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Do I Need a Braking Resistor with My VFD? [Explained]
Braking resistors are often overlooked when sizing variable frequency drives (VFDs), but they can be the key to safe, reliable, and efficient motor control. In this guide, we’ll explain when you need a braking resistor, why it’s important, and how to size one using our free Braking Resistor Calculator.
What Does a Braking Resistor Do?
When a VFD-driven motor decelerates, it acts like a generator—feeding energy back into the drive. If the drive can’t absorb the energy fast enough, the DC bus voltage rises. A braking resistor diverts that excess energy as heat, preventing voltage spikes, drive shutdowns, or damage.
How to Know If You Need One
- High-inertia loads: Heavy rollers, conveyors, large pulleys or machine masses.
- Frequent or fast deceleration: Applications like packaging lines, lifts, or servomotors.
- Limited regeneration capability: Lower-end VFDs might trip or overload without proper braking.
If you’re experiencing drive overvoltage faults, need shorter stopping times, or want to extend drive life, a braking resistor may be essential.
How to Size a Braking Resistor
- Open the Braking Resistor Calculator.
- Input motor rating, load inertia, deceleration time, and drive model.
- The calculator returns the power (kW) and resistance (Ω) required.
- Match it with drives that support those values (e.g., ABB ACS355, Danfoss FC‑302, Delta MS300).
When a Resistor Isn’t Necessary
- Light, low-inertia loads and slow deceleration.
- VFDs equipped with regenerative control or an internal braking option.
- Systems with external regenerative modules or dynamic braking options via DC bus absorption.
Typical Applications That Require Braking
| Application | Why It Needs a Braking Resistor |
|---|---|
| Conveyor systems with heavy loads | High momentum causing overvoltage without brake |
| Elevators and hoists | Rapid-stop safety requirements |
| Packaging machinery | Frequent stop/start cycles generating excess energy |
Braking Resistor Calculator
Having trouble with values? Let our calculator do the math. Enter your specifics here.
Why Buy From Drive Outlet Megastore?
- ✓ 3,000+ OEM VFDs and accessories in stock (ABB, Danfoss, Delta)
- ✓ Next-day UK delivery
- ✓ 2-Year warranty on all components
If you’re not sure about your application, grab our calculator or contact our experts for free sizing support.
