Pwm rectifier including capacitance calculation unit

Abstract

A PWM rectifier includes a main circuit unit carrying out AC-DC power conversion by PWM-control, a PWM control unit PWM-controlling the main circuit unit, a DC voltage detection unit detecting a DC voltage across a smoothing capacitor connected to the DC-side of the main circuit unit, a DC voltage storage unit storing respective DC voltages at the start and end times of an initial boost period during which the smoothing capacitor having been charged to an AC voltage peak value is further charged to a higher voltage, an input power calculation unit calculating input power flowing in from the AC-side based on an AC voltage and current, an integral power calculation unit calculating integral power from the input power over the initial boost period, and a capacitance calculation unit calculating the capacitance of the smoothing capacitor based on the respective DC voltages and the integral power.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a PWM rectifier that converts AC power to DC power by controlling a switching element using a PWM signal (pulse width modulation signal).DESCRIPTION OF THE RELATED ART[0002]A motor control device for driving a motor in a machine tool, an industrial machine, a forming machine, an injection molding machine, or various types of robots uses AC power, to which AC power input from the AC power supply side is once converted to DC power and the DC power is further converted, as drive power of a motor attached to each drive shaft is known. A motor control device includes a rectifier configured to rectify AC power supplied from the AC power supply side and output DC power and an inverter connected to a DC link on the DC side of the rectifier and configured to carry out bilateral power conversion between DC power at the DC link and drive power for a motor or AC power which is regenerative power, and controls the speed, torque, or rot...

AI Technical Summary

Benefits of technology

The aim of this invention is to create a cheap and small PWM rectifier that can accurately measure the capacity of a smoothing capacitor that is connected to the DC output side in parallel.

Problems solved by technology

A rectifier using a diode rectification method has an advantage of low price but also has a disadvantage that power line harmonics or reactive power increases.

When the capacitance of the smoothing capacitor 5 decreases, a problem is caused in that a ripple current which flows through the DC link increases and fluctuation of the DC voltage grows.

If it is not possible to measure the capacitance of the smoothing capacitor accurately, timing for the replacement of the smoothing capacitor may be missed and a substantial ripple current and DC voltage fluctuation may be caused to the DC link.

Alternatively, a case in which a smoothing capacitor which has not reached the end of a lifespan thereof is replaced unnecessarily may be caused.

For example, since, in the invention disclosed in Japanese Unexamined Patent Publication No. 05-76180, the capacitance of the smoothing capacitor to be estimated depends on the peak value of the AC voltage on the AC power supply side and the resistance value of the charging resistor, it is not possible to measure the capacitance of the smoothing capacitor accurately when the peak value of the AC voltage and the resistance value of the charging resistor contain errors.

For example, according to the invention disclosed in International Publication No. 2010 / 055556, although, when the AC power supply is a single-phase AC power supply, no substantial error is caused even if the inflowing current to the smoothing capacitor is approximated to the absolute value of the AC current on the AC power supply side, such an approximation is not possible when the AC power supply is a three-phase AC power supply and it is thus not possible to know the inflowing current to the smoothing capacitor accurately.

Although it may be possible to calculate the capacitance of the smoothing capacitor accurately by disposing a DC current detection unit at the preceding stage or the succeeding stage of an initial charging circuit and measuring the inflowing current to the smoothing capacitor, newly disposing the DC current detection unit leads to an increase in cost and space.

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