Determining RPM - will this work

[Reno]

While working on my batteries my mill is just sitting there. So fooling around I connected a DC motor to the rectified output just to feel like I was doing something. Anyway this got me thinking I one was to monitor the voltage of the windmill and relating to the rated DC voltage and Speed of a PM DC motor would this be a way to detemine RPM at a certain wind speed?

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!

[Reno]

Forgot a question, is the amperage (rating) linear with the rpm on a pmdc motor

thanks

[Chagrin]

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.

If you have access to the AC output of the genny you can use the frequency setting of a multimeter to get an exact answer. The AC frequency will be a multiplier of the genny's speed based on the number of coils/magnets you're using. This might also be possible by looking at the DC ripple - I'm not sure.

[Ungrounded Lightning Rod]

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.)