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A braking resistor is used in motion systems where a motor in controlled by a drive. Typically, these are high-speed servomotors controlled by a servodrive. When accelerating, electrical energy is converted to a mechanical force to move a mass. When decelerating, that kinetic energy is converted (via the motor) back into electrical energy, and must go somewhere. To brake quick enough, the energy must be dumped into a power resistor, to be dissipated as heat.
A thyristor (also known as a silicon-controlled rectifier or SCR) is a diode which has a gate terminal on it. Current at the gate allows the thyristor to go into a conducting mode, allowing current to pass.
Modern drives will detect when their DC bus voltage reaches a certain threshold (from the energy being returned from the motor during braking) and begin triggering the thyristor periodically (typically PWM, pulse-width modulation) to disperse this energy as heat.
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