Brake resistors

During the deceleration of a three-phase motor and its load, part of the stored kinetic energy is converted by the motor into electrical energy and fed back to the frequency inverter. If a large inertia decelerates in a short time, the energy supplied is too much to be absorbed by the capacitors in the DC intermediate circuit. This will increase the voltage in the intermediate circuit and this can result in an Over Volt (OU) trip.

Consider, for example, the following applications:

• A (fire) fan (with a large inertia) must be stopped quickly or change its direction of rotation
• Closing a bridge
• Slacking a crane
• Dynamic applications with short acceleration and deceleration times
• Mixers with oscillating load

The regenerative energy can be dissipated in braking resistors that are controlled by a braking circuit. This circuit measures the intermediate circuit voltage to determine when the internal braking transistor should operate.

To select the correct braking resistor, the necessary ohmic value must be determined (this depends on the controller used) and it must be determined how much energy must be dissipated at which duty cycle of the resistor.