VFD Sizing & Braking Resistor Calculator
Determine the correct VFD rating (Normal Duty vs Heavy Duty) for your application and calculate the minimum Ohms and continuous Watts required for your braking resistor.
VFD SELECTION GUIDE
Note: Constant torque applications require "Heavy Duty" (HD) rating. For 400V class VFDs, the braking chopper usually activates around 750V DC.
Calculation Rules
Conveyors and hoists are Constant Torque loads. They require high starting torque and 150% overload capacity for 60 seconds. Pumps and fans only need 110% overload (Normal Duty/Variable Torque).
Continuous Power = Motor kW × (Duty Cycle / 100). Minimum Ohms = (Chopper Voltage)² / Peak Power. Using 750VDC for the chopper on a 400V system.
FAQ
Why do I need a larger VFD for a conveyor?
Conveyors and hoists are Constant Torque loads. They require high starting torque and 150% overload capacity for 60 seconds. Pumps and fans only need 110% overload (Normal Duty/Variable Torque).
How is the braking resistor calculated?
Continuous Power = Motor kW × (Duty Cycle / 100). Minimum Ohms = (Chopper Voltage)² / Peak Power. Using 750VDC for the chopper on a 400V system.