What is a braking resistor?
(For AC drives, motor controllers, VFD etc.)
When an electric motor needs to stop quickly or slow down, the energy it generates is converted into DC inside the inverter. This energy is then dissipated as heat through the braking resistor, which is connected to the drive’s brake terminals with proper ohmic and wattage values. The braking resistor prevents the DC bus voltage from rising dangerously.
Why Use a Braking Resistor?
A motor keeps rotating due to kinetic energy even when power is cut, acting like a generator. To stop it quickly or reduce speed safely, the inverter channels this energy to the resistor and converts it into heat.
1- Ohmage: Must not be less than the catalogue value. If higher, braking takes longer.
2- Power: Cannot exceed the motor’s rated power, since the motor cannot generate more than its own power.
Please note: For the braking resistor topic, please use the animation below. As you will notice, the real responses of inertia and speed variables are not standard. The DC bus must also be carefully considered in my opinion. In other words, selecting a braking resistor cannot be done blindly.
Applications
Used in cranes, elevators, presses, conveyors, escalators, trolleys, textile reel systems, mining machines, wind turbines, and even hybrid vehicles. Braking resistors are essential in most motor drive applications.
Economical Selection
Normally, braking power ≈ motor power. But since braking times are usually short, smaller resistors can be used and overloaded briefly. This reduces cost without performance loss.
Resistor Technologies
Aluminum Heatsink: Compact, 0–15kW, fast cooling, vibration resistant.
Sheet Metal: Up to 500kW, stainless/galvanized body, custom IP levels.
Fan + Thermal Protected: For heavy duty applications (crane, press, compressor). Fan activates when internal temperature (e.g. 80°C) is exceeded, ensuring safe and long life.
Advantages:
• Reliable and long-lasting operation
• Protection against overheating
• Adjustable temperature levels
• Full control for the user
Editor’s Note: A braking resistor is not just an accessory, it is a protector of the DC bus and the entire drive. In real life, where elevators, cranes and conveyors make frequent stops, the correct selection of resistance directly determines the reliability of the system.
Installation Tip: Place the resistor where airflow is good and away from flammable materials. Loose terminals can cause local overheating, so always finish with a torque wrench. For closed panels, fan + thermal protection options are strongly recommended.
Parameter Insight: In some drives, the brake chopper function is disabled by default. Make sure it is enabled and adjust the DC bus threshold according to your application. If you often see an OV (Over Voltage) alarm, the problem may be: wrong ohm value, insufficient power, or a delayed activation parameter.
Quick Formula: For a rough selection: Resistor Power ≈ Motor Power × (Braking Time / Cycle Time). If you notice smell, discoloration or overheating in practice, choose the next higher resistor rating. When in doubt, check the options on the Braking Resistors Page.
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