Is fan speed proportional to PWM duty cycle?
No. Speed rises monotonically with duty on a healthy fan, but the map is rarely linear: most fans have a minimum duty floor (commonly 10–20%) below which they stall or run rough, a steep mid-range, and a flat top near 90–100% where speed saturates. Never assume 50% duty = 50% speed — measure the map and build the control table from it.
The shape of a typical map
Illustrative map for a 12 V fan (your model will differ): 0–15% duty = stopped or unstable; 20% = ~1,800 rpm; 40% = ~3,100 rpm; 60% = ~4,100 rpm; 80% = ~4,800 rpm; 100% = ~5,200 rpm. The mid-range is steep — small duty changes there cause large speed (and noise) steps — while the top flattens because the motor approaches its voltage limit at the rated speed.
| Duty | Typical speed (% of max) | Control note |
|---|---|---|
| 0–15% | 0 — stall floor | Set as off or min-speed region |
| 20% | ≈35% | Min reliable speed |
| 50% | ≈75% | Steep region — biggest gains |
| 100% | 100% | Voltage-limited saturation |
How to measure the map
With the fan on a stable supply, step duty 0→100% in 10% increments, waiting ~10 s at each step for speed to settle, and record tach-derived RPM. Then step downward — some fans show hysteresis at the stall floor (they keep spinning at a duty where a cold start would not begin). Repeat at your rail's minimum voltage: the whole map shifts down, and the stall floor rises.
- Measure up and down; log hysteresis at the floor.
- Repeat at worst-case supply voltage, not only nominal.
- Note the acoustic step points — they often align with map slope changes.
Designing the control table
Map thermal states to duties through measured speed, not assumed proportionality: pick the duty that achieves the required mass flow (from your system-curve work), add the floor as a permanent minimum if the product must never fully stop the fan, and rate-limit duty slew (a few percent per second) to avoid audible steps and current spikes. If the platform allows, calibrate each unit's map at end-of-line — maps vary unit to unit by a few percent.
Send MEGA Tech your airflow and static pressure target, voltage, size limit, control interface, quantity and environment. We reply with a selection proposal and a sample plan — selection by operating condition, samples validated on your unit.
WhatsApp +86 135 7056 7086 · Email inquiry@cnmegatech.com · Custom fan development