Method for controlling brake chopper, brake chopper and frequency converter

A brake chopper comprising:

• controllable switches;
• two or more brake resistors, the brake resistors being connected in series with the switches to be controlled, each of the brake resistors and the respective one of the switches being connected between a positive and a negative rail of a direct current (DC) voltage intermediate circuit;
• means for determining a magnitude of the voltage of the DC voltage intermediate circuit in relation to a first voltage limit and a second voltage limit;
• and
• means for switching the brake resistors to the DC voltage intermediate circuit in a periodically alternating manner by alternately switching each switch, during a switching period, between a respective off-period and an on-period of each switch when the magnitude of the voltage of the DC intermediate circuit is determined to be above the first voltage limit and below the second voltage limit,
• wherein the length of the respective on-period of each switch is based on the determined magnitude of the voltage of the DC intermediate circuit.

A brake chopper for a DC intermediate circuit, the brake chopper comprising:

• controllable switches;
• brake resistors being respectively connected in series with a corresponding one of the controllable switches;
• and
• a switch controller configured to determine a voltage of the DC intermediate circuit in relation to a first voltage limit and a second voltage limit, and to switch the brake resistors to the DC intermediate circuit in a periodically alternating manner by alternately switching each switch, during a switching period, between a respective off-period and an on-period of each switch when the magnitude of the voltage of the DC intermediate circuit is determined to be above the first voltage limit and below the second voltage limit,
• wherein the length of the respective on-period of each switch is based on the determined magnitude of the voltage of the DC intermediate circuit.

A frequency converter comprising:

• a DC voltage intermediate circuit;
• and
• a brake chopper including:
• controllable switches;
• brake resistors being respectively connected in series with a corresponding one of the controllable switches, each of the brake resistors and the respective one of the switches being connected between a positive and a negative rail of a direct current (DC) voltage intermediate circuit;
• and
• a switch controller configured to determine a voltage of the DC intermediate circuit in relation to a first voltage limit and a second voltage limit, and to switch the brake resistors to the DC intermediate circuit in a periodically alternating manner by alternately switching each switch, during a switching period, between a respective off-period and an on-period of each switch when the magnitude of the voltage of the DC intermediate circuit is determined to be above the first voltage limit and below the second voltage limit,
• wherein the length of the respective on-period of each switch is based on the determined magnitude of the voltage of the DC intermediate circuit.