuit having six active current devices which can be turned off, for example IGBT transistors, to convert the DC voltage in an intermediate voltage circuit into a three-phase voltage system.
Such a converter system having an intermediate voltage circuit which, inter alia, owing to its very wide frequency and amplitude control range, is preferably used for main drives and servo drives in machine tools, robots and production machines, is shown in
FIG. 1
in which the converter UR is connected to a three-phase mains system N via a filter F and an energy-storage inductor whose inductance is L
K
. The converter U\R has the described supply E, an intermediate voltage circuit with the energy-storage capacitance C
ZK
, and an output inverter W.
FIG. 1
shows a regulated supply E which is operated such that it is controlled by switching components (for example a three-phase bridge circuit composed of IGBT transistors), resulting in the arrangement, shown in
FIG. 1
, experiencing excitation A
1
. The inverter W is likewise controlled via further switching components, for example, by means of a three-phase bridge circuit having six IGBT transistors. The fact that switching operations also take place in the inverter also represents excitation A
2
of the system. The capacitor C
ZK
in the voltage intermediate circuit is connected between the positive intermediate circuit rail P600 and the negative intermediate circuit rail M600. The inverter is connected on the output side via a line LT, having a protective-ground conductor PE and a shield SM, to a motor M in the form of a three-phase machine.
The fixed-frequency three-phase mains system N now supplies the intermediate circuit capacitor C
ZK
via the input converter E and via the filter F and the energy-storage inductor L
K
by means of the regulated supply, with the input converter E (for example a pulse-controlled converter) operating together with the energy-storage inductor L
K
as a step-up converter. Once current flows into the energy-storage inductor L
K
, it is connected to the intermediate circuit and drives the current against the greater voltage into the capacitor C
ZK
. The intermediate circuit voltage may therefore even be greater than the peak value of the mains system voltage. This combination effectively represents a DC voltage source. The inverter W uses this DC voltage in the described manner to form a three-phase voltage system, in which case, in contrast to the sinusoidal voltage of a three-phase generator, the output voltage does not have the profile of an ideal sinusoidal oscillation, but also has harmonics in addition to the fundamental, since it is produced electronically via a bridge circuit.
However, in addition to the described elements in such an arrangement, it is also necessary to take account of the fact that parasitic capacitances occur which assist the formation of system oscillations in such a converter system. Thus, in addition to the filter F with a discharge capacitance C
F
, the input converter E, the inverter W and the motor M also have discharge capacitances C
E
, C
W
and C
M
to ground. Furthermore, there is also a capacitance C
PE
in the line LT to the protective-ground conductor PE, and a capacitance C
SM
in the line LT to the grounded shield SM.
It has now been found that these system oscillations are excited to a particularly pronounced extent in the supply E. Depending on the control method chosen for the supply, 2 or 3 phases of the mains system N are in this case short-circuited, in order to pass current to the energy-storage inductor L
K
. If all three phases U, V, W are short-circuited, then either the positive intermediate circuit rail P600 or the negative intermediate circuit rail M600 is hard-connected to the star point of the supplying mains system (generally close to ground potential depending on the zero phase-sequence system component). If two phases of the mains system N are short-circuited, then the relevant intermediate circuit rails P600 and M600 are hard-connected to an inductive voltage divider between the two mains system phases. Depending on the situation relating to the mains system voltages, this voltage is in the vicinity of ground potential (approximately 50-60 V). Since the intermediate circuit capacitance C
ZK
is generally large (continuous voltage profile), the other intermediate circuit rail is 600 V lower or higher and can thus also break down the remaining phase of the mains system. In both cases, the intermediate circuit is particularly severely deflected from its “natural” balanced steady-state position (±300 V with respect to ground), which represents a particularly severe excitation for system oscillation.
With respect to the production of undesirable system oscillations, the frequency band below 50 to 100 kHz which is relevant for the application area allows a resonant frequency to be calculated based on concentrated elements. In this case, the discharge capacitances C
F
to ground in the filter F are generally so large that they do not have a frequency-governing effect. In this case, it can be assumed that dominant excitation to oscillations takes place upstream of the described capacitances, and that the filter discharge capacitance C
F
can be ignored.
The resonant frequency f
res
(sys) of this system, which is referred to as f
sys
in the following text, is thus given by:
f
sys
=
1
2
π
L
∑
·
C
∑
(
1
)
where
L
&Sgr;
=L
K
+L
F
(2)
where L
k
represents the dominant component and L
F
the unbalanced inductive elements acti
Raith Sebastian
Segger Bernd
Baker & Botts LLP
Ro Bentsu
Siemens Aktiengesellschaft
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