经验总结-制动电阻计算公式.doc

Calculating brake resistor sizes Dynamic braking resistors (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 DBR. 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 DBR 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 DBR with appropriate cooling provision is needed. The DBR 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 DBR 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 DBR 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 DBR. What is the stopping energy? The DBR turns the stop energy into heat. Both types of energy are measured in Joules (J); one Joule is a very small quantity, so we usually talk about kJ or MJ. In order to design a braking system we have to consider both the amount of heat (in Joules) and the rate at which it is generated. This is Joules/second, usually kn