A braking resistor in a servo drive is used to safely dissipate excess electrical energy generated when the motor slows down or is driven by an external load. During deceleration, the motor acts like a generator and sends energy back into the drive’s DC bus. If this energy is not removed, the DC bus voltage rises too high, which can trigger an overvoltage fault, damage the drive, or force it to shut down. The braking resistor converts that regenerative energy into heat. This allows the servo system to stop faster, maintain control during rapid deceleration, and handle loads with high inertia more effectively. It is especially useful in applications such as machine tools, elevators, conveyors, robotics, and winding systems, where motors frequently stop, reverse, or must control heavy moving masses. In simple terms, the braking resistor protects the drive and improves performance by providing a place for unwanted regenerated energy to go. Without it, the drive would have limited ability to absorb that energy, which could reduce stopping performance and reliability. Some servo drives have built-in braking transistors that switch the resistor on and off as needed, while others require an external braking resistor. The resistor must be properly sized so it can absorb the expected energy and withstand the heat generated during braking.

Size it from the energy the motor/regenerated load can put back into the drive, and the resistor’s ability to absorb that energy without overheating. 1) Find the available regenerative energy For a deceleration event: E = 1/2 J ω1² − 1/2 J ω2² where J is total reflected inertia (motor + load, at motor shaft) and ω is speed in rad/s. For stopping to zero, E = 1/2 J ω². 2) Determine how often that energy occurs If the stop happens every t seconds, average regen power is: Pavg = E / t If the duty cycle is intermittent, use the repeated-event average over time. 3) Check peak braking power During decel, the resistor must also survive peak instantaneous power: Ppeak ≈ Vdc² / R More precisely, the drive’s maximum braking current and chopper limit determine the true peak. 4) Choose resistor value Use the servo drive manual first. The resistor must be within the allowed ohms range: Rmin prevents overcurrent in the braking transistor. Rmax ensures enough current flows to clamp the DC bus voltage. If unsure, select the manufacturer’s recommended resistor. 5) Check resistor power rating Continuous rating must exceed Pavg. For short-duty braking, use pulse/energy ratings from the resistor datasheet, not just watts. Verify thermal rise, mounting, airflow, and ambient temperature. 6) Verify drive compatibility Confirm DC bus voltage, braking transistor current limit, duty cycle limit, and any external resistor wiring requirements. Rule of thumb: size from the worst-case stop energy, then validate against the drive’s minimum resistance and the resistor’s pulse energy rating.

Use a regenerative braking unit when the energy from slowing the load is significant, frequent, or important to recover. In those cases, a braking resistor simply turns that energy into heat, while a regenerative unit sends it back to the supply or DC bus, improving efficiency and reducing cooling needs. Choose regeneration instead of a braking resistor when: - The drive decelerates often or the machine cycles rapidly. - The load has high inertia, such as centrifuges, elevators, hoists, test stands, large fans, or long conveyors. - You want lower energy costs or have many braking events per hour. - Heat dissipation is a problem, or the cabinet/environment cannot easily handle resistor heat. - You need better thermal stability and less wear on braking components. - Utility regulations, sustainability goals, or facility power management make energy recovery valuable. - The supply system can accept returned energy, or you are using a DC bus shared by multiple drives. Use a braking resistor instead when: - Braking is occasional or the recovered energy is small. - Cost, simplicity, and easy installation matter most. - The drive’s supply cannot accept regenerated power. - You only need to control DC bus overvoltage, not recover energy. Rule of thumb: if braking is infrequent and inexpensive to dump as heat, a resistor is usually best. If braking is repeated, energy-rich, or heat-sensitive, regeneration is often the better long-term choice.

A brake chopper is an electronic circuit used in drives, such as variable-frequency drives (VFDs) and servo systems, to safely handle excess energy when a motor slows down or is driven by a load. When a motor decelerates, it can act like a generator. The regenerated energy flows back into the DC bus of the drive and causes the DC bus voltage to rise. If this voltage gets too high, it can damage the drive or trigger a fault. The brake chopper prevents this by switching a braking resistor into the circuit whenever the DC bus voltage exceeds a preset limit. It works like this: a voltage sensor monitors the DC bus. If the voltage rises above the threshold, the chopper transistor, usually an IGBT, turns on and connects the resistor across the DC bus. The resistor converts the extra electrical energy into heat. When the voltage drops back to a safe level, the transistor turns off. This on-off action can happen many times per second, keeping the bus voltage under control. In simple terms, the brake chopper is a controlled dump switch for regenerative energy. It does not stop the motor by itself; instead, it allows the drive to decelerate loads quickly and safely without overvoltage problems. It is commonly used in cranes, elevators, conveyors, centrifuges, and any application with frequent stopping or overhauling loads.

A servo drive faults on overvoltage during deceleration because the motor acts like a generator when it is slowing down. The kinetic energy stored in the moving load is converted back into electrical energy and pushed into the drive’s DC bus. If the load is heavy, moving fast, or decelerating too quickly, more regenerated energy is returned than the drive can safely absorb. This causes the DC bus voltage to rise. When it exceeds the drive’s overvoltage threshold, the drive trips to protect its power electronics, capacitors, and insulation. Common reasons include: 1. Deceleration too abrupt or short decel time 2. High-inertia load or large vertical load 3. Downhill or overhauling load driving the motor 4. Regenerative resistor missing, undersized, or faulty 5. Drive’s internal bus capacitors already weak or the supply voltage is too high In simple terms, the motor is “pumping” energy back into the drive faster than the drive can dispose of it. The drive faults to prevent damage. Typical fixes are to increase deceleration time, add or size a braking resistor/regenerative unit correctly, reduce load inertia if possible, and check supply voltage and drive settings. For vertical axes, a counterbalance or brake may also be needed.