Operation of Brake Resistors in Inverters

To slow the motor down more quickly than if it were allowed to simply run-on after removing power.

The momentum left in the motor is used to generate electricity. The resistor acts as a load slowing the motor down faster.

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit (resistor controlled by a transistor) to dissipate the braking energy. With 4-quadrants recifiers (active-front-end), the VFD is able to brake the load by applying a reverse torque and reverting the energy back to the network.

In Variable Frequency Drives the incoming power is converted to DC. and then turned back into AC at variable frequencies to achieve speed regulation. When a motor is used as a brake it is turned into a generator, which dumps this generated energy back onto the DC buss. If too much energy is produced the drive will fault out on Buss Overvoltage. The braking resistor is used by the drive and controlled by a transistor to take the excess energy and dissipate it in the form off heat in the braking resistor. On applications that do not use braking or ramp to stop function this resistor is not required.

Here is some additional information:

Where the motor is required to stop quickly, a load is applied to the motor during stopping sequence. There are two means the load is applied: (1) using dynamic braking resistor (for single quadrant drives) and (2) using regenerative drive (four-quadrant drives).

So, if you don't see the dynamic breaking resistor in the inverter, it could be either the inverter is regenerative (four-quadrant drives) or the drive doesn't have the dynamic breaking system at all.

- MS