A VFD braking resistor is not a wattage-only purchase. During deceleration, regenerative energy raises the drive DC-bus voltage; the brake chopper diverts that energy into the resistor as heat. A useful RFQ must therefore describe the drive, the stopping event and the cooling conditions, not just an ohmic value.
1. Start with the drive and minimum permitted resistance
Send the VFD manufacturer, complete model, supply voltage and brake-chopper arrangement. The selected resistance must stay within the drive maker's permitted range. Too low a resistance can demand excessive chopper current; too high a resistance may not provide the required braking torque. Use the drive manual as the governing reference.
2. Define peak energy and average heating
Provide motor power, load type, maximum speed, target deceleration time and estimated moving inertia when available. Peak braking power describes the short event, while average resistor power depends on energy per stop and the total cycle. Danfoss guidance expresses braking duty cycle as braking time divided by the process period. This is why "one stop every 60 seconds" is materially different from repeated rapid indexing, even when the peak power is identical.
3. Describe the real duty cycle
State braking time, cycle time, stops per hour and whether braking is occasional, repetitive or emergency-only. Do not substitute a catalog wattage for this operating profile. Longer or more frequent braking can overheat a resistor designed for intermittent service.
4. Include installation and cooling conditions
Tell the supplier whether the resistor will be mounted inside a cabinet or in free air, the ambient temperature, available airflow, mounting orientation, cable length and space envelope. Keep hot surfaces away from combustible materials and temperature-sensitive components. Final clearances, cable sizing, grounding and enclosure requirements must follow the drive and resistor instructions.
5. Specify thermal protection and sample checks
ABB drive guidance calls for system protection against resistor thermal overload and describes using the resistor thermal switch as a drive fault input. Your RFQ should identify the required thermal switch, fault response and terminal style. Before approving a sample, verify cold resistance, insulation, lead and terminal construction, mounting dimensions, and a representative braking cycle while monitoring temperature. Commissioning should be performed by qualified electrical personnel.
RFQ information checklist
Send the VFD model and voltage, motor and load data, required stopping time, braking time and cycle time, minimum allowed resistance, peak/average power calculation, ambient and cooling, enclosure and mounting limits, cable/terminal needs, and thermal-protection requirement. LORIDA can review these inputs for a braking-resistor sample proposal; final selection must be checked against the drive manufacturer's manual and validated in the actual system.
Technical references: Danfoss Brake Resistor Design Guide; ABB ACS380-E hardware manual, resistor braking section.
