Braking resistors: when you need one and how to size it
A VFD cannot push energy back into the supply. When you ask a spinning spindle to slow down, that energy goes into the DC bus capacitor, the bus voltage rises, and the drive trips on overvoltage. A braking resistor is where the energy goes instead.
Do you need one?
- Probably not if your stops are gentle and you can live with a coast-down of several seconds. Longer deceleration times cost nothing.
- Yes if you need a fast, repeatable stop — an automatic tool change, a rigid tapping cycle, an interlocked guard, or a job where cycle time matters.
- Definitely if the drive trips on overvoltage every time you decelerate, and lengthening the deceleration time is not acceptable.
Router spindles are light for their power and are often fine without one. Large spindles with heavy toolholders are not.
Braking resistor sizing
The drive's minimum allowable resistance overrides everything this produces. Enter it from your manual.
Two limits, and which one bites
| Resistance | Braking | Risk |
|---|---|---|
| Too high | Weak — the bus still rises and you still trip | None to the drive; you just do not get the stop you wanted |
| Correct | Energy leaves fast enough to hold the bus down | — |
| Too low | Strong | More current than the braking transistor is rated for. This is the failure that costs a drive |
So the sequence is: calculate, then check against the drive's stated minimum, then take the higher value. Never the calculated one if it is lower.
Power rating and duty
A resistor's peak power and its continuous rating are different things. Braking is intermittent, so what matters is the average: peak power multiplied by the proportion of time it is actually dissipating. Under-rating shows up as a resistor that glows, smells, and eventually opens — at which point the overvoltage protection is gone and the next fast stop trips the drive.
Two practical notes:
- Mount it where it can shed heat and where nothing flammable is near it. It is designed to get hot.
- Use the thermal contact if it has one. Some braking resistors include a thermal switch precisely so a stalled deceleration cannot cook them silently.
Wiring
- It goes on the dedicated brake terminals — see the diagram. Not across the DC bus generally, not on the output.
- Measure the resistor before you fit it. A resistor with the wrong value, or one that has failed open, presents identically to “my brake resistor is not working”.
- Screened or at least routed away from signal cable: it switches the full bus voltage.
Parts
Braking resistors
Questions
Do I need a braking resistor on a CNC spindle?
Only if you need to stop faster than the spindle coast-stops, or you do rigid tapping and frequent reversals. A router spindle cutting wood or plastic almost never needs one.
Why does the drive trip with overvoltage every time I stop?
The spindle is acting as a generator and pumping its kinetic energy back into the DC bus. Either the deceleration time is too short for the load inertia, or there is nowhere for that energy to go.
Can I use any resistor of the right resistance?
No. The resistance must not be below the minimum your drive allows, and the power rating has to cover the peak braking energy at your duty cycle. Both are maker-specific — take them from your own drive manual. The calculator on this page works out what you need once you have those two limits.
Sources and method
Three kinds of statement appear on this page, and each is labelled where it occurs:
- Derived — the energy and power arithmetic is shown on the page; the resistance you can actually fit is bounded by the drive's own stated minimum, which wins over any calculation.
- Rule of thumb — workshop practice, not a standard. Given with the range it is meant to hold over, and it loses to your manual wherever the two disagree.
- Maker-specific — parameter numbers, connector pin numbers, minimum brake resistance, maximum cable length, bearing speed limits. Not published here. They move between firmware revisions of the same model, and a wrong one is worse than none.
Standards and documents
- IEC 61800-5-1 — adjustable speed drives: safety requirements
- IEC 60204-1 — electrical equipment of machines
- The manual for your own drive and spindle — the only valid source for maker-specific values