Dynamic Braking

Calculating braking resistor sizes (part 1 of 2)

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 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.

What is the stopping energy?

The braking resistor 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 known as watts, and for the same reason usually measured in kW or MW. We therefore need to know the quantity of energy per stop, and the stop frequency. Energy per stop: determines the braking resistor peak power Energy per stop + frequency: determines the braking resistor average power We all have a good idea of what any given length, weight or time interval represents; this is usually not so for energy. By way of illustration here are some everyday examples: Man on a bike stopping: 2kJ Lift with four people in it: 25kJ Car stopping from 50mph: 250kJ Flywheel 600mm x 300mm thick, 1500rpm: 375kJ 40’ container lowered on to a ship: 2MJ Eddie Stobart’s lorry from 65mph: 15MJ London Underground train from 50mph: 50MJ