Brake resistor duty cycle is defined as the share of a repeating cycle during which the resistor is actually conducting current and dumping regenerative energy, calculated as Braking Time ÷ (Braking Time + Rest Time), with the result expressed as a percentage [S1][S2].
Filnor's published sizing method formalises the same definition as DC = Braking Time / Cycle Time, then feeds the percentage directly into the resistor wattage formula, with separate cases for normal braking versus overhauling loads [S2]. The method assumes the drive bus voltage, motor horsepower, and the drive-maker's minimum ohmic value are already known inputs.
What the Duty Cycle Variable Actually Represents
The duty cycle is not a thermal property of the resistor itself; it is a description of the load profile the resistor must survive over a repeating cycle. ES&E's knowledge base states the formula as Duty Cycle = (Braking Time / (Braking Time + Rest Time)) × 100%, with a worked example of 2 s brake + 8 s rest producing a 20% duty cycle [S1].
Filnor's procedure uses the equivalent expression DC = Braking Time / Cycle Time, which gives the same decimal fraction, and the cycle time in their calculator is typically a fixed window of 10 s, 30 s, 60 s, or 120 s depending on the application class [S2][S5]. For a continuous-brake hoist, the duty cycle can approach 100% and the resistor wattage must be rated for the full peak braking power, not a derated average [S3].
Step-by-Step Sizing Workflow (Filnor Method)
The published 7-step procedure starts with motor wattage MW = HP × 746, then peak wattage PW = MW × BT, where BT is the braking torque factor (1.0 for 100% torque, 1.5 for 150% torque) [S2]. Resistance is then R = (DC bus voltage)² / PW, the value that must be greater than or equal to the drive-maker's minimum ohms to avoid tripping the brake transistor.
Average resistor wattage is then DBRW = (PW × DC) / 2 for normal braking, capped at a 60 s maximum on-time, while overhauling-load regeneration uses DBRW = PW × DC because the energy return is roughly twice that of a normal stop [S2]. Current through the resistor is BI = √(PW/R) and DBI = √(DBRW/R) for steady-state and average current respectively. A reference calculator on Frizlen's site uses the same load-cycle parameters (Pbr, Rmin, tBr, total cycle tDC) and applies a 20% safety margin on the resistance to compensate for tolerance and temperature coefficient drift [S5].
Short-Cut Formula, Worked Numbers, and a Comparison of Duty Profiles
For AC VFD sizing, Industrial Monitor Direct (2026-03) condenses the workflow into Watts = Deceleration Torque × (Start RPM + End RPM) × Duty Cycle × 0.0712, which folds the inertia term and the duty factor into a single resistor wattage figure [S4]. The 0.0712 coefficient bundles the unit conversions for the rotational kinetic energy term (1/2 × J × ω²) so the result lands directly in watts for a given RPM span.
Four duty profiles that come up repeatedly on hoist, conveyor, centrifuge, and downhill-conveyor applications can be lined up against the same criteria:
• Low-duty intermittent stop, e.g. 2 s brake / 58 s rest = 3.3% DC, average wattage ≈ 1.7% of peak; suited to NEMA 1 indoor wirewound units in the 300 W to 1 kW range [S1][S2].
• Moderate cyclic stop, e.g. 10 s brake / 50 s rest = 16.7% DC, average wattage ≈ 8.3% of peak; demands 1.5 to 3 kW steel-grid or flat resistors with thermal switch option [S2].
• High-duty cyclic, e.g. 20 s brake / 40 s rest = 33% DC, average wattage ≈ 16.7% of peak; pushes into 3 to 7 kW assemblies, often with NEMA 3R enclosures for outdoor cabinets [S2].
• Continuous overhauling, e.g. 60 s brake / 0 s rest = 100% DC; resistor must be rated at full peak wattage, typically a steel-grid or tubular design with forced-air cooling [S3].
Edge Cases, Failure Modes, and the Thermal-Switch Question
Rexel's basic-rules write-up recommends counting the number of brake cycles in any 2 min window and multiplying by the deceleration time to get the duty fraction on (cycles × deceleration period) / 120 s, which is simply a sliding-window restatement of DC = tBrake / tCycle [S6]. Rockwell's PFLEX-AT001 application technique instead uses average duty cycle = (t3 - t2) / t4, where t2 and t3 bracket the linear power decay window of a single stop and t4 is the full cycle, then integrates the linearly decreasing power to get average watts [S7].
Two common errors show up in field sizing: underspecifying the resistance (which lets peak current exceed the drive's brake-transistor rating and trips the IGBT), and overspecifying the wattage while leaving the ohmic value at the drive's minimum (which forces the resistor to overheat because the energy per pulse is unchanged). A resistor's thermal switch should be wired back to the drive enable or a line-side contactor, so a thermal trip removes the drive from the run command rather than just dumping the IGBT [S3]. For an overview of how a brake resistor fits into a broader drive stop architecture, see the brake resistor fundamentals page and the VFD duty motor reference.
Selection Criteria: Who Needs the Full Calculation and Who Can Skip It
Engineers specifying a brake resistor need the full duty-cycle calculation when the cycle time is short (under 2 min), the inertia is high (large fans, centrifuges, hoists), the load is overhauling, or the duty cycle exceeds roughly 25%; below that threshold most vendors allow you to pick a resistor wattage at or above the peak braking wattage and call it done [S3]. Conversely, for one-shot emergency stops or rare decel events the duty cycle can effectively be ignored and the resistor sized purely on peak power and ohmic value.
The Rockwell PFLEX-AT001 calculator adds the integration step (linearly decreasing power from peak to zero across the deceleration window) for cases where the resistor conducts only briefly at full power before tapering; using a flat average instead of that integral can under-rate the resistor by 20 to 40% on high-inertia loads [S7]. For more on the rotating-machine side of the same energy-balance problem, see electromagnetic brake and clutch brake references.
Trackable Signals After a Sizing Change
Two practical follow-ups confirm whether the calculated duty cycle was realistic in service: log the drive's DC bus voltage during a known stop and compare the energy returned to (½ × J × ω²) minus losses, and trend the brake-resistor case temperature with the thermal switch wired to a discrete input so a trip event is timestamped against the cycle counter on the VFD. [S1]
For related coverage, see Chemical Industry 2026 Cycle: Trough Confirmed, Recovery Uneven by Region.