Why are DC choppers used

Brake chopper

Basics

A brake chopper is used to monitor the intermediate circuit voltage in a frequency converter. This monitoring is necessary because overvoltages can occur in the intermediate circuit. They arise when the motor brakes. The so-called braking resistor, which converts the excess energy in the intermediate circuit into thermal energy, is connected to the chopper. As an electronic switch (chopper), a resistor is switched on before the intermediate circuit voltage reaches a level that is dangerous for the components. As soon as the intermediate circuit voltage decreases again and becomes lower than the switch-on voltage, but higher than the mains voltage, the chopper switches the resistor off again. The process is repeated as soon as the voltage rises again.

How the brake chopper works

If a drive system has a speed v, it also has kinetic energy. If the system is braked, the excess energy flows back into the intermediate circuit in the form of generator current via the output stage. If this voltage exceeds a threshold voltage, the chopper is switched on electronically. The excess energy is then almost completely converted into thermal energy via the connected braking resistor. The limiting element is the braking resistor, as it converts a large amount of energy into heat. The losses in the brake chopper are not relevant.

Example circuit with brake chopper (source: Fundamentals of drive technology, Sew Eurodrive)

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