Whilst working on a project to control and monitor an EC fan from a Raspberry Pi, I have encountered an anomaly that has me completely confused; the fan is still outputting pulses on the tacho line proportional to the input power when the actual fan itself is disconnected from its built-in control circuitry.
I've so far managed to implement an RPM monitor by listening to the pulses coming out of the fan's tacho connection, however the maximum RPM observed was less than the manufacturer claimed the max RPM of the fan was so I decided to double check how many pulses appeared on the tacho line per revolution of the fan blade. Whilst doing this, I disconnected the three wire connection (red, black, yellow) between the fan motor and its controller board. I then turned the bundled fan controller up and saw my RPM monitor respond as though the fan has spun up, which it had not.
- With the fan motor disconnected, adjusting the fan speed controller causes the tacho to respond as though the fan was turning.
- RPM ramps up and down with the same profile as a genuine fan tacho signal, i.e. has ramp up and ramp down as if the momentum of a physical fan blade were involved.
- With the fan motor reconnected to the control board, but the blades clamped stationary, the tacho line outputs no pulses, even when the speed controller is altered up and down.
My initial suspicion was that somehow the controller board was 'faking' the tacho signal, but then you have to wonder why would the manufacturer go to such lengths when a hall effect sensor is such a cheap and standard component, and how would the physical characteristics of a real fan be so accurately emulated if the signal was being faked?
I really hope someone who knows about these fans can shed some light.
Edit: Before anyone asks, yes the fan is absolutely, and without shadow of doubt, not turning.