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ARRANGEMENT FOR USING AN AC FED FREQUENCY CONVERTER AS A COMMON DC INVERTER

IP.com Disclosure Number: IPCOM000248406D
Publication Date: 2016-Nov-25
Document File: 6 page(s) / 1M

Publishing Venue

The IP.com Prior Art Database

Related People

Lauri Artturi Peltonen: AUTHOR [+2]

Abstract

The disclosure relates to using an AC fed frequency converter having a thyristor input bridge and a DC choke as a DC inverter by modifying the frequency converter such that it can be used from a DC supply. The DC chokes are removed and the terminals are short-circuited so that the diodes become antiparallel to the thyristors. The remaining DC- terminal is wired to the module input terminal. The three power modules of the AC bridge can be replaced with a single one with a higher current rating in order to avoid current sharing problems. The charging circuit stays in parallel to the thyristor, thus allowing charging when the thyristor is turned off.

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ARRANGEMENT FOR USING AN AC FED FREQUENCY CONVERTER AS A COMMON DC INVERTER

The disclosure relates to using an AC fed frequency converter as a DC inverter by suitably modifying the frequency converter such that it can be used from a DC supply.

A frequency converter can be used to control an AC-motor by modifying the frequency and/or amplitude of the voltage supplied to the motor. Therefore typical frequency converters have an input bridge able to convert an AC voltage from the mains to a DC voltage and an output bridge able to convert the DC voltage to an AC voltage of different frequency and amplitude.

Typically, when starting the frequency converter, the DC link voltage should be gradually increased to the operating voltage in order to prevent fuses from blowing and other damage to the equipment. If AC voltage was directly connected to the input bridge and the DC link capacitors charged quickly, the charging current would be limited only by the parasitic resistances in the current path. Thus, often a charging circuit is connected over the input bridge in order to slowly charge the DC voltage, and only after the DC voltage is high enough, the input bridge is connected and normal operation current flows through the bridge. It is also important that only the charging current flows through the charging circuit and not the main current, since the charging circuit can typically handle a large current only for a short time.

Often such a charging circuit consists of a contactor or a relay which completely disconnects the input bridge until the charging is done. Another solution is to use thyristors that can be turned on only after the charging is done.

It is also common that the input bridge has either AC or DC chokes to shape the current taken from the grid and to lower the harmonic current noise caused by the frequency converter.

On complex motor/frequency converter setups it is often beneficial to have at least one more powerful supply unit consisting of the input bridge and a choke and many inverters that connect together via the DC link. All the modules may have their own DC capacitors and output bridges. In this configuration, the inverters are often equipped with disconnector switches that can be used to disconnect some inverters from the system. Also, depending on the configuration, the charging circuit may be integrated in the supply unit, or built in the modules (thus charging from DC instead of AC).

It is often economical to use the same devices (called modules) as frequency converters and inverters. In these cases, the module may be equipped with an input bridge for the AC connection and/or a charging circuit for the DC connection.

In some cases it is necessary for the module to switch back to the charging mode, i.e. stop the current flow from the input bridge (or a DC supply) and only allow current flow through the charging circuit. These cases could include main grid voltage dips or opening of the disconnector...