Example Using 100V DC motor rated at 3000 rpm
Voltage at rectifier connected to motor 10V DC
wind speed 10 mph
therefore 10mph wind turns mill at 300rpm
thoughts please!
The voltage is not perfectly linear, but it would be possible to approximate the rotational speed if you were able to create some form of a "lookup table". It's not the accurate way to go.
Well, actually:
- The voltage
- Across the armature of
- A DC motor
- with NO LOAD
IS linear with rotational speed.
In the sample case, if you unhooked the load and measured the voltage you could get the RPM.
Note that the rated voltage/RPM relationship is a little off - because the motor designed to spin at 3000 RPM when fed 100V would, at 3000 RPM, be designed to produce a back-EMF that is somewhat lower, so that back-EMF plus resistive losses totals to 100V. So you'd have to calibrate it to find out what the actual back-EMF is at 3000 RPM.
But if your calibration said that at 3000 RPM it produced 95v, then at 300 RPM it would produce 9.5 volts, and so on. So you only need to calibrate it at one speed.
The motor is behaving as a "tachometer generator" - exactly the sort of instrument you use for measuring RPM.
The problem is that, when in service. the motor IS under load. So the voltage measured is the back EMF from the rotation minus the I*R losses from the resistance of the armature, wiring, brushes, etc. (Under heavy current loads the brushes may not be linear, and the resistance of the windings changes with temperature.) You COULD try to figure out that loss, and compensate. But if you're really interested in measuring the speed under load, hook up a SECOND PM motor to drive your meter. (It can be a tiny one, since it only has to power the meter.)