Vacon Oyj

Hannu Saren

Risto Komulainen

Jaakko Ollila

Julius Luukko

A method and an apparatus for measuring the output currents of a frequency converter, preferably a vector-controlled frequency converter, which comprises a mains bridge () connectable to an alternating-current network and a load bridge () connectable to an alternating-current load and between these a direct-voltage intermediate circuit, by using current samples obtained from a current measuring sensor in the intermediate voltage circuit, wherein a sample is taken from the signal of the current measuring sensor substantially simultaneously with a point of change of the output voltage vector and upon the lapse of a predetermined delay after the point of change after the switching effects have settled.

Measurement of the current of a frequency converter

A method of protecting a frequency converter against overvoltage by controlling an AC motor during braking or during light load and a frequency converter, wherein the frequency converter is a voltage-controlled PWM frequency converter operating in single quadrant (Q) and comprising at least one uncontrolled rectifier bridge () to be connected to a AC supply line (U, U, U), a direct-voltage intermediate circuit and at least one controlled inverter bridge () for feeding an AC motor () with an alternating voltage (U, U, U) of varying magnitude and frequency, in which method: the angle (&#x3b8;) between the output voltage vector (U) and current vector (I), i.e. the power factor, is measured, the angle (&#x3b8;) between the output voltage vector (U) and current vector (I), i.e. the power factor, is controlled on the basis of the measurement, and the angle (&#x3b8;) between the output voltage vector (U) and current vector (I), i.e. the power factor, is controlled so that the angle does not exceed a preset threshold value, preferably approximately &#x3c0;/2.

Frequency converter overvoltage protection

A modulation method for a pulse width modulator in the inverter section () of a frequency converter provided with a voltage intermediate circuit, said frequency converter comprising a single-phase mains bridge () to be connected to a single-phase alternating current source (U, N), a direct-voltage intermediate circuit and a controlled load bridge () for feeding a three-phase alternating voltage (U, U, U) of varying frequency and amplitude into a load (), for controlling the semiconductor switches of the inverter section, in which low-capacitance direct-voltage intermediate circuit the waveform of the intermediate-circuit voltage in a normal load situation follows the absolute value of the supply network voltage, and wherein the modulation is implemented using an estimate value of the intermediate-circuit voltage to control the semiconductor switches of the inverter section.

Control of an inverter pulse-width modulator

A space-vector pulse-width modulation method for a frequency converter provided with a voltage intermediate circuit, said frequency converter comprising a mains bridge () to be connected to an alternating current source (U, U, U), a direct-voltage intermediate circuit and a controlled load bridge () for supplying a variable-frequency alternating voltage (U, U, U) to a load (), and in which method the modulator performing the modulation is given a reference. The load bridge modulator is given a reference consisting of the references for the flux linkage changes produced by the voltage vectors, the flux linkage change is calculated on the basis of measured intermediate-circuit voltage, and the currently active voltage vector is kept turned on until the reference value is reached.

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Lappeenranta, Finland

Hannu Saren