An undersized braking resistor announces itself twice: first with overvoltage trips, later with smoke. Sizing one properly takes ten minutes.
The calculation
Braking power is the kinetic (or potential) energy leaving the mechanics divided by the braking time. For a rotating mass: P = J × ω × (dω/dt), evaluated at the worst point — usually the start of braking from top speed. For hoists, the potential-energy term m × g × v dominates and is continuous during lowering, not a pulse.
Two ratings matter and are not interchangeable:
- Peak power — the resistor (and chopper) must absorb the worst-case braking pulse. Resistance low enough to sink the current, but never below the drive’s minimum permitted resistance.
- Continuous power — the thermal rating, set by duty cycle. A resistor absorbing 30 kW for 3 seconds every 30 seconds needs roughly a 3 kW continuous rating plus margin, not a 30 kW one.
The duty-cycle trap: cycle times shorten in production (“we sped up the line”) and the resistor sized for 10 % ED quietly runs at 25 %. If a resistor enclosure discolors, that is the smell test failing — resize before it fails hard.
Installation notes
Mount where the heat can leave (not sealed above the drive), respect creepage on the high-voltage DC connections, and wire the thermal contact into the drive fault chain — a resistor’s thermostat is its only advocate.
FAQ
Can I parallel resistors? Yes, respecting the minimum-resistance limit of the chopper; parallel halves resistance and doubles power. Keep sets identical.
When is regen the better answer? Frequent braking with meaningful energy — see our regenerative vs resistor comparison; the resistor’s job is turning your product into room heating.
Zone Otomasyon sizes braking systems as part of every retrofit quote. Send us your cycle data.