Crane VFD and DBR Sizing for Hoist and Travel Motions
Cranes don't just consume energy; they generate it. Here is how to handle regenerative power so your drives never trip with a suspended load.
Want to calculate resistor ohms instantly?
Skip the electrical engineering formulas and use our free calculator to size your VFD and Dynamic Braking Resistors (DBR) instantly.
The Over-Voltage Problem in Cranes
If you put a standard Variable Frequency Drive (VFD) on a conveyor belt, it runs beautifully. If you put that same VFD on a crane hoist, it will likely fault out with an "Over-Voltage" error on its very first heavy lift.
Why? Because of Regenerative Energy.
When a crane is lowering a heavy load, gravity is pulling the load down. The electric motor is no longer consuming electricity to turn the gears; instead, the gears are turning the motor. The motor effectively becomes a generator.
This generated electricity flows backward into the VFD's internal DC bus. If this energy is not immediately removed, the voltage will spike, and the VFD will shut down to protect itself. When a VFD shuts down mid-air, the mechanical brake slams shut, causing massive mechanical shock to the crane.
How Do We Solve This? (The DBR)
To prevent the VFD from exploding from over-voltage, we must bleed off the excess electricity. We do this by connecting a Dynamic Braking Resistor (DBR) to the VFD's braking chopper circuit.
A DBR is simply a giant grid of resistive wire that converts electricity into heat. It literally burns off the falling load's potential energy so the crane can lower smoothly.
Heat Dissipation
Because DBRs convert megawatts of energy into pure heat, they can reach temperatures exceeding 400°C (750°F) during continuous lowering. They must be mounted outside the electrical panel, in well-ventilated stainless steel enclosures, away from flammable materials.
Sizing the Dynamic Braking Resistor
Sizing a DBR requires two main calculations: Resistance (Ohms) and Power (Watts).
1. Calculating Ohms (\( \Omega \))
If the resistance is too high, the electricity won't flow into the resistor fast enough, and the drive will trip on Over-Voltage. If the resistance is too low, too much current will flow, and you will blow the VFD's braking IGBT transistor.
Every VFD manual provides a minimum allowable resistance (e.g., 20 Ohms). Your DBR must be equal to or slightly above this minimum value.
2. Calculating Power (Watts)
The wattage rating dictates how much heat the resistor can safely dissipate without melting. A hoist lowering a load 10 meters requires a much larger wattage rating than a hoist lowering a load 2 meters.
We determine this using the Cyclic Duration Factor (CDF) or Braking Duty Cycle.
- Travel Motions (Bridge/Trolley): Generally only require 10% to 20% braking duty, since they only generate power when decelerating sideways.
- Hoist Motions: Require 40% to 50% braking duty, because they generate power the entire time they are lowering a load.
Sizing the VFD (Heavy Duty vs Normal Duty)
You cannot buy a "Normal Duty" pump/fan VFD for a crane.
Cranes require Heavy Duty (HD) or Constant Torque (CT) ratings. When a hoist lifts a load off the ground, the motor requires up to 150% to 200% of its normal current for the first 2-3 seconds to overcome static inertia.
If you use a 10 kW Normal Duty drive on a 10 kW crane motor, it will trip on "Over-Current" the moment you try to lift a heavy load. You must always size crane VFDs based on the Heavy Duty current rating (which usually derates the drive by one size class).
Automate Your DBR Calculations
Calculating regenerative wattage based on lowering speeds, mechanical efficiency, and duty cycles is a tedious process.
We built a tool that automates this. Simply input your motor kW and whether it's for Hoist or Travel, and our calculator will give you the exact VFD size, minimum Ohms, and required continuous Watts for your Braking Resistor.