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VFD Braking Resistors: What They Do, When You Need One, and How to Size It
A braking resistor (dynamic brake resistor) lets a VFD stop a motor faster than it would coast, without tripping on overvoltage. It turns the energy the motor sends back during braking into heat.
Why drives need them
When a drive slows a spinning load, the motor briefly becomes a generator. That energy flows back into the drive's DC bus and raises its voltage. A small amount is absorbed by losses, but with a heavy, fast-spinning or overhauling load the bus voltage climbs until the drive trips on overvoltage.
A brake chopper, a transistor built into many smaller drives and available as an add-on for larger ones, watches the bus. When it rises past a threshold, the chopper switches the braking resistor across the bus, dumping the excess energy as heat and holding the voltage in a safe range.
Do you need one?
You probably need a braking resistor if:
- The drive trips on overvoltage during deceleration.
- The load has high inertia and must stop quickly: large fans, flywheels, centrifuges, saws.
- The load is overhauling, driving the motor: lowering hoists, downhill conveyors, unwinders.
- The machine starts and stops frequently with short cycle times.
You may not need one if a longer deceleration time is acceptable, or if the load can simply coast to a stop.
Alternatives
| Option | Good for | Limitation |
|---|---|---|
| Longer decel time | Most applications; free | Slower stops |
| Bus regulator / overvoltage control | Occasional overvoltage trips | Stretches deceleration automatically; stopping time not guaranteed |
| Coast to stop | Loads where stopping time does not matter | No control over stop |
| DC injection braking | Holding or final stop at low speed | Heats the motor; not for regular high-energy braking |
| Braking resistor | Fast or frequent stops, overhauling loads | Produces heat; must be sized and mounted properly |
| Regenerative unit | Continuous regeneration, large systems | Higher cost |
How to size a braking resistor
Two numbers matter: resistance (ohms) and power rating (watts).
- Resistance: must be no lower than the minimum the drive manual specifies, or the chopper transistor can be damaged. Lower resistance gives more braking power, up to that limit.
- Peak power: the energy to remove divided by the stopping time.
- Average power: the energy per stop times the number of stops per unit of time. The resistor's continuous watt rating must handle this.
Most drive makers publish tables of recommended resistors by drive size and braking duty (for example 10% duty), which is the easiest place to start.
Worked example
A flywheel with a total inertia of 2 kg·m² spins at 1,800 rpm (188.5 rad/s) and must stop in 5 seconds, once a minute.
- Kinetic energy: ½ × 2 × 188.5² ≈ 35.5 kJ per stop.
- Peak braking power: 35.5 kJ ÷ 5 s ≈ 7.1 kW.
- Average power: 35.5 kJ every 60 s ≈ 590 W.
Motor and drive losses absorb some energy, so this is conservative. Choose a resistor at or above the drive's minimum ohms that can handle the peak power for 5 seconds and around 600 W continuous, then check it against the manufacturer's table.
Installation tips
- Resistors get hot. Mount them where heat can escape, away from wiring and combustibles, often outside the enclosure.
- Use the resistor's thermal switch, if it has one, to stop the drive or open the supply on overheating.
- Keep the leads short and follow the drive manual's wiring diagram.
Parts
We carry matched resistors such as the Unitronics UMI-S0050 for UMI drives and Allen-Bradley resistors like the AK-R2-091P500 for PowerFlex. Send us the drive model and duty and we will help match one. Call 1-800-724-4209 or request a quote. See also how a VFD works.