Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Patent
1997-10-14
1999-10-19
Wong, Peter S.
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
363 98, 363 37, 318811, H02M 112, H02M 324, G01R 1900
Patent
active
059699582
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method for measuring phase currents in an inverter, where controlled semiconductor switches invert direct current by pulse width modulation from an intermediate circuit to a three-phase alternating voltage, and where the phase currents are measured by measuring the intermediate circuit current when an active switch state creates an unambiguous relation between the intermediate circuit current and a phase current.
The measuring principle is known from EP 0 502 226 A1, among others. That document describes a detection circuit, which by detecting the phase voltages selects the periods of time when said unambiguous relation between the intermediate circuit current and a phase current exists. The detection circuit controls an integration circuit of the track-and-hold type with three channels, which are connected alternately to a sensor for the intermediate circuit current. One of the channels is controlled to integrate the contributions of the individual phase currents to the intermediate circuit current when there is a potential applied on the inverter output, that is that a voltage is applied that is different from zero. The resulting signal represents the effective current in a connected three-phase a.c. machine. The two other channels are controlled so that the integrated signals periodically follow each its own phase current, and these signals are combined with data on the switch state into an individual reconstruction of the three phase currents.
The known circuit and the measuring method realised in it involve quite extensive analog signal processing. Due to the principle of integration applied, the circuit has a low upper cut-off frequency, and therefore it will be less suitable for inverters with high dynamics, as for example for servo control applications.
Seen in relation to this, it is the object of the present invention to provide a method for measuring phase currents in an inverter which allows a higher upper cut-off frequency, and which can be realised with less circuitry complications than known before.
SUMMARY OF THE INVENTION
According to the present invention this object is realised by measuring the value of the intermediate circuit current in two separate settings of the same active switch state in a modulation period, and by creating a resulting mean value of the measured values.
This measuring method is optimally adapted to the time symmetry, which is normally realised by the control of the semiconductor switches, and which will now be explained quite briefly. At the usual vectorial observation of the of the eight possible switch states in a standard three-phase inverter bridge, these are divided into two zero vectors where no voltage is applied on the phase conductors, and six active vectors where a voltage is applied on the phase conductors. An arbitrary application of voltage on the phase conductors may then be realised by a pulse width modulated addition of two of these active vectors.
The modulation is normally accomplished as two-sided modulation, where in a modulation period one jumps for example from one zero vector to a first active vector, then to a second active vector and then to the second zero vector, and then back via the second active vector and the first active vector to the first zero vector. This sequence can always be carried out in such a manner that the transition between the various vectors or switch states requires only activation or deactivation of a single switch, whereby the sequence minimises the switch losses.
From this it is seen that the first active vector or switch state occurs twice in each modulation period. The second active switch state also occurs twice in each modulation period. Furthermore, the active switch states lie timewise mirror-symmetrically distributed in relation to the centre of the modulation period.
The first active switch state enables measuring of a first phase current by measuring of the intermediate circuit current, and the second active switch state enables measuring
REFERENCES:
patent: 4520298 (1985-05-01), Abbondanti
patent: 5309349 (1994-05-01), Kwan
patent: 5623192 (1997-04-01), Yamamoto
J.ae butted.ger Jens Ulrik
Jensen Johnny Wahl
Nielsen S.o slashed.ren Ejnar
Th.o slashed.gersen Paul Bach
Danfoss
Patel Rajnikant B.
Wong Peter S.
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