Dynamic Braking Resistors for AC Drives

AC drive manufacturers give recommendations for brake resistor for normal usage.

Obviously if you are stopping and starting a lot the resistor needs to have its operating wattage increased this has to be calculated for the No of stops and energy absorbed in watts or joules.

You will find these tables on manufacturers sites.

Here is a link

Generally, the resistor is rated for 1) its resistance and 2) its power dissipation capacity.

The resistance is normally determined by the brake switch (transistor) that switches the DC Link in the middle of the inverter to divert motor regenerated energy into the resistor. So, if you have a certain size (power) inverter, this transistor is normally sized to conduct a current that will equate to a peak value of 150-200% of notional inverter rating to deliver good braking performance potential.

If the braking circuit is integrated, the above is normal, if separate, you can select a rating closer to design braking requirement if this differs from motoring by much e.g. a mill may require high motoring power but occasionally may slightly regenerate under certain air and milling product conditions. Here you may possibly use a smaller brake unit.

The rated power of the resistor is really a thermal question for dissipation capacity. You can overload for short duty with a rest / 'cooling' period, as the power and duty cycle goes up, so does your resistor power capacity, until 100% duty means you have same thermal capacity as the regenerative load.

Although Energy = 1/2 x mass x velocity2, you also need to consider peak power as at the highest speed you have maximum power and must transmit this to the resistor or the drive will trip on Overvoltage.

Does this explain a little or please advise the context of your question with a potential application may make it weasier to understand?