Free deterministic sizing tool
Calculate torque, pole count, diameter, and active weight for a 100 RPM axial flux permanent magnet generator before you request a low-speed AFPM RFQ.
Last reviewed on July 23, 2026. Deterministic first-pass sizing only; final diameter, poles, winding, and cooling require electromagnetic and thermal validation.
Because Power = Torque × RPM, generating power at 100 RPM requires 30 times more torque than generating the same power at 3000 RPM.
Evidence: Calculator torque outputs explicitly demonstrate this inverse relationship.
Direct-drive generators require large outer diameters to achieve sufficient air-gap surface area for torque production.
Evidence: The default 5 kW / 100 RPM case returns about 477 Nm and 1,172 mm OD under the stated sizing heuristic.
To generate 50Hz or 60Hz AC at 100 RPM, the generator needs 60 or 72 poles respectively, which further drives the need for a large circumference.
Evidence: Poles = 120 × frequency / RPM.
| Variable | Formula | Use | Limitation |
|---|---|---|---|
| Required Torque | Nm = kW × 1000 / (100 × 2π / 60) | Calculates the sheer mechanical force needed to turn the generator. | Pure physics formula, assumes 100% mechanical transfer. |
| Pole Count | Poles = 120 × Hz / 100 | Determines the minimum number of magnets for direct AC grid-tie frequency. | DC rectified systems can use arbitrary pole counts. |
| Estimated Diameter | D(mm) ≈ 150 × ∛(Torque) | Provides a realistic physical size for the generator package. | Heuristic based on ~20 kNm/m³ torque density; actual size depends on specific stator cooling and air gap. |
| Metric | Result | Decision Use |
|---|---|---|
| Power input | 5 kW at 100 RPM | Use as a compact first-pass feasibility case before changing duty cycle or voltage. |
| Required shaft torque | 477 Nm | This is already a high-torque package, even before overload and safety margins. |
| Estimated outer diameter | 1,172 mm | Plan for a large direct-drive rotor or compare against a geared generator path. |
| 50Hz / 60Hz pole count | 60 / 72 poles | Direct AC output at 100 RPM needs many poles; rectified DC can choose a different pole plan. |
| Estimated active weight | 33 kg | Treat as magnetic active mass only; housing, shaft, bearings, and sealing add weight. |
| Application | Useful Input | Likely Fit | Next Action |
|---|---|---|---|
| Direct-drive wind turbines | 1-100 kW, 10-100 RPM | Excellent fit for gearless wind power, eliminating gearbox maintenance. | Match wind torque curve against generator cut-in speed. |
| Micro-hydro systems | 1-50 kW, 100-300 RPM | Good fit for low-head water turbines (e.g. Archimedes screw or waterwheel). | Determine if sealed housing (IP67/IP68) is required. |
| Low-speed dynamometer / test bench | 10-200 kW, 0-100 RPM | Provides smooth, high-torque absorption for testing slow-moving equipment. | Validate torque ripple / cogging requirements. |
Why is a 100 RPM generator so big?
Power is the product of torque and speed. At very low speeds like 100 RPM, you must generate massive torque to achieve significant power. Torque requires magnetic surface area (air gap volume), which dictates a large diameter.
Can I get 50Hz directly from 100 RPM?
Yes, but it requires exactly 60 poles (30 magnet pairs). A 60-pole rotor needs a large circumference just to physically fit the magnets, which forces the diameter to be large.
Are there gearless alternatives?
You can use a gearbox to step up the 100 RPM to 1500 RPM or 3000 RPM, which allows using a much smaller, standard generator. However, this introduces maintenance, noise, and mechanical losses.
Does this calculator apply to radial flux?
The torque and pole count formulas are universal physics. The diameter and weight heuristics are specific to axial flux permanent magnet (AFPM) topologies, which typically have a higher torque density (flatter, larger diameter) than radial flux.
| Source | Use on This Page |
|---|---|
| U.S. Department of Energy wind turbine operation reference | Used for wind-generator context and direct-drive system framing. |
| Displayed method table on this page | Torque and pole-count equations are disclosed so engineering teams can reproduce the first-pass screen. |
Use this when the package diameter is fixed and you need to estimate output from speed, cooling, and DC bus voltage.
Use this to compare low-speed generator torque with a compact AFPM motor operating-point screen.
Share torque, speed, voltage, package limits, cooling method, environment, and prototype quantity for engineering review.
Send the calculated torque, target voltage or rectifier plan, maximum outer diameter, axial length, cooling method, environment, and duty cycle so engineering can screen whether a direct-drive AFPM generator is practical.
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