By the EconoTest Engineering Team · Test bench manufacturer, Shanghai
Key Takeaways
- Size the dynamometer to the motor’s peak torque, never its continuous rating — transients routinely reach 2–3× rated torque.
- Zero-speed torque requirements decide the technology: hysteresis and magnetic powder brakes load at 0 RPM; eddy current brakes cannot.
- Speed above 18,000 RPM points to an eddy current dynamometer (up to 50,000 RPM); four-quadrant testing requires an AC dynamometer.
- Accuracy class drives cost: ±0.2% FS for R&D and certification, ±0.5% FS for production pass/fail testing.
What Is Dynamometer Selection?
Dynamometer selection is the process of matching a load device’s torque, speed, power, accuracy and cooling characteristics to the motor or engine you need to test. A correctly sized dynamometer measures every operating point of the unit under test without being overloaded or running in its low-accuracy region.
This guide gives you a 7-step process used by our application engineers. For a side-by-side technology comparison, see our eddy current vs hysteresis vs powder dynamometer guide.
Step 1 — Define Peak Torque, Not Rated Torque
The dynamometer’s rated torque must equal or exceed the peak torque of the motor under test. Servo and EV traction motors deliver short-duration torque far above their continuous rating, and tests such as locked rotor, starting torque and overload capacity intentionally drive the motor to these limits. If your peak torque falls between two models, select the larger one.
Step 2 — Define Maximum Test Speed
The dynamometer maximum speed must exceed the highest speed in your test plan with adequate margin, including overspeed tests. Technology sets hard ceilings here: EconoTest eddy current dynamometers reach 50,000 RPM, hysteresis dynamometers reach 30,000 RPM (KB) or 25,000 RPM (HB), AC dynamometers cover 8,000–18,000 RPM, and magnetic powder dynamometers operate at 2,000–5,000 RPM. Above 50,000 RPM, a dedicated high-speed motor test bench (up to 100,000 RPM) is required.
Step 3 — Check Zero-Speed and Low-Speed Torque Requirements
If your test plan includes starting torque, cogging torque, locked rotor or static load holding, the dynamometer must produce full braking torque at 0 RPM. Hysteresis and magnetic powder dynamometers do this natively because their torque does not depend on rotor motion. Eddy current dynamometers generate no braking force at standstill — a frequent and expensive selection error.
Step 4 — Absorption Only, or Four-Quadrant?
Absorption dynamometers only brake. If you need to drive the unit under test — efficiency mapping in all four quadrants, regenerative braking simulation, hybrid powertrain work, back-driving a gearbox — specify a four-quadrant AC dynamometer (5–2,000 Nm, 0.75–250 kW). Its regenerative converter also returns braking energy to the grid, which materially lowers the running cost of long endurance tests.
Step 5 — Verify Continuous Power Absorption
Torque and speed define instantaneous load; the cooling system defines how long the dynamometer can sustain it. Compare your duty cycle against the model’s continuous power rating — for example, ECW eddy current models absorb from 120 W (ECW05KB) to 15,000 W (ECW300KB) continuously, and ECF magnetic powder models from 80 W (ECF2) to 15,000 W (ECF2000). Short-term ratings are higher, but endurance testing must respect the continuous figure.
Step 6 — Match the Cooling to Your Facility
All eddy current models are water-cooled. Magnetic powder models below 20 Nm are air-cooled; 20 Nm and above require water. Hysteresis models use air, air+water, water+blower or air+blower cooling depending on size. If your lab has no chilled water supply, this constraint alone can decide the technology.
Step 7 — Choose the Accuracy Class
EconoTest dynamometers offer ±0.2% FS (HB hysteresis series, AC dynamometers) and ±0.2–0.5% FS (eddy current, magnetic powder, KB hysteresis). Specify ±0.2% where efficiency-class certification or R&D-grade data is required; ±0.5% is typically sufficient — and more economical — for production line pass/fail testing.
Quick Selection Matrix
| Requirement | Recommended technology |
|---|---|
| Full torque at 0 RPM, ≤30 Nm, precision | Hysteresis (EC/ECO series) |
| High speed, up to 50,000 RPM | Eddy current (ECW series) |
| High torque to 2,000 Nm, ≤5,000 RPM, production line | Magnetic powder (ECF series) |
| Four-quadrant, efficiency mapping, energy recovery | AC dynamometer (EC5A–EC2000A) |
| Above 50,000 RPM or above 250 kW | High-speed motor test bench (up to 100,000 RPM / 800 kW) |
Unsure between two models? Send your motor datasheet to our engineers — RFQ responses within 24 hours via the contact page.
Frequently Asked Questions
How do I choose between a hysteresis, eddy current and magnetic powder dynamometer?
Choose by speed and zero-speed requirements: hysteresis dynamometers (0.1–30 Nm, up to 30,000 RPM) deliver full torque at zero speed and suit precision servo and BLDC testing; eddy current dynamometers (0.5–300 Nm, up to 50,000 RPM) suit high-speed testing but cannot load at zero RPM; magnetic powder dynamometers (2–2,000 Nm, 2,000–5,000 RPM) deliver full torque at zero speed and suit high-torque, low-speed production testing.
What dynamometer accuracy do I need for motor testing?
For R&D and efficiency certification, choose ±0.2% full scale or better (EconoTest HB hysteresis series and AC dynamometers achieve ±0.2%). For production line pass/fail testing, ±0.5% full scale (KB series) is usually sufficient and more cost-effective. Always express accuracy as a percentage of full scale and size the dynamometer so your measurements fall in the upper portion of its range.
Should the dynamometer torque rating match my motor’s rated torque or peak torque?
Size the dynamometer to the motor’s peak torque, not its continuous rated torque. Electric motors routinely deliver 2–3× rated torque during transients, and an undersized dynamometer cannot capture peak torque events and may be overloaded during locked rotor or starting torque tests.
When do I need a four-quadrant AC dynamometer instead of an absorption dynamometer?
You need a four-quadrant AC dynamometer when the test plan includes motoring the unit under test (driving it rather than only braking it): efficiency mapping across all operating quadrants, EV and hybrid powertrain simulation, regenerative braking tests, or back-driving gearboxes. If you only ever apply braking load, an absorption dynamometer (hysteresis, eddy current or magnetic powder) is more economical.
What information should I send a supplier to get the right dynamometer quote?
Provide six data points: motor type (PMSM, BLDC, ASM, servo, etc.), rated and peak torque, maximum test speed, rated power, required test items (efficiency map, cogging torque, durability, etc.), and available cooling utilities (water supply or air only). EconoTest engineers return a model recommendation within 24 hours.
