VFD & DBR Sizing Calculator

Calculate Heavy Duty inverter current and required Dynamic Braking Resistor (DBR) watts for hoist and travel motions.

Read the Crane VFD & DBR Sizing Guide
kW

Hoist motions require 100% braking power due to gravity (regenerative load). Travel motions only require ~50% to decelerate inertia.

%

Standard is 40% ED for typical industrial cranes. Defines the continuous wattage rating of the resistor.

Recommended Sizes

VFD (Inverter)
--
Minimum Heavy Duty (HD) Rating
Dynamic Braking Resistor
Target Resistance - Ω
Peak Braking Power - kW
Continuous Power Rating - kW

* Always check the VFD manual. Your calculated Ohms (Ω) must be greater than or equal to the VFD's minimum allowed resistance limit, but low enough to absorb the peak power.

How It Works

When a crane motor decelerates a load or lowers a load against gravity, the motor acts as a generator. It sends electrical energy back into the VFD. If this energy is not burned off, the VFD's DC bus voltage will spike, causing an "Overvoltage Fault" and tripping the drive.

1. VFD Heavy Duty Rating

Crane motions (especially hoists) require high starting torque. Always select a VFD based on its Heavy Duty (HD) rating (150% overload for 60 seconds), not its Normal Duty (ND) rating. For hoists, it is common to upsize the VFD by one kW bracket above the motor's rating for extra safety margin.

2. DBR Sizing Physics

The Dynamic Braking Resistor (DBR) burns off the regenerated energy as heat.

  • Hoist Motions: Gravity pulls the load down continuously. The peak braking power is roughly 100% of the motor's power.
  • Travel Motions: The resistor only needs to absorb energy while slowing down the crane's inertia. Peak braking power is roughly 50% of the motor's power.
  • Resistance (Ohms): Calculated using Ohm's Law (R = V² / P). For a 400V drive, the braking transistor turns on around 750V DC.
  • Continuous Wattage: The physical size of the resistor box. It is calculated by multiplying the Peak Power by your Duty Cycle (ED%).