CALCULATING BRAKE RESISTOR SIZES

DBR’s for inverters and DC drive systems

A drive motor can also act as a generator. If the drive system is built so as to allow reverse power to flow then this power can be fed into a resistor, thus taking energy out of the system and causing whatever is driving the motor to slow down.

The rate of braking is determined by how fast the energy is put into the braking resistor. The DC link capacitance of any inverter drive can itself absorb 3-5% of the regenerated power. For non-critical applications these losses, together with the mechanical losses in the drive system, may provide enough braking. Higher powers, up to 100% or more of the motor’s full load torque rating, can be absorbed and then dissipated by a braking resistor connected across the DC bus. Where the braking power is only a few tens or hundreds of watts a resistor mounted internally to the drive itself may be suitable, but above these levels the amount of heat generated means that a separately mounted braking resistor with appropriate cooling provision is needed.

The braking resistor is switched on by a separate control unit, activated by a sensor which is monitoring the voltage level of the DC bus and switching on the braking resistor when this voltage rises above some preset trigger level as a result of the reverse power flowing into the drive. There may be temperature sensing in the braking resistor to prevent overloading of the drive. All the energy is used in heating the resistor; some is dissipated at once, the rest after the stop while the resistor cools. This is why we must know the characteristics of the duty cycle before we can specify the right size for the braking resistor.