Hi, I am using a DC motor as an alternator to measure the power out of a steam turbine, I am measuring the Voltage and Current from the Alternator with it connected to a USB-6009 DAQ. I am hoping to measure the RPM using the Frequency of the Alternators Voltage (So I don't waste an I/O port).
The Voltage Signal is particularly messy so am not sure of the best way to clean it up for a clean RPM reading. If anyone has an Idea of the best way to do this it would be really appreciated!
What does the type DC motor tell you ?
You migth get some spikes from the commutator but see the motor as a DC current source .... and depending of the motor maybe a voltage or current ripple.
With the motor shortcutted , if you turn it by hand , do you feel force ripple?
you can use the analog input to measure a frequency, but a simple CNY70 with a reflector (or anything else that gives you 1 or n pulses per revolution) migth be a better sensor 😉
and withsome data in the graph hit edit-make current values default and post your vi. That way we have some data to play with 😄
You are using a DC motor as a generator not an alternator.
An alternator (like in your car) is actually an AC generator with diodes built in so it outputs a (usually 3 phase) rectified DC voltage.
So the frequency of the ripple will vary with rotational speed, the ripple frequency will also be 6x the RPM of the shaft. (3 phase rectified ripple)
A DC generator outputs a DC voltage that varies depending on the speed of the generator, so it's frequency will be zero hertz.
You might be able to approximate the speed by measuring the voltage level, but I doubt it will be linear or accurate.
One more thing to think about. While in theory a DC motor "spun backwards" becomes a DC generator.
You will find that not all DC motors make good DC generators, especially if they use field coils instead of permanent magnets.
Basically, what RTS said, you cannot expect an accurate RPM meassure from the data you are currently obtaining. If you really need it, it would be wise to invest in a RPM sensor with an analog (voltage or current) output an read that instead, there are some rather cheap ones that use a frequency-to-voltage converter that give a decent (roughly +-5%) acuraccy.
Do you use the dc motor only for the RPM measurement or do you want to mainly measure the output power? with what type of load??
Depending on the type of DC motor you maybe can do it, however a simple cny70 and a reflector or a magnet with a hall element on a counter input would give a better RPM signal.
Have a look at the spectrogram examples (STFT) (reading the help files help 😉 ) and analyse your signal with that tool. If there is an RPM information in the signal, you should be able to see it. (play with the colour settings, use db scale) Next step would be to try to make a robust algorythm to extract that information.
From an engineering point of view a dedicated sensor (with a dedicated input) gives a clear RPM signal , is easy and fast to realize. Your approach needs more software, more signal analyses skills ... so from an academic viewpoint it's a nice challenge.