When the speed reference of a frequency converter is reduced, the motor that is controlled by the converter acts as a generator and brakes. When a motor acts as a generator, it supplies energy back to the frequency converter. This energy is collected in the intermediate circuit of the converter.
The function of a brake resistor is to provide a load on the intermediate circuit during braking, thereby ensuring that the braking power – the energy from the motor – is absorbed by the brake resistor. If a brake resistor was not used, the intermediate circuit voltage of the frequency converter would continue to increase, until it cuts out for protection. The advantage of using a brake resistor is it enables the braking of a heavy load – for instance a conveyor belt – quickly.
Brake resistors can form an integral part of the frequency converter or they can be placed outside the frequency converter, in which case the frequency converter needs to come with terminals that allow you to connect a brake resistor. Built-in solutions may save you some space, but placing the brake resistor outside the converter also offers a couple of important advantages:
- The size and type of the brake resistor can be selected as required, depending on for instance the braking cycle
- The brake resistor can be placed outside the panel or cabinet that holds the frequency converter, reducing the need for (extra) cooling
- The braking energy can be (re-)used
- There is no overheating of the electronic components in the frequency converter, even if the brake resistor is overloaded
The requirements for brake resistors vary in different applications. Critical data includes:
- Brake duty cycle, resistance and brake resistor power capability
- Drive minimum resistance
Always consult a frequency converter expert before selecting a brake resistor.